E–H 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.
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