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FLJR/SRHS Biology - High School

Terms

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abdomen
In vertebrates, the portion of the trunk containing visceral organs other than heart and lungs; in arthropods, the posterior portion of the body, made up of similar segments and containing the reproductive organs and part of the digestive tract.
abiotic
Nonliving; specifically, the nonliving components of an ecosystem, such as temperature, humidity, the mineral content of the soil, etc.
abscisic acid (ABA)
A plant hormone that generally acts to inhibit growth, promote dormancy, and help the plant tolerate stressful conditions.
abscission
In plants, the dropping of leaves, flowers, fruits, or stems at the end of a growing season, as the result of formation of a two-layered zone of specialized cells (the abscission zone) and the action of a hormone (ethylene).
absorption
The movement of water and dissolved substances into a cell, tissue, or organism.
absorption spectrum
The range of a pigment's ability to absorb various wavelengths of light.
abyssal zone
The portion of the ocean floor where light does not penetrate and where temperatures are cold and pressures intense.
acclimatization
Physiological adjustment to a change in an environmental factor.
accommodation
The automatic adjustment of an eye to focus on near objects.
acetylcholine
One of the most common neurotransmitters; functions by binding to receptors and altering the permeability of the postsynaptic membrane to specific ions, either depolarizing or hyperpolarizing the membrane.
acetyl CoA
The entry compound for the Krebs cycle in cellular respiration; formed from a fragment of pyruvate attached to a coenzyme.
acid
A substance that increases the hydrogen ion concentration in a solution.
acid precipitation
Rain, snow, or fog that is more acidic than pH 5.6.
acoelomate
A solid-bodied animal lacking a cavity between the gut and outer body wall.
acrosome
An organelle at the tip of a sperm cell that helps the sperm penetrate the egg.
ACTH
Abbreviation of adrenocorticotropic hormone.
actin
A globular protein that links into chains, two of which twist helically about each other, forming microfilaments in muscle and other contractile elements in cells.
action potential
A rapid change in the membrane potential of an excitable cell, caused by stimulus-triggered, selective opening and closing of voltage-sensitive gates in sodium and potassium ion channels.
activation energy
The energy that must be possessed by atoms or molecules in order to react.
active site
The specific portion of an enzyme that attaches to the substrate by means of weak chemical bonds.
active transport
The movement of a substance across a biological membrane against its concentration or electrochemical gradient, with the help of energy input and specific transport proteins.
adaptation
(1) The evolution of features that make a group of organisms better suited to live and reproduce in their environment. (2) A peculiarity of structure, physiology, or behavior that aids the organism in its environment.
adaptive peak
An equilibrium state in a population when the gene pool has allele frequencies that maximize the average fitness of a population's members.
adaptive radiation
The emergence of numerous species from a common ancestor introduced into an environment, presenting a diversity of new opportunities and problems.
adenosine diphosphate (ADP)
A nucleotide consisting of adenine, ribose, and two phosphate groups; formed by the removal of one phosphate from an ATP molecule.
adenosine monophosphate (AMP)
A nucleotide consisting of adenine, ribose, and one phosphate group; can be formed by the removal of two phosphates from an ATP molecule; in its cyclic form, functions as a "second messenger" for a number of vertebrate hormones and neurotransmitters.
adenosine triphosphate (ATP)
An adenine-containing nucleoside triphosphate that releases free energy when its phosphate bonds are hydrolyzed. This energy is used to drive endergonic reactions in cells.
adenylyl cyclase
An enzyme that converts ATP to cyclic AMP in response to a chemical signal.
ADH
Abbreviation of antidiuretic hormone.
adhesion
The tendency of different kinds of molecules to stick together.
