[词汇] SAT II 生物词汇表

A
abiotic
Nonliving materials in the environment—such as elements, sunlight, and soil—that influence
and are influenced by living (biotic) entities on the planet.
acetylcholine
A neurotransmitter released by neurons to excite an action potential or trigger a muscle to
contract.
acids
Hydrogen ion (H+) donors. Acids are very important in the chemical reactions of life because
they are highly reactive. Acids have pH values below 7. They are the opposite of bases.
actin
Protein filaments that, along with myosin, allow muscles to contract.
active site
The part of an enzyme that interacts with, or binds to, a substrate.
active transport
The movement of molecules across a cell membrane from a region of lower concentration to a
region of higher concentration. Because active transport involves moving the molecule against
the natural flow of the concentration gradient, the process requires energy.
adaptive radiation
The evolutionary process by which ancestral forms of an organism are diversified through
adaptation to new environments.
adenosine triphosphate (ATP)
The energy storage molecule for the cell. ATP consists of an adenosine molecule bonded to
three phosphate groups. Each phosphate bond contains energy; by breaking these bonds, the
cell can get the energy it needs for chemical reactions. Cells build ATP during cell respiration,
using the raw material of glucose.
adrenal glands
Two glands, the adrenal cortex and adrenal medulla, located on the kidney.
aerobic respiration
A form of cell respiration requiring oxygen (as opposed to anaerobic respiration, which does
not need oxygen). Aerobic respiration is much more efficient than anaerobic respiration; it

produces 36 ATP for every molecule of glucose. Aerobic respiration proceeds in three stages:
glycolysis, the Krebs cycle, and the electron transport chain.
allele
A specific form or possible version of a gene having multiple versions. Alleles may be
dominant or recessive.
allelic frequency
The frequency with which a particular allele for a certain characteristic appears among all
possible alleles for that characteristic in a population.
alternation of generations
The fluctuation between the diploid (sporophyte) and haploid (gametophyte) life stages that
occur in plants.
amino acid
The monomer of a protein. A central carbon attached to an amino group (–NH2), a carboxyl
group (–COOH), and a hydrogen atom (–H). The fourth group is variable and defines the
amino acid’s chemical identity.
anaerobic respiration
A form of cell respiration that does not use oxygen (as opposed to aerobic cell respiration).
Anaerobic respiration is less efficient than the aerobic variety and produces just 2 ATP per
molecule of glucose. Anaerobic respiration has two stages: glycolysis and fermentation.
analogous trait
A trait that is morphologically and functionally similar to that of a different species but that
arose from a distinct, ancestral condition.
anaphase
The stage of mitosis in which sister chromosomes are separated and pulled to opposite ends
of the cell by microtubules; the fourth stage of the first meiotic division (meiosis I), during which
maternal and paternal homologous pairs are separated on microtubules; the fourth stage of the
second meiotic division (meiosis II), during which either maternal or paternal sister chromatids
are separated on microtubules.
angiosperm
A vascular flowering plant in which seeds are enclosed inside protective ovaries, such as fruit
or flowers. Angiosperms can be monocots or dicots.
anther
Pollen-producing structure at the top of the stamen, the male reproductive organ of flowers.
anticodon
The sequence of three nucleotides on tRNA that pairs with a codon of mRNA at the A site of a
ribosome during translation.
antigen
A protein coat on the surface of red blood cells; a red blood cell may have a protein coat of
type A, B, or AB. If the cell has no antigens, it is called type O. The presence of a foreign
antigen in a body will cause blood to clot.
aorta
The largest artery in the body; carries oxygenated blood from the left ventricle of the heart.
aphotic zone

Literally, zone without light. The aphotic zone is part of the marine pelagic zone and begins
600 feet below the surface of the ocean. Only chemosynthetic organisms, scavengers, and
predators are able to survive in this habitat.
artery
Vessel that carries blood away from the heart and has thick, elastic, muscular walls that can
dilate or contract to control blood pressure within the vessels. Blood in arteries is oxygenated,
with the exception of the blood in the pulmonary artery.
autonomic nervous system
The involuntary half of the peripheral nervous system. The autonomic nervous system is in two
antagonistic parts: the sympathetic and parasympathetic nervous systems. Their interactions
control smooth and cardiac muscle, glands, and organs and processes such as heartbeat, the
movements of the digestive tract, and the contraction of the bladder.
autosome
Any chromosome that is not a sex chromosome. Humans have 44 autosomes, in 22
homologous pairs. The two sex chromosomes are the twenty-third pair of chromosomes.
autotroph
An organism that can produce the organic molecules and energy necessary for life through the
processes of photosynthesis or chemosynthesis. Autotrophs do not rely on other organisms for
food. In a food web, autotrophs are producers.
auxin
One in a class of plant hormones that stimulates (among other things) cell elongation,
secondary tissue growth, and fruit development.
B
base
An ion or compound that removes H+ ions from solution. Often bases are substances that
release hydroxide ions (OH–). Bases have pH values above 7. They are the opposite of acids.
bile
An emulsifier of fats secreted by the liver and stored in the gallbladder for release in the small
intestine.
binary fission
Asexual reproduction found in prokaryotes in which a cell divides into two equal daughter cells
by a nonmitotic process.
biomass
The amount of living matter in a given ecosystem. Because only 10 percent of energy is
transferred between trophic levels, the biomass of lower trophic levels is greater than the
biomass of subsequent trophic levels: biomass of producers > biomass of primary consumers
> biomass of secondary consumers > biomass of tertiary consumers.
biome
A particular geographic area with a common climate and characteristic plant and animal life.
There are six major terrestrial biomes and two aquatic biomes. The six terrestrial biomes are
tropical rain forest, savanna, desert, temperate deciduous forest, taiga, and tundra. The two

