Biology questions and answers A

Biology A

A

 What is ABO blood group ?

Blood group antibodies (A, B, AB, O) that may destroy red blood cells bearing the antigen to which they are directed; also called “agglutinins.” These red-cell antigens are the phenotypic expression of inherited genes, and the frequency of the four main groups varies in populations throughout the world. The antigens of the ABO system are an integral part of the red-cell membrane as well as all cells throughout the body and are the most important in transfusion practice.

What is abortion ?

The termination of gestation before the fetus can survive on its own.

What is abscisic acid (ABA) ?

A plant hormone (C15H20O4) and weak acid that generally acts to inhibit growth, induces dormancy, and helps the plant tolerate stressful conditions by closing stomata. Abscisic acid was named based on a belief that the hormone caused the abscission (shedding) of leaves from deciduous trees during the fall.

At times when a plant needs to slow down growth and assume a resting (dormant) stage, abscisic acid is produced in the terminal bud, which slows down growth and directs the leaf primordia to develop scales that protect the dormant bud during winter. Because the hormone also inhibits cell division in the vascular cambium, both primary and secondary growth are put on hold during winter.

This hormone also acts as a stress agent that helps a plant deal with adverse conditions. For example, ABA accumulates on leaves and causes stomata to close, reducing the loss of water when a plant begins to wilt

In 1963, abscisic acid was first identified and characterized by Frederick Addicott and colleagues. In 1965, the chemical structure of ABA was defined, and in 1967, it was formally called abscisic acid.

What is absorption spectrum ?

Different pigments absorb light of different wavelengths. For example, chlorophyll effectively absorbs blue and red. The absorption spectrum of a pigment is produced by examining, through the pigment and an instrument called a spectroscope, a continuous spectrum of radiation. The energies removed from the continuous spectrum by the absorbing pigment show up as black lines or bands and can be graphed.

What is abyssal zone ?

The portion of the ocean floor below 1,000–2,000 m (3,281–6,561 ft.), where light does not penetrate and where temperatures are cold and pressures are intense. It lies seaward of the continental slope and covers approximately 75 percent of the ocean floor. The temperature does not rise above 4°C. Because oxygen is present, a diverse community of invertebrates and fishes do exist, and some have adapted to harsh environments such as hydrothermal vents of volcanic creation. Food-producing organisms at this depth are chemoautotrophic prokaryotes and not photosynthetic producers.

What is acclimatization ?

Acclimatization is the progressive physiological adjustment or adaptation by an organism to a change in an environmental factor, such as temperature, or in conditions that would reduce the amount of oxygen to its cells. This adjustment can take place immediately or over a period of days or weeks. For example, the human body produces more erythrocytes (red blood cells) in response to low partial pressures of oxygen at high altitudes; short-term responses include shivering or sweating in warmblooded animals.

What is accommodation ?

The automatic reflex adjustment that allows the focal length of the lens of an eye to change to focus on an object. The lens shape, more convex for near objects and less convex for distant objects, is caused by ciliary muscles acting on the elastic property of the lens.

What is acetylcholine (ACh) ?

One of the most common neurotransmitters of the vertebrate nervous system, ACh is a chemical (CH3COOCH2CH2N+(CH3)3) that transmits impulses between the ends of two adjacent nerves or neuromuscular junctions. Released by nerve stimulation (exciting or inhibiting), it is confined largely to the parasympathetic nervous system, where it diffuses across the gap of the synapse and stimulates the adjacent nerve or muscle fiber. It rapidly becomes inactive by the enzyme cholinesterase, allowing further impulses to occur.

 What is acetyl CoA ?

A compound formed in the mitochondria when the thiol group (–SH) of coenzyme A combines with an acetyl group (CH3CO–). It is important in the Krebs cycle in cellular respiration and plays a role in the synthesis and oxidation of fatty acids.

A compound formed in the mitochondria when the thiol group (–SH) of coenzyme A combines with an acetyl group (CH3CO–). It is important in the Krebs cycle in cellular respiration and plays a role in the synthesis and oxidation of fatty acids.

What is Acid ?

A chemical capable of donating a HYDRON (proton, H+) or capable of forming a covalent bond with an electron pair. An acid increases the hydrogen ion concentration in a solution, and it can react with certain metals, such as zinc, to form hydrogen gas. A strong acid is a relatively good conductor of electricity. Examples of strong acids are hydrochloric (muriatic), nitric, sulfuric, while examples of mild acids are sulfurous and acetic (vinegar). The strength of an acidic solution is usually measured in terms of its pH (a logarithmic function of the H+ ion concentration). Strong acid solutions have low pHs (typically around 0–3), while weak acid solutions have pHs in the range 3–6.

What is acidity constant ?

The equilibrium constant for splitting off a HYDRON from a BRØNSTED ACID.

What is acid-labile sulfide ?

Refers to sulfide LIGANDs, e.g., the BRIDGING LIGANDs in IRON–SULFUR PROTEINS, which are released as H2S at acid pH.

What is acid precipitation ?

Because pure precipitation (e.g., rain) is slightly acidic (due to the reaction between water droplets and carbon dioxide, creating carbonic acid) with a potential pH of 5.6, acid precipitation refers to precipitation with a pH less than 5.6. Acid precipitation includes rain, fog, snow, and dry deposition. Anthropogenic (man-made) pollutants (carbon dioxide, carbon monoxide, ozone, nitrogen and sulfur oxides, and hydrocarbons) react with water vapor to produce acid precipitation. These pollutants come primarily from burning coal and other fossil fuels. Sulfur dioxide, which reacts readily with water vapor and droplets (i.e., has a short residence time in the atmosphere as a gas), has been linked to the weathering (eating away) of marble structures and the acidification of freshwater lakes (consequently killing fish). Natural interactions within the biosphere can also lead to acid precipitation.

What is acoelomate ?

A solid-bodied animal lacking a body cavity, the space between the gut (digestive tract) and body wall. Simple animals do not have a body cavity as higher animals do; this body cavity is called a coelom in mammals and contains the gut (a cavity by itself), heart, and lungs, for example. Acoelomates are bilateral animals and are triploblastic (have three layers: ectoderm, endoderm, and mesoderm). They can move forward and have a degree of cephalization (centralization of neural and sensory organs in the head).

Representative phyla of acoelomates are the Platyhelminthes: flatworms that include the Turbellaria (nonconfined flatworms such as planarians), Monogenea (monogeneans), Trematoda (trematodes, or flukes), and Cestoidea (tapeworms). There are more than 20,000 species of flatworms living in wet environments such as marine or freshwater bodies and damp terrestrial areas.

What is aconitase ?

A name for citrate (isocitrate) hydro-LYASE (aconitate hydratase), which catalyzes the interconversion of citrate, cis-aconitate ((Z)-prop-1-ene-1,2,3-tricarboxylate), and isocitrate. The active ENZYME contains a catalytic [4FE-4S] CLUSTER.

What is acrosome ?

The acrosome is a special area or compartment that is located at the tip of the head of a sperm cell. It contains special digestive enzymes that on contact with the egg help the sperm head penetrate the egg for fertilization. Directly behind the acrosome is the haploid nucleus (single set of unpaired chromosomes) that contains the genetic material.

What is actin ?

A globular protein found in muscle tissue as thin filaments and in microfilaments that form portions of cell cytoskeletons. Actin links into chains, and paired chains twist helically around each other, forming microfilaments in muscle and other contractile elements in cells. Actin and myosin filaments interact to initiate muscle contraction.

Tropomyosin and troponin are two protein molecules associated with actin filaments in muscle. Tropomyosin runs along the length of the actin filament and covers the area of the actin molecule that interacts with myosin when at rest. On the other hand, when a muscle is contracted, tropomyosin is replaced with troponin as it binds to calcium ions. Troponin is located at regular intervals along the actin filament and allows actin to interact with myosin.

What is actinomorphic ?

Symmetrical over more than one vertical plane; e.g., flowers that can be separated into symmetrical halves along any plane.

What is action potential ?

A localized rapid change in voltage that occurs across the membrane of a muscle or nerve cell when a nerve impulse is initiated. It is caused by a physicochemical change in the membrane during the interaction of the flow and exchange of sodium and potassium ions.

What is Active tranport ?

 The movement of a substance across a biological membrane, such as living cells, against a concentration (diffusion) gradient with the help of metabolic energy, usually provided by ATP (adenosine triphosphate). Active transport serves to maintain the normal balance of ions in cells, in particular ions of sodium and potassium, which play a vital role in nerve and muscle cells. Because a molecule is “pumped” across the membrane against its gradient with the help of metabolic energy, it is referred to as “active” transport.

The sodium–potassium “pump” that exchanges sodium (Na+) for potassium (K+) across the plasma membrane of animal cells is an example of the active transport mechanism.

It is the carriage of a solute across a biological membrane from low to high concentration that requires the expenditure of metabolic energy

What is adaptive radiation ?

The process where a population of plants or animals evolves into a number of different ones over time, usually as a response to multiplying and living under different environmental conditions. Subpopulations from the common ancestor develop as a response to adapting to the new environmental conditions, and new species evolve from this original parent stock.

Impressive rapid adaptive radiations have occurred over time after mass extinctions caused by cataclysmic episodes on the Earth. Plate tectonics, volcanism, and possible Earth-comet-asteroid collisions all have wiped the landscape clean, allowing survivors and new species to rapidly fill the voids of these new adaptive zones.

What is address-message concept ?

 Refers to compounds in which part of the molecule is required for binding (address) and part for the biological action (message).

What is adenosine 5’-triphosphate (ATP) ?

Key NUCLEOTIDE in energy-dependent cellular reactions, in combination with Mg(II). The reaction: ATP + water ➔ ADP + phosphate is used to supply the necessary energy

What is adenylyl cyclase ?

e An enzyme, embedded in the plasma membrane, that converts ATP to cyclic adenosine monophosphate (cyclic AMP, or cAMP) in response to a chemical signal. It is activated when a signal molecule binds to a membrane receptor. Cyclic AMP acts as a second messenger, relaying the signal from the membrane to the metabolic machinery of the cytoplasm.

What is adrenal glands ?

A pair of small triangular endocrine glands (one above each kidney in animals) that are ductless and secrete hormones into the blood. The glands are composed of two portions. The adrenal cortex, which forms an outer shell on each and is controlled by the pituitary gland, responds to endocrine signals in reacting to stress and homeostatic conditions by (a) secreting steroid hormones (corticosteroid, cortisol, and aldosterone) that deal with carbohydrate metabolism and with salt and water balance (electrolyte metabolism), such as the reabsorption of water by the kidneys, and (b) releasing androgens (male sex hormone) and estrogens (female sex hormone).

The adrenal cortex surrounds the central medulla, is controlled by the nervous system, and responds to nervous inputs resulting from stress and produces adrenaline and noradrenaline, hormones that increase blood sugar level and reduce body fat. The adrenal glands are also known as the suprarenal glands.

What is adrenodoxin ?

