abiotic Refers to the nonliving components of an environment, such as temperature and precipitation.
acid A substance that increases the hydrogen ion concentration of solutions.
activation energy The energy required for a chemical reaction to proceed. Enzymes accelerate reactions by reducing their activation energy.
active site The part of an enzyme that binds to a substrate.
active transport The energy-requiring process by which solutes are pumped from an area of lower concentration to an area of higher concentration with the help of transport proteins.
adaptation The process by which populations become better suited to their environment as a result of natural selection.
adaptive radiation The spreading and diversification of organisms that occur when they colonize a new habitat.
adenosine triphosphate (ATP) The molecule that cells use to power energy-requiring functions; the cell’s energy “currency.”
adhesion Water molecules sticking to other surfaces through hydrogen bonding.
adult (somatic) stem cells Stem cells located in tissues that help maintain and regenerate those tissues.
aerobic respiration A series of reactions that occurs in the presence of oxygen and converts energy stored in food into ATP.
alga (plural: algae) A uni- or multicellular photosynthetic protist.
allele frequency The relative proportion of an allele in a population.
alleles Alternative versions of the same gene that have different nucleotide sequences.
amino acid The building block, or monomer, of a protein.
amniocentesis A procedure that removes fluid surrounding the fetus to obtain and analyze fetal cells to diagnose genetic disorders.
anabolic reaction Any chemical reaction that combines simple molecules to build more-complex molecules.
anecdotal evidence An informal observation that has not been systematically tested.
aneuploidy An abnormal number of one or more chromosomes (either extra or missing copies).
angiosperm A seed-bearing flowering plant with seeds typically contained within a fruit.
animal A eukaryotic multicellular organism that obtains nutrients by ingesting other organisms.
annelid A segmented worm, such as an earthworm.
antibiotics Chemicals that either kill bacteria or slow their growth by interfering with the function of essential bacterial cell structures.
anticodon The part of a tRNA molecule that binds to a complementary mRNA codon.
apoptosis Programmed cell death; often referred to as cellular suicide.
aquifer Underground layers of porous rock from which water can be drawn for use.
Archaea One of the two domains of prokaryotic life; the other is Bacteria.
arthropod An invertebrate having a segmented body, a hard exoskeleton, and jointed appendages.
atom The smallest unit of an element that cannot be chemically broken down into smaller units.
autosomes Paired chromosomes present in both males and females; all chromosomes except the X and Y chromosomes.
autotrophs Organisms such as plants, algae, and certain bacteria that capture the energy of sunlight by photosynthesis.
Bacteria One of the two domains of prokaryotic life; the other is Archaea.
base A substance that reduces the hydrogen ion concentration of solutions.
benign tumor A noncancerous tumor that will not spread throughout the body.
bilateral symmetry The pattern exhibited by a body plan with right and left halves that are mirror images of each other.
binary fission A type of asexual reproduction in which one parental cell divides into two.
biocapacity The amount of Earth’s biologically productive area—cropland, pasture, forest, and fisheries—that is available to provide resources and absorb wastes to support life.
biodiversity The number of different species and their relative abundances in a specific region or on the planet as a whole.
biofuels Renewable fuels made from living organisms (e.g., plants and algae).
biogeography The study of how organisms are distributed in geographical space.
biological species concept The definition of a species as a population whose members can interbreed to produce fertile offspring.
biome A large geographic area defined by its characteristic plant life, which in turn is determined by temperature and levels of moisture.
biotic Refers to the living components of an environment.
blastocyst The stage of embryonic development in which the embryo is a hollow ball of cells. Researchers can derive embryonic stem cell lines during the blastocyst stage.
blood vessels The components of the cardiovascular system that transport blood throughout the body.
body mass index (BMI) An estimate of body fat based on height and weight.
bottleneck effect A type of genetic drift that occurs when a population is suddenly reduced to a small number of individuals, and alleles are lost from the population as a result.
bryophyte A nonvascular plant that does not produce seeds.
