Cell membrane structureEdexcel A-Level Biology A: Revision notes

All cell membranes are built on a phospholipid bilayer. Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails. In water the molecules arrange into two layers, with the heads facing the watery cytoplasm and tissue fluid and the tails facing each other inside the membrane.

The hydrophobic core means:

• small, non-polar molecules (oxygen, carbon dioxide) can dissolve in it and diffuse across
• ions and polar molecules (such as glucose) cannot pass through easily

This makes the membrane partially permeable.

• Cholesterol fits between the phospholipid tails, regulating fluidity: it restricts movement at high temperature and prevents tight packing at low temperature, and it makes the membrane less leaky.
• Channel proteins form water-filled pores for ions and polar molecules.
• Carrier proteins change shape to move specific molecules across.
• Glycoproteins and glycolipids (carbohydrates attached to proteins or lipids) project from the outer surface and act as receptors (for hormones) and in cell recognition.

The fluid mosaic model (Singer and Nicolson, 1972) describes the membrane as:

• fluid: phospholipids and many proteins can move sideways within the layer
• a mosaic: proteins of different types are scattered through the bilayer, like tiles in a mosaic

Some proteins are intrinsic (spanning the bilayer) and some are on the surface (extrinsic). The membrane is about 7 nm thick.

Scientific models are interpretations of data and change as new evidence emerges.

• Davson and Danielli (1935): a phospholipid layer sandwiched between two continuous layers of protein. It was a static model.
• Freeze-fracture electron microscopy showed protein molecules embedded within the bilayer.
• Frye and Edidin (1970): labelled proteins in a fused mouse and human cell mixed over 40 minutes at 37 °C, but hardly at 1 °C. Proteins therefore move within the membrane, which is fluid, and movement slows as temperature falls.

These observations led to the replacement of the earlier model by the fluid mosaic model.

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

1. Cut equal discs of beetroot with a cork borer and rinse them.
2. Place one in each tube containing the same volume of different ethanol concentrations (or the same volume of water at different temperatures).
3. Leave for a fixed time, then measure the absorbance of each solution in a colorimeter.

Higher absorbance means more pigment has leaked, so the membrane is more permeable. Ethanol dissolves the phospholipids, and high temperatures disrupt the bilayer and denature membrane proteins.

Control the disc size, volume, time and temperature, rinse the discs first, and repeat for a mean.