aquatic biomes are marine and freshwater. Each biome is characterized by specific climax
communities.
blood
The liquid that carries nutrients and oxygen to the cells and carries carbon dioxide and
nitrogenous wastes away. The liquid fluid of blood is called plasma. Red blood cells contain
hemoglobin, an iron-containing protein that binds oxygen. White blood cells fight disease.
Platelets clot to prevent extreme blood loss resulting from injury.
bone
Rigid structures composed of living cells rooted in a matrix of calcium, phosphate salts, and
collagen fibers. Bones are the primary component of most vertebrate skeletons.
brain
The center of the central nervous system. The brain coordinates the processes of the body. It
is composed of various distinct regions, all of which have different functions, including the
cerebrum, cerebellum, medulla oblongata, and hypothalamus.
bryophyte
A lower terrestrial plant (often a moss or liverwort) that lacks a vascular system and is
dependent on environmental moisture for reproductive and nutritive functions.
budding
Asexual reproductive process in which a small portion of the cell membrane and cytoplasm
receive a nucleus and pinch off from the parent cell.
buffer
Solutions that resist change in pH even when acids and bases are added.
bulb
Roughly spherical underground bud containing additional buds that can develop asexually into
new plants.
C
Calvin cycle
Light-independent phase of photosynthesis, where carbon dioxide is fixed to a three-carbon
compound used to form glucose. ATP and NADH are consumed in this cycle. Also called the
Calvin-Benson cycle or the dark reactions.
capillary
Tiny blood vessels able to branch through the body and deliver oxygen and nutrients to every
cell.
carbon
The central element of life. Carbon has the ability to form bonds with up to four other elements
or molecules at the same time.
carrying capacity
The maximum number of individuals in a population that can be sustained in a given
environment. As populations become increasingly concentrated, competition for food and
space, predation, and disease all determine carrying capacity.
cartilage

A firm but flexible substance, found in regions of vertebrate skeletons, such as the ribs, that
need to bend.
cell
The smallest unit of life, consisting of a solution of organic molecules enclosed by a plasma
membrane.
cell cycle
A process in which cells reproduce. First the cell replicates its DNA and then divides into two
daughter cells. The two main phases of the cell cycle are interphase and mitosis.
cell membrane
The phospholipid bilayer that surrounds all cells, regulating the passage of molecules in and
out of the cell.
cellular respiration
The process in which the cell burns glucose to create ATP with the aid of oxygen. Cells have
two different methods of turning food into usable fuel: aerobic respiration and anaerobic
respiration.
cell theory
The doctrine that every living organism is composed of cells and that all cells come only from
other preexisting cells.
cell wall
A rigid structure that surrounds the outer membrane of some cells and helps maintain their
shape. In plants the cell wall contains cellulose; in fungi it contains chitin; in prokaryotes it
typically contains peptidoglycan.
cellulose
A complex carbohydrate that constitutes the cell walls of plants and protist molds.
central nervous system (CNS)
The brain and the spinal cord. The CNS acts as the central command center of the body.
Mostly made up of interneurons.
centriole
A structure in the cell that plays an important role in cell replication. During prophase, the
centrioles migrate to the poles of the cell and form the mitotic spindle, which allows the
chromosomes to be organized and split when the cell divides.
cerebellum
Part of the brain. Makes sure that movements are coordinated and balanced.
cerebrum
Part of the brain. Controls all voluntary movement, sensory perception, speech, memory, and
creative thought.
chemical cycles
The cycles in which inorganic elements move through the biotic and abiotic aspects of an
ecosystem. The two most important chemical cycles are the carbon and nitrogen cycles.
chemosynthesis
Synthesizing organic compounds by energy derived from chemical reactions rather than from
the energy of the sun. Chemosynthetic organisms are autotrophs.
chitin

A rough polysaccharide that constitutes the cell wall of fungi and exoskeleton of arthropods.
chlorophyll
A pigment located within a chloroplast that absorbs light in plant cells, helping to convert light
energy into biological energy through the process of photosynthesis.
chloroplast
A double-membrane-bound organelle that contains chlorophyll and is found in plant cells.
Chloroplasts are responsible for mediating photosynthesis.
chromatin
The stringy web of genetic material and histone proteins found in the nucleus of eukaryotic
cells. During cell division, each strand of DNA coils to form a chromosome.
chromosome
A physical structure composed of a single long strand of DNA (and associated proteins),
containing along its length many genes. The human genome consists of 46 chromosomes
contained within the nucleus of each cell.
cilia
Short, hairlike projections found on eukaryotic cells that can help the cell move or can sweep
food particles toward the mouth.
circadian rhythms
Behavior cycles that depend on time of day.
circulatory system
System of organs and blood that brings nutrients and oxygen to cells and carries away wastes.
In higher vertebrates, the system has a pulmonary and systemic circuit. The pulmonary circuit
carries blood to the lungs to be oxygenated, while the systemic circuit carries oxygenated
blood to the body. Vertebrates have a closed circulatory system, while arthropods have an
open system.
citric acid cycle
See Krebs cycle.
climax community
A combination of plant and animal forms that dominate mature ecological communities. Climax
communities are unique and shaped by various factors, including temperature, rainfall, and soil
acidity.
codominance
A phenomenon in which two alleles of the same gene are fully expressed in the phenotype
when both are present in a heterozygote. Blood type is an example of codominance.
codon
A three-nucleotide sequence in a DNA or mRNA molecule. Each codon specifies a single
amino acid.
coenzyme
A compound that regulates activity by binding to an enzyme to tell it when to catalyze a
reaction.
cold-blooded
Animals that are unable to retain heat produced by metabolic activities. Also known as
ectothermic. The metabolism of cold-blooded animals is greatly influenced by climate and

temperature.
community
The many populations that interact in a given geographical locale constitute ecological
communities. Communities exhibit particular interactions such as competition, symbiosis,
predation, and food relationships. They also undergo ecological succession.
competition
The struggle for survival between organisms or populations that use similar resources and
occupy similar niches. Interspecific competition refers to competition between populations that
may drive a population out of a community or push it to evolve a different niche to reduce
competition. Intraspecific competition refers to competition between individuals of the same
species.
consumer
Consumers are heterotrophic organisms within the food web of a community. In the trophic
levels of the food chain, primary consumers consume producers, secondary consumers
consume primary consumers, and tertiary consumers consume secondary consumers.
contractile vacuole
An organelle often found in protozoa that pumps excess water out of the cell to keep the cell
from bursting in a hypotonic environment (like freshwater).
corpus luteum
After releasing its ovum (ovulation), the follicle becomes the corpus luteum, which produces
progesterone for the continued buildup of the uterine wall.
crossing-over
The exchange of genetic information between homologous chromosomes during meiosis I.
Crossing-over can disrupt the normal linkage between genes on the same chromosome.
cytokinesis
The final part of mitosis, in which a cell with duplicated contents splits into two independent
cells.
cytoplasm
The entire content of the cell outside the nucleus, including the membrane-bound organelles
and the cytosol.
cytoskeleton
A system of protein filaments found throughout the cytoplasm of eukaryotic cells, which
provides structural support for the cell. The cytoskeleton also helps with the movement of
organelles within the cell. It is composed of microfilaments and microtubules.
cytosol
The main component of the cytoplasm. It is a grayish, gel-like liquid containing the nucleus,
organelles, and cytoskeleton.
D
Darwin, Charles
English naturalist (1809–1882) who proposed the modern theory of evolution through natural
selection. Darwin traveled aboard the HMS Beagle to the Galápagos Islands, where his

