Biology Glossary

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.