Chapter 6

gap junction

A 2.7-nanometer gap between plasma membranes of two animal cells, spanned by protein channels. Gap junctions allow chemical substances or electrical signals to pass from cell to cell.

secondary active transport

A form of active transport that does not use ATP as an energy source; rather, transport is coupled to ion diffusion down a concentration gradient established by primary active transport.

tight junction

A junction between epithelial cells in which there is no gap between adjacent cells.

glycolipid

A lipid to which sugars are attached.

integral membrane protein

A membrane protein embedded in the bilayer of the membrane. (Contrast with peripheral membrane protein.)

gated channel

A membrane protein that changes its three-dimensional shape, and therefore its ion conductance, in response to a stimulus. When open, it allows specific ions to move across the membrane.

uniporter

A membrane transport protein that carries a single substance in one direction. (Contrast with antiporter, symporter.)

symporter

A membrane transport protein that carries two substances in the same direction. (Contrast with antiporter, uniporter.)

antiporter

A membrane transport protein that moves one substance in one direction and another in the opposite direction. (Contrast with symporter, uniporter.)

fluid mosaic model

A molecular model for the structure of biological membranes consisting of a fluid phospholipid bilayer in which suspended proteins are free to move in the plane of the bilayer.

exocytosis

A process by which a vesicle within a cell fuses with the plasma membrane and releases its contents to the outside. (Contrast with endocytosis.)

endocytosis

A process by which liquids or solid particles are taken up by a cell through invagination of the plasma membrane. (Contrast with exocytosis.)

receptor protein

A protein that can bind to a specific molecule, or detect a specific stimulus, within the cell or in the cell's external environment.

glycoprotein

A protein to which sugars are attached.

aquaporin

A transport protein in plant and animal cell membranes through which water passes in osmosis.

primary active transport

Active transport in which ATP is hydrolyzed, yielding the energy required to transport an ion or molecule against its concentration gradient. (Contrast with secondary active transport.)

selective permeability

Allowing certain substances to pass through while other substances are excluded; a characteristic of membranes.

desmosome

An adhering junction between animal cells.

ion channel

An integral membrane protein that allows ions to diffuse across the membrane in which it is embedded.

channel protein

An integral membrane protein that forms an aqueous passageway across the membrane in which it is inserted through which specific solutes may pass.

transmembrane protein

An integral membrane protein that spans the phospholipid bilayer.

sodium-potassium (Na+-K+) pump

Antiporter responsible for primary active transport; it pumps sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients. Also called a sodium-potassium ATPase.

ligand

Any molecule that binds to a receptor site of another (usually larger) molecule.

cell recognition

Binding of cells to one another mediated by membrane proteins or carbohydrates.

passive transport

Diffusion across a membrane; may or may not require a channel or carrier protein. (Contrast with active transport.)

phagocytosis

Endocytosis by a cell of another cell or large particle.

pinocytosis

Endocytosis by a cell of liquid containing dissolved substances.

receptor-mediated endocytosis

Endocytosis initiated by macromolecular binding to a specific membrane receptor.

hypertonic

Having a greater solute concentration. Said of one solution compared with another. (Contrast with hypotonic, isotonic.)

hypotonic

Having a lesser solute concentration. Said of one solution in comparing it to another. (Contrast with hypertonic, isotonic.)

isotonic

Having the same solute concentration; said of two solutions. (Contrast with hypertonic, hypotonic.)

integrin

In animals, a transmembrane protein that mediates the attachment of epithelial cells to the extracellular matrix.

carrier protein

In facilitated diffusion, a membrane protein that binds a specific molecule and transports it through the membrane.

diffusion

Random movement of molecules or other particles, resulting in even distribution of the particles when no barriers are present.

cell junctions

Specialized structures associated with the plasma membranes of epithelial cells. Some contribute to cell adhesion, others to intercellular communication.

membrane potential

The difference in electrical charge between the inside and the outside of a cell, caused by a difference in the distribution of ions.

active transport

The energy-dependent transport of a substance across a biological membrane against a concentration gradient—that is, from a region of low concentration (of that substance) to one of high concentration. (See also primary active transport, secondary active transport; Contrast with facilitated diffusion, passive transport.)

turgor pressure

The hydrostatic pressure of an enclosed solution in excess of the surrounding atmospheric pressure.

peripheral membrane protein

Membrane protein not embedded in the bilayer. (Contrast with integral membrane protein.)

freeze-fracturing

Method of tissue preparation for transmission and scanning electron microscopy in which a tissue is frozen and a knife is then used to crack open the tissue; the fracture often occurs in the path of least resistance, within a membrane.

cell adhesion molecules

Molecules on animal cell surfaces that affect the selective association of cells into tissues during development of the embryo.

osmosis

Movement of water across a differentially permeable membrane, from one region to another region where the water potential is more negative.

facilitated diffusion

Passive movement through a membrane involving a specific carrier protein; does not proceed against a concentration gradient. (Contrast with active transport, diffusion.)

heterotypic

Pertaining to adhesion of cells of different types. (Contrast with homotypic.)

homotypic

Pertaining to adhesion of cells of the same type. (Contrast with heterotypic.)