Calorie 1,000 calories or 1 kilocalorie (kcal); the capital “C” in Calorie indicates “kilocalorie.” The Calorie is the common unit of energy used in food nutrition labels.
calorie The amount of energy required to raise the temperature of 1 g of water by 1°C.
cancer A disease of unregulated cell division: cells divide inappropriately and accumulate, in some instances forming a tumor.
capsule A sticky coating surrounding some bacterial cells that adheres to surfaces.
carbohydrate An organic molecule made up of one or more sugars. A one-sugar carbohydrate is called a monosaccharide; a carbohydrate with multiple linked sugars is called a polysaccharide.
carbon cycle The movement of carbon atoms as they cycle between organic molecules and inorganic CO2.
carbon fixation The conversion of inorganic carbon (e.g., CO2) into organic forms (e.g., sugars).
carbon footprint A measure of the total greenhouse gases produced by human activities.
carcinogen Any chemical agent that causes cancer by damaging DNA. Carcinogens are a type of mutagen.
carrier An individual who is heterozygous for a particular gene of interest, and therefore can pass on the recessive allele without showing any of its effects.
carrying capacity The maximum population size that a given environment or habitat can support given its food supply and other natural resources.
catabolic reaction Any chemical reaction that breaks down complex molecules into simpler molecules.
catalysis The process of speeding up the rate of a chemical reaction (e.g., by enzymes).
cell The basic structural unit of living organisms.
cell cycle The ordered sequence of stages that a cell progresses through in order to divide during its life; stages include preparatory phases (G1, S, G2) and division phases (mitosis and cytokinesis).
cell cycle checkpoint A cellular mechanism that ensures that each stage of the cell cycle is completed accurately.
cell division The process by which a cell reproduces itself; cell division is important for normal growth, development, and repair of an organism.
cell membrane A phospholipid bilayer with embedded proteins that forms the boundary of all cells.
cell theory The concept that all living organisms are made of cells and that cells are formed by the reproduction of existing cells.
cell wall A rigid structure enclosing the cell membrane of some cells that helps the cell maintain its shape.
cellular differentiation The process by which a cell specializes to carry out a specific role.
centromere The specialized region of a chromosome where the sister chromatids are joined; critical for proper alignment and separation of sister chromatids during mitosis.
chemical energy Potential energy stored in the bonds of biological molecules.
chemotherapy The treatment of disease, specifically cancer, by the use of chemicals.
chlorophyll The pigment present in the green parts of plants that absorbs photons of light energy during the “photo” reactions of photosynthesis.
chloroplast The organelle in plant and algae cells where photosynthesis occurs.
chromosome A single, large DNA molecule wrapped around proteins. Chromosomes are located in the nuclei of most eukaryotic cells.
citric acid cycle A set of reactions that takes place in mitochondria and helps extract energy (in the form of high-energy electrons) from food; the second stage of aerobic respiration.
coding sequence The part of a gene that specifies the amino acid sequence of a protein. Coding sequences determine the identity, shape, and function of proteins.
codominance A form of inheritance in which both alleles contribute equally to the phenotype.
codon A sequence of three mRNA nucleotides that specifies a particular amino acid.
coenzyme A small organic molecule, such as a vitamin, required to activate an enzyme.
cofactor An inorganic substance, such as a metal ion, required to activate an enzyme.
cohesion Water molecules sticking to water molecules through hydrogen bonding.
commensalism A type of symbiotic relationship in which one member benefits and the other is unharmed.
community A group of interacting populations of different species living together in the same area.
competition An interaction between two or more organisms that rely on a common resource that is not available in sufficient quantities.
competitive exclusion principle The concept that when two species compete for resources in an identical niche, one is inevitably driven to extinction.
complementary Fitting together; two strands of DNA are said to be complementary in that A always pairs with T, and G always pairs with C.
conservation of energy The principle that energy cannot be created or destroyed, but can be transformed from one form to another.
consumers Heterotrophs that eat other organisms lower on the food chain to obtain energy.
continuous variation Variation in a population showing an unbroken range of phenotypes rather than discrete categories.
control group The group in an experiment that experiences no experimental intervention or manipulation.
convergent evolution The process by which organisms that are not closely related evolve similar adaptations as a result of independent episodes of natural selection.
correlation A consistent relationship between two variables.
covalent bond A strong chemical bond resulting from the sharing of a pair of electrons between two atoms.
cytokinesis The physical division of a cell into two daughter cells.
cytoplasm The gelatinous, aqueous interior of all cells.
cytoskeleton A network of protein fibers in eukaryotic cells that provides structure and facilitates cell movement.