A [2FE-2S] FERREDOXIN involved in electron transfer from NADPH+ (the reduced form of NADP [nicotinamide adenine dinucleotide phosphate, a coenzyme]), via a REDUCTASE, to CYTOCHROME P-450 in the adrenal gland

 Who was Adrian, Edgar Douglas ?

Adrian, Edgar Douglas (1889–1977) British Physiologist Edgar Douglas Adrian was born on November 30, 1889, in London to Alfred Douglas Adrian, a legal adviser to the British Local Government Board. He attended the Westminster School, London, and in 1908 enrolled at Trinity College, Cambridge. At Cambridge University, he studied physiology, receiving a bachelor’s degree in 1911.

In 1913 he entered Trinity College, studied medicine, did his clinical work at St. Bartholomew’s Hospital, London, and received his M.D. in 1915.

In 1929 he was elected Foulerton professor of the Royal Society and in 1937 became professor of physiology at the University of Cambridge until 1951, when he was elected master of Trinity College, Cambridge. He was chancellor of the university from 1968 until two years before his death.

He spent most of his research studying the physiology of the human nervous system, particularly the brain, and how neurons send messages. In 1932 he shared the Nobel Prize in physiology or medicine for his work on the function of the neuron. He is considered one of the founders of modern neurophysiology.

He wrote three books, The Basis of Sensation (1927), The Mechanism of Nervous Action (1932), and The Physical Basis of Perception (1947), and was knighted baron of Cambridge in 1955. He died on August 4, 1977, and is buried at Trinity College.

 What is Acrobic ?

Any organism, environmental condition, or cellular process that requires atmospheric oxygen. Aerobic microorganisms, called aerobes, require the presence of oxygen for growth. An aerobe is capable of using oxygen as a terminal electron acceptor and can tolerate oxygen levels higher than that present in the air (21 percent oxygen). They have a respiratory type of metabolism, and some aerobes are also capable of growing anaerobically with electron accepters other than oxygen.

 What is affinity ?

The tendency of a molecule to associate with another. The affinity of a DRUG is its ability to bind to its biological target (RECEPTOR, ENZYME, transport system, etc.). For pharmacological receptors, it can be thought of as the frequency with which the drug, when brought into the proximity of a receptor by diffusion, will reside at a position of minimum free energy within the force field of that receptor.

For an AGONIST (or for an ANTAGONIST), the numerical representation of affinity is the reciprocal of the equilibrium dissociation constant of the ligand–receptor complex, denoted KA, calculated as the rate constant for offset (k–1) divided by the rate constant for onset (k1).

 What is Age structure ?

The relative number of individuals of each age in a population, or the composition of a country by age groups. Since generations coexist over a time period, an age structure develops and is important in foreseeing the growth rate of an entire population. Except for the “baby boom” generation of the late 1940s–50s, the United States has a pretty even age distribution.

 What is Agnatha ?

 A superclass family of jawless vertebrate fish that probably originated during the late Precambrian or early Cambrian. This superclass is the source of the oldest vertebrate fossils, dating some 465 million years ago during the Paleozoic era. Early agnathans, such as the now extinct ostracoderms, were encased in bony plates. While most were small, no larger than 20 in., they lacked paired fins and had circular mouths or slits with no jaws, although there were exceptions that had paired fins. Agnathans were most likely bottomdwelling mud suckers or suspension feeders, taking in food through the mouth and then trapped in the gill, which also functioned as the area for gas exchange.

Only about 60 species comprising two classes of agnathans exist today, while the rest declined and disappeared during the Devonian period. Two classes, the Myxini (hagfishes) and Cephalaspidormorphi (lampreys), are all that remain.

Both hagfishes and lampreys lack paired appendages and lack body armor. Hagfishes are scavengers, living only in salt water and feeding on dead fish or marine worms, and lack a larval stage. Lampreys use their round mouth and a rasping tongue to latch on the side of a fish, penetrate the skin, and ingest its blood. As larvae, they live in freshwater streams, are suspension feeders, and migrate to the sea and lakes when they become adults. Some species only feed while in the larval stage, reproduce, and die. The agnathans are considered the most primitive living vertebrates known today.

 What is agonist ?

An endogenous substance or a DRUG that can interact with a RECEPTOR and initiate a physiological or a pharmacological response characteristic of that RECEPTOR (contraction, relaxation, secretion, ENZYME activation, etc.).

 What is agonistic behavior ?

This behavior usually involves two animals in a competitive contest, which can be in the form of combat, threat, or ritual, for food, a sexual partner, or other need. The end result is one becoming a victor while the other surrenders or becomes submissive, both exhibiting different traits. When one surrenders, it stops the combat because the continued battle could end up injuring both. Likewise, any future combat between the two individuals will likely end with the same result as the first and will not last as long.

Many animal social groups are maintained by agonistic behavior where one individual becomes dominant, others become subdominant, and so on down the line, each controlling the others in a dominance hierarchy or “pecking order.” This dominant behavior can be used to control access to food or mates. Chickens, gorillas, and wolves are good examples of social groups maintained by dominance

Agonistic behavior is used to defend territories, areas that a dominant individual will defend for feeding, mating, rearing, or any combination of these activities.

What is AIDS (acquired immunodeficiency syndrome) ?

 AIDS was first reported in 1981 in the United States and has since become a major epidemic, killing nearly 12 million people and infecting more than 30 million others worldwide. The disease is caused by HIV, a virus that destroys the body’s ability to fight infections and certain cancers

HIV-infected white blood cell. Scanning electron micrograph (SEM) of the abnormal surface of a white blood cell infected with HIV virus that causes AIDS. Glycosaminoglycan (GAG) gene expression for cell surface proteins is abnormal. The HIV virus’s genetic material controls the cell, forcing it to express abnormal structural proteins. Normally these surface GAG proteins are tiny spheres, but here the cell’s outer surface is formed from large irregular rods. HIV causes AIDS (acquired immunodeficiency syndrome). AIDS destroys white blood cells, leading to a weakened immune system. Magnification: × 6600 at 4.5 × 5.5 inch size.

AIDS is the name given to the late stages of HIV infection, first discovered in 1981 in Los Angeles, California. By 1983 the retrovirus responsible for it, the human immunodeficiency virus (HIV), was first described, and since then millions around the world have died from contracting the disease. It is thought to have originated in central Africa from monkeys or to have developed from contaminated vaccines used in the world’s first mass immunization for polio.

AIDS is acquired mostly by sexual contact either through homo- or heterosexual practice by having unprotected sex via vaginal or anal intercourse. The routes of infection include infected blood, semen, and vaginal fluid. The virus can also be transmitted by blood by-products, through maternofetal infection (where the virus is transmitted by an infected mother to the unborn child in the uterus), or by maternal blood during parturition, or by breast milk consumption upon birth. Intravenous drug abuse also is a cause.

The virus destroys a subgroup of lymphocytes, essential for combating infections, known as the helper T cells, or CD4 lymphocytes, and suppresses the body’s immune system, leaving it prone to infection.

Infection by the virus produces antibodies, but not all those exposed develop chronic infection. For those that do, AIDS or AIDS-related complex (ARC) bring on a variety of ailments involving the lymph nodes, intermittent fever, loss of weight, diarrheas, fatigue, pneumonia, and tumors. A person infected, known as HIV-positive, can remain disease-free for up to 10 years, as the virus can remain dormant before fullblown AIDS develops.

While HIV has been isolated from bodily fluids such as semen to breast milk, the virus does not survive outside the body, and it is considered highly unlikely that ordinary social contact can spread the disease. However, the medical profession has developed high standards to deal with handling blood, blood products, and body fluids from HIV-infected people.

In the early discovery stage of the disease, AIDS was almost certainly fatal, but the development of antiviral drugs, such as zidovudine (AZT), didanosine (ddl), zalcitabine (ddc), lamivudine (3TC), stavudine (DAT), and protease inhibitors used in combination with the others, has showed promise in slowing or eradicating the disease. Initial problems with finding a cure have to do with the fact that glycoproteins encasing the virus display a great deal of variability in their amino acid sequences, making it difficult to prepare a specific AIDS vaccine.

During the 1980s and 1990s, an AIDS epidemic brought considerable media coverage to the disease, especially as well-known celebrities such as actors Rock Hudson and Anthony Perkins, Liberace, and others died from it. Hudson was the first to admit having the disease in 1985. During the 1980s and 1990s, the homosexual community became active in lobbying for funds to study the disease, as it early on was considered simply a “gay” disease. ACT UP, acronym for the AIDS Coalition to Unleash Power, began as a grassroots AIDS organization associated with nonviolent civil disobedience in 1987. ACT UP became the standard-bearer for protest against governmental and societal indifference to the AIDS epidemic. Public attitude changed when heterosexuals became infected, and greater education on the causes of the disease became more widespread, initiated by celebrities such as Elizabeth Taylor and the American Foundation for AIDS Research, where fundraising activities made national news coverage.

There have been significant advances in the treatment for HIV/AIDS by attacking the virus itself, strengthening the immune system, and controlling AIDS-related cancers and opportunistic infections. At present, there is still no cure or vaccine.

What is albumin ?

A type of protein, especially a protein of blood PLASMA, that transports various substances, including metal ions, drugs, and XENOBIOTICs.

What is aldehydes ?

Aldehydes are organic chemicals that contain the –CHO (aldehyde) group, a carbonyl group (C=O) that has the carbon and hydrogen atom bound. They are the result of the oxidation of alcohols and, when further oxidized, form carboxylic acids. Methanal (formaldehyde) and ethanal (acetaldehyde) are common examples.

What is aldosterone ?

An adrenal steroid hormone, derived from cholesterol, that is secreted by the adrenal cortex and acts on the distal tubules of the kidney to control the elimination of salts and water by the kidneys. The presence of the pituitary hormone ACTH, changing levels of sodium and potassium, and variations in blood volume stimulate the production of aldosterone by the cortex.

Aldosterone is a component of the renninangiotensin-aldosterone system (RAAS) that is a complex feedback system that functions in homeostasis.

What is algae (singular, alga) ?

A large and diverse group of photosynthetic organisms formerly called simple plants but now members of their own phyla, the Protoctista, that also includes the slime molds and protozoa. Algae, some 17,000 species or more, live in aquatic and moist inland regions. They do not have roots, stems, or leaves and have no vascular water-conducting systems. They reproduce by spores, and in some species the spores are mobile with the use of flagella. They range from simple single cells (e.g., Euglena) to “plants” many feet long (e.g., kelps such as Macrocytis) and make up marine seaweed and much of the plankton that provide food for other species.

Unusual growth outbursts result in “algal blooms” or “red tides” and occur when there is an increase in nutrient levels in a body of water.

