absorption: the process in which nutrients enter cells of the villi, then move across the cells and enter blood vessels.

acids: compounds that release hydrogen ions (H+) when the compounds are placed in water.

actin: a protein filament within the sarcomeres of muscle cells.

action potential: occurs when a neuron is displaying a nerve impulse.

active site: the portion of an enzyme that interacts with the substrate.

active transport: the movement of molecules across a membrane from a region of low concentration to a region of high concentration that requires the expenditure of energy (ATP).

adenosine diphosphate (ADP): a product of adenosine triphosphate (ATP) breakdown.

adenosine triphosphate (ATP): the chemical substance that serves as the currency of energy in cells.

adrenal glands: two glands lying atop the kidneys that produce a family of steroids.

aerobic: organisms that require oxygen for their metabolism.

algae: a large number of photosynthetic organisms that are generally unicellular and not classified as plants.

alleles: different forms of the same gene.

alveoli: microscopic air sacs that are surrounded by a rich network of blood vessels in mammalian lungs that function in gas exchange; the air sacs are at the end of the bronchioles.

amino acids: the building blocks of proteins.

amoeba: single-celled organisms with no distinct shape; members of the phylum Sarcodina.

anabolism: the process of synthesizing large molecules by joining smaller molecules together.

anaerobic: organisms that thrive in an oxygen-free environment.

anaphase: a phase during mitosis in which chromatids separate to become visible chromosomes and migrate to opposite poles.

anaphase I: a phase during meiosis in which homologous chromosomes separate.

anaphase II: a phase during meiosis II in which the centromeres divide and the chromosomes separate from one another.

androgens: hormones, such as testosterone, produced from the testes that promote secondary male characteristics.

Animalia: the kingdom that includes the animals.

antibodies: proteins synthesized by plasma cells that are released into the circulation to the antigen site and destroy the microorganisms by chemically reacting with them.

antibody-mediated immunity: the process by which antibodies bind to antigens and destroy the microorganisms in a highly specific manner.

anticodon: the complementary codon present on a tRNA molecule.

antigens: the immune-stimulating polysaccharides on the surface of cells.

aorta: the major artery of the human circulatory system that receives blood from the left ventricle.

appendix :a small fingerlike process that may be a vestige of larger organs functional in human ancestors.

archaebacteria: ancient bacteria that have a different ribosomal structure, membrane composition, and cell wall composition than modern bacteria.

arteries: the channels through which fluid flows away from the heart.

atom: the smallest part of an element that can enter into various combinations with atoms of other elements.

atrium: a thin-walled receiving chamber in which blood accumulates in fishes.

auditory nerve: the nerve within the ear that carries impulses to the brain for interpretation.

autonomic nervous system: a subdivision of the peripheral nervous system, which is divided into the sympathetic and parasympathetic nervous systems.

autosomes: the 22 pairs of human chromosomes that are not sex chromosomes.

autotrophic: certain bacteria that synthesize their own foods.

axon: the long extension of a neuron.

bacilli: the rod-shaped bacteria (singular, bacillus).

bark: the structure of vascular plants formed between the phloem and the cork.

bases: compounds that attract hydrogen atoms when placed in water.

basophils: the white blood cells that function in allergic responses.

bicuspid (mitral) valve: the valve that leads into the left ventricle of the human heart.

binomial name: the scientific name of an organism, which contains two elements.

biomass: the total dry weight of food at each level of the food pyramid.

biome: a group of communities dominated by a particular climax community, such as deserts, forests, and prairies.

biosphere: the blanket of living things that surrounds the substratum of the earth.

blastocyst: a hollow ball of cells resulting after the morula has passed through the Fallopian tubes and enters the female uterus.

blood clotting: the process in which platelets adhere to the walls of damaged blood vessels, setting off a series of processes leading to the formation of a patchy mesh at the injury site.

blue-green algae: cyanobacteria; members of the kingdom Monera that are photosynthetic and are found in the soil and in freshwater or saltwater environments.

B lymphocytes: white blood cells within the lymph nodes; stimulated by microorganisms or other foreign materials in the blood.

Bowman's capsule: an enlarged cuplike structure below the nephron in the human kidney.

bronchi: two large tubes at the lower end of the trachea (singular, bronchus).

bronchioles: the branches formed from the bronchi.

capillaries: the microscopic blood vessels between the arteries and the veins.

carbohydrates: the primary energy source for living things; composed of carbon, hydrogen, and oxygen.

cardiac muscle: the involuntary muscle found in the heart; contains actin and myosin filaments.

carnivores: animals that eat other animals.

carrying capacity: a situation when a population has reached the maximum size that the environment can support.

catabolism: the breakdown or digestion of large, complex molecules.

cecum: a blind sac that is the meeting point of the small and large intestines.

cerebellum: a portion of the hindbrain that serves as a coordinating center for motor activity.

cell body: the main portion of the nerve cell.

