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. See also artery.
Artery 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.
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.
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 (karyo-gamy) 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 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.
A tailless whip scorpion (arthropod) from a cave in the Bahamas is an example of a troglodyte, an animal that lives underground.
Assimilation To transform food and other nutrients into a part of the living organism.
Associative learning 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 A carbon atom covalently bonded to four different atoms or groups of atoms.
Asymmetric synthesis A traditional term for stereo-selective synthesis. A chemical reaction or reaction sequence in which one or more new elements of chi-rality are formed in a substrate molecule and which produces the stereoisomer^ (enantiomeric or diastere0is0meric) products in unequal amounts.
Asymmetry parameter In nuclear quadrupole resonance spectroscopy, the parameter, n, is used for describing nonsymmetric fields. It is defined as n = (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.
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.
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.
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 ender-gonic 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.
ATP synthase (proton translocating ATPase) A protein complex (a chemiosmotic enzyme) that synthesizes adenosine triphosphate (ATP) from adeno-sine 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 model According to autogenesis ("self-generating"), eukaryotic cells evolved by the specialization of internal membranes originally derived from prokaryotic plasma membranes. This is another word for spontaneous generation or abiogenesis.
Autoimmune disease An immunological disorder in which the immune system turns against itself. Autoim-munity 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 pem-phigoid (a blistering disease).
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 auto-nomic 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.
Autophytic The process whereby an organism uses photosynthesis to make complex foods from inorganic substances.
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 spe-ciation, where a new species can evolve in the geographical midst of its parent species because of reproductive isolation.
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.
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.
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.
Aves The vertebrate class of birds, characterized by feathers and other flight adaptations, such as an active metabolism, and distinguished by having the body more or less completely covered with feathers and the forelimbs modified as wings. Birds are a monophyletic lineage that evolved once from a common ancestor, and all birds are related through that common origin. There are about 30 orders of birds, about 180 families, and about 2,000 genera with 10,000 species.
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.
Azurin An electron transfer protein, containing a type 1 copper site, that is isolated from certain bacteria.
Bacteria One of two prokaryotic (no nucleus) domains, the other being the archaea. Bacteria are microscopic, simple, single-cell organisms. Some bacteria are harmless and often beneficial, playing a major role in the cycling of nutrients in ecosystems via aerobic and anaerobic decomposition (saprophytic), while others are pathogenic, causing disease and even death. Some species form symbiotic relationships with other organisms, such as legumes, and help them survive in the environment by fixing atmospheric nitrogen. Many different species exist as single cells or colonies, and they fall into four shapes based on the shape of their rigid cell wall: coccal (spherical), bacillary (rod-shaped), spirochetal (spiral/helical or corkscrew), and vibro (comma-shaped). Bacteria are also classified on the basis of oxygen requirement (aerobic vs. anaerobic).
Photomicrograph of Streptococcus (Diplococcus) pneumoniae bacteria, using Gram’s stain technique. Streptococcus pneumoniae is one of the most common organisms causing respiratory infections such as pneumonia and sinusitis, as well as bacteremia, otitis media, meningitis, peritonitis, and arthritis.
In the laboratory, bacteria are classified as gram-positive (blue) or gram-negative (pink) following a laboratory procedure called a Gram’s stain. Gram-negative bacteria, such as those that cause the plague, cholera, typhoid fever, and salmonella, for example, have two outer membranes, which make them more resistant to conventional treatment. They can also easily mutate and transfer these genetic changes to other strains, making them more resistant to antibiotics. Gram-positive bacteria, such as those that cause anthrax and liste-riosis, are more rare and are treatable with penicillin but can cause severe damage by either releasing toxic chemicals (e.g., clostridium botulinum) or by penetrating deep into tissue (e.g., streptococci). Bacteria are often called germs.
Bacteriochlorin (7,8,17,18-tetrahydroporphyrin) A reduced porphyrin with two pairs of nonfused saturated carbon atoms (C-7, C-8 and C-17, C-18) in two of the pyrrole rings.
Bacterium (plural, bacteria) A single-celled pro-karyotic microorganism in the bacteria domain.