Cyanophyta, or blue-green algae, are now classified as cyanobacteria. The phylum Cyanophyta also includes chloroxybacteria. Cyanophytes contain phycocyanin, a photosynthetic pigment giving them a blue color. The red pigment phycoerythrin is also almost always present. They are diverse and can live as single cells or as colonies or large filaments. Some are nitrogen fixers in soil and others, like lichens, display symbiosis with a fungus, usually with a member of Ascomycota. The fungus provides the host plant for the algae cells that are distributed throughout and provide food to the fungus while the fungus protects the algae cells. While many lichen have a different fungi component, they often have the same algae species. Some common species of lichens are British soldiers (Cladonia cristatella), pixie cups (Cladonia grayi), cedar lichen (Vulpicidia viridis), wrinkled shield lichen (Flavoparmelia caperata), and reindeer lichen (Cladina subtenuis).

The chloroxybacteria, or green-grass bacteria, contain both chlorophylls a and b but do not contain the red or blue pigments of the blue-green algae. They are nonmotile, aerobic organisms.

Other phyla of algae include the Bacillariophyceae, comprising the diatoms; Charophyceae, fresh- or brackish- water algae that resemble bryophytes; Chlorophyceae, or green algae believed to be the progenitor of plants; Chrysophyceae, or yellow-green algae; Dinophyceae, unicellular algae with two flagella; Phaeophyceae, or brown algae; and Rhodophyceae, or red algae.

The Bacillariophyceae or diatoms are unicellular algae that are found in single, colonial, or filamentous states. Under the microscope they often are beautifully symmetrical, as their cell walls, or frustules, are composed of silica and are bivalved, one of which overlaps the other, and the frustule is often punctated and ornamented. The two orders, Centrales and Pennales, occupy two different environments. The centric diatoms (Centrales) are circular in shape with radial symmetry and live mostly in marine environments. The pennate diatoms (Pennales) are elliptical in shape, have bilateral symmetry, and are found in freshwater environments.

Deposits of fossil diatoms known as diatomaceous earth have been mined and used for years in paints, abrasives, and other products such as chalk. The famous White Cliffs of Dover in England (rising to 300 feet) are composed of massive amounts of diatoms— coccoliths—that were laid down some 790 million years ago when Great Britain was submerged by a shallow sea.

The Charophyceae, also known as stoneworts, and which resemble bryophytes, live in fresh and brackish water especially rich in calcium, where they become stiff and lime encrusted. The stoneworts consist of a complex branched thallus with an erect stemlike structure and many whorls of short branches.

The Chlorophyceae (or green algae) are the closest to plants in pigment composition and structure and are related based on a common ancestor. More than 7,000 species live in freshwater and marine environments as unicellular parts of plankton, in damp soil and even snow as colonies or filaments, symbiotically with other eukaryotes, or mutually with fungi as lichens.

The Chrysophyceae or golden algae, named because of their yellow and brown carotene and xanthophyll pigments, are typically cells with two flagella at one end of the cell, and many live among freshwater and marine plankton. While most are unicellular, a few are colonial.

The Dinophyceae or dinoflagellates are algae that are unicellular with two flagella of unequal length contained in channels on the cell surface. They can change shape with different water temperatures and are very tolerant of chemical and physical conditions.

The Phaeophyceae or brown algae are the largest and most complex of the algae. All members are multicellular, and the majority live in marine environments, especially common in cool water along temperate coastlines. Many of the marine seaweeds are brown algae.

The Rhodophyceae or red algae are more recent and have lost their flagellate stages in their life cycle. Some species are actually black and not red, as those that live deeper in waters, because of different levels of the pigment phycoerythrin. While most are found in warm coastal waters of the tropical oceans, some also live in freshwater and on land in the soil. Most rhodophytes are multicellular, sharing seaweed status with brown algae.Because the various forms of algae are now assigned to different phyla, the words alga and algae are used informally and have no taxonomic status.

 What is allantois ?

During the embryonic stage of mammals, birds, and reptiles, the allantois, a small sac, is one of four extra-embryonic membranes (along with amnion, yolk sac, chorion) and serves several functions, such as a repository for the embryo’s nitrogenous waste (chiefly uric acid) in reptiles and birds (in the egg). The allantois provides oxygen to mammals, birds, and reptiles, as well as food in mammals (via the placenta).

The membrane of the allantois works with the chorion in respiratory functions, allowing the exchange of gases between the embryo and surrounding air. In humans, it is involved in the development of the urinary bladder.

 What is allele ?

An allele is one of two or more alternative forms of a gene that can exist at a single locus. Each allele therefore has a unique nucleotide sequence and may lead to different phenotypes for a given trait. If the alleles for a gene are identical, the organism is called homozygous. If the alleles are different, the organism is heterozygous.

If two alleles are different, one becomes dominant and is fully expressed in appearance in the organism, while the other is recessive and has no noticeable effect on the appearance of the organism.

This is shown in the color of your eyes, determined by the genes inherited from your parents. The gene for brown eyes is dominant and overrides genes for other eye colors. The gene for blue eyes is recessive and will appear when there are no genes for other eye colors. A person with brown eyes may have a recessive, or “hidden,” gene for blue eyes. Therefore, two brown-eyed parents may each give a recessive gene for blue eyes to their child, who would then have blue eyes. Gray, green, and other eye colors result from a complex mixing of different eye color genes.

What is Allen’s Rule ?

In warm-blooded animals, the warmer the climate, the longer the appendages (ears, legs, wings) as compared with closely related taxa from colder areas.

 What is allometric growth ?

The variation in the relative rates of growth of various parts of the body, which helps shape the organism. In other words, it is the pattern of growth whereby different parts of the body grow at different rates with respect to each other. Allometry is the study of relative growth and of changes in proportion with increase in size. For example, human arms and legs grow at a faster rate than the body and head, making adult proportions strikingly different from those of infants. Another striking example is the male fiddler crab Uca pugnax. In small males, the two claws are of equal weight, each weighing about 8 percent of the total crab weight. However, as the crab enlarges, its large crushing claw grows more rapidly, eventually constituting about 38 percent of the crab’s weight.

In 1932, Sir Julian Huxley described a simple mathematical method for the detection and measurement of the allometric growth. In order to compare the relative growth of two components (one of which may be the whole body), they are plotted logarithmically on x- and y-axes:

log y = log b + k log x

The slope of the resulting regression is called the allometric growth ratio, often designated as k.

k = 1, both components are growing at the same rate.

k < 1, the component represented on the y-axis is growing more slowly than the component on the xaxis.

k > 1, the y-axis component is growing faster than the x-axis component

Another formula for measuring allometric growth is Y=bxa, where Y is equal to the mass of the organ, x = mass of the organism, a = growth coefficient of the organ, and b = a constant.

Yet another formula for measuring allometric growth is Y = bxa/c, where a and c are the growth rates for two body parts.

Allometric growth studies have also been applied to animal husbandry, archaeology, and urban systems studies.

What is allometry ?

The study of relative growth and of changes in proportion with increase in size.

 What is allopatric speciation ?

One of two methods of speciation (the other is sympatric), allopatric speciation happens when the ancestral population becomes segregated by a geographical barrier. The Karner blue butterfly (Lycaeides melissa samualis) became allopatric from its parent the Melissa blue butterfly (Lycaeides melissa melissa) when the climate changed and restricted various populations along its range to northeastern pine barrens environments several thousand years ago. As populations become isolated, the isolated gene pools accumulate different genetic traits by microevolution. Small populations are more likely to evolve into separate species than larger isolated populations. Several populations of the Karner blue butterfly are now separated from each other by human-made development and may be evolving into separate subspecies or species, even though geographically they are isolated by only a few miles in some cases.

Conditions that favor allopatric speciation are when one population becomes isolated at the fringe of the parent population’s range. This splinter population, called a peripheral isolate, is likely to become allopatric because the gene pool of the isolate may already be different, since living on the border of the range encourages the expression of the extremes of any genotypic CLINEs that existed in the original population. Furthermore, if the population is small enough, a FOUNDER EFFECT will occur, giving rise to a gene pool that is not that of the parent population.

Genetic drift will also occur until the peripheral isolate becomes a larger population and will continue to change the gene pool at random until the population grows. New mutations or combinations of existing alleles that are neutral in adaptive value now may become fixed in the population by chance, causing genotypic or phenotypic divergence from the parent population. For example, the Karner blue butterfly has a row of orange spots on the top of the hindwing, whereas, the ancestral parent, the Melissa blue butterfly, has orange spots on the top of both front and hindwing, a phenotypic variation.

Another factor in causing allopatric speciation is that evolution via natural selection may take a different road in the peripheral population. The isolate will allopatric speciation 11 encounter selection factors that are different from and perhaps more severe than that experienced by the parent because the isolate is living in an environment slightly, or completely, different from that of the parent. These small isolated populations are not guaranteed to become new species, as they are more often likely to become extinct, yet it is clear in evolutionary history that allopatric speciation does occur.

 What is allopolyploid ?

A type of polyploid (having a nucleus that contains more than two sets of chromosomes) species, often a plant, resulting from two different species interbreeding and combining their chromosomes. Hybrids are often sterile because they do not have sets of homologous chromosomes, making pairing nonexistent unless two diploid hybrids double the chromosome numbers, resulting in a fertile allotetraploid that now contains two sets of homologous chromosomes. Plant breeders find that this is beneficial, since it is possible to breed the advantages of different species into one. Triticale (a “new” grain created by crossing rye and durum wheat) is an allopolyploid that was developed from wheat and rye. Some crops are naturally allopolyploid, such as cotton, oats, tall fescue, potatoes, wheat, and tobacco. It is estimated that half of all angiosperms (flowering plants) are polyploid.

What is allosteric binding ?

allosteric binding sites A type of binding site contained in many ENZYMEs and RECEPTORs. As a consequence of the binding to allosteric binding sites, the interaction with the normal ligand (ligands are molecules that bind to proteins) may be either enhanced or reduced. Ligand binding can change the shape of a protein.

What is allosteric effector ?

Specific small molecules that bind to a protein at a site other than a catalytic site and that modulate (by activation or INHIBITION) the biological activity.

STRATE for catalysis, where a small regulatory molecule binds and affects that catalytic activity. This effector molecule may be structurally unrelated to the substrate, or it may be a second molecule of substrate. If the catalytic activity is enhanced by binding, the effector is called an activator; if it is diminished, the effector is called an INHIBITOR.

What is allosteric regulation ?

The regulation of the activity of allosteric ENZYMEs.

What is allosteric site ?

A specific receptor site on an enzyme molecule not on the active site (the site on the surface of an enzyme molecule that binds the substrate molecule). Molecules bind to the allosteric site and change the shape of the active site, either enabling the substrate to bind to the active site or prevent the binding of the substrate.

The molecule that binds to the allosteric site is an inhibitor because it causes a change in the three-dimensional structure of the enzyme that prevents the substrate from binding to the active site.

What is allozyme ?

An enzyme form, a variant of the same enzyme (protein) that is coded for by different alleles at a single locus.

What is alpha helix ?

Most proteins contain one or more stretches of amino acids that take on a particular shape in three-dimensional space. The most common forms are alpha helix and beta sheet.