ADP
Abbreviation of adenosine diphosphate.
adrenal gland
An endocrine gland located adjacent to the kidney in mammals; composed of two glandular portions: an outer cortex, which responds to endocrine signals in reacting to stress and effecting salt and water balance, and a central medulla, which responds to nervous inputs resulting from stress.
adrenaline
A hormone, produced by the medulla of the adrenal gland, that increases the concentration of glucose in the blood, raises blood pressure and heartbeat rate, and increases muscular power and resistance to fatigue; also a neurotransmitter across synaptic junctions. Also called epinephrine. See Epinephrine.
adrenocorticotropic hormone (ACTH)
A hormone, produced by the anterior lobe of the pituitary gland, that stimulates the production of cortisol by the adrenal cortex.
adventitious
Referring to a structure arising from an unusual place, such as roots growing from stems or leaves.
aerobic
Containing oxygen; referring to an organism, environment, or cellular process that requires oxygen.
afferent
Bringing inward to a central part, applied to nerves and blood vessels.
agar
A gelatinous material prepared from certain red algae that is used to solidify nutrient media for growing microorganisms.
age structure
The relative number of individuals of each age in a population.
agnathan
A member of a jawless class of vertebrates represented today by the lampreys and hagfishes.
agonistic behavior
A type of behavior involving a contest of some kind that determines which competitor gains access to some resource, such as food or mates.
AIDS (acquired immunodeficiency syndrome)
The name of the late stages of HIV infection; defined by a specified reduction of T cells and the appearance of characteristic secondary infections.
aldehyde
An organic molecule with a carbonyl group located at the end of the carbon skeleton.
aldosterone
An adrenal hormone that acts on the distal tubules of the kidney to stimulate the reabsorption of sodium (Na+) and the passive flow of water from the filtrate.
aleurone layer
The outermost cell layer of the endosperm of the grains (seeds) of wheat and other grasses; when acted upon by gibberellin, the aleurone layer releases enzymes that digest the stored food of the endosperm into small nutrient molecules that can be taken up by the embryo.
alga pl. algae
A photosynthetic, plantlike protist.
alkaline
Pertaining to substances that increase the relative number of hydroxide ions (OH–) in a solution; having a pH greater than 7; basic; opposite of acidic.
all-or-none event
An action that occurs either completely or not at all, such as the generation of an action potential by a neuron.
allantois
One of four extraembryonic membranes; serves as a repository for the embryo's nitrogenous waste.
allele
An alternative form of a gene.
allele frequency
The proportion of a particular allele in a population.
allergic reaction
An inflammatory response triggered by a weak antigen (an allergen) to which most individuals do not react; involves the release of large amounts of histamine from mast cells.
allometric growth
The variation in the relative rates of growth of various parts of the body, which helps shape the organism.
allopatric speciation
A mode of speciation induced when the ancestral population becomes segregated by a geographical barrier.
allopolyploid
A common type of polyploid species resulting from two different species interbreeding and combining their chromosomes.
allosteric site
A specific receptor site on an enzyme molecule remote from the active site. Molecules bind to the allosteric site and change the shape of the active site, making it either more or less receptive to the substrate.
allozymes
Slightly different versions of the same enzyme, distinguishable via gel electrophoresis.
alpha helix
A spiral shape constituting one form of the secondary structure of proteins, arising from a specific hydrogen-bonding structure.
alternation of generations
A life cycle in which there is both a multicellular diploid form, the sporophyte, and a multicellular haploid form, the gametophyte; characteristic of plants.
alternative splicing
In alternative splicing, the same pre-mRNA molecule, which consists of introns and exons, is spliced in different ways to produce mature mRNAs of different lengths and different functionality.
altruistic behavior
The aiding of another individual at one's own risk or expense.
alveolus pl. alveoli
(1) One of the deadend, multilobed air sacs that constitute the gas exchange surface of the lungs. (2) One of the milk-secreting sacs of epithelial tissue in the mammary glands.
amino acid
An organic molecule possessing both carboxyl and amino groups. Amino acids serve as the monomers of proteins.
amino group
A functional group that consists of a nitrogen atom bonded to two hydrogen atoms; can act as a base in solution, accepting a hydrogen ion and acquiring a charge of +1.
aminoacyl—tRNA synthetases
A family of enzymes, at least one for each amino acid, that catalyze the attachment of an amino acid to its specific tRNA molecule.
ammonification
The process by which decomposers break down proteins and amino acids, releasing the excess nitrogen in the form of ammonia (NH3) or ammonium ion (NH4+).
amniocentesis
A technique for determining genetic abnormalities in a fetus by the presence of certain chemicals or defective fetal cells in the amniotic fluid, obtained by aspiration from a needle inserted into the uterus.
amnion
The innermost of four extraembryonic membranes; encloses a fluid-filled sac in which the embryo is suspended.
amniote
A vertebrate possessing an amnion surrounding the embryo; reptiles, birds, and mammals are amniotes.
amniotic egg
A shelled, water-retaining egg that enables reptiles, birds, and egg-laying mammals to complete their life cycles on dry land.
amoeboid
Moving or feeding by means of pseudopodia (temporary cytoplasmic protrusions from the cell body).