decomposer An organism such as a fungus or bacterium that digests and uses the organic molecules in dead organisms as sources of nutrients and energy.
deoxyribonucleic acid (DNA) The molecule of heredity, common to all life forms, that is passed from parents to offspring.
dependent variable The measured result of an experiment, analyzed in both the experimental and control groups.
descent with modification Darwin’s term for evolution, combining the ideas that all living things are related and that organisms have changed over time.
differential gene expression The process by which genes are “turned on,” or expressed, in different cell types.
diploid Having two copies of every chromosome.
directional selection A type of natural selection in which organisms with phenotypes at one end of a spectrum are favored by the environment.
distribution pattern The way that organisms are distributed in geographic space, which depends on resources and interactions with other members of the population.
diversifying selection A type of natural selection in which organisms with phenotypes at both extremes of the phenotypic range are favored by the environment.
DNA polymerase An enzyme that “reads” the sequence of a DNA strand and helps to add complementary nucleotides to form a new strand during DNA replication.
DNA profile A visual representation of a person’s unique DNA sequence.
DNA replication The natural process by which cells make an identical copy of a DNA molecule.
domain The highest category in the modern system of classification; there are three domains—Bacteria, Archaea, and Eukarya.
dominant allele An allele that can mask the presence of a recessive allele.
double helix The spiral structure formed by two strands of DNA nucleotides bound together.
ecological footprint A measure of how much land and water area is required to supply the resources a person or population consumes and to absorb the wastes it produces.
ecology The study of the interactions between organisms and between organisms and their nonliving environment.
ecosystem The living and nonliving components of an environment, including the communities of organisms present and the physical and chemical environment with which they interact.
electron A negatively charged subatomic particle with negligible mass.
electron transport chain A process that takes place in mitochondria and produces the bulk of ATP during aerobic respiration; the third stage of aerobic respiration.
element A chemically pure substance that cannot be chemically broken down; each element is made up of and defined by a single type of atom.
embryo An early stage of development reached when a zygote undergoes cell division to form a multicellular structure.
embryonic stem cells Stem cells that make up an early embryo and which can differentiate into nearly every cell type in the body.
emigration The movement of individuals out of a population.
endoplasmic reticulum (ER) A network of membranes in eukaryotic cells where proteins and lipids are synthesized.
endoskeleton A solid internal skeleton found in many animals, including humans.
endosymbiosis The scientific theory that free-living prokaryotic cells engulfed other free living prokaryotic cells billions of years ago, forming eukaryotic organelles such as mitochondria and chloroplasts.
energy The ability to do work, including building complex molecules.
enzyme A protein that speeds up the rate of a chemical reaction.
epidemiology The study of patterns of disease in populations, including risk factors.
essential amino acids Amino acids the human body cannot synthesize and must obtain from food.
essential nutrients Nutrients that cannot be made by the body, so must be obtained from the diet.
eukaryote Any organism of the domain Eukarya; eukaryotic cells are characterized by the presence of a membrane-enclosed nucleus and organelles.
eukaryotic cells Cells that contain membrane-bound organelles, including a central nucleus.
evolution Change in allele frequencies in a population over time.
exoskeleton A hard external skeleton covering the body of many animals, such as arthropods.
experiment A carefully designed test, the results of which will either support or rule out a hypothesis.
experimental group The group in an experiment that experiences the experimental intervention or manipulation.
exponential growth The unrestricted growth of a population increasing at a constant growth rate.
extinction The elimination of all individuals in a species; extinction may occur over time or in a sudden mass die-off.
facilitated diffusion The process by which large or hydrophilic solutes move across a membrane from an area of higher concentration to an area of lower concentration with the help of transport proteins; facilitated diffusion does not require an input of energy.
falsifiable Describes a hypothesis that can be ruled out by data that show that the hypothesis does not explain the observation.
fermentation A series of chemical reactions that takes place in the absence of oxygen and converts some of the energy stored in food into ATP. Fermentation produces far less ATP than does aerobic respiration.
fern The first true vascular plants; ferns do not produce seeds.
fitness The relative ability of an organism to survive and reproduce in a particular environment.
flagella (singular: flagellum) Whiplike appendages extending from the surface of some bacteria, used in movement of the cell.