revolutionary observations took shape.
decomposer
Organisms that consume waste products and dead organic material and constitute part of the
food web, which also includes producers and consumers. Also called saprophytes.
Decomposers liberate inorganic elements such as nitrogen and carbon and allow those
elements to move back into their respective chemical cycles. Examples of decomposers are
bacteria and fungi.
dehydration synthesis
A common biochemical reaction in which a new compound is formed by the joining of two
compounds to release water. Occurs in the synthesis of polysaccharides and polypeptides.
The reverse of hydrolysis.
deoxyribonucleic acid (DNA)
A type of nucleic acid polymer built from sugar-phosphate backbones and nitrogenous bases.
DNA’s sugar, deoxyribose, has one fewer oxygen atom than ribose, found in RNA. The
nitrogenous bases adenine, cytosine, guanine, and thymine are used in DNA.
dicot
A flowering plant (angiosperm) that possesses two cotyledons during embryonic development.
Usually has taproots, flower parts in multiples of fours and fives, and branching veins in leaves.
diffusion
The transport or natural drift of molecules traveling from an area of higher concentration to an
area of lower concentration. Diffusion does not require outside energy from the cell.
digestive system
The system of organs that converts food to usable nutrients through mechanical and chemical
breakdown. Important components of the system are the alimentary canal, glands, esophagus,
stomach, small intestine, large intestine, and rectum.
diploid number
The total number of chromosomes present in a somatic cell. The diploid number is twice the
haploid number. In humans, the diploid number is 46.
disaccharide
A sugar compound consisting of two carbohydrate monomers.
dominant
Refers to an allele that controls the phenotype even when a different allele is also present, as
in a heterozygote. Can also refer to the trait or phenotype produced by a dominant allele. Also
known as Mendel’s law of dominance, based on Gregor Mendel’s observations that when two
purebred individuals with different forms of the same trait are mated, only one of the two forms
appears in the first generation of offspring. Mendel called the apparent form dominant and the
suppressed form recessive.
E
ear
Sensory organ capable of detecting sound.
ecological succession

The progression of plant life and attendant animal life in a given geographic location, from
pioneer plant to climax community.
ecology
The study of the interactions and relationships of populations with each other and their abiotic
environments.
ecosystem
A community of organisms and its abiotic environment.
ectotherm
See cold-blooded.
egg
The female gamete in sexual reproduction; also called an ovum.
electron microscope
An instrument that uses an electron beam to form clear and highly magnified images of
microscopic structures. Electron microscopes cannot take pictures of living organisms.
electron transport chain
The final stage of aerobic respiration. The electron transport chain establishes an
electrochemical gradient across the inner mitochondrial membrane that powers the synthesis
of ATP in oxidative phosphorylation.
embryo
Before birth, the maturing cells that will grow into a fully formed organism.
endocrine system
Control system of the body that functions by releasing hormones into the bloodstream.
endocytosis
Process by which liquids or small solid particles are taken into a cell in the form of small
vesicles that are produced through the invagination of the cell membrane.
endoplasmic reticulum
A network of membrane-bound tubes and sacs in the cytoplasm. The endoplasmic reticulum is
a major site of protein and lipid synthesis.
endoskeleton
An interior skeleton found in vertebrates made of bone and cartilage.
endotherm
See warm-blooded.
energy pyramid
Energy in a community can be depicted as a pyramid of food or biomass. The availability of
food, biomass, and energy from the trophic level of producers up through each subsequent
level on the food web is approximately 10 percent of that available in the previous trophic level.
enzymes
Biological catalysts made from proteins. Enzymes have attachment locations for substrates
called active sites.
estrogen
Hormone that stimulates the growth of the uterine lining during pregnancy and that develops
and maintains the female secondary sex characteristics, such as the development of
mammary glands, a narrower waist and wider hips, axillary and pubic hair, and a higher-

pitched voice.
eukaryote
An organism whose cells have membrane-bound intracellular organelles, including a nucleus
containing multiple chromosomes. Eukaryotes, unlike prokaryotes, can undergo sexual
reproduction via meiosis. Compared to prokaryotes, eukaryotes are more complex and arose
later in evolutionary history. Protists, fungi, plants, and animals are all eukaryotic organisms.
excretory system
The organ system that filters blood and removes nitrogenous wastes from the body in the form
of urea or uric acid. In humans, the two kidneys are the vital organs of blood filtration. In
annelids, nephridia fill the filtering role; Malpighian tubules do the same in arthropods. In
humans, other important structures of the system are the ureters, the urinary bladder, and the
urethra.
exocytosis
Process by which molecules are secreted from the cell. Exocytosis occurs when a vesicle
fuses with the cell membrane and releases its contents to the outside.
exoskeleton
A rigid, chitinous protective structure that surrounds the bodies of arthropods and provides
support.
eye
Sensory organ capable of detecting light.
F
F1 generation
The first generation of offspring from a cross between two varieties or individuals. In Mendel’s
experiments, all the F1 offspring were heterozygous hybrids with a dominant phenotype.
F2 generation
The second generation of offspring in a breeding experiment; the offspring from a mating
between two F1 hybrids. In Mendel’s monohybrid experiments, the ratio of dominant to
recessive phenotypes in the F2 generation was 3:1.
facilitated diffusion
Diffusion of molecules that cannot pass through the cell membrane independently, but rather
through permeable protein channels embedded in the membrane. Facilitated diffusion does
not require outside energy.
FADH2
A molecule that stores energy for harvest by the electron transport chain.
Fallopian tube
Duct that connects the ovaries with the uterus; the passage through which the ovulated egg
travels from the ovaries to the uterus.
fermentation
The second stage of anaerobic respiration, which produces the NAD+ necessary for glycolysis.
There are two types of fermentation: alcoholic fermentation and lactic acid fermentation. Yeast
engage in alcoholic fermentation, while muscle cells lacking oxygen produce lactic acid.