AMP
Abbreviation of adenosine monophosphate.
Amphibia
The vertebrate class of amphibians, represented by frogs, salamanders, and caecilians.
amphipathic molecule
A molecule that has both a hydrophilic region and a hydrophobic region.
anabolic steroids
Synthetic chemical variants of the male sex hormone testosterone; they produce increased muscle mass but also suppress testosterone production, leading to shrinkage of the testes, growth of the breasts, and premature baldness; long-term use increases the risk of kidney and liver damage and of liver cancer.
anabolism
Within a cell or organism, the sum of all biosynthetic reactions (that is, chemical reactions in which larger molecules are formed from smaller ones).
anaerobic
Lacking oxygen; referring to an organism, environment, or cellular process that lacks oxygen and may be poisoned by it.
anagenesis
A pattern of evolutionary change involving the transformation of an entire population, sometimes to a state different enough from the ancestral population to justify renaming it as a separate species; also called phyletic evolution.
analogy
The similarity of structure between two species that are not closely related; attributable to convergent evolution.
analogous
Applied to structures similar in function but different in evolutionary origin, such as the wing of a bird and the wing of an insect.
anaphase
The third stage of mitosis, beginning when the centromeres of duplicated chromosomes divide and sister chromotids separate from each other, and ending when a complete set of daughter chromosomes are located at each of the two poles of the cell.
androgen
The principal male steroid hormones, such as testosterone, which stimulate the development and maintenance of the male reproductive system and secondary sex characteristics.
aneuploidy
A chromosomal aberration in which certain chromosomes are present in extra copies or are deficient in number.
angiosperm
A flowering plant, which forms seeds inside a protective chamber called an ovary.
anion
A negatively charged ion.
annual
A plant that completes its entire life cycle in a single year or growing season.
antennae
Long, paired sensory appendages on the head of many arthropods.
anterior
Referring to the head end of a bilaterally symmetrical animal.
anther
The terminal pollen sac of a stamen, inside which pollen grains with male gametes form in the flower of an angiosperm.
antheridium pl. antheridia
In plants, the male gametangium, a moist chamber in which gametes develop.
anthocyanin
Natural water-soluble pigments of blue, purple or red which are dissolved in the cell-sap vacuole of plant cells.
anthropoid
A higher primate; includes monkeys, apes, and humans.
antibiotic
A chemical that kills bacteria or inhibits their growth.
antibody
An antigen-binding immunoglobulin, produced by B cells, that functions as the effector in an immune response.
anticodon
A specialized base triplet on one end of a tRNA molecule that recognizes a particular complementary codon on an mRNA molecule.
antidiuretic hormone (ADH)
A hormone important in osmoregulation.
antigen
A foreign macromolecule that does not belong to the host organism and that elicits an immune response.
aorta
The major artery in blood-circulating systems; the aorta sends blood to the other body tissues.
aphotic zone
The part of the ocean beneath the photic zone, where light does not penetrate sufficiently for photosynthesis to occur.
apical dominance
Concentration of growth at the tip of a plant shoot, where a terminal bud partially inhibits axillary bud growth.
apical meristem
Embryonic plant tissue in the tips of roots and in the buds of shoots that supplies cells for the plant to grow in length.
apomorphic character
A derived phenotypic character, or homology, that evolved after a branch diverged from a phylogenetic tree.
apoplast
In plants, the nonliving continuum formed by the extracellular pathway provided by the continuous matrix of cell walls.
apoptosis
Programmed cell death brought about by signals that trigger the activation of a cascade of "suicide" proteins in the cells destined to die.
aposematic coloration
The bright coloration of animals with effective physical or chemical defenses that acts as a warning to predators.
aquaporin
A transport protein in the plasma membranes of a plant or animal cell that specifically facilitates the diffusion of water across the membrane (osmosis).
aqueous solution
A solution in which water is the solvent.
arboreal
Tree-dwelling.