Alpha helix is spiral shaped, constituting one form of the secondary structure of proteins, arising from a specific hydrogen-bonding structure; the carbonyl group (–C=O) of each peptide bond extends parallel to the axis of the helix and points directly at the –N–H group of the peptide bond four amino acids below it in the helix. A hydrogen bond forms between them [–N–H . . . . . O=C–] and plays a role in stabilizing the helix conformation. The alpha helix is right-handed and twists clockwise, like a corkscrew, and makes a complete turn every 3.6 amino acids. The distance between two turns is 0.54 nm. However, an alpha helix can also be left-handed. Most enzymes contain sections of alpha helix.

The alpha helix was discovered by Linus Pauling in 1948.

What is alternation of generations ?

A life cycle in plants where there is both a multicellular diploid form (the sporophyte generation) and a multicellular haploid form (the gametophyte generation).

Gametophytes produce haploid gametes that fuse zygotes that are forming. These zygotes then develop into diploid sporophytes. Meiosis in the sporophytes produces haploid spores, with division by meiosis giving rise to the next generation of gametophytes.

Alternation of generations occurs in plants and certain species of ALGAE. Ferns and fern allies (such as the club moss) are common examples that display alternation of generations. The above ground parent fern plant (the diploid sporophyte, or spore-bearing plant) has two full sets of chromosomes (two of each kind of chromosome). It sheds its single-celled haploid spores, having one set of chromosomes (one of each kind), which fall to the ground, and these in turn grow into a different plant, the gametophyte or prothallus, also haploid. The gametophyte has special bodies within the plant called archegonia (female cells) and antheridia (male cells). Here sexual fertilization takes place, and a new diploid sporophyte then grows.

There are four main groups of plants considered to be “fern allies,” a diverse group of vascular plants that are neither flowering plants nor ferns and that reproduce by shedding spore to initiate an alternation of generations. These are the Lycophyta (Lycopsida, the club mosses; Selaginellopsida, the spike mosses; and Isoëtopsida, the quillworts); the Archeophyta (Sphenopsida, the horsetails and scouring-rushes; Psilopsida, the whiskbrooms; and Ophioglossopsida, the adder’s-tongues and grape-ferns); the Pteridophyta (ferns); and Spermatophyta (flowering plants).

In some examples of alternation of generations—for example, in certain algae species such as in some green or brown forms—the alternation of generations takes on two different approaches. Where the sporophytes and gametophytes are structurally different, the two generations are heteromorphic. If the sporophytes and gametophytes look the same and have different chromosome pairs, the generations are said to be isomorphic.

What is altruistic behavior ?

The aiding of another individual at one’s own risk or expense. This can be in the form of one animal sending out a distress call to warn others of impending trouble, although putting itself in danger by giving out its location. Strangers coming to the rescue of other strangers, such as victims in an accident, hurricane, or earthquake, is another example of altruistic behavior.

What is alveolus (plural, alveoli) ?

Latin for “hollow cavity.” There are several definitions for alveolus. It is a thin, multilobed air sac that exchanges gases in the lungs of mammals and reptiles at the end of each bronchiole, a very fine respiratory tube in the lungs. An alveolus is lined with many blood capillaries where the exchange of carbon dioxide and oxygen takes place.

It is also the name given to the socket in the jawbone in which a tooth is rooted by means of the periodontal membrane, the connective tissue that surrounds the root and anchors it.

Furthermore, it is the term used to describe a single hexagonal beehive cell found in a honeycomb. It is also the term that refers to the milk-secreting sacs of the mammary gland.

 What is ambidentate ?

LIGANDs, such as (NCS)–, that can bond to a CENTRAL ATOM through either of two or more donor atoms.

What is amicyanin ?

An ELECTRON TRANSFER PROTEIN containing a TYPE 1 COPPER site, isolated from certain bacteria.

What is amino acid ?

An organic molecule possessing both acidic carboxylic acid (–COOH) and basic amino (–NH2) groups attached to the same tetrahedral carbon atom.

Amino acids are the principal building blocks of proteins and enzymes. They are incorporated into amino acid 13 proteins by transfer RNA according to the genetic code while messenger RNA is being decoded by ribosomes. The amino acid content dictates the spatial and biochemical properties of the protein or enzyme during and after the final assembly of a protein. Amino acids have an average molecular weight of about 135 daltons. While more than 50 have been discovered, 20 are essential for making proteins, long chains of bonded amino acids.

Some naturally occurring amino acids are alanine, arginine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine.

The two classes of amino acids that exist are based on whether the R-group is hydrophobic or hydrophilic. Hydrophobic or nonpolar amino acids tend to repel the aqueous environment and are located mostly in the interior of proteins. They do not ionize or participate in the formation of hydrogen bonds. On the other hand, the hydrophilic or polar amino acids tend to interact with the aqueous environment, are usually involved in the formation of hydrogen bonds, and are usually found on the exterior surfaces of proteins or in their reactive centers. It is for this reason that certain amino acid R-groups allow enzyme reactions to occur

The hydrophilic amino acids can be further subdivided into polar with no charge, polar with negatively charged side chains (acidic), and polar with positively charged side chains (basic).

While all amino acids share some structural similarities, it is the side groups, or “R”-groups as they are called, that make the various amino acids chemically and physically different from each other so that they react differently with the environment. These groupings, found among the 20 naturally occurring amino acids, are ionic (aspartic acid, arginine, glutamic acid, lysine, and histidine), polar (asparagine, serine, threonine, cysteine, tyrosine, and glutamine), and nonpolar amino acids (alanine, glycine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, and proline).

Amino acids are also referred to as amphoteric, meaning they can react with both acids and alkali, which makes them effective buffers in biological systems. A buffer is a solution where the pH usually stays constant when an acid or base is added.

In 1986 scientists found a 21st amino acid, selenocysteine. In 2002 two teams of researchers from Ohio State University identified the 22nd genetically encoded amino acid, called pyrrolysine, a discovery that is the biological equivalent of physicists finding a new fundamental particle or chemists discovering a new element.

Amino acid supplements are widely used in exercise and dietary programs.

What is amino acid residue (in a polypeptide) ?

When two or more amino acids combine to form a peptide, the elements of water are removed, and what remains of each amino acid is called amino acid residue. Amino acid residues are therefore structures that lack a hydrogen atom of the amino group (–NH–CHR–COOH), or the hydroxy moiety of the carboxy group (NH2–CHR–CO–), or both (–NH–CHR–CO–); all units of a peptide chain are therefore amino acid residues. (Residues of amino acids that contain two amino groups or two carboxy groups may be joined by isopeptide bonds, and so may not have the formulas shown.) The residue in a peptide that has an amino group that is free, or at least not acylated by another amino acid residue (it may, for example, be acylated or formylated), is called N-terminal; it is the Nterminus. The residue that has a free carboxy group, or at least does not acylate another amino acid residue (it may, for example, acylate ammonia to give –NH–CHR–CO–NH2), is called C-terminal.

list of amino acid

A	Ala	Alanine
B	Asx	Asparagine or aspartic acid
C	Cys	Cysteine
D	Asp	Aspartic acid
E	Glu	Glutamic acid
F	Phe	Phenylalanine
G	Gly	Glycine
H	His	Histidine
I	iie	Isoleucine
k	Lys	Lysine
L	Leu	Leucine
M	Met	Methionine
N	Asn	Asparagine
P	Pro	Proline
Q	Gln	Glutamine
R	Arg	Arginine
S	Ser	Serine
T	Thr	Threonine
V	Val	Valine
W	Trp	Tryptophan
Y	Tyr	Tyrosine
Z	Glx	Glutamine or glutamic acid

 What is aminoacyl-tRNA synthetases (aaRSs) ?

When ribosomes pair a tRNA (transfer ribonucleic acid) with a codon (three bases in a DNA or RNA sequence), an amino acid is expected to be carried by the tRNA. Since each tRNA is matched with its amino acid before it meets the ribosome, the ribosome has no way of knowing if the match was made. The match is made by a family of enzymes called aminoacyl-tRNA synthetases. These enzymes charge each tRNA with the proper amino acid via a covalent ester bond, allowing each tRNA to make the proper translation from the genetic code of DNA into the amino acid code of proteins. Cells make at least 20 different aminoacyl-tRNS synthetases, one for each of the amino acids.

Aminoacyl-tRNA synthetases belong to two classes, depending on which amino acid they specify. Class I enzymes usually are monomeric and attach to the carobxyl of their specific amino acid to the 2’ OH of adenosine 76 in the tRNA molecule. Class II enzymes are either dimeric or tetrameric and attach to their amino acids at the 3’ OH. These enzymes catalyze first by activating the amino acid by forming an aminoacyl-adenylate. Here the carboxyl of the amino acid is linked to the alpha-phosphate of ATP, displacing pyrophosphate. After the corrected tRNA is bound, the aminoacyl group of the aminoacyladenylate is transferred to the 2’ or 3’ terminal OH of the tRNA.

Recent studies have shown that aminoacyl-tRNA synthetases can tell the difference between the right and the wrong tRNA before they ever start catalysis, and if the enzyme binds aminoacyl-adenylate first, it is even more specific during tRNA binding. Previous studies have also proved that aminoacyl-tRNA synthetases reject wrong tRNAs during catalysis. Other research has shown that specific aaRSs play roles in cellular fidelity, tRNA processing, RNA splicing, RNA trafficking, apoptosis, and transcriptional and translational regulation. These new revelations may present new evolutionary models for the development of cells and perhaps opportunities for pharmaceutical advancements.

What is amino group (–NH2) ?

A functional group (group of atoms within a molecule that is responsible for certain properties of the molecule and reactions in which it takes part), common to all amino acids, that consists of a nitrogen atom bonded covalently to two hydrogen atoms, leaving a lone valence electron on the nitrogen atom capable of bonding to another atom. It can act as a base in solution by accepting a hydrogen ion and carrying a charge of +1. Any organic compound that has an amino group is called an amine and is a derivative of the inorganic compound ammonia, NH3. A primary amine has one hydrogen atom replaced, such as in the amino group. A secondary amine has two hydrogens replaced. A tertiary amine has all three hydrogens replaced. Amines are created by decomposing organic matter.

 What is amniocentesis ?

Amniocentesis is the removal of about two tablespoons of amniotic fluid via a needle inserted through the maternal abdomen into the uterus and amniotic sac. This is done to gain information about the condition, and even the sex, of the fetus. The fluid contains cells from the fetus and placenta.

Some women have a greater chance of giving birth to a baby with a chromosome problem, and amniocentesis can provide the answers if performed at about 16 weeks gestation (second trimester) or later. Chromosome analysis and alpha-fetoprotein (AFP) tests are two such tests, and these check for chromosome abnormalities such as Down’s syndrome and whether there are any openings in the fetal skin, such as in the spine, that could lead to neural-tube defects like spina bifida or anencephaly, or inherited disorders such as cystic fibrosis.

While the procedure is relatively safe, some problems that can occur are miscarriage (1 in 200, or 0.5 percent chance), cramping, and infections (less than 1 in 1,000).