fetus
The name given to an embryo after it has developed organs.
flagellum
A whiplike structure projecting from the surface of some cells and single-celled organisms;
coordinated waving of the flagellum allows the organism to swim. Prokaryotic flagella are made
of a single helical chain of flagellin proteins; eukaryotic flagella are made of multiple chains of
microtubule proteins.
fluid-mosaic model
Theory describing the cell membrane as a dynamic structure with proteins floating, yet partially
embedded, in a sea of phospholipids.
food chain
A linear relationship of predators and prey.
food web
Many connected food chains that exhibit the relationships of all predators to all prey constitute
a food web.
fossil record
The grouping of fossilized remains according to relative and absolute age.
G
gallbladder
An organ that stores the bile produced by the liver and releases it to the small intestine during
digestion.
gamete
A haploid sex cell (either an egg or sperm cell); male and female gametes join during
fertilization to create a diploid zygote. Gametes are created out of germ cells and are passed
down to offspring.
gametophyte
A haploid plant or plant structure that produces haploid gametes through mitosis.
ganglion
A simple cluster of nerve cells that acts as a coordinating center. In more sophisticated
organisms, ganglia evolved into a brain and spinal cord.
gene
The fundamental unit of heredity, composed of a stretch of DNA. In general, a single gene
encodes the information needed to produce one kind of protein. Each gene resides in a
specific spot on a chromosome.
gene flow
The movement of genes, within a population or between populations, through mating.
genetic code
The series of codons that make up an organism’s DNA.
genotype
The entire set of specific alleles present in an organism or cell: the genetic information that
(together with the environment) defines the phenotype. Often refers only to the alleles
controlling a particular trait of interest.
germ cell
Cells that lead to the production of gametes. Produced by meiosis.
glucose
A monosaccharide with the chemical formula C6H12O6. Used as the raw material for cellular
respiration.
glycolysis
The first step of aerobic and anaerobic respiration. Glycolysis produces ATP while converting
glucose to pyruvate, which is the raw material for the rest of aerobic respiration.
Golgi apparatus
A series of membrane-bound sacs in the eukaryotic cytoplasm. The Golgi apparatus takes
proteins produced by the endoplasmic reticulum and packages and secretes them to various
destinations inside and outside of the cell.
gonads
Sex organs that produce gametes. The gonads also release sex hormones, such as
testosterone, estrogen, and progesterone. In humans, the male gonads are the testes; in
females, the ovaries.
grafting
An artificial form of vegetative propagation in which parts of two young plants are joined
together, first by artificial means and then by tissue regeneration.
Gram staining
A process by which components of bacterial cell walls are bound to Gram’s stain. Depending
on the amount of peptidoglycan in their cell walls, bacteria stain differently and are classified
as Gram-negative or Gram-positive.
guard cells
Epidermal plant cells found in pairs surrounding the stomata of leaves. By increasing or
decreasing their size, guard cells regulate gas exchange by opening and closing individual
stoma.
gymnosperm
A vascular nonflowering plant (commonly known as a conifer) in which seeds are “naked”—
collected in a cone and not protected by an ovary. The dispersion of their spermatozoids often
relies on wind.
H
haploid number
The number of homologous pairs in a cell. Equal to half the diploid number. Gametes, cells
that are passed on to offspring, contain the haploid number of chromosomes. In humans, the
haploid number is 23.
heart
The muscular organ that pumps blood through the circulatory system. Mammals and birds
have a four-chambered heart, with a left atrium and ventricle and a right atrium and ventricle.
The right half of the heart receives deoxygenated blood from the body and pumps it to the

lungs, while the left half receives oxygenated blood from the lungs and pumps it to the body.
heredity
The genetic transmission of traits from parents to offspring, so that offspring resemble their
parents. Traits transmitted this way are called hereditary traits.
heterotrophs
Organisms that can only get the organic molecules and energy necessary for life through the
consumption of other organic matter. In the food web, all consumers and decomposers are
heterotrophs. Heterotrophs can be herbivores, carnivores, or omnivores.
heterozygous
A situation in which an individual (heterozygote) possesses two dissimilar alleles for the same
gene. The opposite is homozygous.
homologous chromosomes
Chromosomes containing the same series of genes; they may or may not carry the same
alleles. Humans receive one set of 23 paternal chromosomes from their male parent and
another set of 23 maternal chromosomes from their female parent. Each set matches up to the
other for a total of 23 different pairs of homologous chromosomes. During meiosis,
homologous pairs line up and are separated. In males, the X and Y chromosomes act as a
homologous pair, although they are only partially homologous.
homologous trait
A trait found in different species that are morphologically and functionally similar and that
comes from the same ancestral condition. A whale’s fin and a human’s arm are homologous
structures.
homozygous
A situation in which an individual (homozygote) has the exact same allele on both homologous
chromosomes. Mating of two individuals with the same homozygous genotype will produce
only offspring with that same identical genotype. The two identical alleles may be dominant or
recessive (e.g., RR or rr). The opposite of homozygous is heterozygous.
hormones
A chemical messenger that can be made of either peptides or lipids. Secreted by glands in one
part of the body, hormones affect glands or organs in another part.
hybrid
A genetic mixture; the offspring of two genetically different parents. Hybrids are usually
heterozygous for a variety of genes.
hydrogen bond
A weak bond between hydrogen and a set of other elements, including oxygen. Hydrogen
bonds are a subset of dipole-dipole interactions.
hydrolysis
A common biochemical reaction in which the bond between two molecules is split by the
addition of a water molecule. Hydrolysis is the process that breaks down polymers and dimers.
The reverse is dehydration synthesis.
hydrophilic
Having an affinity for water; usually polar molecules. For the SAT II Biology, this is principally
important in relation to the phospholipid bilayer.