Archaea
One of two prokaryotic domains, the other being the Bacteria.
archegonium pl. archegonia
In plants, the female gametangium, a moist chamber in which gametes develop.
archenteron
The endoderm-lined cavity, formed during the gastrulation process, that develops into the digestive tract of an animal.
Archezoa
Primitive eukaryotic group that includes diplomonads, such as Giardia; some systematists assign kingdom status to archezoans.
arteriole
A very small artery. See also artery.
artery
A vessel that carries blood away from the heart to organs throughout the body.
arteriosclerosis
A cardiovascular disease caused by the formation of hard plaques within the arteries.
artificial selection
The selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits.
ascus pl. asci
A saclike spore capsule located at the tip of the ascocarp in dikaryotic hyphae; defining feature of the Ascomycota division of fungi.
asexual reproduction
A type of reproduction involving only one parent that produces genetically identical offspring by budding or by the division of a single cell or the entire organism into two or more parts.
assimilation
The energy-requiring process by which plant cells convert nitrate ions (NO3–) taken up by the roots of plants into ammonium ions (NH4+), which can then be used in the synthesis of amino acids and other nitrogenous compounds.
associative learning
The acquired ability to associate one stimulus with another; also called classical conditioning.
assortative mating
A type of nonrandom mating in which mating partners resemble each other in certain phenotypic characters.
asymmetric carbon
A carbon atom covalently bonded to four different atoms or groups of atoms.
atmospheric pressure
The weight of the Earth's atmosphere over a unit area of the Earth's surface.
atom
The smallest unit of matter that retains the properties of an element.
atomic number
The number of protons in the nucleus of an atom, unique for each element and designated by a subscript to the left of the elemental symbol.
atomic weight
The total atomic mass, which is the mass in grams of one mole of the atom.
ATP
Abbreviation of adenosine triphosphate, the principal energy-carrying compound of the cell.
ATP synthase
A cluster of several membrane proteins found in the mitochondrial cristae (and bacterial plasma membrane) that function in chemiosmosis with adjacent electron transport chains, using the energy of a hydrogen-ion concentration gradient to make ATP. ATP synthases provide a port through which hydrogen ions diffuse into the matrix of a mitrochondrion.
atrioventricular node
A group of slow-conducting fibers in the atrium of the vertebrate heart that are stimulated by impulses originating in the sinoatrial node (the pacemaker) and that conduct impulses to the bundle of His, a group of fibers that stimulate contraction of the ventricles.
atrioventricular valve
A valve in the heart between each atrium and ventricle that prevents a backflow of blood when the ventricles contract.
atrium pl. atria
A chamber that receives blood returning to the vertebrate heart.
autogenesis model
According to this model, eukaryotic cells evolved by the specialization of internal membranes originally derived from prokaryotic plasma membranes.
autoimmune disease
An immunological disorder in which the immune system turns against itself.
autonomic nervous system
A subdivision of the motor nervous system of vertebrates that regulates the internal environment; consists of the sympathetic and parasympathetic divisions.
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.
autosome
A chromosome that is not directly involved in determining sex, as opposed to the sex chromosomes.
autotroph
An organism that obtains organic food molecules without eating other organisms. Autotrophs use energy from the sun or from the oxidation of inorganic substances to make organic molecules from inorganic ones.
auxin
A class of plant hormones, including indoleacetic acid (IAA), having a variety of effects, such as phototropic response through the stimulation of cell elongation, stimulation of secondary growth, and the development of leaf traces and fruit.
auxotroph
A nutritional mutant that is unable to synthesize and that cannot grow on media lacking certain essential molecules normally synthesized by wild-type strains of the same species.
Aves
The vertebrate class of birds, characterized by feathers and other flight adaptations.
axillary bud
An embryonic shoot present in the angle formed by a leaf and stem.
axis
An imaginary line passing through a body or organ around which parts are symmetrically aligned.
axon
A typically long extension, or process, from a neuron that carries nerve impulses away from the cell body toward target cells.