folate A B vitamin also known as folic acid, folate is an essential nutrient, necessary for basic bodily processes such as DNA replication and cell division.
food chain A linked series of feeding relationships in a community in which organisms further up the chain feed on ones below.
food web A complex interconnection of feeding relationships in a community.
fossil fuel A carbon-rich energy source, such as coal, petroleum, or natural gas, formed from the compressed, fossilized remains of once-living organisms.
fossil record An assemblage of fossils arranged in order of age, providing evidence of changes in species over time.
fossils The preserved remains or impressions of once-living organisms.
founder effect A type of genetic drift in which a small number of individuals leaves one population and establishes a new population; by chance, the newly established population may have lower genetic diversity than the original population.
fungus (plural: fungi) A unicellular or multicellular eukaryotic organism that obtains nutrients by secreting digestive enzymes onto organic matter and absorbing the digested product.
gametes Specialized reproductive cells that carry one copy of each chromosome (that is, they are haploid). Sperm are male gametes; eggs are female gametes.
gel electrophoresis A laboratory technique that separates fragments of DNA by size.
gene A sequence of DNA that contains the information to make at least one protein.
gene expression The process of using DNA instructions to make proteins.
gene flow The movement of alleles from one population to another, which may increase the genetic diversity of a population.
gene pool The total collection of alleles in a population.
gene therapy A treatment that aims to cure human disease by replacing defective genes with functional ones.
genetic code The set of rules relating particular mRNA codons to particular amino acids.
genetic drift Random changes in the allele frequencies of a population between generations; genetic drift tends to have more dramatic effects in smaller populations than in larger ones.
genetic engineering The process of assembling new genes with novel combinations of regulatory and coding sequences.
genetically modified organism (GMO) An organism that has been genetically altered by humans.
genome One complete set of genetic instructions encoded in the DNA of an organism.
genotype The particular genetic makeup of an individual.
germ-line mutation A mutation occurring in gametes; passed on to offspring.
global hectare A unit of measurement representing the biological productivity (both resource-providing and waste-absorbing capacity) of an average hectare of Earth.
global warming An increase in Earth’s average temperature.
glycogen A complex animal carbohydrate, made up of linked chains of glucose molecules, that stores energy for short term use.
glycolysis A series of reactions that breaks down sugar into smaller units; glycolysis takes place in the cytoplasm and is the first stage of both aerobic respiration and fermentation.
Golgi apparatus An organelle made up of stacked membrane enclosed discs that packages proteins and prepares them for transport.
gonads Sex organs: ovaries in females, testes in males.
Gram-negative Refers to bacteria with a cell wall that includes a thin layer of peptidoglycan surrounded by an outer lipid membrane that does not retain the Gram stain.
Gram-positive Refers to bacteria with a cell wall that includes a thick layer of peptidoglycan that retains the Gram stain.
greenhouse effect The normal process by which heat is radiated from Earth’s surface and trapped by gases in the atmosphere, helping to maintain Earth at a temperature that can support life.
greenhouse gas Any of the gases in Earth’s atmosphere that absorb heat radiated from Earth’s surface and contribute to the greenhouse effect; for example, carbon dioxide and methane.
growth rate The difference between the birth rate and the death rate of a given population; also known as the rate of natural increase.
gymnosperm A seed-bearing plant with exposed seeds typically held in cones.
habitat The physical environment where an organism lives and to which it is adapted.
half-life The time it takes for one-half of a sample of a radioactive isotope to decay.
haploid Having only one copy of every chromosome.
Hardy-Weinberg equation A mathematical formula that calculates the frequency of genotypes and phenotypes one would expect to find in a nonevolving population.