Amniocentesis can also be performed during the second and third trimesters to determine fetal lung maturity, to verify the health of the fetus in cases of Rh sensitivity, and to identify any infections.

First used in 1882 to remove excess amniotic fluid, it is often used in late pregnancy to test for anemia in fetuses with Rh disease and to check if the fetal lungs are advanced enough for delivery to occur.

amnion

The amnion is a thin, but tough, transparent membranous sac and innermost of the four extra embryonic membranes (allantois, yolk sac, chorion) that encloses the embryo of reptiles, birds, and mammals. These membranes hold the amniotic fluid and form a protective layer for the fetus, insulating it from bacteria and infection.

amniotes

Any of the vertebrates such as reptiles, birds, and mammals that have an amnion surrounding the embryo.

amniotic egg

A calcium based or leathery shelled water-retaining egg that enables reptiles, birds, and egg-laying mammals, such as the monotremes (duckbilled platypus and two species of echidna, spiny anteaters), to complete their life cycles on dry land.

amoebic dysentery

Dysentery caused by a protozoan parasite (Entamoeba histolytica), mostly caused by poor sanitary conditions and transmitted by contaminated food or water.

amphibian

Cold-blooded, or ectothermic, vertebrates in the class Amphibia. These include the frogs and toads (order Anura, or Salientia), salamanders and newts (order Urodela, or Caudata), and the caecilians, limbless amphibians (order Apoda, or Gymnophiona). There are more than 11,000 species of amphibians, and they are believed to be the first vertebrate species to live on land.

Located between the fish and reptiles on the evolutionary scale, they are the most primitive of the terrestrial vertebrates and undergo a metamorphosis from water-breathing limbless larva (tadpole) to land-loving, or partly terrestrial, air-breathing four-legged adult.

Eggs are typically deposited in water or a wet protected place, although some do lay eggs in dry places. The eggs are not shelled and do not possess the membranes that are common in reptiles or higher vertebrates. Adults have moist skins with no scales or small scales, and they are specialized in living habitats. Each has its own evolutionary adaptations from the jumping ability (over 17 feet in some cases) of frogs and toads, to the limbless caecilians, to the long tails of the salamanders and newts. For example, frogs can enter aestivation, a period of dormancy similar to hibernation, when experiencing long periods of heat or drought conditions, and they can breathe through their skin in a process called cutaneous gas exchange. The most poisonous frog known, Phyllobates terribilis, only needs 0.00000007 ounce of skin secretion to kill a predator, while an antibiotic secreted from the African clawed frog (Xenopus laevis) may someday be used to treat burns and cystic fibrosis.

Over the last 50 years, many species of amphibians around the world have declined markedly in numbers; some species have become extinct. In many instances, these declines are attributable to adverse human influences acting locally, such as deforestation, draining of wetlands, and pollution.

However, in 1988, herpetologists (scientists who study amphibians) from many parts of the world reported declines in amphibian populations in protected, or pristine, habitats such as national parks and nature reserves, where such local effects could not be blamed. This suggested that there may be one or more global factors that are affecting climatic and atmospheric changes and adversely affecting amphibians, such as increased UV-B radiation, widespread pollution, acid rain, and disease. In effect, the decline could be the result of human-induced changes to the global ecosystem and could have far-reaching consequences for human survival.

 What is amphipathic molecule ?

A molecule that has both a hydrophilic (water soluble, polar) region and a hydrophobic (water hating, nonpolar) region. The hydrophilic part is called the head, while the hydrophobic part is called the tail. Lipids (phospholipids, cholesterol and other sterols, glycolipids [lipids with sugars attached], and sphingolipids) are examples of amphipathic molecules.

Amphipathic molecules act as surfactants, materials that can reduce the surface tension of a liquid at low concentrations, and are used in wetting agents, demisters, foaming agents, and emulsifiers.

 What is anabolism ?

The processes of metabolism that result in the synthesis of cellular components from precursors of low molecular weight.

 What is anaerobic ?

Any organism or environmental or cellular process that does not require the use of free oxygen. Certain bacteria such as Actinomyces israeli, Bacteroides fragilis, Prevotella melaninogenica, Clostridium difficile, and Peptostreptococcus are anaerobes.

In effect, an anaerobic organism does not need oxygen for growth. Many anaerobes are even sensitive to oxygen. Obligate (strict) anaerobes grow only in the absence of oxygen. Facultative anaerobes can grow either in the presence or in the absence of oxygen.

 What is anagenesis ?

A pattern of evolutionary change along a single, unbranching lineage involving the transformation of an entire population, sometimes so different from the ancestral population that it can be called a separate species. Examples would be one taxon replacing another or the transformation of a single ancestral species into a single descendant species. Anagenesis is also known as phyletic evolution and is the opposite of CLADOGENESIS.

In medicine, it refers to the regeneration of tissue or structure.

 What is analog ?

A DRUG whose structure is related to that of another drug but whose chemical and biological properties may be quite different.

analogy

The similarity of structure between two species that are not closely related; usually attributed tolarity in underlying structure (or origin) are called analogous structures. For example, birds and bats each have their forelimbs modified as wings. They are analogous because they evolved independently after the earliest birds and bats diverged from their common ancestor, who did not have wings. However, the details of their structures are quite different. convergent evolution. Structures that resemble each other due to a similarity in function without any simi

anation

Replacement of the LIGAND water by an anion in a COORDINATION entity.

 What is androgens ?

Steroid sex hormones, such as testosterone secreted by the testes in males, and others secreted by the adrenal cortex in humans and higher animals, as well as by the adrenal glands and ovaries in mammals. Androgens stimulate the development and maintenance of the male reproductive system such as sperm production, sexual behavior, and muscle development. Secondary sex characteristics such as the growth of pubic hair in females is also a product of androgens, as is the deepening of the voice at puberty.

Testosterone is present in a number of forms, such as free testosterone, as testosterone bound to a protein, sex hormone binding globulin (SHBG), and as dihydrotestosterone. Testosterone and synthetic androgens (anabolic steroids) have been used for infertility, athletic enhancement, erectile dysfunction, and libido problems, but their use can cause side effects such as muscle weakness, muscle atrophy, little facial and body hair, and even changes in the size of the genitalia. Prolonged use can damage the liver, and their use is banned in many sports.

Other androgens are androsterone (excreted in urine), which reinforces masculine characteristics; dihydrotestosterone, which is a metabolite synthesized mainly in the liver from free testosterone by the enzyme 5-alpha-reductase and which levels are proportionally correlated to sex drive as well as erectile capabilities; and dehydroepiandrosterone, which are adrenal androgens that have been linked to puberty and aging.

 What is androgynous ?

Term applied to flowering plants that have both staminate and pistillate flowers, or to cryptograms (ferns, mosses, fungi, algae) where the antheridia and archegonia are together.

 What is anemia ?

Condition in which there is a reduction in the number of red blood cells or amount of HEMOGLOBIN per unit volume of blood below the reference interval for a similar individual of the species under consideration, often causing pallor and fatigue.

 What is aneuploidy ?

Aneuploidy is the gain or loss of individual chromosomes from the normal diploid set of 46 and is the most common cytogenetic abnormality caused when homologous chromosomes fail to separate during the first division of meiosis.

When a loss of a chromosome occurs, it is called monosomy and is rarely seen in live births, since most monosomic embryos and fetuses are lost to spontaneous abortion at very early stages of pregnancy. One exception to this is the loss of an X chromosome, which produces Turner syndrome in about one out of every 5,000 female births.

The more common gain of a single chromosome is called trisomy and has been associated with various cancers. A common autosomal trisomy is Down’s syndrome in humans.

Another form of aneuploidy is called nullisomy, which is the loss of both pairs of homologous chromosomes and is almost always fatal to humans, since humans have no extra disposable chromosomes in the genome.

Tetrasomy is the gain of an extra pair of homologous chromosomes and is a rare chromosomal aberration. It can cause metopic craniosynostosis, facial anomalies, cranial asymmetry, atrioseptal defects, hydronephrosis, flexion contractures of the lower limbs, sensorineural hearing loss, and mental retardation.

 What is angiosperm ?

 A flowering plant. There are close to 250,000 species of flowering plants, second in abundance only to insects. All have three basic organs (roots, stems, and leaves) and represent the most abundant and advanced terrestrial plants, which include trees, herbaceous plants, herbs, shrubs, all grasses, and some aquatic plants. Angiosperms are the source of most of the food on which human beings and other mammals rely and of many raw materials and natural products that provide the infrastructure for modern civilizations.

Angiosperms are divided into two large groups. The dicotyledonea, or dicotyledons (also called magnoliopsida), the larger of the two groups, includes trees and shrubs and herbaceous plants. Dicots have two seed leaves (cotyledons) in the embryo. The smaller of the two groups is the monocotyledoneae, or monocotyledons (also called liliopsida), that include rice, corn, palms, bananas, coconuts, grasses, lilies, orchids, and garden plants. Monocots have a single seed leaf in the embryo

The life cycles of the angiosperms have several advantages over those of conifers, or gymnosperms, the only other group of seed-bearing plants, and from which scientists believe the angiosperms evolved during the Cretaceous era some 145 million years ago. They reproduce via flowers instead of cones; their ovules are embedded in female sporophylls instead of being exposed on a bare ground surface (e.g., apple); the gametophyte is reduced; and seeds are enclosed in fruits that develop from the ovary or related structures.

Angiosperms have a true flower that is either a highly modified shoot with modified stem and leaves or a condensed and reduced compound strobilus (conelike structure) or inflorescence (flower cluster). Floral parts are in the form of sepals, petals, stamens, and carpels, while the ovules—the structure that develops in the plant ovary and contains the female gametophyte—are contained within the megasporophylls that are sealed in most angiosperm families. Pollination is facilitated by wind, water, or many animals. Self-pollination as well as parthenogenesis, a process by which embryonic development is initiated directly from an unfertilized cell, are common. Double fertilization occurs in all members of the phylum to produce the unusual stored food tissue called endosperm. Sexual reproduction in flowering plants occurs by this process of double fertilization in which one fertilization event forms an embryo, and a second fertilization event produces endosperm, a polyploid embryo-nourishing tissue found only in the angiosperms. Seeds are dispersed through a variety of forms such as fruits, follicles, capsules, berries, drupes, samaras, nuts, and achenes. Angiosperm is a combination of the Latin word angi- (enclosed) and the Greek word sperma (seed).

 What is anion ?

An atom or molecule that has a negative charge; a negatively charged ion.

 What is anisotropy ?

The property of molecules and materials to exhibit variations in physical properties along different molecular axes of the substance.

 What is anther ?

In angiosperms, it is the terminal pollen sac of the stamen. The pollen grains with male gametes form inside the anther. It is the pollen that fertilizes the ovules. The anther is the primary male reproductive structure at the apex of the flower’s stamen, the male sexual organ.

 What is antheridium ?

