Lipid Functions: Storage, Insulation, Membranes and Hormones

Revision notes

Energy sources and stores

Triglycerides store considerable chemical energy because their hydrocarbon chains can be oxidised. Per gram, lipids generally yield more energy than carbohydrates. They are insoluble in water, so storage droplets have relatively little direct osmotic effect. Their compact storage is useful in adipose tissue and many seeds.

Fatty acids can support ATP production through metabolic pathways linked to respiration. This does not mean every tissue can use every lipid at the same rate or that lipids supply the fastest ATP during explosive exercise. Carbohydrate and lipid stores have different physiological advantages.

Insulation and protection

Subcutaneous adipose tissue reduces heat transfer from the body. Adipose around organs cushions and protects them. These are physical functions distinct from energy storage, even when the same tissue performs all three. Insulation reduces the rate of heat loss; it does not generate heat by itself. Waxes can form water-resistant protective coatings on biological surfaces.

Phospholipid membranes

Phospholipids are amphipathic: their phosphate-containing heads interact with water, while their fatty-acid tails avoid it. In water they can form bilayers with heads facing the aqueous surroundings and tails facing inward. This creates a barrier to many ions and polar solutes while allowing some small non-polar molecules to cross more readily.

Membrane proteins provide selective transport and signalling. Cholesterol in animal membranes helps regulate fluidity and permeability in a temperature-dependent way. A membrane is therefore a dynamic mixture, not a solid sheet of stored triglyceride.

Steroid hormones

Steroid hormones, including cortisol and sex steroids, are derived from cholesterol. Their lipid-soluble character permits diffusion across many cell membranes, and they commonly act through intracellular receptors to alter gene expression. Contrast these with peptide hormones such as insulin, which act through cell-surface receptors. Avoid saying that all hormones are lipids or that all lipids act as hormones.

Connect structure to role

A non-polar energy-rich triglyceride is suitable for storage; an amphipathic phospholipid is suitable for bilayers; a steroid’s ring structure supports a different signalling role. The word lipid covers chemically related but functionally diverse substances.

Worked application and data skills

Using illustrative food-energy values of 37 kJ g⁻¹ for lipid and 17 kJ g⁻¹ for carbohydrate, 10 g supplies 370 kJ and 170 kJ respectively. The ratio is 370/170 ≈ 2.18. These are comparison values, not a claim that all cellular pathways transfer the same fraction into ATP.

Common misconceptions

Insulation slows heat loss rather than creating heat. Triglycerides do not form the standard membrane bilayer. Insulin is not a steroid.

Examination tips

Use a named lipid class and connect its properties to the required function.

Check your understanding

Five original questions. Use Tab to move between controls, arrow keys within a radio group, and Enter or Space on buttons. Feedback appears after Check Answer. Try Again clears the question. Attempts are not saved.

Sources and scope

Scope: Unit 1 B5.2. Original BTECRevision notes, schematics and questions. Independent resource; not endorsed by Pearson.

Official Pearson specification. Scientific review references are listed in the accompanying QA report.