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.
