320 ❯ Glossary
corpus callosum Bridge that connects the two
hemispheres of the brain.
cortex Outer region of the kidney or adrenal gland.
cortisol Stress hormone released in response to phys-
iological challenges.
cotyledon Structure that provides nutrients for a
developing angiosperm plant.
cri-du-chat syndrome This syndrome occurs with a
deletion in chromosome 5 that leads to mental
retardation, unusual facial features, and a small
head. Most die in infancy or early childhood.
crossover Also referred to as “crossing over.” When
the homologous pairs match up during prophase I
of meiosis, complementary pieces from the two
homologous chromosomes wrap around each
other and are exchanged between the chromo-
somes. This is one of the mechanisms that allows
offspring to differ from their parents.
cryptic coloration Those being hunted adopt a col-
oring scheme that allows them to blend in to the
colors of the environment.
cuticle Waxy covering that protects terrestrial plants
against water loss.
cutinWaxy coat that protects plants.
cyclic light reactionsPathway that produces only
ATP and uses only photosystem I.
cyclin Protein that accumulates during interphase;
vital to cell cycle control.
cystic fibrosis (CF)A recessive disorder that is the
most common lethal genetic disease in the United
States. A defective version of a gene on chromo-
some 7 results in the excessive secretion of a thick
mucus, which accumulates in the lungs and diges-
tive tract. Left untreated, children with CF die at a
very young age.
cytokinesis The physical separation of the newly
formed daughter cells during meiosis and mitosis.
Occurs immediately after telophase.
cytokininPlant hormone that promotes cell division
and leaf enlargement, and slows down the aging of
leaves.
cytoskeleton Provides support, shape, and mobility
to cells.
death rateNumber of deaths per time period.
deceptive markingsPatterns that can cause a preda-
tor to think twice before attacking. For example,
some insects may have colored designs on their
wings that resemble large eyes, making individuals
look more imposing than they are.
decomposerSeedetritivore.
dehydration reaction A reaction in which two
compounds merge, releasing H 2 O as a product.
deletion A piece of the chromosome is lost in the
developmental process.
demographersScientists who study the theory and
statistics behind population growth and decline.
dendriteOne of many short, branched processes of
a neuron that help send the nerve impulses toward
the cell body.
denitrification The process by which bacteria use
nitrates and release N 2 as a product.
density-dependent inhibition When a certain den-
sity of cells is reached, cell growth will slow or
stop. This is because there are not enough raw
materials for the growth and survival of more cells.
density-dependent limiting factors Factors related
to population size that come into play as popula-
tion size approaches or passes the carrying capacity.
Examples of density-dependent limiting factors
include food, waste, and disease.
density-independent limiting factors Factors that
limit population growth that have nothing to do
with the population size, such as natural disasters
and weather.
depolarizationThe electric potential becomes less
negative inside the cell, allowing an action poten-
tial to occur.
desertThe driest land biome on earth, which expe-
riences a wide range of temperatures from day to
night and exists on nearly every continent.
detritivore Also known as decomposer.A consumer
that obtains its energy through the consumption
of dead animals and plants.
dicot (dicotyledon) An angiosperm plant that has
two cotyledons.
diffusion The movement of molecules down their
concentration gradients without the use of energy.
It is a passive process during which molecules
move from a region of higher concentration to a
region of lower concentration.
dihybrid cross The crossing of two different characters
(BbRr×BbRr). A dihybrid cross between heterozy-
gous gametes gives a 9:3:3:1 phenotype ratio in the
offspring.
diploid (2n) An organism that has two copies of each
type of chromosome. In humans, this refers to the
pairs of homologous chromosomes.