cell cycle: many repetitions of cellular growth and reproduction; divided into interphase and mitosis.

cell-mediated immunity: the process in which the T lymphocytes interact with the microorganisms cell-to-cell and destroy them.

cells: the fundamental units of living things.

cellular respiration: the process by which animals and other organisms obtain the energy available in carbohydrates.

cell wall: a strong membrane outside the plasma membrane present in certain cells, such as bacteria and plants.

centriole: a cylinder-like organelle that assists in chromosomal migration during mitosis.

centromere: the place of attachment of the two homologous chromatids during prophase in mitosis.

cerebrum: the portion of the forebrain that controls higher mental activity, such as learning, memory, logic, creativity, and emotion.

cervix: the opening at the lower end of the uterus.

chemiosmosis: the subdivision of cellular respiration in which the energy given off by electrons is used to pump protons across a membrane and synthesize ATP.

chemoreceptors: the specialized receptor cells that transmit smell and taste.

chlorophyll: green pigment that makes up a photosystem that absorbs energy from the sun during photosynthesis.

chloroplast: an organelle within green plants in which photosynthesis occurs.

chordates: animals with rods along their backs, including reptiles, amphibians, birds, and mammals.

chromatid: homologous chromosomes joined to each other at the centromere; present during the prophase of mitosis.

chromatin: compacted DNA and protein.

chromosomes: linear units of DNA.

chyme: a soupy liquid formed in the stomach from the churning of the bolus with gastric juices.

circulatory system: the transport system in animals.

class: a grouping of similar orders.

cocci: spherical bacteria (singular, coccus).

cochlea: a snail-like series of coiled tubes within the skull that assist hearing.

coenzymes: organic molecules that act as cofactors, such as NAD and FAD.

cofactors: ions or molecules that associate with enzymes and are required for enzymatic reactions to take place.

commensalism: a relationship in which one population receives a benefit from an association while the other is neither benefited nor harmed.

community: a situation in which populations of organisms each contain a habitat and a niche.

comparative anatomy: comparing the anatomical structures of modern day organisms with fossils to yield clues to the type of organisms that roamed earth long ago.

comparative biochemistry: the comparison of biochemical processes of modern day organisms with fossils and ancient species; modern biochemistry indicates there is a biochemical similarity in all living things.

compound: a collection of molecules.

cone cells: cells of the eye that detect color.

consumers: the organisms within an ecosystem that meet their energy needs by feeding on the producers.

cork: a tough tissue that combines with the phloem to become the bark of vascular plants.

coronary arteries: the arteries that supply the heart muscle with blood.

corpus luteum: the mass of cells derived from the female follicle that secretes progesterone.

cortex: the outer portion of the adrenal gland.

corticosteroids: the steroids secreted from the adrenal glands.

cristae: the folds of the inner mitochondrial membrane.

crossing over: a process during prophase I in which segments of DNA from one chromatid in the tetrad pass to another chromatid in the tetrad.

cyanobacteria: see blue-green algae.

cytochromes: molecules that accept and release electrons in an electron transport system.

cytokinesis: the process during mitosis in which the cytoplasm divides into two separate cells; also called cytoplasmic division.

cytoplasm: semiliquid substance that composes the foundation of the cell and contains the organelles.

cytoskeleton: an organelle within cells consisting of an interconnected system of fibers, threads, and interwoven molecules that give structure to the cell.

cytosol: see cytoplasm.

decomposers: the organisms of decay; usually bacteria and fungi.

dendrites: the short extensions of the neuron.

deoxyribonucleic acid: see DNA.

deoxyribose: the five-carbon carbohydrate attached to purine or pyrimidine bases within DNA molecules.

dermal tissue: the tissue that functions to protect the plant from injury and water loss and covers the outside of the plant.

diffusion: the movement of molecules through a membrane from a region of high concentration to low concentration.

diploid: cells having two sets of chromosomes.

diploid nuclei: contained within a mass of cytoplasm within cellular slime molds.

disaccharides: sugars composed of two molecules.

division: see phylum.

DNA: deoxyribonucleic acid; a double helix nucleotide molecule containing deoxyribose, nitrogenous base, and a phosphate group; contains the genetic information from which amino acids are determined.

DNA fingerprinting: a technique that uses electrophoresis to match DNA molecules to one another for identification purposes.

DNA polymerase: the enzyme that joins all the nucleotide components to one another to form a long strand of nucleotides.