B cell
A type of lymphocyte that develops in the bone marrow and later produces antibodies, which mediate humoral immunity.
Bacteria
One of two prokaryotic domains, the other being the Archaea.
bacteriophage
A virus that parasitizes a bacterial cell.
bacterium pl. bacteria
A prokaryotic microorganism in Domain Bacteria.
balanced polymorphism
A type of polymorphism in which the frequencies of the coexisting forms do not change noticeably over many generations.
bark
All tissues external to the vascular cambium in a plant growing in thickness, consisting of phloem, phelloderm, cork cambium, and cork.
Barr body
A dense object lying along the inside of the nuclear envelope in female mammalian cells, representing an inactivated X chromosome.
basal body
A eukaryotic cell organelle consisting of a 9 + 0 arrangement of microtubule triplets; may organize the microtubule assembly of a cilium or flagellum; structurally identical to a centriole.
basal metabolic rate (BMR)
The minimal number of kilocalories a resting animal requires to fuel itself for a given time.
base
A substance that reduces the hydrogen ion concentration in a solution. See Alkaline.
basement membrane
The floor of an epithelial membrane on which the basal cells rest.
base-pair substitution
A point mutation; the replacement of one nucleotide and its partner from the complementary DNA strand by another pair of nucleotides.
base-pairing principle
In the formation of nucleic acids, the requirement that adenine must always pair with thymine (or uracil) and guanine with cytosine.
basidium pl. basidia
A reproductive appendage that produces sexual spores on the gills of mushrooms. The fungal division Basidiomycota is named for this structure.
Batesian mimicry
A type of mimicry in which a harmless species looks like a different species that is poisonous or otherwise harmful to predators.
behavior
All of the acts an organism performs, as in, for example, seeking a suitable habitat, obtaining food, avoiding predators, and seeking a mate and reproducing.
behavioral ecology
A heuristic approach based on the expectation that Darwinian fitness (reproductive success) is improved by optimal behavior.
benthic zone
The bottom surfaces of aquatic environments.
biennial
A plant that requires two years to complete its life cycle.
bilateral symmetry
Characterizing a body form with a central longitudinal plane that divides the body into two equal but opposite halves.
bilateria
Members of the branch of eumetazoans possessing bilateral symmetry.
bile
A yellow secretion of the vertebrate liver, temporarily stored in the gallbladder and composed of organic salts that emulsify fats in the small intestine.
binary fission
The type of cell division by which prokaryotes reproduce; each dividing daughter cell receives a copy of the single parental chromosome.
binomial
The two-part Latinized name of a species, consisting of genus and specific epithet.
biochemical pathway
An ordered series of chemical reactions in a living cell, in which each step is catalyzed by a specific enzyme; different biochemical pathways serve different functions in the life of the cell.
biodiversity hotspot
A relatively small area with an exceptional concentration of species.
bioenergetics
The study of how organisms manage their energy resources.
biogeochemical cycles
The various nutrient circuits, which involve both biotic and abiotic components of ecosystems.
biogeography
The study of the past and present distribution of species.
biological clock
Proposed internal factor(s) in organisms that governs functions that occur rhythmically in the absence of external stimuli.
biological magnification
A trophic process in which retained substances become more concentrated with each link in the food chain.
biological species
A population or group of populations whose members have the potential to interbreed.
biomass
The dry weight of organic matter comprising a group of organisms in a particular habitat.
biome
One of the world's major communities, classified according to the predominant vegetation and characterized by adaptations of organisms to that particular environment.
biosphere
The entire portion of Earth that is inhabited by life; the sum of all the planet's communities and ecosystems.
biosynthesis
Formation by living organisms of organic compounds from elements or simple compounds.
biotechnology
The industrial use of living organisms or their components to improve human health and food production.
biotic
Pertaining to the living organisms in the environment.
bipedal
Walking upright on two feet.
blade
(1) The broad, expanded part of a leaf. (2) The broad, expanded photosynthetic part of the thallus of a multicellular alga or a simple plant.
blastocoel
The fluid-filled cavity that forms in the center of the blastula embryo.
blastocyst
An embryonic stage in mammals; a hollow ball of cells produced one week after fertilization in humans.
