What Is the Structure and Function of Lipids?
Learn how triglyceride and phospholipid structures suit energy storage and membrane formation in AQA A-level Biology, with key terms and exam mistakes explained.
Lipids have different structures suited to different functions. In AQA A-level Biology, the two key types are triglycerides, which are mainly energy stores, and phospholipids, which form cell membranes. Both contain glycerol and fatty acids, but replacing one fatty acid changes how the molecule behaves in water.
Triglycerides
A triglyceride contains one glycerol molecule joined to three fatty acids. Each fatty acid joins to glycerol by a condensation reaction, forming an ester bond and releasing water. A fatty acid has a long hydrocarbon chain. If the chain has no carbon-to-carbon double bonds, it is saturated; if it has at least one, it is unsaturated.
Triglycerides are effective energy stores because their fatty acids contain many carbon-to-hydrogen bonds and can be broken down during respiration to release energy. Their high energy content per unit mass makes them a compact store, particularly useful when organisms need to carry reserves. They are also insoluble in water, so storing them does not lower a cell’s water potential. Triglycerides can provide thermal insulation and cushioning around organs.
Phospholipids
A phospholipid has glycerol joined to two fatty acids and a phosphate-containing group in place of the third fatty acid. Its phosphate-containing head is hydrophilic (attracted to water), while its fatty acid tails are hydrophobic (repelled by water).
In water, phospholipids form a bilayer: the heads face the watery surroundings on either side, and the tails point inwards. This makes phospholipids well suited to forming cell-surface and organelle membranes. The hydrophobic centre acts as a barrier to many water-soluble substances. Phospholipid molecules can move within the bilayer, helping the membrane remain flexible rather than forming a rigid wall.
Exam mistake to avoid: do not describe a phospholipid as having three fatty acids. That describes a triglyceride; a phospholipid has two fatty acids and a phosphate-containing group.