DNA replication: the process by which cells replicate or synthesize their DNA; takes place during S phase of the cell division cycle.

domestic breeding: a process of directed evolution that brings about new forms that differ from ancestral stock.

dominant: an allele that expresses itself.

ductless glands: glands that have no ducts, such as the endocrine glands.

duodenum: the first 10 to 12 inches of the small intestine in which most of the chemical digestion takes place.

eardrum: the tympanic membrane that receives vibrations from the outer ear.

ecosystems: systems formed from the interactions between communities and their physical environments.

ectoderm: one of three germ layers that develops into the skin and nervous system.

egg: the haploid cell within the female ovary.

elements: the fundamental building blocks of matter within all living things.

embryo: forms when all the organs of the body have taken shape.

embryology: the study of embryonic development.

endergonic reaction: chemical reactions in which energy is obtained and trapped from the environment.

endocrine glands: glands throughout the animal body that secrete hormones, which help coordinate body systems.

endocytosis: the process in which a small patch of plasma membrane encloses particles that are near the cell surface.

endoderm: one of three germ layers that develops into the gastrointestinal tract.

endoplasmic reticulum (ER): an organelle comprised of a series of membranes extending throughout the cytoplasm; two types exist, rough and smooth ER.

endoskeleton: an internal support system in the echinoderms and most vertebrates that may include a framework of bones and cartilage that serves as a point of attachment for muscle.

endosperm: the female tissue that encloses the seed within the angiosperms.

entropy: the degree of disorder or randomness of a system.

environmental fitness: an individual's ability to adapt to an environment and reproduce.

enzymes: proteins that catalyze the chemical reactions within cells.

eosinophils: white blood cells whose functions are uncertain.

epididymis: the tube in which sperm cells mature.

epiglottis: a thin flap of tissue that folds over the opening to the mammalian trachea during swallowing and prevents food from entering the trachea.

epinephrine: a hormone produced in the adrenal medulla that increases heart rate, blood pressure, and the blood supply to skeletal muscle.

erythrocytes: the red blood cells; disk-shaped cells produced in the bone marrow that have no nucleus; their cytoplasm is filled with hemoglobin to transport oxygen.

erythropoetin: a hormone produced by the kidney cells that functions in the production of red blood cells.

esophagus: a thick-walled muscular tube located behind the windpipe that extends through the neck and chest to the stomach.

estrogen: a hormone produced by the ovaries that stimulates the development of secondary female characteristics.

eubacteria: modern bacteria.

eukaryotes: cells that contain a nucleus and internal cellular bodies called organelles.

evolution: changes that occur within populations and organisms that make individuals able to adapt to their external environment.

exergonic reaction: a chemical reaction in which energy is released.

exocrine glands: glands, such as the salivary glands, that deliver their enzymes via ducts.

exoskeleton: the hard, protective, outer covering of arthropods and mollusks.

facilitated diffusion: the movement of molecules across a membrane from a region of high concentration to a region of low concentration that is assisted by proteins.

Fallopian tubes: the passageways that egg cells enter after release from the ovaries; also called oviducts.

family: similar genera classified together.

fats: lipids composed of a glycerol and fatty acids.

fatty acids: long chains of carbon atoms with carboxyl groups at one end.

feeding pattern: the pattern in which animals obtain their nutrients.

fermentation: an anaerobic process in which energy can be released from glucose even though oxygen is not available; occurs in yeast cells.

fertilized egg cell: an egg cell that has been fertilized by a sperm cell.

fetus: results from a developing embryo at about eight weeks when the embryo is somewhat human looking and the remaining development consists chiefly of growth and maturation.

flavin adenine dinucleotide (FAD): a coenzyme that functions in the production of ATP.

food chain: the transfer of food energy from producers to consumers.

food pyramid: a way of expressing the availability of food in an ecosystem at a successive number of trophic levels.

food web: many interwoven food chains.

forebrain: a portion of the brain that consists of the cerebrum, thalamus, hypothalamus, and limbic system.

Fungi: a kingdom that includes the yeasts, molds, mildews, and mushrooms.

G1 phase: a phase within interphase of the cell division cycle that prepares cells for DNA replication.

G2 phase: a phase within interphase of the cell division cycle that prepares cells for mitosis.

gametes: sex cells of parent organisms; usually haploid cells.

gastrin: a hormone produced by digestive glands to influence digestive processes.

gene: the functional segment of chromosomes.

gene flow: a mechanism of evolution that results when individuals migrate from one group to another and contribute their genes to the gene pool of the new population.

gene linkage: the concept of transfer of a linkage group.

gene linkage map: a map that pinpoints the location of genes based on their connection to certain marker gene sequences.

gene pool: the collection of genes within a population; as changes in the gene pool occur, a population evolves.

genetic drift: a mechanism of evolution that occurs when a small group of individuals leaves a population and establishes a new one in a geographically isolated region.

genome: the set of all genes that specify an organism's traits.

genotype: the gene composition of a living organism.

genus: a grouping of similar species (plural, genera).

geographic distribution: the distribution of species in geographical areas.

geotropism: the turning of a plant away from or toward the earth.

gills: structures that allow fish to exchange gases with their environment.

glial cells: the cells of the nervous system that support, protect, and nourish the neurons.

glomerulus: a ball of capillaries that comprises Bowman's capsule in the human kidney.

glottis: a slitlike structure at the opening to the mammalian trachea.

glucagon: a hormone produced in the pancreas that stimulates the breakdown of glycogen to glucose in the liver.

glucose: a carbohydrate with the chemical formula C6H12O6 that serves as the primary carbon source of living things.

glycogen: a polysaccharide composed of thousands of glucose units that serves as the storage form of glucose in the human liver.

glycolysis: the subdivision of cellular respiration in which glucose molecules are broken down to form pyruvic acid molecules.

