The cell membrane separates the internal cell environment from the external environment. Materials needed for cell growth are brought into a cell while toxic substances are kept out of the cell. The exchange of materials is strictly controlled by the membrane.

• the membrane is partially permeable

• the exchange of materials occurs by diffusion, osmosis, and active transport

The cell surface membrane and tonoplast are two of the many membranes in a cell. Membranes are important for compartmentalization in a cell.

Components of the Membrane

The cell surface membrane contains phospholipids that form a bilayer, proteins such as glycolipids and glycoproteins, and cholesterol.

Phospholipid: contains a hydrophilic phosphate head and two hydrophobic fatty acid tails

Proteins:

• intrinsic membrane proteins: contained in the phospholipid bilayer; go right through the layer (transmembrane)

  • transmembrane: provide a hydrophilic pathway to allow ions and polar molecules to pass through the membrane (carrier and channel proteins)

• integral membrane proteins: permanently attached to the membrane

• extrinsic membrane proteins: found on one side of the membrane, often on the surface

  • act as receptors/antigens

• peripheral membrane proteins: temporarily attached to the membrane

• glycolipids/glycoproteins: containing a short carbohydrate chain

  • they are responsible for cell stability, cell signalling, endocytosis, and cell-to-cell recognition

Cholesterol: a steroid that maintains the flexibility and permeability of the cell membrane

The fluid mosaic model of the membrane was first outlined in 1972 by S.J Singer and Garth L. Nicholson.

• The fluid mosaic model describes cell membranes as ‘fluid‘ due to:

  • the movement of the phospholipids and proteins via diffusion

  • the movement of the phospholipids within their own layers

  • the different types of proteins are interspersed and move within the layer, though some may be fixed

• The fluid mosaic model describes cell membranes as a ‘mosaic‘ as their scattered pattern produced by the proteins within the bilayer forms a ‘mosaic-like‘ image when viewed from above

The fluid mosaic model is used to explain the passive and active movement between cells and their surroundings, cell-to-cell interactions, and cell signalling.