Students should be able to: recognise, from diagrams, saturated and…

The idea

This is the applied version of the lipid content, and its two halves are tested in different ways. To recognise fatty acids from a diagram, scan the hydrocarbon chain for a carbon-carbon double bond: no double bond means saturated, and the chain is drawn straight; one or more means unsaturated, and the chain is usually drawn with a kink at the double bond. Do not count carbon or hydrogen atoms unless asked, because the double bond is the feature being tested. To explain properties, run each answer from structure to consequence. Three hydrophobic tails and no polar group make a triglyceride non-polar and insoluble, so it is stored in droplets without affecting water potential and yields about twice the energy per gram of carbohydrate when respired. A phospholipid has two tails and a hydrophilic phosphate-containing head, so it is amphipathic and forms a bilayer in water.

What earns the marks

• Identifying a fatty acid with no carbon-carbon double bonds in the R group as saturated.
• Identifying a double bond or kink in the chain as showing an unsaturated fatty acid.
• The triglyceride being hydrophobic and insoluble because it has three non-polar hydrocarbon tails.
• The phospholipid having a hydrophilic phosphate-containing head and hydrophobic tails, so it forms a bilayer.
• Explaining that in a bilayer the hydrophobic tails face inwards and the hydrophilic heads face outwards towards water.

Common mistakes and how to put them right

• Counting hydrogen atoms on a skeletal diagram instead of looking for a carbon-carbon double bond.

  Put it right: The presence or absence of a C=C in the hydrocarbon chain determines saturation.

• Mistaking the C=O of the carboxyl group for evidence of an unsaturated R group.

  Put it right: The carboxyl group is present in all fatty acids, so look for C=C within the hydrocarbon chain.

• Describing structure without giving a consequence, so an explain question is answered as a describe question.

  Put it right: Link each structural feature to a property or role.

• Saying triglycerides form membranes because they contain fatty acids.

  Put it right: Triglycerides are storage lipids, while phospholipids form membranes.

• Stating that a phospholipid is hydrophobic overall, then trying to explain why it faces water.

  Put it right: Phospholipids are amphipathic, with a hydrophilic head and hydrophobic tails.

Exam tips

• In recognition questions, circle the double bond on the diagram before writing; it prevents describing the wrong molecule.
• Answer property questions in the form structure, therefore property, therefore use - each link can carry a mark.
• When a diagram shows a phospholipid bilayer, describe the arrangement by referring to the hydrophobic tails facing inwards and the hydrophilic heads facing the aqueous environment.