Hardy-Weinberg equilibrium The principle that, in a nonevolving population, both allele and genotype frequencies remain constant from one generation to the next.
heat The kinetic energy generated by random movements of molecules or atoms.
herbivory Predation on plants, which may or may not kill the plant preyed on.
heterotrophs Organisms, such as humans and other animals, that obtain energy by eating organic molecules that were produced by other organisms.
heterozygous Having two different alleles.
homeostasis The maintenance of a relatively stable internal environment, even when the external environment changes.
hominid Any living or extinct member of the family Hominidae, the great apes—humans, orangutans, gorillas, chimpanzees, and bonobos.
homologous chromosomes A pair of chromosomes that both contain the same genes. In a diploid cell, one chromosome in the pair is inherited from the mother, the other from the father.
homology Anatomical, genetic, or developmental similarity among organisms due to common ancestry.
homozygous Having two identical alleles.
human chorionic gonadotropin (hCG) A hormone produced by an early embryo that helps maintain the corpus luteum until the placenta develops.
humoral immunity The type of adaptive immunity that fights free-floating pathogens infections and other foreign substances in the circulation and lymph fluid.
hydrogen bond A weak electrical attraction between a partially positive hydrogen atom and an atom with a partial negative charge.
hydrophilic “Water-loving”; hydrophilic molecules dissolve in water.
hydrophobic “Water-fearing”; hydrophobic molecules will not dissolve in water.
hypertonic Describes a solution surrounding a cell that has a higher concentration of solutes than the cell.
hypha (plural: hyphae) A long, threadlike structure through which fungi absorb nutrients.
hypothalamus A master coordinator region of the brain responsible for a variety of physiological functions.
hypothesis A tentative explanation for a scientific observation or question.
hypotonic Describes a solution surrounding a cell that has a lower concentration of solutes than the cell.
immigration The movement of individuals into a population.
inbreeding Mating between closely related individuals. Inbreeding does not change the allele frequency within a population, but it does increase the proportion of homozygous individuals to heterozygotes.
inbreeding depression The negative reproductive consequences for a population associated with having a high frequency of homozygous individuals possessing harmful recessive alleles.
incomplete dominance A form of inheritance in which heterozygotes have a phenotype that is intermediate between homozygous dominant and homozygous recessive.
independent assortment The principle that alleles of different genes are distributed independently of one another during meiosis.
independent variable The variable, or factor, being deliberately changed in the experimental group.
induced pluripotent stem cell A pluripotent stem cell that was generated by manipulation of a differentiated somatic cell.
inflammation An innate defense that is activated by infection or local tissue damage; characterized by redness, swelling, and pain.
ingestion The act of taking food into the mouth.
inorganic Describes a molecule that lacks a carbon-based backbone and C-H bonds.
insect A six-legged arthropod with three body segments: head, thorax, and abdomen.
interphase The stage of the cell cycle in which cells spend most of their time, preparing for cell division. There are three distinct sub-phases: G1, S, and G2.
invertebrate An animal lacking a backbone.
ion An electrically charged atom, the charge resulting from the loss or gain of electrons.
ionic bond A strong electrical attraction between oppositely charged ions formed by the transfer of one or more electrons from one atom to another.
isotonic Describes a solution surrounding a cell that has the same solute concentration as the cell.
karyotype The chromosomal makeup of cells. Karyotype analysis can be used to detect trisomy 21 prenatally.
keystone species Species on which other species depend, and whose removal has a dramatic impact on the community.
kinetic energy The energy of motion or movement.
kinetochore Proteins located at the centromere that provide an attachment point for microtubules of the mitotic spindle.
light energy The energy of the electromagnetic spectrum of radiation that is visible to the human eye.
lipids Organic molecules that generally repel water.
logistic growth A pattern of growth that starts off fast and then levels off as the population reaches the carrying capacity of the environment.
lysosome An organelle in eukaryotic cells filled with enzymes that can degrade worn-out cellular structures.
macromolecules Large organic molecules that make up living organisms; they include carbohydrates, proteins, and nucleic acids.
macronutrients Nutrients, including carbohydrates, proteins, and fats, that organisms must ingest in large amounts to maintain health.
malignant tumor A cancerous tumor that spreads throughout the body.
malnutrition The medical condition resulting from the lack of any essential nutrient in the diet. Malnutrition is often, but not always, associated with starvation.
mammals Members of the class Mammalia; all members of this class have mammary glands and a body covered with hair.
mass extinction An extinction of between 50% and 90% of all species that occurs relatively rapidly.
matter Anything that takes up space and has mass.
meiosis A specialized type of nuclear division that generates genetically unique haploid gametes.
melanin Pigment produced by a specific type of skin cell that gives skin its color.
messenger RNA (mRNA) The RNA copy of an original DNA sequence made during transcription.
metabolism All the chemical reactions taking place in the cells of a living organism that allow it to obtain and use energy.
