Lipid

In the syllabus and the exam

Lipids are examined through the ethanol emulsion test, which produces a cloudy white emulsion when a fat or oil is present, and through their role as the most energy-dense biomolecule, storing about twice as much energy per gram as carbohydrate or protein, a comparison that appears often in questions about food reserves and balanced diet. Candidates are expected to know that lipids are made of one glycerol molecule joined to three fatty acid molecules, that they are used for long-term energy storage, insulation against heat loss, and protection of internal organs, and that excess lipid intake is linked to conditions such as obesity and cardiovascular disease, connecting this term to lifestyle-disease questions elsewhere in the syllabus. A common error is confusing the ethanol emulsion test with the iodine test for starch, or forgetting that the ethanol test requires the sample to first be dissolved in ethanol before water is added to produce the visible cloudy emulsion, since skipping the ethanol step or adding water first gives an incorrect result. Candidates sometimes also state that lipids are the body's main energy source used first during normal activity, when in the syllabus's hierarchy of energy sources carbohydrate is used first, with lipid reserved mainly as a longer-term store. Candidates are sometimes asked to compare lipid with carbohydrate as an energy store in the context of hibernating or migrating animals, which rely heavily on fat reserves precisely because lipid packs more energy into a given mass, reducing the weight the animal must carry or burn.

Example

Cooking oil and the fat under human skin are both lipids, made of fatty acids and glycerol, which the body uses to store energy and to insulate against heat loss.

Related terms

CarbohydrateProtein

Source:SRC-DSKP-EN