Golgi apparatus: an organelle within eukaryotic cells comprised of a series of flattened sacs; the site of protein and lipid processing and packaging; also called Golgi bodies.

Graafian follicle: a cluster of cells within the ovary that is derived from egg cells and secretes female hormones called estrogens.

ground tissue: the tissue of the vascular plant that is responsible for storing the carbohydrates produced by the plant.

gymnosperms: vascular plants having naked seeds, such as the conifers.

haploid: cells containing one copy of each chromosome.

hemoglobin: a red pigment that binds oxygen and carbon dioxide molecules and carries them through the bloodstream.

herbivores: animals that eat plants.

heterotrophic: species that acquire food from organic matter.

heterozygous: two different alleles that are present for a particular characteristic.

hindbrain: the portion of the brain that consists of the medulla, pons, and cerebellum.

histones: nuclear proteins that coil DNA molecules.

homeostasis: the process in which the internal environment exists at a steady-state equilibrium despite changes in the external environment.

homeothermic: animals that can maintain a constant body temperature.

homozygous: two identical alleles that are present for a particular characteristic.

hormones: biochemical substances produced within plant or animal cells, or glands, that exert a particular effect.

hydrostatic skeleton: a water-based skeleton present in many animals (such as the earthworm) that lack structures, such as bone, for muscles to pull against.

hypothalamus: the portion of the forebrain that serves as the control center for hunger, thirst, body temperature, and blood pressure.

hypothesis: the proposal of a solution to the question within the scientific method.

ileum: the final 12 feet of the small intestine.

immune response: the stimulation of B and T lymphocytes.

incomplete dominance: an allele combination in which two characteristics blend and both alleles can express themselves; one example is red, white, and pink snapdragons.

inner cell mass: a group of cells that continues to develop at one end of the blastocyst.

interneuron: a type of neuron that connects sensory and motor neurons and carries stimuli in the brain and spinal cord.

interphase: the cell division cycle phase in which the cell spends most of its time; includes G1, S phase (DNA replication), and G2.

invertebrates: the most primitive of the chordates; lack a backbone.

involuntary muscle: see smooth muscle and cardiac muscle.

islets of Langerhans: clusters of cells that make up the endocrine portion of the pancreas.

jejunum: the second 10 inches of the small intestine.

kinetochore: a region of DNA that has remained undivided during prophase of mitosis; binds to the spindle fibers that eventually pull apart the sister chromatids.

kingdom: the largest and broadest category of the classification system.

Krebs cycle: the subdivision of cellular respiration in which pyruvic acid is broken down and the energy in its molecules is used to form high-energy compounds.

larynx: the voicebox of mammals, formed from several folds of cartilage at the upper end of the trachea.

left atrium: the chamber of the human heart that receives oxygen-rich blood via the pulmonary vein.

left ventricle: the chamber of the human heart in which oxygen-rich blood enters through the bicuspid valve that leads into the aorta.

lens: the portion of the eye that focuses the light on the retina.

leukocytes: the white blood cells produced in the bone marrow that have various functions in the body, such as immune reaction.

lichens: associations between the cyanobacteria and the fungi.

ligaments: the tough, fibrous tissues that link bones to one another.

limbic system: a collection of structures that ring the edge of the brain and apparently function as centers of emotion.

lipid: an organic molecule used to form cellular and organelle membranes, the sheaths surrounding nerve fibers, and certain hormones; includes fats as an energy source.

liver: the organ that helps to process the products of human digestion and removes excess glucose from the bloodstream, converting it to a polymer called glycogen for storage.

loop of Henle: the segment of the human kidney after the proximal tubule.

lungs: the organ where oxygen diffuses into the blood to join with hemoglobin in the red blood cells.

lymph: a watery fluid derived from plasma that seeps out of the blood system capillaries and mingles with the cells.

lymph nodes: capsule-like bodies that contain cells that filter the lymph and phagocytize foreign particles.

lymphatic system: the extension of the circulatory system consisting of capillaries called lymph vessels, a fluid called lymph, and structures called lymph nodes.

lymphatic vessels: a series of vessels that return the lymph fluid to the circulatory system.

lymphocytes: the white blood cells that are essential components of the immune system.

lysosome: an organelle within eukaryotic cells; a droplike sac filled with enzymes used for digestion within the cell.

mammals: milk-producing animals.

marsupials: the mammals whose embryos develop within the mother's uterus for a short period of time before birth.

medulla: the inner portion of the adrenal glands; a swelling at the tip of the hindbrain that serves as the passageway for nerves extending to and from the brain.

