Phospholipid structure

A phospholipid is a lipid made of a glycerol molecule joined to two fatty acids and a phosphate-containing group. The phosphate end is the head, which is hydrophilic (attracted to water). The two fatty acids are the tails, which are hydrophobic (repelled by water). That split personality is why phospholipids line up into the bilayer that makes every cell membrane.

The structure of a phospholipid

AQA's A-level Biology specification describes it like this: in phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group. The fatty acids join the glycerol by ester bonds, each made in a condensation reaction that releases a molecule of water.

Phospholipids vs triglycerides

Explaining how these different structures give different properties is one of AQA's named skills for lipids.

Why phospholipids form a bilayer

In water, phospholipids arrange themselves so the heads touch the water and the tails keep away from it. With water on both sides of a membrane, the best arrangement is two layers back to back: heads facing outwards on each side, tails tucked into the middle. This phospholipid bilayer forms on its own, without needing energy.

The bilayer in cell membranes

Cell membranes are built on this bilayer, with proteins, cholesterol and other molecules scattered through it, a structure called the fluid mosaic model. The bilayer explains several things about membranes:

• A barrier to water-soluble substances. Ions and polar molecules can't easily cross the hydrophobic core, so they need channel or carrier proteins.
• Small non-polar molecules pass through, such as oxygen and carbon dioxide, by diffusion.
• Flexibility. The phospholipids can move sideways, so the membrane is fluid and can change shape.
• Self-sealing. A small tear closes up, because exposed tails are pushed back together by the water.

The Cell Biology course has an interactive bilayer in its section on the cell membrane, followed by diffusion, osmosis and active transport. For the cells themselves, see animal and plant cells.

Saturated and unsaturated tails

A saturated fatty acid has only single bonds between its carbon atoms, so its tail is straight. An unsaturated fatty acid has at least one C=C double bond, which puts a kink in the tail, as in the drawing above. Kinked tails can't pack as closely together, so membranes with more unsaturated phospholipids are more fluid. AQA expects you to recognise saturated and unsaturated fatty acids from diagrams.

Testing for lipids

The emulsion test is AQA's test for lipids. Shake the sample with ethanol, then pour the liquid into water. A milky white emulsion shows that a lipid is present.

Common mistakes

• Saying phospholipids have three fatty acids. They have two; the third is replaced by the phosphate group.
• Swapping hydrophilic and hydrophobic. Hydrophilic means water-loving: the head. Hydrophobic means water-fearing: the tails.
• Drawing the bilayer with heads in the middle. The tails point inwards; the heads face the water.
• Saying the bilayer stops everything. Small non-polar molecules cross it easily.

To test biological molecules under time pressure, try today's A-level Biology: Year 1 short timed paper. Each paper mixes topics.

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