The multicellular male sex organ or gametangium where motile male gametes (sperm) are formed and protected in algae, fungi, bryophytes (mosses, liverworts, etc.), and pteridophytes (ferns).

anthrax

x Bacterial disease of animals and humans caused by contamination with spores from Bacillus anthracis through inhalation or skin entry (cutaneous); can be used as an agent of bioterrorism.

anti

In the representation of STEREOCHEMICAL relationships, anti means “on opposite sides” of a reference plane, in contrast to syn, which means “on the same side.”

antibiotic

A chemical agent that is produced synthetically or by an organism that is harmful to another organism. It is used to combat disease, either topically or by ingestion, in humans, animals, and plants. It can be made from a mold or bacterium and kills or slows the growth of other microbes, in particular bacteria. Penicillin, one of the most famous antibiotics, was accidentally discovered by the British bacteriologist SIR ALEXANDER FLEMING in 1928.

Antibiotic resistance can occur when antibiotics are used repetitively. While most of the targeted bacteria are killed by a dose of antibiotics, some escape death, and these remaining bacteria have or develop a genetic resistance to the antibiotic. Unfortunately, this resistance trait can be passed on to their offspring.

 What is antibody ?

A soluble immunoglobulin blood protein produced by the B cells, white blood cells, that develop in the bone marrow (also known as B lymphocytes, plasma cells) in response to an antigen (a foreign substance). Antibodies are produced in response to disease and help the body fight against a particular disease by binding to the antigen and killing it, or making it more vulnerable to action by white blood cells. They help the body develop an immunity to diseases.

Each antibody has two light (L) and two heavy (H) immunoglobulin polypeptide chains linked together by disulfide bonds, with two antigen-binding sites. There are more than 1,000 possible variations, yet each antibody recognizes only one specific antigen. Antibodies are normally bound to a B cell, but when an antibody encounters an antigen, the B cell produces copies of the antibody with the assistance of helper T cells (a lymphocyte that undergoes a developmental stage in the thymus). The released antibodies then go after and bind to the antigen, either killing it or marking it for destruction by phagocytes.

There are five immunoglobulins: IgC, IgA, IgM, IgD, and IgE.

IgA, or immunoglobulin A, comprises about 10–15 percent of the body’s total immunoglobulins and is found in external secretions such as saliva, tears, breast milk, and mucous, both intestinal and bronchial. They are secreted on the surface of the body as a first defense against bacteria and viral antigens in an attempt to prevent them from entering the body.

IgM or immunoglobulin M antibodies are produced in response to new or repeat infections and stay in the body for a short time after infection. They make up from 5 to 10 percent of the total immunoglobulins and are the first to show up in the serum after an antigen enters. IgM is produced during the primary immune response. It is the IgMs that capture and bind antigens to form large insoluble complexes that are cleared from the blood.

IgG or immunoglobulin G (gamma globulin) antibodies remain in the body for long periods of time after infection and are the most common type, comprising about 80 percent of the body’s total immunoglobulins. They are in the serum and are produced in substantial quantities during the secondary immune response, and along with IgM activate the complement system, which results in the destruction of the membrane of pathogens. The IgGs act by agglutinating, by opsonising, by activating complement-mediated reactions against cellular pathogens, and by neutralizing toxins.

IgE or immunoglobulin E is associated with mast cells, which are basophils, a type of granular white blood cell that has left the bloodstream and entered a tissue. Mast cells release histamine and heparin, chemicals that mediate allergic reactions. Not surprisingly, IgE is responsible for immediate hypersensitivity (allergic) reactions and immune defense against parasites.

IgD or immunoglobulin D is a specialized immunoglobulin, but its function is currently unknown. It is found in small amounts in the serum.

 What is anticodon ?

A specialized sequence of three nucleotides on a tRNA (transfer ribonucleic acid) molecule. The anticodon associates with a complementary triplet of bases—the codon—on an mRNA (messenger RNA) molecule during protein synthesis.

The tRNA molecule acts like a “ferry” whose job is to “pick up a passenger” (read the code from the mRNA) and then “shuttle it” (dock to the corresponding amino acid) into place. The other end of the tRNA molecule has an acceptor site where the tRNA’s specific amino acid will bind.

The 20 amino acids in the table below can create 64 different tRNA molecules, 61 for tRNA coding and three codes for chain termination (pairing up with “stop codons” that end the mRNA message), and each amino acid can create more than one set of codons.

 What is antidiuretic hormone(ADH) ?

antidiuretic hormone (ADH) Also known as vasopressin, ADH is a nine–amino acid peptide secreted from the posterior pituitary gland. The hormone is packaged in secretory vesicles with a carrier protein called neurophysin within hypothalamic neurons, and both are released upon hormone secretion. The single most important effect of antidiuretic hormone is to conserve body water by reducing the output of urine. It binds to receptors in the distal or collecting tubules of the kidney and promotes reabsorption of water back into the circulation.

The release of ADH is based on plasma osmolarity, the concentration of solutes in the blood. For example, loss of water (e.g., sweating) results in a concentration of blood solutes, so plasma osmolarity increases. Osmoreceptors, neurons in the hypothalamus, stimulate secretion from the neurons that produce ADH. If the plasma osmolarity falls below a certain threshold, the osmoreceptors do nothing and no ADH is released. However, when osmolarity increases above the threshold, the osmoreceptors stimulate the neurons and ADH is released.

 What is antigen ?

A foreign substance, a macromolecule, that is not indigenous to the host organism and therefore elicits an immune response.

 What is antimetabolite ?

A structural ANALOG of an intermediate (substrate or COENZYME) in a physiologically occurring metabolic pathway that acts by replacing the natural substrate, thus blocking or diverting the biosynthesis of physiologically important substances.

  What is antisense molecule ?

An OLIGONUCLEOTIDE or ANALOG thereof that is complementary to a segment of RNA (ribonucleic acid) or DNA (deoxyribonucleic acid) and that binds to it and inhibits its normal function.

An antisense molecule is the noncoding strand in double-stranded DNA. The antisense strand serves as the template for mRNA synthesis

 What is aphotic zone ?

The deeper part of the ocean beneath the photic zone, where light does not penetrate sufficiently for photosynthesis to occur.

 What is apical dominance ?

Concentration of growth at the tip of a plant shoot, where a terminal bud partially inhibits axillary bud growth. It is thought to be caused by the apical bud producing a great deal of IAA (auxin), which is transported from the apical bud to the surrounding area and causes lateral buds to stay dormant.

 What is apical meristem ?

Embryonic plant tissue (meristematic cells) in the tips of roots and in the buds of shoots that supplies cells via mitosis for the plant to grow in length.

 What is apomixis ?

The ability of certain plants to reproduce clones of themselves, i.e., the scaly male fern group, Dryopteris affinis (Lowe) Fraser-Jenkins.

 What is apomorphic character ?

A phenotypic character, or homology, in which the similarity of characters found in different species is the result of common descent, i.e., the species evolved after a branch diverged from a phylogenetic tree.

Two characters in two taxa are homologues if they are the same as the character that is found in the ancestry of the two taxa or they are different characters that have an ancestor/descendant relationship described as preexisting or novel. The ancestral character is termed the plesiomorphic character, and the descendant character is termed the apomorphic character. Examples are the flippers of whales and human arms.

 What is apoplast ?

The cell-wall continuum of an organ or a plant; in a plant it includes the xylem. The movement of substances via cell walls is called apoplastic transport.

 What is apoprotein ?

A protein without its characteristic PROSTHETIC GROUP or metal.

 What is apoptosis ?

Cells die by injury or commit “suicide.” Apoptosis is a programmed cell death (PCD) brought about by signals that trigger the activation of a flood of “suicide” proteins in the cells destined to die. The destined cells then go through a number of molecular and morphological changes until they finally die. PCD is important in proper development in mitosis and cells that may be threatening to the host organism. It can be induced by a variety of stimuli, such as ligation of cell surface receptors, starvation, growth factor/survival factor deprivation, heat shock, hypoxia, DNA damage, viral infection, and cytotoxic/chemotherapeutical agents. Apoptosis is a word of Greek origin meaning “falling off or dropping off.” There is a Web site devoted to the topic at http://www.celldeath.de/mainfram.htm

 What is aposematic coloration ?

The bright coloration of animals with effective physical or chemical defenses that acts as a warning to experienced predators. The larvae of the monarch butterfly and Phymateus morbillosus, a foaming grasshopper from South Africa, are two examples. The warning coloration alerts the predator, who may have eaten a similar-looking animal and was sickened by it, to avoid it. This also helps those species that mimic others in appearance, such as the viceroy butterfly and the monarch butterfly.

 What is aquaporins (AQPs) ?

The aquaporins are a family of proteins known for facilitating water transport. An aquaporin is a transport protein in the plasma membranes of a plant or animal cell that specifically facilitates the diffusion of water across the membrane (osmosis).

Aquaporin-1, or CHIP-28, discovered in 1992 by Peter Agre, is the major water channel of the red blood cells. In the kidneys, it is involved in the reabsorption of most of the waste filtered through the glomeruli. It is also thought to influence the movement of CO2 across the cell membrane, since it is present in most cells that have high levels of CO2. Aquaporin-2, or WCH-CD, is a water channel that makes the principal cells of the medullary collecting duct in the kidneys more permeable to water. Lack of a functional aquaporin-2 gene leads to a rare form of nephrogenic diabetes insipidus. There are many more aquaporins that have been discovered in the more water-permeable parts of the body, such as the moist surface tissues of the alveoli in the lung, the kidney tubules, the choroid plexus of the brain where cerebrospinal fluid is produced, the ciliary epithelium of the eye where aqueous humor is formed, and the salivary and lacrimal tear glands. Aquaporins are believed to be involved in mechanisms defending against brain edema, congestive heart failure, and many other clinical entities.

 What is aquation ?

The incorporation of one or more integral molecules of water into another chemical species with or without displacement of one or more atoms or groups.

 What is aqueous solution ?

A solution in which water is the solvent or dissolving medium, such as salt water, rain, or soda.

 What is Archaea ?

a One of two prokaryotic (no nucleus) domains, the other being the Bacteria. Archaeans include organisms that live in some of the most extreme environments on the planet and resemble bacteria. They are single-cell organisms that, with bacteria, are called prokaryotes. Their DNA is not enclosed in a nucleus. Bacteria and archaea are the only prokaryotes; all other life forms are eukaryotes. Archaeans are among the earliest forms of life that appeared on Earth billions of years ago, and it is believed that the archaea and bacteria developed separately from a common ancestor nearly 4 billion years ago.

Some archaeans are “extremophiles,” that is, they live near rift vents in the deep sea at temperatures well over 100°C (212°F). Others live in hot springs (such as the hot springs of Yellowstone National Park, where some of archaea were first discovered) or in extremely alkaline or acid waters. They have been found inside the digestive tracts of cows, termites, and marine life, where they produce methane. They also live in the anoxic muds of marshes and at the bottom of the ocean and in petroleum deposits deep underground. They are also quite abundant in the plankton of the open sea and even have been found in the Antarctic. They survive in these harsh conditions by using a variety of protective molecules and enzymes.