meiosis: the process by which the chromosome number is halved during gamete formation.

menstruation: the process by which the endometrium is released in females.

meristematic tissue: the growth tissue; the location of most cell division of vascular plants.

mesoderm: one of three germ layers that develops to become the muscles and other internal organs.

metabolism: the rapid turnover of chemical materials; involves the release or use of chemical energy.

metaphase: the stage during mitosis in which the pairs of chromatids line up on the equatorial plate.

metaphase I: the phase during meiosis in which tetrads align on the equatorial plate (as in mitosis).

metaphase II: the phase during meiosis II in which the chromatid pairs gather at the center of the cell prior to separation.

midbrain: a portion of the brain that lies between the hindbrain and the forebrain that consists of a collection of crossing nerve tracts.

minerals: types of nutrients that include phosphorus, sulfur, potassium, magnesium, and zinc.

mitochondrion: the organelle that is the site of energy production in eukaryotic cells.

molecule: a precise arrangement of atoms of different elements.

Monera: the kingdom that includes the bacteria and the cyanobacteria; prokaryotic organisms.

monocytes: some of the white blood cells that function in phagocytosis.

monosaccharides: sugars that are composed of single molecules.

monotremes: the egg-laying mammals that produce milk.

morula: a solid mass of cells that develops about six days after fertilization of an egg cell.

motor neuron: a type of neuron that transmits impulses from the brain and spinal cord to muscles or glands.

mRNA: messenger RNA; the RNA molecules that receive the genetic code in the DNA and carry the code into the cytoplasm where protein synthesis takes place.

multiple alleles: a condition in which more than two alleles exist for a characteristic; one example is A, B, AB, and O blood types.

muscle contraction: a process in which actin and myosin proteins move within a sarcomere.

mutation: a random change in the gene pool of a population that gives rise to new alleles and is the source of variation in a population.

mutualism: a living arrangement in which both partners benefit.

myelin sheath: a fatty layer of material that covers the axons of nerve cells.

myofibrils: microscopic filaments that make up a muscle cell.

myosin: a protein microfilament that comprises the sarcomere of muscle cells.

natural selection: the concept that random, small variations take place in living things that lead to the gradual development of a species.

nephron: the functional and structural unit of the kidney that produces urine and is the primary unit of homeostasis in the human body.

nerve chord: also called a spinal cord; a hollow structure that extends the length of the animal just above the notochord.

nerve impulse: an electrochemical event that occurs within the neuron.

nerve roots: the 31 pairs of projections that extend out along each side of the spinal cord; the sites of axons of the sensory and motor neurons.

nerves: bundles of axons bound together.

neuroglia: the glial cells together with the extracellular tissue.

neuron: a nerve cell.

neurotransmitter: a chemical substance that accumulates in the synapse and increases the membrane permeability of the next dendrite.

neutrophils: the white blood cells that function in phagocytosis.

nicotinamide adenine dinucleotide (NAD): a coenzyme that functions during respiration to produce ATP.

nicotinamide adenine dinucleotide phosphate (NADP): a coenzyme that functions during photosynthesis to produce ATP.

nitrogenous base: the nitrous molecules that make up DNA (and RNA) molecules; two major types are purines and pyrimidines.

nonvascular plants: the plants that do not have specialized tissues to transport fluids.

norepinephrine: a hormone produced in the adrenal medulla that intensifies the effects of epinephrine.

notochord: a flexible rod of tissue extending the length of an animal that provides internal support.

nucleic acids: large molecules comprised of nucleotides.

nucleoli: the small organelles that make up the nucleus; the site for ribosomal synthesis, assembly, and packaging (singular, nucleolus).

nucleotide: the unit that makes up nucleic acid; contains a nitrogen base, a phosphate group, and a carbohydrate molecule.

nucleus: the organelle within eukaryotic cells that contains the genetic material, DNA.

Okazaki: fragments new sections of DNA that are placed along the lagging strand during DNA replication and are joined together by DNA ligase to produce a new DNA strand.

olfactory nerve: the nerve that carries the impulse from the nose to the brain for interpretation.

omnivores: animals that consume both plants and animals.

oocytes: the developed oogonia in a female after the age of puberty.

oogonia: primitive egg cells that accumulate in the ovaries before a female is born.

optic nerve: the nerve that carries impulses from the eye to the brain.

order: a grouping of similar families.

organelles: microscopic bodies within the cytoplasm that perform distinct functions.

osmosis: the movement of water molecules across a membrane from a region of high concentration to a region of low concentration.

ovary: an endocrine gland that secretes estrogens. In plants, the structure of the pistil where the ovules are enclosed.

oviducts: see Fallopian tubes.

ovulation: the process by which an egg cell is released from the follicle and swept into the Fallopian tube where it moves toward to uterus.

ovules: the protective structures that contain egg cells produced by the female.

