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 "downhill" diffusion of one substance to the "uphill" transport 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 generating voltage across the 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. Walled cells are flaccid in isotonic 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.

glycoprotein

A protein covalently attached to a carbohydrate.

hypertonic

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

hypotonic solution

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

integral protein

Typically transmembrane proteins with hydrophobic regions that completely span the hydrophobic interior of the membrane.

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 the cytoplasm and extracellular fluid in all cells, 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 control of water balance in organisms living in hypertonic, hypotonic, or 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

Protein appendages loosely bound to the surface of the membrane and not embedded in the lipid bilayer.

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.

plasmolyze

The shrinkage of a cell due to water loss.

proton pump

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

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.

transport protein

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

turgid

Firm. Walled cells become turgid as a result of the entry of water from a hypotonic environment.