metastasis The spread of cancer cells from one location in the body to another.
micronutrients Nutrients, including vitamins and minerals, that organisms must ingest in small amounts to maintain health.
microtubules Hollow protein fibers that are key components of the cytoskeleton and make up the fibers of the mitotic spindle.
mineral An inorganic chemical element required by organisms for normal growth, reproduction, and tissue maintenance; examples are calcium, iron, potassium, and zinc.
mitochondria (singular: mitochondrion) Membrane-bound organelles responsible for important energy conversion reactions in eukaryotes.
mitochondrial DNA (mtDNA) The DNA in mitochondria that is inherited solely from mothers.
mitosis The segregation and separation of duplicated chromosomes during cell division.
mitotic spindle The structure that separates sister chromatids during mitosis.
molecule Atoms linked by covalent bonds.
mollusk A soft-bodied invertebrate, generally with a hard shell (which may be tiny, internal, or absent in some mollusks).
monomer One chemical subunit of a polymer.
monosaccharide The building block, or monomer, of a carbohydrate.
multifactorial inheritance An interaction between genes and the environment that contributes to a phenotype or trait.
multipotent Describes a cell with the ability to differentiate into a limited number of cell types in the body.
mutagen Any chemical or physical agent that can damage DNA by changing its nucleotide sequence.
mutation A change in the nucleotide sequence of DNA.
mutualism A type of symbiotic relationship in which both members benefit; a “win-win” relationship.
mycelium (plural: mycelia) A spreading mass of interwoven hyphae that forms the often subterranean body of multicellular fungi.
NAD+ An electron carrier. NAD+ can accept electrons, becoming NADH in the process.
natural resources Raw materials that are obtained from the earth and are considered valuable even in their relatively unmodified, natural form.
natural selection Differential survival and reproduction of individuals in response to environmental pressure that leads to change in allele frequencies in a population over time.
neutron An electrically uncharged subatomic particle in the nucleus of an atom.
niche The space, environmental conditions, and resources that a species needs in order to survive and reproduce.
nitrogen fixation The conversion of atmospheric nitrogen into a form that plants can use for growth.
nonadaptive evolution Any change in allele frequency that does not by itself lead a population to become more adapted to its environment; the causes of nonadaptive evolution are mutation, genetic drift, and gene flow.
nondisjunction The failure of chromosomes to separate accurately during cell division; nondisjunction in meiosis leads to aneuploid gametes.
nonrenewable resources Natural resources like fossil fuels that cannot easily be replaced.
nuclear envelope The double membrane surrounding the nucleus of a eukaryotic cell.
nucleic acids Organic molecules made up of linked nucleotide subunits; DNA and RNA are examples of nucleic acids.
nucleotide The building block, or monomer, of a nucleic acid.
nucleus (atomic) The dense core of an atom.
nucleus (eukaryotic) The organelle in eukaryotic cells that contains the genetic material.
nutrients Components in food that the body needs to grow, develop, and repair itself.
obese Having 20% more body fat than is recommended for one’s height, as measured by a body mass index equal to or greater than 30.
oncogene A mutated and overactive form of a proto-oncogene. Oncogenes drive cells to divide continually.
organelles The membrane-bound compartments of eukaryotic cells that carry out specific functions.
organic Describes a molecule with a carbon-based backbone and at least one C-H bond.
osmosis The diffusion of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration.
overweight Having a BMI between 25 and 29.9.
paleontologist A scientist who studies ancient life by examining the fossil record.
pancreas An organ that secretes the hormones insulin and glucagon, as well as digestive enzymes.
parasitism A type of symbiotic relationship in which one member benefits at the expense of the other.
pathogen Infectious agents including certain viruses, bacteria, fungi, and parasites. Many pathogens trigger an immune response.
pedigree A visual representation of the occurrence of phenotypes across generations.
peer review A process in which independent scientific experts read scientific studies before they are published to ensure that the authors have appropriately designed and interpreted the study.
peptidoglycan The macromolecule found in all bacterial cell walls that confers rigidity.
pH A measure of the concentration of H1 in a solution.
phenotype The visible or measurable features of an individual.
phospholipid A type of lipid that forms the cell membrane.
photons Packets of light energy, each with a specific wavelength and quantity of energy.