hydrophobic
Having a reluctance to mix with water; usually nonpolar molecules. The fatty acids that form
the interior pocket between the two layers of the cell membrane are hydrophobic.
hydrostatic skeleton
A fluid skeleton in many soft-bodied invertebrates, including annelids, that allows an organism
to change shape but not volume.
hypertonic
A situation in which the concentration of solutes in a solution is higher than what it contains.
For example, a sodium solution of 10 percent would be hypertonic to an animal cell (with a
sodium concentration of about 0.9 percent), causing water to leave the cell by osmosis.
hypothalamus
Part of the brain responsible for temperature regulation, controlling hunger and thirst, and
managing water balance. It also helps generate emotion.
hypotonic
A situation in which the concentration of solutes in a solution is lower than what an organism
contains. An example is a paramecium in pond water: the organism has more solutes than its
environment, so water flows into the cell by osmosis. Paramecia have evolved contractile
vacuoles to keep from exploding.
I
imprinting
Instinctual behavior in which social bonds are formed during early development. Typically used
with the example of a newly hatched bird or other animal identifying and treating the first
moving object it sees as its mother.
incomplete dominance
Occurs when two different alleles of the same gene are both partially expressed in a
heterozygote. The resulting phenotype is intermediate between the homozygous phenotypes
of the two alleles. Color in a four o’clock flower is an example of incomplete dominance.
independent assortment
Mendel’s conclusion that during gamete formation, the segregation of one pair of genes has no
influence over the segregation of another pair of hereditary units. Two different traits will thus
be inherited independently of each other. This law holds true, except for linked genes. It is
often called Mendel’s second law.
inheritance of acquired traits
A crucial, and incorrect, aspect of Lamarck’s theory of evolution. Lamarck believed that the
traits a parent acquired during its lifetime would be passed on to its offspring.
instincts
Behavior that is hardwired into the brain of an organism; behavior that does not have to be
learned.
insulin
Hormone secreted by the pancreas that reduces blood sugar levels. A lack of insulin can result

in diabetes.
interphase
The phase of the eukaryotic cell cycle in which the cell prepares for division, primarily by
replicating its DNA. After interphase, the cell enters mitosis.
intertidal zone
The most shallow zone in a marine habitat. Periodically dry or wet with the changing tides.
Algae, sponges, mollusks, starfish, and crabs inhabit this zone. Also called the littoral zone.
ion
An atom or molecule that has lost or gained an electron and consequently has a positive or
negative charge.
isotonic
A situation in which the concentration of solutes in a solution is equivalent to what it contains.
There is no concentration gradient in isotonic solutions and no net flow of solutes or water.
K
karyotype
A photograph of the chromosomes from an individual cell, usually lined up in homologous
pairs, according to size. Missing, extra, or abnormal chromosomes can then be easily
identified, aiding in the diagnosis of genetic disorders.
kidney
The organ of blood filtration in humans. The functional unit of the kidney is the nephron.
kingdom
The broadest category of biological taxonomy.
Krebs cycle
A metabolic pathway in aerobic organisms in which acetyl-CoA combines with oxaloacetic acid
to form citric acid. For this reason, the Krebs cycle is also known as the citric acid cycle. The
cycle produces energy in the forms of ATP, NADH, and FADH2.
L
Lamarckism
An evolutionary theory (proved false by Darwin) stating that species change over time by the
use and disuse of structures and the inheritance of acquired traits.
learned behavior
Behavior that an organism picks up over the course of its life. Three types of learned behavior
are habituation, conditioning, and associative learning.
Leeuwenhoek, Antonie van
Dutch clothing salesman (1632–1723) who made the first observations of bacteria and
protozoa using single-lens microscopes of his own design.
ligament
Connective tissue between bones.
linked genes

Genes that are located close together on the same chromosome. Linked genes will not
undergo independent assortment during gamete formation, constituting an exception to the law
of independent assortment. Crossing-over will disrupt the linkage of two genes on the same
chromosome if they are far enough apart.
lymphatic system
A means of returning blood fluid, lymph, that has escaped from capillaries back into the
bloodstream. Defended against foreign bodies by lymphocytes.
lymphocyte
White blood cell that is specialized to kill specific invading bacteria. Three types of
lymphocytes work in coordination: B cells, helper T cells, and killer T cells.
M
Malpighian tubules
The organ of blood filtration in arthropods.
medulla oblongata
Part of the brain responsible for the control of involuntary functions such as breathing,
cardiovascular regulation, and swallowing.
meiosis
A type of cellular reproduction that results in the formation of four haploid cells from one diploid
cell. Contains two cellular divisions, meiosis I and meiosis II, that follow only one round of DNA
replication. Meiosis produces germ cells.
Mendel, Gregor
Gregor Mendel (1822–1884) was an Austrian monk and scientist. Through a series of
experiments with pea plants, he discovered the basic laws of heredity, including dominance,
segregation, and independent assortment.
menstrual cycle
A 28-day hormone sequence that defines the production, ovulation, and menstruation of eggs
in the female reproductive system. If fertilization of the egg occurs, the menstrual cycle stops.
mesophyll
The internal tissue of a leaf between the epidermal cells; specialized for photosynthesis.
Contains the palisade and spongy layer.
messenger RNA (mRNA)
An RNA molecule that specifies the amino acid sequence of a protein. In transcription,
messenger RNA molecules copy the genetic information stored in DNA. The mRNA then bring
the recipes for proteins from the nucleus to ribosomes in the cytoplasm.
metaphase
The second stage of mitosis in which microtubules align the chromosomes in the center of the
cell along the metaphase plate; the stage of meiosis I and II during which the chromosomes
align at the center of the cell.
minerals
Inorganic molecules required by the body to carry out life processes. Important minerals are
iron, a necessary component of hemoglobin; iodine, which is essential for making thyroid