Pacinian corpuscles: the touch and pain receptors on the skin, muscles, and tendons.

paleontology: the science of locating, cataloging, and interpreting the life forms that existed in past millennia.

pancreas: a large, glandular organ lying near the stomach that produces many of the enzymes used to digest food.

parasites: organisms that attack living things and cause disease.

parasitism: a type of symbiosis in which one population benefits while the other is harmed.

parasympathetic nervous system: a subdivision of the autonomic nervous system that returns the body to normal after an emergency.

parathyroid glands: glands located on the posterior surfaces of the thyroid gland that produce parathyroid hormone.

pathogenic: organisms that cause human disease.

PCR: polymerase chain reaction; a technique used to amplify a gene of interest.

peptides: small proteins.

peripheral nervous system: a collection of nerves that connect the brain and spinal cord to other parts of the body and the external environment.

peristalsis: a rhythmic series of muscular contractions that propels the bolus along.

peroxisome: cytoplasmic body containing enzymes for digestion.

phagocytes: cells that attack and engulf invading microorganisms.

phagocytosis: occurs when the vesicle formed from endocytosis contains particulate matter; the process by which cells or microorganisms are engulfed by another cell.

pharynx: the cavity at the rear of the mouth that the nasal chambers open into; the throat.

phenotype: the expression of genes and the physical characteristics that result.

phloem: structures of vascular plants that transport sugars and other nutrients from the leaves to the other parts of the plant.

phosphate group: a group derived from a molecule of phosphoric acid that connects the DNA molecules to one another.

phosphate ion: a product of adenosine triphosphate (ATP) together with ADP.

photosystem: the site within the chloroplast in which sunlight is captured; includes the pigment molecules, proton pumps, enzymes, coenzymes, and cytochromes.

phototropism: the bending and turning of the plant stem toward a light source.

phyla: related classes grouped together (singular, phylum).

physical map: a map that locates a gene of interest precisely by showing the actual number of base pairs between genes on a chromosome.

pineal gland: a human endocrine gland in the midbrain that regulates mating behaviors and day-night cycles.

pinocytosis: when the vesicle formed from endocytosis contains droplets of fluid.

pistil: the structure of the flower that contains a stigma, a style, and an ovary.

pith: the structure at the center of the stem of vascular plants.

pituitary gland: a gland at the base of the brain consisting of the anterior and posterior lobes that secretes several hormones.

placenta: the structure that supplies the fetus with nourishment.

placental mammals: mammals that have a nutritive connection between the embryo and the mother's uterine wall.

plant hormones: hormones that regulate the growth and development of many plants.

plasma: a straw-colored liquid composed primarily of water; the fluid portion of blood.

plasma cells: large antibody-producing cells derived from B lymphocytes when stimulated.

plasma membrane: also called a cell membrane; a membrane composed of lipids, proteins, and phospholipids.

plasmid: small circular DNA molecules often used as vectors to transform specific genes into cells.

platelets: small disk-shaped blood fragments produced in the bone marrow that serve as the starting material for blood clotting.

polygenic inheritance: the condition in which some characteristics are determined by an interaction of genes on several chromosomes or at several places on one chromosome; one example is human skin color.

polymerase chain reaction: see PCR.

polysaccharides: complex carbohydrates formed by linking multiple monosaccharides.

pons: the portion of the hindbrain below the medulla and the midbrain that acts as a bridge between various portions of the brain.

population: an interbreeding group of individuals of one species occupying a defined geographic area.

predation: a relationship in which one population within a community may capture and feed upon another population.

producers: organisms within an ecosystem that trap energy (by photosynthesis).

progesterone: a hormone produced by the corpus luteum that regulates the buildup of tissue in the endometrium and inhibits the contractions of the uterus.

prokaryotes: cells that do not contain a nucleus or internal organelles; include bacteria, cyanobacteria, and archaebacteria.

prophase: the first phase of mitosis; involves chromosomal condensation, nuclear membrane breakdown, and the migration of centrioles to opposite poles.

prophase I: the first phase of meiotic division, during which crossing over takes place.

prophase II: the phase during meiosis II in which the chromatin material condenses and each chromosome contains two chromatids attached by the centromere.

prostaglandins: the hormones secreted by various tissue cells that produce their effects on smooth muscles, on various glands, and in reproductive physiology.

proteinoids: the primitive polymers formed by the unison of amino acids; able to act as enzymes and catalyze organic reactions.

proteins: long chains of amino acid units that are the main molecules from which living things are constructed.

Protista: a kingdom that includes protozoa, one-celled algae, and slime molds.

protocells: the first cells.

protons: positively charged particles within the nucleus of an atom.

pulmonary artery: the artery of the human circulatory system that pumps the blood from the right ventricle to the lungs for gas exchange.

pulmonary vein: the vein of the human circulatory system that returns oxygen-rich blood from the lungs to the left atrium.

purine: a type of nitrogenous base present in DNA molecules containing two fused rings of carbon and nitrogen atoms; two examples in DNA are adenine (A) and guanine (G).

