Fluid mosaic membranes

Biology AS · Cell membranes and transport. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A cell surface membrane is a phospholipid bilayer with other molecules set in it. In water, the hydrophilic phosphate heads face the watery solutions on each side and the hydrophobic fatty acid tails point inwards, away from water. The hydrophobic middle is a barrier to ions and polar molecules.

The model is called fluid because phospholipids and many proteins can move sideways within their layer. It is called mosaic because proteins are scattered among the phospholipids like tiles in a pattern.

Each component has a role: transport, stability, recognition or receiving signals from other cells.

Rules to remember

Phospholipids: form the bilayer; small non-polar molecules (oxygen, carbon dioxide) diffuse through it; ions and large polar molecules cannot
Cholesterol: lies between the phospholipid tails; regulates fluidity, gives mechanical stability and reduces permeability to ions
Channel proteins have a water-filled pore for specific ions; carrier proteins bind a specific molecule and change shape to move it across
Glycoproteins and glycolipids have carbohydrate chains on the outer surface only; they act in cell recognition (cell surface antigens) and as receptors
More unsaturated fatty acid tails (kinked, loosely packed) make a membrane more fluid
Cell signalling: a cell secretes a ligand → the ligand is transported to target cells → it binds to a specific cell surface receptor → the target cell responds
Only cells with a receptor of complementary shape to the ligand respond

Common mistake

Drawing or describing carbohydrate chains on the inside of the membrane. They are found only on the outer surface, facing away from the cytoplasm.

Worked example

A mouse cell and a human cell have membrane proteins labelled with different coloured dyes. The cells are fused. At first each colour covers one half of the new cell. After 40 minutes both colours are evenly mixed over the whole surface. What does this show?

The proteins began in two separate halves of the membrane.
For the colours to mix, the proteins must have moved sideways through the membrane.
Proteins can only move like this if the phospholipids around them are also free to move.

Answer: It shows that the membrane is fluid: proteins and phospholipids are not fixed in place but move sideways within the bilayer.

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