Chapter 5 vocabulary

amphipathic molecule

Having both a hydrophilic region and a hydrophobic region.

aquaporin

A channel protein in the plasma membrane of a plant, animal, or microorganism cell that specifically facilitates osmosis, the diffusion of free water across the membrane.

concentration gradient

A region along which the density of a chemical substance increases or decreases.

diffusion

The random thermal motion of particles of liquids, gases, or solids. In the presence of a concentration or electrochemical gradient, diffusion results in the net movement of a substance from a region where it is more concentrated to a region where it is less concentrated.

endocytosis

Cellular uptake of biological molecules and particulate matter via formation of vesicles from the plasma membrane.

exocytosis

The cellular secretion of biological molecules by the fusion of vesicles containing them with the plasma membrane.

facilitated diffusion

The passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport proteins, requiring no energy expenditure.

flaccid

Limp. Lacking turgor (stiffness or firmness), as in a plant cell in surroundings where there is a tendency for water to leave the cell. (A walled cell becomes flaccid if it has a higher water potential than its surroundings, resulting in the loss of water.)

fluid mosaic model

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

gated channel

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

glycoprotein

A protein with one or more covalently attached carbohydrates.

hypertonic

Referring to a solution that, when surrounding a cell, will cause the cell to lose water

hypotonic

Referring to a solution that, when surrounding a cell, will cause the cell to take up water.

integral protein

A transmembrane protein with hydrophobic regions that extend into and often completely span the hydrophobic interior of the membrane and with hydrophilic regions in contact with the aqueous solution on one or both sides of the membrane (or lining the channel in the case of a channel protein).

ion channel

A transmembrane protein channel that allows a specific ion to diffuse across the membrane down its concentration or electrochemical gradient.

glycolipid

A lipid with one or more covalently attached carbohydrates.

isotonic

Referring to a solution that, when surrounding a cell, causes no net movement of water into or out of the cell.

ligand

A molecule that binds specifically to another molecule, usually a larger one.

membrane potential

The difference in electrical charge (voltage) across a cell’s plasma membrane due to the differential distribution of ions. Membrane potential affects the activity of excitable cells and the transmembrane movement of all charged substances.

osmoregulation

Regulation of solute concentrations and water balance by a cell or organism.

osmosis

Regulation of solute concentrations and water balance by a cell or organism.

passive transport

The diffusion of a substance across a biological membrane with no expenditure of energy.

peripheral protein

A protein loosely bound to the surface of a membrane or to part of an integral protein and not embedded in the lipid bilayer.

phagocytosis

A type of endocytosis in which large particulate substances or small organisms are taken up by a cell. It is carried out by some protists and by certain immune cells of animals (in mammals, mainly macrophages, neutrophils, and dendritic cells).

plasmolysis

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

proton pump

An active transport protein in a cell membrane that uses ATP to transport hydrogen ions out of a cell against their concentration gradient, generating a membrane potential in the process.

selective permeability

A property of biological membranes that allows them to regulate the passage of substances across them.

sodium-potassium pump

A transport protein in the plasma membrane of animal cells that actively transports sodium out of the cell and potassium into the cell.

tonicity

The ability of a solution surrounding a cell to cause that cell 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

Swollen or distended, as in plant cells. (A walled cell becomes turgid if it has a lower water potential than its surroundings, resulting in entry of water.)

cytolysis

or osmotic lysis, occurs when a cell bursts due to an osmotic imbalance that has caused excess water to diffuse into the cell. Water can enter the cell by diffusion through the cell membrane or through selective membrane channels called aquaporins, which greatly facilitate the flow of water.

receptor-mediated endocytosis

a form of endocytosis in which receptor proteins on the cell surface are used to capture a specific target molecule. The receptors, which are transmembrane proteins, cluster in regions of the plasma membrane known as coated pits.

pinocytosis

an active, energy consuming process where extracellular fluid and solutes are taken up into a cell via small vesicles. It is a type of endocytosis, which refers to the uptake of substances by a cell. Phagocytosis and receptor-mediated endocytosis are other types of endocytosis.