photosynthesis The process by which plants and other autotrophs use the energy of sunlight to make energy-rich molecules using carbon dioxide and water.
phylogenetic tree A branching diagram of relationships showing common ancestry.
phylogeny The evolutionary history of a group of organisms.
pili (singular: pilus) Short, hairlike appendages extending from the surface of some bacteria, used to adhere to surfaces.
pistil The female reproductive structure of a flower, made up of a stigma, style, and ovary.
placebo A fake treatment given to control groups to mimic the experience of the experimental groups.
plant A multicellular eukaryote that has cell walls, carries out photosynthesis, and is adapted to living on land.
plate tectonics The movement of Earth’s upper mantle and crust, which influences the geographical distribution of landmasses and organisms.
pluripotent Describes a cell with the ability to differentiate into nearly any cell type in the body.
polar molecule A molecule in which electrons are not shared equally between atoms, causing a partial negative charge at one end and a partial positive charge at the other; for example, water.
pollen Small, thick-walled plant structures that contain cells that develop into sperm.
pollination The transfer of pollen from male to female plant structures so that fertilization can occur.
polygenic trait A trait whose phenotype is determined by the interaction among alleles of more than one gene.
polymer A molecule made up of individual subunits, called monomers, linked together in a chain.
polymerase chain reaction (PCR) A laboratory technique used to replicate, and thus amplify, a specific DNA segment.
population A group of organisms of the same species living and interacting in a particular area.
population density The number of organisms per unit area.
population genetics The study of the genetic makeup of populations and how the genetic composition of a population changes.
potential energy Stored energy.
predation An interaction between two organisms in which one organism (the predator) feeds on the other (the prey).
prion An infectious agent made only of protein.
producers Autotrophs (photosynthetic organisms) that form the base of every food chain.
prokaryote A usually unicellular organism whose cell lacks internal membrane-bound organelles and whose DNA is not contained within a nucleus.
prokaryotic cells Cells that lack internal membrane-bound organelles.
protein An organic molecule made up of linked amino acid subunits.
protist A eukaryote that cannot be classified as a plant, animal, or fungus; usually unicellular.
proton A positively charged subatomic particle in the nucleus of an atom.
proto-oncogene A gene that codes for a protein that helps cells divide normally.
punctuated equilibrium Periodic bursts of species change as a result of sudden environmental change.
Punnett square A diagram used to determine probabilities of offspring having particular genotypes, given the genotypes of the parents.
radial symmetry The pattern exhibited by a body plan that is circular, with no defined left and right sides.
radiation therapy The use of ionizing (high-energy) radiation to treat cancer.
radiometric dating The use of radioactive isotopes as a measure for determining the age of a rock or fossil.
randomized clinical trial A controlled medical experiment in which subjects are randomly chosen to receive either an experimental treatment or a standard treatment (or a placebo).
recessive allele An allele that reveals itself in the phenotype only if a masking dominant allele is not present.
recombinant gene A genetically engineered gene.
recombination An event in meiosis during which maternal and paternal chromosomes pair and physically exchange DNA segments.
red blood cells (erythrocytes) A type of blood cell specialized for carrying oxygen.
regulatory sequence The part of a gene that determines the timing, amount, and location of protein production.
relative dating Determining the age of a fossil from its position relative to layers of rock or fossils of known age.
renewable resources Natural resources that are replenished after use as long as the rate of consumption does not exceed the rate of replacement.
reproductive isolation Mechanisms that prevent mating (and therefore gene flow) between members of different species.
ribosome A complex of RNA and proteins that carries out protein synthesis in all cells.
RNA polymerase The enzyme that carries out transcription. RNA polymerase copies a strand of DNA into a complementary strand of mRNA.
root system The belowground parts of a plant, which anchor it and absorb water and nutrients.
sample size The number of experimental subjects or the number of times an experiment is repeated. In human studies, sample size is the number of participants.
saturated fat An animal fat, such as butter; saturated fats are solid at room temperature.
science The process of using observations and experiments to draw conclusions based on evidence.
scientific theory An explanation of the natural world that is supported by a large body of evidence and has never been disproved.
seed The embryo of a plant, together with a starting supply of food, all encased in a protective covering.
semiconservative DNA replication is said to be semiconservative because each newly made DNA molecule has one original and one new strand of DNA.
