Cell Membrane

plasma membrane

Membranes consisting of a phospholipid bilayer found around and within all cells. The cell-surface membrane is the plasma membrane that surrounds cells.

phospholipid

Triglyceride in which one of the three fatty acid molecules is replaced by a phosphate molecule. Phospholipids are important in the structure an functioning of plasma membranes.

bilayer

A membrane consisting of two layers of phospholipids.

channel protein

A protein completely spanning the phospholipid bilayer which form water-filled tubes to allow water-soluble ions to diffuse across the membrane.

carrier protein

A protein completely spanning the phospholipid bilayer which bind to ions or molecules then change shape in order to move these molecules across the membrane.

glycoprotein

Carbohydrate chains attached to a protein (often extrinsic) which are part of the cell surface membrane. They act as recognition sites, help cells to attach to one another and allows cells to recognise one another.

glycolipid

A carbohydrate covalently bonded with a lipid. They act as recognition sites, help maintain stability of the membrane and help cells attach to one another.

cholesterol

Lipid that is an important component of cell-surface membranes because it adds strength. Excess in the blood can lead to atheroma.

permeability

How permeable a substance is depends on the size, polarity and charge of the molecule. If it is small, non-polar and fat soluble it is very permeable and can pass through the cell membrane.

fluid mosaic model

The arrangement of the various molecules of the cell-surface membrane. Fluid because the individual phospholipid molecules can move relative to one another and mosaic because the proteins vary in shape, size and pattern.

diffusion

The net movement of molecules (or ions) from a region of high concentration to a region of low concentration. It is passive.

facilitated diffusion

Diffusion involving the presence of protein carrier molecules to allow the passive movement of substances (normally large, polar or charged molecules) across plasma membrane.

osmosis

The passage of water from a region where there is a higher water potential to a region where there is a lower water potential through a partially permeable membrane.

water potential

The pressure created by water molecules. It is the measure of the extent to which a solution gives out water. The greater the number of water molecules present, the higher (less negative) the water potential. Pure water has a water potential of zero.

isotonic

A solution which has the same water potential as the cell within it.

active transport

Movement of a substance from a region where it is in a low concentration to a region where it is in a high concentration. The process requires the expenditure of metabolic energy in the form of ATP.

co-transport

The transport of one substance coupled with the transport of another substance across a plasma membrane in the same direction through the same protein carrier.