actin filament
cytoskeletal filaments of eukaryotic cells composed of the protein actin; also refers to the thin filaments of muscle cells.
active site
region on the surface of an enzyme where the substrate binds and where the reaction occurs
active transport
use of a plasma membrane carrier protein and energy to move a substance into or out of a cell from lower to higher concentration
aerobic
requiring oxygen
anaerobic
growing or metabolizing in the absence of oxygen
cell theory
one of the major theories of biology; states that all organisms are made up of cells and cells come only from preexisting cells.
cellular respiration
metabolic reactions that use the energy primarily from carbohydrates but also from fatty acid or amino acid breakdown to produce ATP molecules
centrosome
central microtubule organizing center of cells. in animal cells, it contains centrioles.
chromatin
network of fine threads in the nucleus composed of DNA and proteins.
chromosome
chromatin condensed into a compact structure
cilium
short, hair-like projection from the plasma membrane, occurring usually in large numbers.
citric acid cycle
cycle of reactions in mitochondria that begins with citric acid; it breaks down an acetyl group as CO2, ATP, NADH, and FADH2 are given off; also called the Krebs cycle.
coenzyme
nonprotein organic molecule that aids the action of the enzyme to which it is loosely bound
cytoplasm
contents of a cell between the nucleus and the plasma membrane that contains the organelles
cytoskeleton
internal framework of the cell, consisting of microtubules, actin filaments, and intermediate filaments.
diffusion
movement of molecules or ions from a region of higher to lower concentration; it requires no energy and stops when the distribution is equal.
electron transport chain
passage of electrons along a series of membrane-bound carrier molecules from a higher to lower energy level; the energy released is used for the synthesis of ATP.
endomembrane system
a collection of membranous structures involved in transport within the cell
endoplasmic reticulum (ER)
system of membranous saccules and channels in the cytoplasm, often with attached ribosomes.
eukaryotic cell
type of cell that has a membrane-bound nucleus and membranous organelles
facilitated transport
use of a plasma membrane carrier to move a substance into or out of a cell from higher to lower concentration; no energy required.
fermentation
anaerobic breakdown of glucose that results in a gain of two ATP and end products such as alcohol and lactate.
flagellum
slender, long extension that propels a cell through a fluid medium
fluid-mosaic model
model for the plasma membrane based on the changing location and pattern of protein molecules in a fluid phospholipid bilayer
glycolysis
anaerobic breakdown of glucose that results in a gain of two ATP molecules
golgi apparatus
organelle, consisting of saccules and vesicles, that processes, packages, and distributes molecules about or from the cell
intermediate filament
rope-like assemblies of fibrous polypeptides in the cytoskeleton that provide support and strength to cells; so called because they are intermediate in size between actin filaments and microtubules.
lysosome
membrane-bound vesicle that contains hydrolytic enzymes for digesting macromolecules
metabolism
all of the chemical reactions that occur in a cell
microtubule
small cylindrical structure that contains 13 rows of the protein tubulin around an empty central core; present in the cytoplasm, centrioles, cilia, and flagella.
mitochondrion
membrane-bound organelle in which ATP molecules are produced during the process of cellular respiration
NAD+ (nicotinamide adenine dinucleotide)
coenzyme that functions as a carrier of electrons and hydrogen ions, especially in cellular respiration.
nuclear envelope
double membrane that surrounds the nucleus and is connected to the endoplasmic reticulum; has pores that allow substances to pass between the nucleus and the cytoplasm.
nuclear pore
opening in the nuclear envelope that permits the passage of proteins into the nucleus and ribosomal subunits out of the nucleus
nucleolus
dark-staining, spherical body in the cell nucleus that produces ribosomal subunits.
nucleoplasm
semifluid medium of the nucleus, containing chromatin
nucleus
membrane-bounded organelle that contains chromosomes and controls the structure and function of the cell
organelle
small membranous structure in the cytoplasm having a specific structure and function
osmosis
diffusion of water through a selectively permeable membrane
osmotic pressure
measure of the tendency of water to move across a selectively permeable membrane; visible as an increase in liquid on the side of the membrane with higher solute concentration
phagocytosis
process by which amoeboid-type cells engulf large substances, forming an intracellular vacuole
plasma membrane
membrane surrounding the cytoplasm that consists of a phospholipid bilayer with embedded proteins; functions to regulate the entrance and exit of molecules from the cell
polyribosome
string of ribosomes simultaneously translating regions of the same mRNA strand during protein synthesis
product
substance that forms as a result of a reaction
prokaryotic cell
type of cell that lacks a membrane-bounded nucleus and organelles
reactant
substance that participates in a reaction
ribosome
RNA and protein in two subunits; site of protein synthesis in the cytoplasm
selectively permeable
having degrees of permeability; the cell is impermeable to some substances and allows others to pass through at varying rates.
substrate
reactant in a reaction controlled by an enzyme
tonicity
osmolarity of a solution compared with that of a cell. if the solution is isotonic to the cell, there is no net movement of water; if the solution is hypotonic, the cell gains water; and if the solution is hypertonic, the cell loses water
vesicle
small, membrane-bounded sac that stores substances within a cell