Brooker Biology: Chapter 5 - Membrane Structure, Synthesis, and Transport

Plasma Membrane

The biological membrane that separates the internal contents of a cell from its external environment.

phospholipid bilayer

The basic framework of a biological membrane, consisting of two layers of phospholipids.

amphipathic

Refers to molecules containing a hydrophobic (water-fearing) region and a hydrophilic (water-loving) region.

fluid-mosaic model

The accepted model of a biological membrane; its basic framework is the semifluid phospholipid bilayer with a mosaic of proteins. Carbohydrates may be attached to the lipids or proteins.

leaflet

1. Half of a phospholipid bilayer. 2. A portion of a compound leaf.

integral membrane protein

A protein such as a transmembrane protein or a lipid-anchored protein that cannot be released from the membrane unless the membrane is dissolved with an organic solvent or detergent.

transmembrane proteins

A protein that has one or more regions that are physically embedded in the hydrophobic interior of a membrane's phospholipid bilayer.

lipid-anchored protein

A type of integral membrane protein that is attached to the membrane via a lipid molecule.

peripheral membrane protein

A protein that is noncovalently bound to a region of an integral membrane protein that projects out from the membrane or noncovalently bound to the polar head group of a phospholipid.

Fluidity

A property of biological membranes in which individual molecules remain in close association yet have the ability to move rotationally or laterally within the plane of the membrane. Membranes are semifluid.

semifluid

A property of biological membranes in which movement of membrane components occurs only in two dimensions.

lipid raft

In a membrane, a group of lipids, sometimes including associated proteins, that float together as a unit in a larger sea of lipids.

unsaturated

The property of certain lipids that contain one or more C═C double bonds.

lipid exchange proteins

A protein that extracts a lipid from one membrane, diffuses through the cell, and inserts the lipid into another membrane.

Glycosylation

The covalent attachment of a carbohydrate to a protein or lipid, producing a glycoprotein or glycolipid, respectively.

glycolipid

A lipid that has carbohydrate attached to it.

glycoprotein

A protein that has a carbohydrate attached to it.

Membrane transport

The movement of ions or molecules across a biological membrane.

Selective permeability

The property of membranes that allows the passage of certain ions or molecules but not others.

simple diffusion

When a substance moves across a membrane from a region of high concentration to a region of lower concentration by passing directly through the phospholipid bilayer.

facilitated diffusion

A mechanism of passive transport in which a transport protein provides a passageway for a substance to cross a membrane from an area of higher concentration to one of lower concentration.

passive transport

The diffusion of a solute across a membrane in a process that is energetically favorable and does not require an input of energy.

active transport

The transport of a substance across a membrane from an area of low concentration to one of higher concentration with the aid of a transport protein; requires an input of energy.

solutes

A substance dissolved in a liquid.

transmembrane gradient

A situation in which the concentration of a solute is higher on one side of a membrane than on the other.

electrochemical gradient

A dual gradient across a membrane, having both electrical and chemical components; determines the direction in which ions will move.

Isotonic

Condition in which the solute concentrations on both sides of a plasma membrane are equal, which does not cause a cell to shrink or swell.

hypertonic

When the concentration of solutes outside the cell is higher and causes a cell to shrink due to osmosis of water out of the cell.

hypotonic

When the concentration of solutes outside the cell is lower and causes a cell to swell due to the uptake of water via osmosis.

osmosis

The movement of water across a membrane to balance solute concentrations. Water diffuses from a solution that is hypotonic (lower solute concentration) into a solution that is hypertonic (higher solute concentration).

plasmolysis

The shrinkage of algal or plant cytoplasm that occurs when water leaves the cell by osmosis, with the result that the plasma membrane no longer presses on the cell wall.

channel

A transmembrane protein that forms an open passageway for the facilitated diffusion of ions or molecules across a membrane.

gated

A property of many channels that allows them to open and close to control the movement of solutes across a membrane.

transporters

A transmembrane protein that binds a solute and undergoes a conformational change to allow the movement of the solute across a membrane; also called a carrier.

Uniporters

A type of transporter that binds a single ion or molecule and transports it across a membrane.

Symporters

A type of transporter that binds two or more ions or molecules and transports them in the same direction across a membrane; also called a cotransporter.

Antiporters

A type of transporter that binds two or more ions or molecules and transports them in opposite directions across a membrane; also called an exchanger.

Primary active transport`

A type of transport that involves pumps that directly use energy to transport a solute against a gradient.

Pump

A transporter that directly couples its conformational changes to an energy source, such as ATP hydrolysis.

Secondary active transport

A type of membrane transport that involves the utilization of a pre-existing gradient to drive the active transport of another solute.

electrogenic pump

A pump that generates an electrical gradient across a membrane.

Exocytosis

A process in which material inside a cell is packaged into vesicles and excreted into the extracellular environment.

Endocytosis

A process in which the plasma membrane invaginates, or folds inward, to form a vesicle that brings substances or particles into the cell.

receptor-mediated endocytosis

A common type of endocytosis in which a receptor in the membrane is specific for a given cargo.

Pinocytosis

A type of endocytosis that involves the formation of membrane vesicles from the plasma membrane as a way for cells to internalize the extracellular fluid.

Phagocytosis

A type of endocytosis that involves the formation of a membrane vesicle, called a phagosome or phagocytic vacuole, which engulfs a particle such as a bacterium.

biological membrane

Any membrane made by living cells; can be the plasma membrane or an internal membrane that surrounds an organelle.

solvent

The liquid in which a solute is dissolved.

concentration gradient

See transmembrane gradient.

ligand

An ion or molecule that binds to a protein, such as an enzyme, a receptor, or a channel.

aquaporin

A transport protein in the form of a channel that allows the rapid diffusion of water across the cell membrane.