Purkinje fibers: the nerves that transfer amplified impulses to regions of the heart to control its function.

pyrimidine: a type of nitrogenous base in DNA molecules that has one ring containing carbon and nitrogen atoms; two examples in DNA are cytosine (C) and thymine (T).

recessive: the allele overshadowed by the dominant allele.

recombinant DNA: DNA molecules that have been altered in some way during the process of genetic engineering or biotechnology.

red blood cells: also known as erythrocytes; cells that contain hemoglobin to transport oxygen.

reflex arc: the simplest unit of nervous activity; involved in the detection of a stimulus in the environment by sensory nerve endings, followed by impulses that travel via the sensory neurons to the spinal cord.

renal arteries: arteries in which blood enters the kidney.

renal veins: veins in which blood exits the kidney.

responsiveness: the ability of living things to respond to stimuli in the external environment.

resting potential: the inactive state of a neuron in which the cytoplasm is negatively charged with respect to the outside of the cell.

restriction enzymes: catalyze the opening of a DNA molecule at a "restriction" point; many leave dangling ends of DNA molecules at the point where the DNA has been opened.

retina: a single layer containing nerve cells within the eye.

RFLP: restriction fragment length polymorphism; a technique using small bits of DNA fragments linked to various diseases.

rhodopsin: a light-sensitive pigment of the eye that functions in the detection of light.

ribonucleic acid: see RNA.

ribosomes: organelle bodies that may be bound to the ER that are the sites of protein synthesis in eukaryotic cells; the bodies in which amino acids are bound together to form proteins.

right atrium: the chamber of the human heart in which oxygen-poor blood enters through a major vein called the vena cava.

right ventricle: the pumping chamber of the human heart from which blood exits.

RNA: ribonucleic acid; a nucleic acid produced during transcription that is complementary to a DNA strand; similar to DNA in structure but contains the carbohydrate ribose and the pyrimidine uracil rather than thymine.

RNA polymerase: the enzyme that moves along the DNA strand, reads the nucleotides one by one, and synthesizes a complementary mRNA molecule according to the principle of complementary base pairing.

rod cells: the cells of the eye that permit vision in dim light.

roots: the structures of vascular plants that anchor them to the ground and take in water and minerals from the soil.

rough endoplasmic reticulum: ER studded with ribosomes; the site of protein synthesis in eukaryotic cells.

rRNA: ribosomal RNA; RNA molecules that function to manufacture ribosomes.

salivary glands: the parotid glands, the submaxillary glands, and the sublingual glands that secrete saliva into the mouth.

sarcolemma: the muscle cell membrane.

sarcomere: the functional unit of the muscle that contains thin actin filaments and thick myosin filaments.

scientific method: an orderly process of gaining information about the biological world.

scrotum: a pouch outside the male body that contains the testes.

secretin: a hormone produced by digestive glands that influences digestive processes.

seedless vascular plants: the division Pteridophyta that includes the ferns.

semen: a fluid secretion containing sperm and secretions from the prostate gland, seminal vesicles, and Cowper's glands.

semilunar valves: two valves found in the pulmonary artery and the aorta.

seminiferous tubules: coiled passageways in which sperm production takes place.

sensory neurons: neurons that receive stimuli from the external environment.

sensory somatic system: a subdivision of the peripheral nervous system that carries impulses from the external environment and the senses.

sepals: modified leaves that enclose and protect a growing bud in flowers.

serum: plasma from which clotting proteins have been removed.

sex chromosomes: one pair among the 23 pairs of human chromosomes; the X and Y chromosomes.

skeletal muscle: see striated muscle.

slime molds (cellular): amoebalike cells that live independently and unite with other cellular slime molds to form a single, large, flat cell with many nuclei.

slime molds (true): single, flat, very large cells with many nuclei.

small intestine: the site of chemical digestion; includes the duodenum, jejunum, and ileum.

smooth endoplasmic reticulum: ER with no ribosomes attached.

smooth muscle: found in the linings of the blood vessels, along the gastrointestinal tract, in the respiratory tract, and in the urinary bladder; contains few actin and myosin filaments; also called involuntary muscle.

species: a group of individuals that share features and are able to interbreed under natural conditions to yield fertile offspring.

spermatogonia: primitive cells within the seminiferous tubules that undergo a series of changes and then meiosis to yield sperm cells.

sperm cells: haploid cells within the male testes.