hormone; and calcium, which is required by the bones and for many cellular processes.
mitochondria
Double membrane-bound organelles that produce most of the energy in eukaryotic cells
through the process of aerobic (cellular) respiration, which generates ATP.
mitosis
The phase of the eukaryotic cell cycle in which the cell divides. The four steps of mitosis are
prophase, metaphase, anaphase, and telophase.
mitotic spindle
A complex of microtubules that forms between opposite poles of a cell during mitosis. The
mitotic spindle is formed by the centrioles and serves to separate and move chromosomes to
opposite ends of the cell for division.
molecular clock
A molecule or gene sequence that has a constant rate of change through accumulation of
neutral substitutions and is therefore a good measuring stick for the relatedness of different
species.
monocot
A flowering plant (angiosperm) that possesses one cotyledon during embryonic development.
Usually has fibrous roots, flower parts in threes, and parallel veins in leaves.
monomer
Each of the repeating units that make up a polymer.
monosaccharide
A carbohydrate monomer. Glucose and fructose are common examples.
muscle
Structures that create movement in an organism by contracting under a stimulus from a
neuron. There are three types of muscle: skeletal, which is responsible for voluntary
movement; smooth, which is responsible for involuntary movement; and cardiac, which makes
up the heart.
mutation
An error in the sequence of nucleotides in DNA or RNA that in turn affects the production of
proteins. There are two main types of mutations: substitution mutations and frameshift
mutations. A substitution mutation occurs when one nucleotide is replaced by another; these
mutations can range from ineffectual to drastic, depending on how the new nucleotide changes
the protein coded for. Frameshift mutations occur when a nucleotide is either inserted or
deleted into the code; these mutations are always drastic and often fatal, since an insertion or
deletion will affect every codon in a particular genetic sequence by throwing the entire three-bythree
codon frame out of whack.
myelin sheath
A structure that speeds the movement of action potentials along the axon of a neuron. The
sheath is built of Schwann cells, which wrap themselves around the axon of the neuron,
leaving small gaps in between known as the nodes of Ranvier.
myosin
Protein filaments that, along with actin, allow muscles to contract.

N
NADH
An energy-carrying coenzyme produced by glycolysis and the Krebs cycle. NADH carries
energy to the electron transport chain, where it is stored in ATP.
natural selection
The theory, first proposed by Darwin, which holds that organisms produce as many offspring
as possible, which compete for limited resources. Organisms’ characteristics vary, and certain
characteristics will allow organisms to survive and reproduce more effectively. These adaptive
characteristics will be more prevalent in subsequent generations. Natural selection is the
engine of evolution, choosing the most fit genes to pass from one generation to the next.
nephridium
A blood filtration and excretory organ characteristic of segmented worms.
nephron
Tiny, tubule structures responsible for the filtering of blood in the kidneys of vertebrates.
neritic zone
The medium depth zone of the marine biome. Extends to 600 feet beneath the water’s surface
and sits on the continental shelf, hundreds of miles from any shore. Algae, crustaceans, and
fish inhabit this region.
nervous system
Control system of the body that functions by sending impulses through neurons to receive
information and spur muscles to action.
neuron
The functional unit of the nervous system. A neuron is a specialized cell able to carry an action
potential and made up of dendrites, a cell body, and an axon. There are three types of
neurons: sensory, motor, and interneurons.
niche
The unique role a population plays in a community. A niche includes all characteristics that
define the way a population exists in a community, from where the members live to what they
eat, when they sleep, and how they reproduce.
nitrogenous base
One of the nitrogen-containing bases in DNA and RNA nucleotides. There are five nitrogenous
bases in living organisms. DNA is composed of adenine, thymine, guanine, and cytosine. RNA
is composed of adenine, uracil, guanine, and cytosine.
nondisjunction
Occurs when a pair of homologous chromosomes fails to separate during gamete formation.
The offspring produced from these gametes have either one too many or one too few of a
particular chromosome. Nondisjunction is the cause of genetic disorders such as Down
syndrome.
notochord
A longitudinal rod of cells that forms in the least developed chordates and in embryonic stages
of more developed chordates.
nucleolus
A dense, spherical body inside the nucleus of a cell. The nucleolus makes the RNA that is a
structural component of the ribosomes.
nucleotide
The monomer of a nucleic acid. Nucleotides consist of (1) a phosphate group, a group of
atoms containing phosphorus; (2) a sugar; and (3) a nitrogenous base, a compound containing
nitrogen that removes H+ ions from solution. Nucleotides are grouped into two general
classes, depending on the sugar group that they carry: deoxyribonucleic acids (DNA) contain
the sugar deoxyribose and ribonucleic acids (RNA) contain the sugar ribose. Nucleotides are
further divided by the type of nitrogenous base that they carry. DNA is composed of adenine,
thymine, guanine, and cytosine. RNA is composed of adenine, uracil, guanine, and cytosine.
nucleus
The large, central organelle of eukaryotes. The nucleus contains the genetic material of the
cell and controls cellular activities.
O
olfactory epithelium
Region near the top of the nasal cavity with chemoreceptors and neurons that inform the
sense of smell.
oogenesis
The process in which haploid egg cells (ova) form through meiotic division.
organ
A discrete unit of tissues that work together to perform specific functions within the body.
organelle
Specialized membrane-bound structure in a cell that performs a specific function. Examples of
organelles include the endoplasmic reticulum and the mitochondria.
osmosis
Process by which water naturally travels from an area of high water concentration to low water
concentration.
ovary
In animals, the female gonad that produces ova and sex hormones. In plants, the ovaries are
the structure at the base of the pistil that contains the eggs.
ovule
Structure that contains the female gametophyte and gametes; after fertilization, develops into a
seed.
oxidative phosphorylation
Part of the electron transport chain. A process occurring in the mitochondria that results in the
formation of ATP from the flow of electrons across the inner membrane to bind with oxygen.
P
pancreas
A digestive organ that releases enzymes into the small intestine. Also an endocrine gland that