Three groups of archaeans are known and include the Crenarchaeota, those that are extremophiles; the Euryarchaeota, methane producers and salt lovers; and the Korarchaeota, an all-inclusive group that contains a number of types that are little understood today

Archaeans produce energy by feeding on hydrogen gas, carbon dioxide, and sulfur and can even create energy from the sun by using a pigment around the membrane called a bacteriorhodopsin that reacts with light and produces ATP

The archaeans were not discovered as a separate group until the late 1970s.

 What is archaezoa ?

This group is believed to be the first to diverge from the prokaryotes. They lack mitochondria (converts foods into usable energy), though some archaezoans have genes for mitochondrial. They also lack an endoplasmic reticulum (important for protein synthesis) and golgi apparatus (important for glycosylation, secretion), have no peroxisomes (use oxygen to carry out catabolic reactions), and have small ribosomes similar to bacteria.

Archaezoa has three known subgroups: diplomonads, microspoidians, and trichomonads. They are usually found with flagellas in moist/damp environments such as streams, lakes, underground water deposits, and in damp soil.

Some members have been found in harsh environments and can exist in bodies of water that can drop below –20° Fahrenheit and around ocean floor vents that exceed 320°F. These organisms can survive in a variety of environments as long as they are in water.

Many archaezoans are parasites and feed off their host. The species Giardia, which causes abdominal cramps and severe diarrhea, uses a ventral suction cup to attach to the human intestinal epithelium. Some species have chloroplasts that allow them to take in light energy and use it when needed. Some species contain hydrogenosomes, organelles that are similar to mitochondria but do not respire with oxygen. They convert pyruvate into acetate, CO2, and H2, allowing extra ATP synthesis without respiration.

Since they have no mitochondria or plastid, it is believed that they are the intermediate stage between prokaryotes and eukaryotes and are also used as evidence for the evolution of the nucleus before the organelles.

 What is archegonium ?

In plants, the multicellular flaskshaped female gametangium (a moist chamber in which gametes develop in bryophytes, ferns, and gymnosperms).

 What is archenteron ?

The endoderm-lined gut (enteron) hollow cavity formed during the gastrulation process in metazoan embryos. The archenteron is formed by the infolding of part of the outer surface of the BLASTULA and opening to the exterior via the BLASTOPORE. Also called the primitive gut, or gastrocoel in early embryonic development, it is the digestive cavity. The term is Greek for “primitive intestine.”

 What is archipelago ?

A group or chain of islands clustered in a body of water, e.g., the African Bazaruto Archipelago, consisting of five islands: Bazaruto, Magaruque, Santa Carolina, Benguera (Benguerra), and Bangue.

Who was Aristotle ?

Aristotle (384 B.C.E.–322 B.C.E.) Greek Philosopher Aristotle, a Greek philosopher and scientist, has had more influence on the field of science than anyone. His influence, which lasted more than 2,000 years, was due to the fact that he was the first to depart from the old Platonic school of thinking by reasoning that accurate observation, description, inductive reasoning, and interpretation was the way to understand the natural world. Since he was the first to use this method, he is often called the “Father of Natural History.”

Born in 384 B.C.E. in the Ionian colony of Stagirus (now Macedonia), Aristotle was the son of Nicomachus, a physician and grandfather of Alexander the Great. At 17, he became a student in Plato’s academy in Athens and stayed there for more than 20 years as a student and teacher. In 347 B.C.E., he moved to the princedom of Atarneus in Mysia (northwestern Asia Minor), ruled by Hermias, and who presided over a small circle of Plato followers in the town of Assos. Aristotle befriended Hermias, joined the group, and eventually married Hermias’s niece and adopted daughter Pythias.

Around 342 B.C.E., he moved to Mieza, near the Macedonian capital Pella, to supervise the education of 13-year-old Alexander the Great. Aristotle returned to Athens in 335 B.C.E. to teach, promote research projects, and organize a library in the Lyceum. His school was known as the Peripatetic School. After Alexander’s death in 323 B.C.E., Aristotle was prosecuted and had to leave Athens, leaving his school to Theophrastus. He died shortly after at Chalcis in Euboea in 322 B.C.E.

While his writings were immense, one of his works particularly influenced the field of meteorology for over 2,000 years. Meteorologica (meteorology) was written in 350 B.C.E. and comprised four books, although there are doubts about the authenticity of the last one. They deal mainly with atmospheric phenomena, oceans, meteors and comets, and the fields of astronomy, chemistry, and geography

Aristotle attempted to explain the atmosphere in a philosophical way and discussed all forms of “meteors,” a term then used to explain anything suspended in the atmosphere. Aristotle discussed the philosophical nature of clouds and mist, snow, rain and hail, wind, lightning and thunder, rivers, rainbows, and climatic changes. His ideas posited the existence of four elements (earth, wind, fire, and water), each arranged in separate layers but capable of mingling.

Aristotle’s observations in the biological sciences had some validity, but many of his observations and conclusions regarding weather and climate were wrong, and it was not until the 17th century—with the invention of meteorological instruments such as the hygrometer, thermometer, and barometer—that his ideas were disproved scientifically. However, he correctly reasoned that the earth was a sphere, recorded information regarding the bathymetry of seas, correctly interpreted dolphins and whales as mammals, separated vertebrates into oviparous and viviparous, and described and named many organisms, including crustaceans and worms, mollusks, echinoderms, and fish from the Aegean Sea.

Who was Arrhenius, Svante August ?

Arrhenius, Svante August (1859–1927) Swedish Chemist, Physicist Svante August Arrhenius was born in Vik (or Wijk), near Uppsala, Sweden, on February 19, 1859. He was the second son of Svante Gustav Arrhenius and Carolina Christina (née Thunberg). Svante’s father was a surveyor and an administrator of his family’s estate at Vik. In 1860, a year after Arrhenius was born, his family moved to Uppsala, where his father became a supervisor at the university. He was reading by the age of three.

Arrhenius received his early education at the cathedral school in Uppsala, excelling in biology, physics, and mathematics. In 1876, he entered the University of Uppsala and studied physics, chemistry, and mathematics, receiving his B.S. two years later. While he continued graduate classes for three years in physics at Uppsala, his studies were not completed there. Instead, Arrhenius transferred to the Swedish Academy of Sciences in Stockholm in 1881 to work under Erick Edlund to conduct research in the field of electrical theory.

lutions by passing electric current through a variety of solutions. His research determined that molecules in some of the substances split apart, or dissociated from each other, into two or more ions when they were dissolved in a liquid. He found that while each intact molecule was electrically balanced, the split particles carried a small positive or negative electrical charge when dissolved in water. The charged atoms permitted the passage of electricity, and the electrical current directed the active components toward the electrodes. His thesis on the theory of ionic dissociation was barely accepted by the University of Uppsala in 1884, since the faculty believed that oppositely charged particles could not coexist in solution. He received a grade that prohibited him from being able to teach.

Arrhenius published his theories (“Investigations on the Galvanic Conductivity of Electrolytes”) and sent copies of his thesis to a number of leading European scientists. Russian-German chemist Wilhelm Ostwald, one of the leading European scientists of the day and one of the principal founders of physical chemistry, was impressed and visited him in Uppsala, offering him a teaching position, which he declined. However, Ostwald’s support was enough for Uppsala to give him a lecturing position, which he kept for two years.

The Stockholm Academy of Sciences awarded Arrhenius a traveling scholarship in 1886. As a result, he worked with Ostwald in Riga with physicist Friedrich Kohlrausch at the University of Wurzburg, with physicist Ludwig Boltzmann at the University of Graz, and with chemist Jacobus Van’t Hoff at the University of Amsterdam. In 1889, he formulated his rate equation that is used for many chemical transformations and processes, in which the rate is exponentially related to temperature, known as the “Arrhenius equation.”

He returned to Stockholm in 1891 and became a lecturer in physics at Stockholm’s Hogskola (high school) and was appointed physics professor in 1895 and rector in 1897. Arrhenius married Sofia Rudbeck in 1894 and had one son. The marriage lasted a short two years. Arrhenius continued his work on electrolytic dissociation and added the study of osmotic pressure.

In 1896, he made the first quantitative link between changes in carbon dioxide concentration and climate. He calculated the absorption coefficients of carbon dioxide and water based on the emission spectrum of the moon, and he also calculated the amount of total heat absorption and corresponding temperature change in the atmosphere for various concentrations of carbon dioxide. His prediction of a doubling of carbon dioxide from a temperate rise of 5–6°C is close to modern predictions. He predicted that increasing reliance on fossil fuel combustion to drive the world’s increasing industrialization would, in the end, lead to increases in the concentration of CO2 in the atmosphere, thereby giving rise to a warming of the Earth.

In 1900, he published his Textbook of Theoretical Electrochemistry. In 1901 he and others confirmed the Scottish physicist James Clerk Maxwell’s hypothesis that cosmic radiation exerts pressure on particles. Arrhenius went on to use this phenomenon in an effort to explain the aurora borealis and solar corona. He supported the Norwegian physicist Kristian Birkeland’s explanation of the origin of auroras that he proposed in 1896. He also suggested that radiation pressure could carry spores and other living seeds through space and believed that life on earth was brought here under those conditions. He likewise believed that spores might have populated many other planets, resulting in life throughout the universe.

In 1902, he received the Davy Medal of the Royal Society and proposed a theory of immunology. The following year he was awarded the Nobel Prize for chemistry for his work that originally had been perceived as improbable by his Uppsala professors. He also published his Textbook of Cosmic Physics.

l Chemistry in Stockholm in 1905 (a post he held until a few months before his death). He married Maria Johansson and had one son and two daughters. The following year he also had time to publish three books, Theories of Chemistry, Immunochemistry, and Worlds in the Making.

He was elected a foreign member of the Royal Society in 1911, the same year he received the Willard Gibbs Medal of the American Chemical Society. Three years later he was awarded the Faraday Medal of the British Chemical Society. He was also a member of the Swedish Academy of Sciences and the German Chemical Society.

During the latter part of his life his interests included the chemistry of living matter and astrophysics, especially the origins and fate of stars and planets. He continued to write books such as Smallpox and Its Combating (1913), Destiny of the Stars (1915), Quantitative Laws in Biological Chemistry (1915), and Chemistry and Modern Life (1919). He also received honorary degrees from the universities of Birmingham, Edinburgh, Heidelberg, and Leipzig and from Oxford and Cambridge Universities. He died in Stockholm on October 2, 1927, after a brief illness, and is buried at Uppsala.