S phase: the phase within the cell division cycle in which DNA is replicated.

spinal cord: the white cord of tissue passing through the bony tunnel made by the vertebrae.

spiracles: a series of openings on the body surface of terrestrial arthropods that open into tiny air tubes that assist in gas exchange.

spirilla: flexible spiral bacteria (singular, spirillum).

spirochetes: rigid spiral bacteria.

spleen: the site where red blood cells are destroyed; a reserve blood supply for the body.

stamen: the structure of a flower that contains a thin, stemlike filament and an anther.

stomata: the pores within leaves surrounded by guard cells that regulate the rate of gas exchange, which regulates the rate of photosynthesis. (Singular, stoma.)

striated muscle: skeletal muscle fiber that appears to be banded due to the presence of overlapping actin and myosin filaments; also called voluntary muscle.

substrate: the substance changed or acted on by an enzyme.

survival of the fittest: the concept of natural selection that states that the fittest survive and spread their traits through a population.

sutures: the immovable joints where bones come together within the skull.

symbiosis: the relationship between two populations that live together in a close, permanent, and mutually beneficial association.

sympathetic nervous system: a subdivision of the autonomic nervous system that prepares the body for an emergency.

synapse: the fluid-filled space separating the end of the axon from the dendrite of the next neuron or from a muscle cell.

synergism: a type of relationship in which two populations accomplish together what neither could accomplish on its own.

telophase: a phase during mitosis in which the chromosomes arrive at the opposite poles of the cell.

telophase I: the phase during meiosis in which the nucleus reorganizes as the chromosomes become chromatin; cytoplasmic division takes place, resulting in two cells.

telophase II: the phase during meiosis II in which the chromosomes gather at the poles of the cells and form a mass of chromatin; the nuclear envelope develops, the nucleoli reappear, and the cells undergo cytokinesis.

tendons: the connective tissue by which muscles are attached to bones.

testes: endocrine glands that secrete androgens; the male reproductive organs located in the scrotum.

thalamus: a portion of the forebrain that integrates sensory impulses.

theory: a hypothesis that is confirmed through repeated experimentation.

thrombocytes: the starting material for blood clotting; also called platelets.

thylakoids: membranes that make up the grana in chloroplasts; the actual site of photosynthesis within chloroplasts.

thymosins: hormones secreted by the thymus gland that influence the development of the T lymphocytes of the immune system.

thymus gland: an endocrine gland in the neck tissues that secretes thymosins.

thyroid gland: a gland at the base of the neck that produces several hormones, such as thyroxine and calcitonin.

T lymphocytes: white blood cells in the lymph nodes that are stimulated by microorganisms or other foreign material in the blood.

trachea: the windpipe of mammals.

tracheae: the branching network that extends from holes to all parts of an anthropod body to assist in gas exchange.

tracheids: the main conducting vessels of the xylem in most vascular plants.

tracheophytes: vascular plants composed of a xylem and phloem.

transcription: the process in which a complementary strand of mRNA is synthesized according to the nitrogenous base code of DNA.

transgenic animals: animals in which one or more genes have been introduced into the nonreproductive cells.

translation: the process by which the genetic code is transferred to an amino acid sequence in a protein.

tricuspid valve: a valve that passes blood from the right atrium into the right ventricle.

tRNA: transfer RNA; RNA molecules in the cytoplasm of a cell that carry amino acids to the ribosomes for protein synthesis.

trophoblast: a layer of cells that forms after fertilization; projections from the trophoblast form vessels, which merge with maternal blood vessels to form the placenta.

tropism: the bending or turning response of a plant caused by external stimuli.

turgor pressure: the pressure exerted on a plant's guard cells to open.

umbilical cord: the source of attachment of the fetus to the maternal blood supply.

urea: a component of urine that results from amino acid breakdown in the liver.

ureters: tubes that carry waste from the kidneys to the urinary bladder for storage or release.

urethra: the path in which urine flows from the bladder to the exterior; the tube within the penis that carries the sperm.

uric acid: a component of urine that results from nucleic acid breakdown.

urinary bladder: the site where waste products are shipped from the kidney for storage or for release.

urine: the product of the kidney; a watery solution of waste products, salts, organic compounds, uric acid, and urea.

uterus: a muscular organ in the pelvic cavity of female mammals; also called the womb.

vacuole: an organelle found in mature plant cells that stores nutrients and toxic waste.

vagina: a muscular organ in female mammals leading from the cervix to the exterior.

vascular bundles: arrangements of the xylem and phloem in vascular plants.

vascular plants: plants that contain specialized tissues to transport fluids.

vascular plants with protected seeds: angiosperms; the most developed and complex vascular plants.

vascular plants with unprotected seeds: gymnosperms; vascular plants that contain naked seeds, such as the conifers.

vectors: the carriers of DNA genes to be inserted into cells.

veins: channels through which fluid flows toward the heart.

vena cava: the major vein in the human heart; pumps oxygen-poor blood into the right atrium.

ventricle: a pumping chamber for blood to exit from the heart.

vertebrates: animals with backbones.

vessels: the main conducting vessels of the xylem found in the angiosperms.

virus: fragments of nucleic acid surrounded by a protein coat; may attack cells and replicate within the cells, destroying them.

vitamins: organic nutrients essential in trace amounts to the health of animals.

voluntary muscle: see striated muscle.

white blood cells: see leukocytes.

xylem: the structure of vascular plants that conducts water and minerals upward from the roots.

zygote: a fertilized egg cell, which is diploid.