regulates glucose levels in the blood by the release of insulin or glucagon from specialized
cells called islets of Langerhans.
parathyroid
Four small glands embedded on the posterior surface of the thyroid gland. The parathyroid
glands produce a hormone that regulates the level of calcium in the bloodstream.
parthenogenesis
Asexual reproduction in which an unfertilized gamete (usually female) produces female
offspring. Parthenogenesis vastly increases the speed at which a population can grow, though
it results in a loss of genetic diversity among members of the population.
pelagic zone
The open-ocean zone at the greatest depth in a marine habitat. This zone is divided into a
photic (down to 600 feet below the water’s surface) and aphotic zone.
peptide bond
The bond between the amino acids in a protein. Formed by dehydration synthesis.
peripheral nervous system (PNS)
The pathways by which the central nervous system receives sensory information from the
body and sends commands to muscles. The peripheral nervous system is divided into two
halves. The sensory system brings information in from the body, while the motor system sends
commands out to muscles.
peristalsis
The rolling motion of smooth muscle that moves food along the alimentary canal. Includes the
passage from the esophagus to the stomach, the churning action of the stomach, and the
passage through the small intestine.
pH
A scale for measuring the presence or absence of hydrogen ions in solution. Values between 0
and 7 indicate an excess of hydrogen ions. Such solutions are called acids. Values between 7
and 14 indicate the presence of compounds that counteract the effects of hydrogen ions. Such
solutions are called bases. At pH 7, solutions are neutral.
phagocyte
A type of white blood cell that kills invading cells by ingesting them.
phagocytosis
A form of endocytosis in which a cell ingests a solid particle.
phenotype
The entire set of observable characteristics of an organism or cell; the physical traits of an
organism. The genotype, together with environmental factors, defines the phenotype.
phloem
Vascular tissue composed of cells that are living at maturity; transports the products of
photosynthesis throughout the plant body.
phospholipid
Type of lipid found in cell membranes. Phospholipids are made up of a single hydrophilic
phosphate head and two nonpolar hydrophobic lipid tails.
phospholipid bilayer
A double layer of phospholipid molecules that provides the structure of the cell membrane.

Formed naturally from the alignment of the two layers of lipids such that their hydrophobic tails
point inward toward each other and their hydrophilic phosphate heads point outward into the
watery environments inside and outside of the cell.
photic zone
Literally, zone with light. The photic zone is part of the marine pelagic zone and extends to 600
feet below the surface of the ocean. Photosynthetic plankton as well as bony fish, sharks, and
whales inhabit this zone.
photoperiodism
An organism’s response to the length of day and night within a 24-hour period (photoperiod); in
many plants, this phenomenon determines when flowering occurs.
photosynthesis
The process by which plants and other autotrophic organisms convert light energy into organic
materials, such as glucose.
phylogeny
The evolutionary relationships of a genetically similar group of organisms.
pinocytosis
Form of endocytosis during which liquids are taken into the cell through the invagination of the
cell membrane.
pioneer population
The first population to move into a geographic location and begin the process of ecological
succession.
pistil
The female reproductive organ of the flower, composed of a stigma, style, and ovary;
sometimes called the carpel.
pituitary
The “master” gland of the endocrine system. The pituitary releases hormones that control the
other major glands of the endocrine system. Made up of the anterior and posterior pituitary.
Controlled by the hypothalamus.
placenta
A structure that develops in the uterus during pregnancy; filter through which the embryo gains
nutrition from the mother.
plasmids
Circular DNA molecules found in prokaryotes.
pollen
The male gametophyte of gymnosperms and angiosperms.
polymer
A large molecule consisting of the same or similar units attached in a series, forming a chain.
population
A group of interbreeding organisms in a particular locale exhibiting a unique set of
characteristics such as patterns of growth and reproductive strategies.
predation
Term that refers to one organism eating another. Predation covers both carnivorous and
herbivorous consumption.

producers
Autotrophic organisms such as plants, plankton, and chemosynthetic bacteria that are able to
synthesize organic compounds using energy from the sun or chemical reactions. Producers do
not have to consume other organisms to attain energy and are the foundation of every food
web.
progesterone
Hormone that prepares the uterus for embryo implantation and helps to maintain pregnancy.
prokaryote
A single-celled organism that completely lacks membrane-bound intracellular organelles such
as a nucleus or mitochondria; prokaryotes possess only a single circular strand of DNA.
Prokaryotes are simpler than eukaryotes and arose earlier in evolutionary history. All bacteria
are prokaryotes. Taxonomists group all prokaryotes into the kingdom Monera.
prophase
The first stage of mitosis, meiosis I, and meiosis II, during which the chromosomes become
visible and the centrioles move to opposite ends of the cell and begin to form the spindle.
pseudopods
Temporary cytoplasmic protrusions of ameboid cells that function in movement and food
uptake by phagocytosis.
Punnett square
A pictorial method of showing the gene combinations (genotypes) of offspring that might result
from an experimental genetic cross of two parents.
pyruvate
The three-carbon end product of glycolysis. Pyruvate is the raw material of the Krebs cycle.
R
recessive
Refers to an allele that cannot control the phenotype unless it is the only kind of allele present,
as in a homozygote or hemizygote. Also refers to the trait or phenotype produced when only a
recessive allele is present. The opposite of recessive is dominant.
reproductive isolation
The inability of individuals within a species to create offspring with members of any other
species. The mark of a species is its reproductive isolation from all other species.
respiratory system
The organ system responsible for the intake of oxygen and diffusion of that gas into the blood
and the elimination of carbon dioxide from the body. Important structures of the system are the
pharynx, larynx, trachea, bronchi, bronchioles, and lungs. Alveoli in the lungs are the location
of gas exchange with the blood. The movement of the muscular diaphragm allows the lungs to
inhale and exhale.
ribonucleic acid (RNA)
One type of nucleic acid polymer. RNA nucleotides’ sugars contain one more oxygen atom
than DNA nucleotides’ sugars. RNA nucleotides can have the nitrogenous bases adenine,
cytosine, guanine, and uracil.

