A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Triglycerides and Phospholipids: Structure and Properties — mark scheme explained
The short answer
Lipids are a diverse group of biological molecules that include triglycerides and phospholipids. Understanding the structure and properties of these lipids is crucial for grasping their roles in biological systems.
The question
Identify whether the following fatty acid is saturated or unsaturated: CH 3 (CH 2 ) 16 COOH [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Examine the structure of the fatty acid.
- S2
The given fatty acid has a straight, linear hydrocarbon chain with no double bonds.
- S3
Step 2: Determine if it is saturated or unsaturated.
- S4
Since there are no double bonds in the hydrocarbon chain, this fatty acid is saturated.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Examine the structure of the fatty acid.
- S2
The given fatty acid has a straight, linear hydrocarbon chain with no double bonds.
- S3
Step 2: Determine if it is saturated or unsaturated.
- S4
Since there are no double bonds in the hydrocarbon chain, this fatty acid is saturated.
Final answer: Saturated
Common mistakes
- Misidentifying fatty acids as saturated or unsaturated based on their structure. — Always check for double bonds in the hydrocarbon chain. If there are no double bonds, the fatty acid is saturated; if there are one or more double bonds, it is unsaturated.
- Forgetting that triglycerides are formed from glycerol and three fatty acids. — Remember that triglycerides are formed by the condensation reaction between one molecule of glycerol and three molecules of fatty acids, resulting in the formation of ester bonds.
- Not understanding the amphipathic nature of phospholipids. — Practice explaining that phospholipids have a polar head group and two non-polar fatty acid tails. The polar heads face the water, while the non-polar tails face each other to form a bilayer structure.
- Failing to explain why triglycerides are insoluble in water. — Remember that the long hydrocarbon chains of fatty acids do not interact well with water molecules, making triglycerides insoluble in water. This property is crucial for their role as energy storage molecules.
- Incorrectly describing the steps and results of the emulsion test. — Practice the steps: dissolve the sample in ethanol first, then add water and shake gently, and observe a cloudy or milky appearance if lipids are present (because lipid is insoluble in water and comes out of solution). Ensure you can clearly describe each step and result.
- Confusing the functions of triglycerides and phospholipids. — Remember that triglycerides are primarily used for energy storage due to their high energy density, while phospholipids form cell membranes and play a role in signal transduction and membrane fluidity. Practice distinguishing between their functions.
Where the marks go
- Full worked solution (all marking points)2 marks