Biology KS5: Transport Across Membranes

Vocabulary terms and definitions covering the KS5 biology mini-topic of transport across cell membranes.

Phospholipid bilayer

A double layer of phospholipids that forms the majority of biological membranes, arranged with hydrophobic tails facing inwards and phosphate heads on the outside.

Fluid-mosaic model

The arrangement of different molecules including a phospholipid bilayer, cholesterol, and protein molecules to form biological membranes, possessing a flexible structure because phospholipids can move within the membrane.

Cholesterol

A lipid molecule that adds strength and some rigidity by pulling together phospholipid molecules to limit their movement.

Simple diffusion

The net movement of molecules or ions from a region of high concentration to a region of lower concentration until an equilibrium is reached, which can occur across a membrane by movement through the phospholipid bilayer.

Facilitated diffusion

The net movement of molecules or ions across a membrane via a protein channel or carrier protein from a region of high concentration to a region of lower concentration until an equilibrium is reached.

Water potential (Psi)

The pressure exerted by water molecules measured in KPA. A measure of the extent to which water is able to move from a solution to another. The greater the number of water molecules in a given volume, the higher the value of water potential. Pure water has a Psi value of zero; the highest possible value for Psi.

Osmosis

The movement of water from a region of high Psi to a region of lower Psi across a partially permeable membrane until an equilibrium is reached.

Active transport

The movement of a substance from a region of low concentration to a region of higher concentration requiring energy in the form of ATP and a carrier protein (protein pump).

Co-transport

The transport of one substance coupled with the transport of another substance across a plasma membrane in the same direction using a protein carrier (a cotransport or symporter protein).

Microvilli

Tiny finger-like projections of membrane and cytoplasm from the surface of a cell that greatly increase the cell's surface area to increase the rate of exchange.