AP BIO CH 7

active transport

The movement of a substance across a biological membrane against its concentration or electrochemical gradient with the help of energy input and specific transport proteins.

amphipathic molecule

A molecule that has both a hydrophilic region and a hydrophobic region.

aquaporin

A transport protein in the plasma membrane of a plant or animal cell that specifically facilitates the diffusion of water across the membrane (osmosis).

concentration gradient

An increase or decrease in the density of a chemical substance in an area. Cells often maintain concentration gradients of ions across their membranes. When a gradient exists, the ions or other chemical substances involved tend to move from where they are more concentrated to where they are less concentrated.

cotransport

The coupling of the downhilldiffusion of one substance to the uphilltransport of another against its own concentration gradient.

diffusion

The spontaneous tendency of a substance to move down its concentration gradient from a more concentrated to a less concentrated area.

electrochemical gradient

The diffusion gradient of an ion, representing a type of potential energy that accounts for both the concentration difference of the ion across a membrane and its tendency to move relative to the membrane potential.

electrogenic pump

An ion transport protein that generates voltage across a membrane.

endocytosis

The cellular uptake of macromolecules and particulate substances by localized regions of the plasma membrane that surround the substance and pinch off to form an intracellular vesicle.

exocytosis

The cellular secretion of macromolecules by the fusion of vesicles with the plasma membrane.

facilitated diffusion

The spontaneous passage of molecules and ions, bound to specific carrier proteins, across a biological membrane down their concentration gradients.

flaccid

Limp. A walled cell is flaccid in surroundings where there is no tendency for water to enter.

fluid mosaic model

The currently accepted model of cell membrane structure, which envisions the membrane as a mosaic of individually inserted protein molecules drifting laterally in a fluid bilayer of phospholipids.

gated channel

A protein channel in a cell membrane that opens or closes in response to a particular stimulus.

glycolipid

A lipid covalently attached to a carbohydrate.

glycoprotein

A protein covalently attached to a carbohydrate.

hypertonic

In comparing two solutions, referring to the one with a greater solute concentration.

hypotonic

In comparing two solutions, referring to the one with a lower solute concentration.

integral protein

Typically a transmembrane protein with hydrophobic regions that completely spans the hydrophobic interior of the membrane.

ion channel

Protein channel in a cell membrane that allows passage of a specific ion down its concentration gradient.

isotonic

Having the same solute concentration as another solution.

ligand

A molecule that binds specifically to a receptor site of another molecule.

membrane potential

The charge difference between a cell's cytoplasm and the extracellular fluid, due to the differential distribution of ions. Membrane potential affects the activity of excitable cells and the transmembrane movement of all charged substances.

osmoregulation

The regulation of solute and water concentrations in body fluids by organisms living in hyperosmotic, hypoosmotic, and terrestrial environments.

osmosis

The diffusion of water across a selectively permeable membrane.

passive transport

The diffusion of a substance across a biological membrane.

peripheral protein

A protein appendage loosely bound to the surface of a membrane and not embedded in the lipid bilayer.

phagocytosis

A type of endocytosis involving large, particulate substances, accomplished mainly by macrophages, neutrophils, and dendritic cells.

pinocytosis

A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes.

plasmolysis

A phenomenon in walled cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall when the cell loses water to a hypertonic environment.

proton pump

An active transport mechanism in cell membranes that uses ATP to force hydrogen ions out of a cell, generating a membrane potential in the process.

receptor-mediated endocytosis

The movement of specific molecules into a cell by the inward budding of membranous vesicles containing proteins with receptor sites specific to the molecules being taken in; enables a cell to acquire bulk quantities of specific substances.

selective permeability

A property of biological membranes that allows some substances to cross more easily than others.

sodium-potassium pump

A special transport protein in the plasma membrane of animal cells that transports sodium out of the cell and potassium into the cell against their concentration gradients.

tonicity

The ability of a solution to cause a cell within it to gain or lose water.

transport protein

A transmembrane protein that helps a certain substance or class of closely related substances to cross the membrane.

turgid

Very firm. A walled cell become turgid if it has a greater solute concentration than its surroundings, resulting in entry of water.