blastodisc
Disklike area on the surface of a large, yolky egg that undergoes cleavage and gives rise to the embryo.
blastopore
The opening of the archenteron in the gastrula that develops into the mouth in protostomes and the anus in deuterostomes.
blastula
The hollow ball of cells marking the end stage of cleavage during early embryonic development.
blood
A type of connective tissue with a fluid matrix called plasma in which blood cells are suspended.
blood-brain barrier
A specialized capillary arrangement in the brain that restricts the passage of most substances into the brain, thereby preventing dramatic fluctuations in the brain's environment.
blood pressure
The hydrostatic force that blood exerts against the wall of a vessel.
bond energy
The quantity of energy that must be absorbed to break a particular kind of chemical bond; equal to the quantity of energy the bond releases when it forms.
bond strength
The strength with which a chemical bond holds two atoms together; conventionally measured in terms of the amount of energy, in kilocalories per mole, required to break the bond.
book lungs
Organs of gas exchange in spiders, consisting of stacked plates contained in an internal chamber.
botany
The study of plants.
bottleneck effect
Genetic drift resulting from the reduction of a population, typically by a natural disaster, such that the surviving population is no longer genetically representative of the original population.
Bowman's capsule
A cup-shaped receptacle in the vertebrate kidney that is the initial, expanded segment of the nephron where filtrate enters from the blood.
box
A small portion of a gene or protein that appears in many genes or proteins that are related in structure; the box usually has some specific function, sometimes called a "motif", like binding DNA or interacting with specific proteins or other molecules.
brain
The master control center in an animal; in vertebrates, the brain and spinal cord make up the central nervous system.
brainstem
The hindbrain and midbrain of the vertebrate central nervous system. In humans, it forms a cap on the anterior end of the spinal cord, extending to about the middle of the brain.
bryophyte
The mosses, liverworts, and hornworts; a group of nonvascular plants that inhabit the land but lack many of the terrestrial adaptations of vascular plants.
bronchus pl. bronchi
One of a pair of respiratory tubes branching into either lung at the lower end of the trachea; it subdivides into progressively finer passageways, the bronchioles, culminating in the alveoli.
bud
(1) In plants, an embryonic shoot, including rudimentary leaves, often protected by special bud scales. (2) In animals, an asexually produced outgrowth that develops into a new individual.
budding
An asexual means of propagation in which outgrowths from the parent form and pinch off to live independently or else remain attached to eventually form extensive colonies.
buffer
A substance that consists of acid and base forms in solution and that minimizes changes in pH when extraneous acids or bases are added to the solution.
bulb
A modified bud with thickened leaves adapted for underground food storage.
bulbourethral gland
One of a pair of glands near the base of the penis in the human male that secrete fluid that lubricates and neutralizes acids in the urethra during sexual arousal.
bulk flow
The movement of water due to a difference in pressure between two locations.
bundle of His
In the vertebrate heart, a group of muscle fibers that carry impulses from the atrioventricular node to the walls of the ventricles; the only electrical bridge between the atria and the ventricles.
C3 pathway
See Calvin cycle.
C3 plant
A plant that uses the Calvin cycle for the initial steps that incorporate CO2 into organic material, forming a three-carbon compound as the first stable intermediate.
C4 pathway
The set of reactions by which some plants initially fix carbon in the four-carbon compound oxaloacetic acid; the carbon dioxide is later released in the interior of the leaf and enters the Calvin cycle.
C4 plant
A plant that prefaces the Calvin cycle with reactions that incorporate CO2 into four-carbon compounds, the end-product of which supplies CO2 for the Calvin cycle.
calcitonin
A mammalian thyroid hormone that lowers blood calcium levels.
callus
In plants, undifferentiated tissue; a term used in tissue culture, grafting, and wound healing.
calmodulin
An intracellular protein to which calcium binds in its function as a second messenger in hormone action.
calorie (cal)
The amount of heat energy required to raise the temperature of 1 g of water 1°C; the amount of heat energy that 1 g of water releases when it cools by 1°C. The Calorie (with a capital C), usually used to indicate the energy content of food, is a kilocalorie.