sex chromosomes Paired chromosomes that differ between males and females, XX in females, XY in males.
short tandem repeats (STRs) Sections of a chromosome in which DNA sequences are repeated.
simple diffusion The movement of small, hydrophobic molecules across a membrane from an area of higher concentration to an area of lower concentration; simple diffusion does not require an input of energy.
sister chromatids The two identical DNA molecules that make up a duplicated chromosome following DNA replication.
solute A dissolved substance.
solution The mixture of solute and solvent.
solvent A substance in which other substances can dissolve; for example, water.
somatic mutation A mutation that occurs in a body (nongamete) cell; not passed on to offspring.
speciation The genetic divergence of populations, leading over time to reproductive isolation and the formation of new species.
spinal cord A bundle of nerve fibers, contained within the bony spinal column, that transmits information between the brain and the rest of the body.
stabilizing selection A type of natural selection in which organisms near the middle of the phenotypic range of variation are favored by the environment.
stamen The male reproductive structure of a flower, made up of a filament and an anther.
starch A complex plant carbohydrate made of linked chains of glucose molecules; a source of stored energy.
statistical significance A measure of confidence that the results obtained are “real” and not due to chance.
stem cells Immature cells that can divide and differentiate into specialized cell types.
stigma The sticky “landing pad” for pollen on the pistil.
stomata (singular: stoma) Pores on leaves that permit the exchange of oxygen and carbon dioxide with the air and also allow water loss.
style The tubelike structure that leads from the stigma to the ovary.
substrate A molecule to which an enzyme binds and on which it acts.
sustainability The use of Earth’s resources in a way that will not permanently destroy or deplete them; living within the limits of Earth’s biocapacity.
symbiosis A relationship in which two different organisms live together, often interdependently.
taxonomy The process of identifying, naming, and classifying organisms on the basis of shared traits.
testable Describes a hypothesis that can be supported or rejected by carefully designed experiments or observational studies.
tetrapod A vertebrate animal with four true limbs, that is, jointed, bony appendages with digits. Mammals, amphibians, birds, and reptiles are tetrapods.
tissue An organized group of different cell types that work together to carry out a particular function.
totipotent Describes a cell with the ability to differentiate into any cell type in the body.
trans fat A type of vegetable fat that has been hydrogenated, that is, hydrogen atoms have been added, making it solid at room temperature.
transcription The first stage of gene expression, during which cells produce molecules of messenger RNA (mRNA) from the instructions encoded within genes in DNA.
transfer RNA (tRNA) A type of RNA that transports amino acids to the ribosome during translation.
transgenic Refers to an organism that carries one or more genes from a different species.
translation The second stage of gene expression, during which mRNA sequences are used to assemble the corresponding amino acids to make a protein.
transport proteins Proteins involved in the movement of molecules across the cell membrane.
triglycerides A type of lipid found in fat cells that stores excess energy for long-term use.
trisomy 21 Carrying an extra copy of chromosome 21; also known as Down syndrome.
trophic levels Feeding levels, based on positions in a food chain.
tumor A mass of cells resulting from uncontrolled cell division.
tumor suppressor gene A gene that codes for proteins that monitor and check cell cycle progression. When these genes mutate, tumor suppressor proteins lose normal function.
unsaturated fat A plant fat, such as olive oil; unsaturated fats are liquid at room temperature.
vascular plant A plant with tissues that transport water and nutrients through the plant body.
ventricles The chambers of the heart that pump blood away from the heart. The right ventricle pumps blood to the lungs, and the left ventricle pumps blood to the body.
vertebrate An animal with a bony or cartilaginous backbone.
vestigial structure A structure inherited from an ancestor that no longer serves a clear function in the organism that possesses it.
virus An infectious agent made up of a protein shell that encloses genetic information.
vitamin An organic molecule required in small amounts for normal growth, reproduction, and tissue maintenance.
vitamin D A fat-soluble vitamin required to maintain a healthy immune system and to build healthy bones and teeth. The human body produces vitamin D when skin is exposed to UV light.
X-linked trait A phenotype determined by an allele on an X chromosome.
Y-chromosome analysis Comparing sequences on the Y chromosome to examine paternity and paternal ancestry.
zygote A fertilized egg.