 What is arteriosclerosis ?

arteriosclerosis Also known as “hardening of the arteries.” It is a disease whereby the arteries thicken and the inner surfaces accumulate deposits of hard plaques of cholesterol, calcium, fibrin, and other cellular debris. The arteries become inelastic and narrowed, which increases the stress on the heart as it pumps blood through, and complete obstruction with loss of blood supply can occur. This is a common cause for high blood pressure. There are hereditary links that are associated with increased risk of heart attack and stroke. When arteriosclerosis occurs in large arteries, such as the aorta, it is often referred to as atherosclerosis.

artery

Illustration of the arterial system in the human body, shown in a standing figure. The heart and kidneys are also shown. Note the feathery network of blood vessels in the left and right lungs (next to the heart). Arteries are the blood vessels that carry oxygen-rich blood to the body’s tissues. Veins (not shown) carry blood back to the heart. The average adult has about five liters of blood. At rest, this volume of blood passes through the heart each minute.

A blood vessel that carries oxygenated (except the pulmonary artery) blood away from the heart via the right and left ventricles to organs throughout the body. The main trunk of the arterial system in the body is called the aorta. The aortic divisions are the abdominal aorta, thoracic aorta, aortic artery, and ascending aorta. The pulmonary artery carries unoxygenated blood from the heart to the lungs for oxygenation.

arthritis

Inflammation of one or more of the joints in the body.

Arthropoda

An animal phylum where individuals have a segmented body, exoskeleton, and jointed legs.

artificial selection

Artificial selection is the conscious attempt by human beings to alter the environments or traits of other organisms (including their own environment) so as to alter the evolution of these organism’s species. It is used in the selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits or new breeds. Chickens are artificially selected to produce better eggs, and pet fish are selectively bred to produce vibrant colors and other desirable traits.

ascus (plural, asci)

In Ascomycota (blue, green, and red molds), a saclike spore capsule located at the tip of the fruiting body, called the ascocarp in dikaryotic (containing two differing haploid nuclei) hyphae, in which ascopores are found and in which karyogamy is performed, i.e., two (dikaryotic) nuclei fuse (karyogamy) to form diploid nuclei. Asci vary in shape from narrow and elongate to nearly round. While the number of ascospores per ascus is usually eight, numerous other counts of ascospores per ascus are also known.

In medicine ASCUS stands for atypical squamous cells of undetermined significance and means that irregular cells have shown up on a Pap smear

asexual reproduction

asexual reproduction A type of reproduction, without meiosis or syngamy (the fusion of two gametes in fertilization), involving only one parent that produces genetically identical offspring by budding, by the division of a single cell, or by the entire organism breaking into two or more parts. The offspring has the identical genes and chromosomes as the parent. Most plants are capable of asexual reproduction by means of specialized organs called propagules, such as tubers, stolons, gemma cups, and rhizomes.

Asexual reproduction is also known as vegetative reproduction. Examples of organisms that reproduce by asexual reproduction include aspens, dandelions, strawberries, walking fern (Asplenium rhizophyllum), and yeast. While asexual reproduction guarantees reproduction (no dependence on others), it does not allow genetic variation.

assimilation

To transform food and other nutrients into a part of the living organism.

associative learning

g The acquired ability to associate one stimulus with another, such as one linked to a reward or punishment; also called classical conditioning and trial-and-error learning.

assortative mating

A type of nonrandom or preference mating in which mating partners resemble each other in certain phenotypic characteristics. It can be a preference or avoidance of certain individuals as mates based on physical or social traits.

astigmatism

Distorted vision, especially at close distances, resulting from an irregularly shaped cornea.

asymmetric carbon

n A carbon atom covalently bonded to four different atoms or groups of atoms.

asymmetric synthesis

A traditional term for stereoselective synthesis. A chemical reaction or reaction sequence in which one or more new elements of CHIRALITY are formed in a SUBSTRATE molecule and which produces the STEREOISOMERic (ENANTIOMERic or DIASTEREOISOMERic) products in unequal amounts.

asymmetry parameter

In nuclear quadrupole resonance spectroscopy, the parameter, η, is used for describing nonsymmetric fields. It is defined as η = (qxx – qyy)/qzz in which qxx, qyy, and qzz are the components of the field gradient q (which is the second derivative of the time-averaged electric potential) along the x– , y– and z–axes. By convention qzz refers to the largest field gradient, qyy to the next largest, and qxx to the smallest when all three values are different.

 What is atomic number ?

The atomic number is equal to the number of positively charged protons in an atom’s nucleus and determines which element an atom is. The atomic number is unique for each element and is designated by a subscript to the left of the elemental symbol. The atomic number for hydrogen is 1; it has one proton. Elements are substances made up of atoms with the same atomic number. Most of the elements are metals (75 percent) and the others are nonmetals.

 What is atomic weight or mass ?

The total atomic mass (the weighted average of the naturally occurring isotopes), which is the mass in grams of one MOLE of the atom. The atomic weight is calculated by adding the number of protons and neutrons together. The atomic weight of hydrogen is 1.0079 grams per mole.

  What is  ATP (adenosine triphosphate) ?

An adenine (purine base), ribose, and three phosphate units containing nucleoside triphosphate that (a) releases free energy when its phosphate bonds are hydrolyzed and (b) produces adenosine diphosphate (ADP) and inorganic phosphorous. This energy is used to drive ENDERGONIC REACTIONS in cells (chemical reactions that require energy input to begin). ATP is produced in the cristae of mitochondria and chloroplasts in plants and is the driving force in muscle contraction and protein synthesis in animals. It is the major energy source within cells.

 What is ATP synthase (proton translocating ATPase) ?

A protein complex (a chemiosmotic enzyme) that synthesizes adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and enables phosphate coupling with an electrochemical ion gradient across the membrane. It is found in cellular membranes and the inner membrane of mitochondria, the thylakoid membrane of chloroplasts, and the plasma membrane of prokaryotes. The protein consists of two portions: a soluble fraction that contains three catalytic sites and a membrane-bound portion that contains anion channels. It functions in chemiosmosis, the use of ion gradients across membranes, with adjacent electron transport chains, and it uses the energy stored across the photosynthetic membrane (a hydrogen-ion concentration gradient) to add inorganic phosphate to ADP, thereby creating ATP. This allows hydrogen ions (H+) to diffuse into the mitochondrion.

atrioventricular valve

A valve in the heart between each atrium and ventricle. It prevents a backflow of blood when the ventricles contract

atrium (plural, atria)

An upper chamber that receives blood from the veins returning to the vertebrate heart and then pushes the blood to the ventricles, the lower chambers. There is a left and right atrium. Oxygenated blood returns from the lungs into the left atrium and gets pushed down to the left ventricle. The left ventricle pumps the blood out to the rest of the body, transporting the oxygen to parts of the body that need it. Blood returning from its voyage through the body arrives in the right atrium. It then goes into the right ventricle from which it goes through the lungs again to get more oxygen, and the cycle continuously repeats itself.

autacoid

A biological substance secreted by various cells whose physiological activity is restricted to the vicinity of its release; it is often referred to as local HORMONE.

autogenesis mode

According to autogenesis (“selfgenerating”), eukaryotic cells evolved by the specialization of internal membranes originally derived from prokaryotic plasma membranes. This is another word for spontaneous generation or abiogenesis.

 What is autoimmune disease ?

An immunological disorder in which the immune system turns against itself. Autoimmunity can be the cause of a broad spectrum of human illnesses. Autoimmune diseases were not accepted into the mainstream of medicine until the 1950s and 1960s. They are diseases in which the progression from benign autoimmunity to pathogenic autoimmunity happens over a period of time and is determined by both genetic influences and environmental triggers. Examples of autoimmune diseases are idiopathic thrombocytopenic purpura, Graves’ disease, myasthenia gravis, pemphigus vulgaris (cause of pemphigus), and bullous pemphigoid (a blistering disease).

 What is autonomic nervous system (ANS) ?

A division of the nervous system of vertebrates. The nervous system consists of two major subdivisions: the central nervous system (CNS), made up of the brain and spinal cord, and the peripheral nervous system (PNS), which comprises ganglia and peripheral nerves outside the brain and the spinal cord. The peripheral nervous system is divided into two parts: the somatic, which is concerned with sensory information about the environment outside the body as well as muscle and limb position; and the autonomic nervous system that regulates the internal environment of vertebrates. It consists of the sympathetic (fight/flight), parasympathetic (rest/rebuild), and enteric nervous systems. The ANS is involved in the function of virtually every organ system.

The parasympathetic nervous system takes care of essential background operations such as heart/lungs and digestion, while the sympathetic nervous system provides stress-response and procreation strategies and functions. The enteric nervous system takes care of controlling the function of the gut.

The sympathetic nerves form part of the nerve network connecting the organ systems with the central nervous system. The sympathetic nerves permit an animal to respond to stressful situations and helps control the reaction of the body to stress. Examples of the sympathetic reactions are increase in heart rate, decrease in secretion of salivary and digestive glands, and dilation of pupils. The parasympathetic nerves connect both somatic and visceral organs to the central nervous system, and their primary action is to keep body functions normalized. The ANS works to conserve the body’s resources and to restore equilibrium to the resting state.

 What is autophytic ?

The process whereby an organism uses photosynthesis to make complex foods from inorganic substances.

 What is autopolyploid ?

A type of polyploid species resulting from one species doubling its chromosome number to become tetraploid, which may self-fertilize or mate with other tetraploids. This can result in sympatric speciation, where a new species can evolve in the geographical midst of its parent species because of reproductive isolation.

 What is autoreceptor ?

 Present at a nerve ending, a RECEPTOR that regulates, via positive or negative feedback processes, the synthesis and/or release of its own physiological ligand.

 What is autosome

A chromosome that is not directly involved in determining sex, as opposed to the sex chromosomes or the mitochondrial chromosome. Human cells have 22 pairs of autosomes.

What is autotroph ?

Any organism capable of making its own food. It synthesizes its own organic food substances from inorganic compounds using sources such as carbon dioxide, ammonia, and nitrates. Most plants and many protists and bacteria are autotrophs. Photoautotrophs can use light energy to make their food (photosynthesis). Chemoautotrophs use chemical energy to make their food by oxidizing compounds such as hydrogen sulfide (H2S). Heterotrophs are organisms that must obtain their energy from organic compounds

auxins

A group of plant hormones that produce a number of effects, including plant growth, phototropic response through the stimulation of cell elongation (photopropism), stimulation of secondary growth, apical dominance, and the development of leaf traces and fruit. An important plant auxin is indole-3-acetic acid (IAA). (IAA and synthetic auxins such as 2,4-D and 2,4,5-T are used as common weed killers.)

auxotroph

A nutritionally mutant organism that is unable to synthesize certain essential molecules (e.g., mineral salts and glucose) and that cannot grow on media lacking these molecules normally synthesized by wild-type strains of the same species without the addition of a specific supplement like an amino acid.

What is axillary bud ?

An embryonic shoot present in the angle formed by a leaf and stem. Also called the lateral bud.

What is axon ?

A process from a neuron, usually covered with a myelin sheath, that carries nerve impulses away from the cell body and to the synapse in contact with a target cell. The end of the axon contains vesicles (hollow spheres), in which transmitters are stored, and specialized structures forming the synapse.

What is azurin ?

An ELECTRON TRANSFER PROTEIN, containing a TYPE 1 COPPER site, that is isolated from certain bacteria.