Calvin cycle
The second of two major stages in photosynthesis (following the light reactions), involving atmospheric CO2 fixation and reduction of the fixed carbon into carbohydrate.
calyx
Collectively, the sepals of a flower.
CAM photosynthesis
See Crassulacean acid metabolism.
CAM plant
A plant that uses crassulacean acid metabolism, an adaptation for photosynthesis in arid conditions, first discovered in the family Crassulaceae. Carbon dioxide entering open stomata during the night is converted into organic acids, which release CO2 for the Calvin cycle during the day, when stomata are closed.
Cambrian explosion
A burst of evolutionary origins when most of the major body plans of animals appeared in a relatively brief time in geological history; recorded in the fossil record about 545 to 525 million years ago.
capillary
A microscopic blood vessel that penetrates the tissues and consists of a single layer of endothelial cells that allows exchange between the blood and interstitial fluid.
capillary action
The movement of water or any liquid along a surface; results from the combined effect of cohesion and adhesion.
capsid
The protein shell that encloses the viral genome; rod-shaped, polyhedral, or more completely shaped.
capsule
(1) A slimy layer around the cells of certain bacteria. (2) The sporangium of a bryophyte.
carbohydrate
A sugar (monosaccharide) or one of its dimers (disaccharides) or polymers (polysaccharides).
carbon cycle
Worldwide circulation and reutilization of carbon atoms, chiefly due to metabolic processes of living organisms. Inorganic carbon, in the form of carbon dioxide, is incorporated into organic compounds by photosynthetic organisms; when the organic compounds are broken down in respiration, carbon dioxide is released. Large quantities of carbon are "stored" in the seas and the atmosphere, as well as in fossil fuel deposits.
carbon fixation
The incorporation of carbon from CO2 into an organic compound by an autotrophic organism (a plant, another photosynthetic organism, or a chemoautotrophic bacterium).
carbonyl group
A functional group present in aldehydes and ketones, consisting of a carbon atom double-bonded to an oxygen atom.
carboxyl group
A functional group present in organic acids, consisting of a single carbon atom double-bonded to an oxygen atom and also bonded to a hydroxyl group.
carcinogen
A chemical agent that causes cancer.
cardiac muscle
A type of muscle that forms the contractile wall of the heart; its cells are joined by intercalated discs that relay each heartbeat.
cardiac output
The volume of blood pumped per minute by the left ventricle of the heart.
cardiovascular system
A closed circulatory system with a heart and branching network of arteries, capilleries, and veins.
carnivore
An animal, such as a shark, hawk, or spider, that eats other animals.
carotenoids
Accessory pigments, yellow and orange, in the chloroplasts of plants; by absorbing wavelengths of light that chlorophyll cannot, they broaden the spectrum of colors that can drive photosynthesis.
carpel
The female reproductive organ of a flower, consisting of the stigma, style, and ovary.
carrying capacity
The maximum population size that can be supported by the available resources, symbolized as K.
cartilage
A type of flexible connective tissue with an abundance of collagenous fibers embedded in chondrin.
Casparian strip
A water-impermeable ring of wax around endodermal cells in plants that blocks the passive flow of water and solutes into the stele by way of cell walls.
catabolic pathway
A metabolic pathway that releases energy by breaking down complex molecules into simpler compounds.
catabolism
Within a cell or organism, the sum of all chemical reactions in which large molecules are broken down into smaller parts.
catabolite activator protein (CAP)
In E. coli, a helper protein that stimulates gene expression by binding within the promoter region of an operon and enhancing the promoter's ability to associate with RNA polymerase.
catalyst
A substance that lowers the activation energy of a chemical reaction by forming a temporary association with the reacting molecules; as a result, the rate of the reaction is accelerated. Enzymes are catalysts.
category
In a hierarchical classification system, the level at which a particular group is ranked.
cation
An ion with a positive charge, produced by the loss of one or more electrons.
cation exchange
A process in which positively charged minerals are made available to a plant when hydrogen ions in the soil displace mineral ions from the clay particles.
cell
A basic unit of living matter separated from its environment by a plasma membrane; the fundamental structural unit of life.
cell center
A region in the cytoplasm near the nucleus from which microtubules originate and radiate.

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