Cell membrane structureEdexcel International A Level Biology: Revision notes

The cell surface membrane is made mainly of phospholipids and proteins. A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails.

In water, phospholipids form a bilayer: the heads face the watery cytoplasm and tissue fluid on each side, and the tails face each other in the centre. This hydrophobic core is a barrier to charged and polar substances such as ions and glucose, but small, non-polar molecules such as oxygen and carbon dioxide can diffuse through.

Proteins are scattered through the bilayer. Some span the whole bilayer (intrinsic) and form channel and carrier proteins that move specific substances across; others lie on one surface (extrinsic) or act as receptors or enzymes.

Cholesterol fits between phospholipid tails and regulates fluidity, making the membrane less fluid at high temperature and preventing it solidifying at low temperature.

Glycoproteins and glycolipids have carbohydrate chains on the outer surface, used in cell recognition and as receptors.

Cell membranes are partially permeable: they let some substances through easily, others only through proteins, and block others. They are also fluid, because phospholipids and many proteins can move sideways. Membranes also separate the cell from its surroundings and form compartments inside the cell, for example around organelles.

The fluid mosaic model (Singer and Nicolson) describes the membrane as a fluid phospholipid bilayer with proteins scattered through it like the pieces of a mosaic. It replaced the earlier idea that proteins formed continuous layers on both surfaces of a lipid layer.

Evidence includes:

• Cell fusion: labelled proteins from two fused cells mix over the whole surface, so proteins can move. At low temperature they do not mix, because phospholipids move less.
• Freeze-fracture electron micrographs show proteins embedded within the bilayer.

When interpreting data, link the observation (proteins move, proteins in the bilayer) to the part of the model it supports.

Beetroot cells contain a red pigment (betalain) in their vacuoles. If the membranes are damaged, the pigment leaks out.

Method: cut identical discs with a cork borer, rinse them, and place equal numbers in equal volumes of water at different temperatures, or in different ethanol concentrations, for the same time. Remove the discs and measure the absorbance of the liquid with a colorimeter (zeroed with a blank). Higher absorbance means more pigment, so a more permeable membrane.

Temperature: high temperature denatures proteins and makes the bilayer more fluid and disrupted. Ethanol: dissolves lipids in the bilayer and can denature proteins. Control disc size, volume, time and temperature; repeat and calculate means.