ribosome
Organelle responsible for protein synthesis. Ribosomes are located in cytoplasm or
endoplasmic reticulum.
root
The part of a plant beneath the soil; responsible for collecting water and minerals from the soil,
storing nutrients, and securing the plant to the ground. Can be fibrous or a taproot.
root hair
An outgrowth of a plant root that provides an increased surface area for the absorption of
water and dissolved minerals from the soil.
runner
Slender horizontal stem that can form new plants via specialized nodes.
S
saprophytes
See decomposers.
segregation
Mendel’s conclusion that individuals have two copies of each gene, and that these copies
separate randomly during gamete formation, one copy per gamete. This law is true except for
genes on sex chromosomes in males, who have only one copy of each such gene. It is often
called Mendel’s first law.
semicircular canal
Fluid-filled structure within the ear that can detect balance.
sepal
Green, leaflike structure that encloses and protects the unopened flower bud.
sex cells
See gamete.
sex chromosome
Refers to a chromosome involved in defining the sex of an individual. Humans have two sex
chromosomes and 44 autosomes. In females, both sex chromosomes are X chromosomes.
Males have one X chromosome and one Y chromosome.
sex-linked
Refers to a gene located on a sex chromosome or to a trait defined by such a gene. Such traits
will appear with different frequencies in males and females, and males and females will differ
in their ability to transmit the trait to their offspring. In order for a recessive sex-linked trait such
as hemophilia or color blindness to manifest in the phenotype of a woman, it must be inherited
from both mother and father; such diseases will be present in the phenotype of a man if he
inherits it just from his mother.
somatic cell
Any plant or animal cell that is not a germ cell, meaning it is not passed down to offspring. The
class of cell formed during mitosis.
somatic nervous system
One half of the motor system of the peripheral nervous system. Responsible for voluntary, or
conscious, movement. Neurons in this system target skeletal muscles and release the
neurotransmitter acetylcholine.
speciation
The development of a species through evolution. A species forms when its members become
reproductively isolated from all other organisms. Speciation can occur through geographic
separation that eliminates gene flow or through adaptive radiation.
species
A group of organisms defined by their ability to interbreed with only each other.
sperm
The male gamete in sexual reproduction.
spermatogenesis
The process in which haploid sperm cells form through meiotic division.
spinal cord
A long cylinder of nervous tissue that extends along the vertebral column from the head to the
lower back. It controls some autonomic responses and connects the brain to the peripheral
nervous system.
spores
Usually unicellular and microscopic, spores are produced by protist molds, fungi, and plants
and are able to develop into new individuals. Spores are able to survive without food or water
for long periods. Most fungi spend part of their life cycle as hyphae and part as spores.
sporophyte
A diploid plant or plant structure that produces haploid spores through meiosis.
stabilizing selection
When selection pressures favor the average form of a trait.
stamen
The male reproductive organ of the flower, consisting of an anther and filament.
stigma
The top part of the pistil, where pollen grains are received.
stomata
Small epidermal pores, surrounded by two guard cells, through which gases diffuse and water
transpires in and out of a leaf.
stop codon
A codon on mRNA that signals the termination of DNA translation. There are three stop
codons: UAA, UAG, or UGA.
style
The shaft of the pistil that leads from the stigma down into the ovary.
substrate
The starting material that will undergo chemical change in a chemical reaction facilitated by an
enzyme.
symbiosis
A type of interaction within a community that falls into one of three categories: a parasitic
relationship benefits one organism and hurts the other; a commensal relationship benefits one
and does not affect the other; a mutualistic relationship benefits both organisms.

synapse
The gap between two neurons, spanning the space between the axon of one and the dendrites
of the other. In order to pass an impulse across a synapse, neurons must release
neurotransmitters.
T
taste buds
Structures on the tongue that contain chemoreceptors, which respond to four main sensations
—sour, salty, bitter, and sweet—to create the sense of taste.
taxonomy
The study of biological classification.
telophase
The final stage of mitosis before cytokinesis. In telophase, the nuclear envelope re-forms
around separated sister chromatids and kinetochore microtubules disappear. Cell elongation
also occurs during this phase. The final stage of the first meiotic division (meiosis I), during
which chromosomes arrive at the poles of the cell and begin to recondense; the final stage of
the second meiotic division (meiosis II), during which chromosomes arrive at the poles of the
cell, the nuclear envelope begins to re-form, and the chromosomes begin to recondense.
tendon
Connective tissue between bones and muscles.
testes
The male gonads; sperm and testosterone are produced here.
testosterone
A hormone necessary for sperm production in men. Also responsible for developing and
maintaining the secondary sex characteristics of males, starting at puberty.
thyroid
Gland that produces the hormone thyroxine, which increases the metabolism of most of the
cells in the body. Located in the neck.
tissue
A group of closely connected and similar cells that cooperate to generate a specific structure
or specialized function within an organism.
tracheophyte
A terrestrial plant with a vascular system.
trait
Any observable feature or characteristic of an organism.
transfer RNA (tRNA)
An RNA molecule used in protein synthesis as a link helping to convert messenger RNA into
amino acids.
transpiration
The process by which a plant loses water to its environment through evaporation.
trophic level
Steps on a food/biomass pyramid that are defined by organisms within a community that are

the same distance from the primary producers in a food web.
tropism
Long-term growth of a plant toward or away from a stimulus.
tuber
Fleshy underground storage structure composed of an enlarged portion of the stem that has
on its surface buds capable of producing new plants.
U
uterus
Structure in the female reproductive system in which the embryo develops.
V
vascular cambium
Tissue that produces new vascular cells; lies between the xylem and phloem in dicot stems.
vascular tissue
A conductive component (either xylem or phloem) of the vascular system that transports food
and nutrients throughout the plant body.
vegetative propagation
A form of asexual reproduction in which plants produce genetically identical offshoots (clones)
of themselves, which then develop into independent plants.
vein
A blood vessel that carries blood back to the heart. The blood in veins is not oxygenated, with
the exception of the pulmonary vein.
vertebrate
An animal that has a skeletal rod of bone or cartilage running up its back, surrounding the
nerve cord. Segments of this rod are called vertebrae. All vertebrate animals belong to
subphylum Vertebrata, part of phylum Chordata.
vestigial structures
Bodily structures that developed in the past but no longer serve any function for an organism.
villi
Fingerlike projections in the small intestine that increase surface area and maximize the
absorption of nutrients.
vitamins
Complex molecules that usually serve as coenzymes, assisting in physiological processes.
W
warm-blooded
Animals that have developed fat and fur, hair, or feathers in order to retain heat produced by
metabolic activities. Also known as endothermic. Warm-blooded animals are able to thrive in
various climates, because they are minimally affected by environmental fluctuations in
temperature.

X
xylem
Vascular tissue composed of cells that are dead at maturity; transports water and dissolved
minerals upward from the roots to the shoot.
Z
zygote
In sexual reproduction, the diploid product of the fusion of the father’s haploid sperm cell and
the mother’s haploid ovum (egg) cell. The single-celled zygote divides billions of times to form
a whole individual.