A-Level · Biology · AQA · Mark scheme decoded

AQA A-Level Biology: Triglycerides and Phospholipids: Structure and Properties — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Biology specificationlast verified 3 July 2026

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.]

2 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For identification questions, marks are typically awarded for correctly naming structures or components. For explanation and description questions, marks are given for clear and concise answers that demonstrate a deep understanding of the topic. Ensure you provide all necessary details and avoid vague statements.

What the command words demand

Identify: Recognize and name specific structures or components from given information.
Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Describe: Give a clear account of the steps or characteristics of a process or structure.
Determine: Find or derive a specific value or quantity based on given information.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed answers, and check your work.

Examine the structure of the fatty acid.1 mark
The given fatty acid has a straight, linear hydrocarbon chain with no double bonds.
Determine if it is saturated or unsaturated.1 mark
Since there are no double bonds in the hydrocarbon chain, this fatty acid is saturated.

Final answer: Saturated

Work through every step correctly and you earn all 2 marks.

Another worked example

Explain why phospholipids form a bilayer in aqueous environments.

4 marks

Describe the structure of a phospholipid.1 mark
A phospholipid has a polar head group and two non-polar fatty acid tails.
Explain the behavior of the polar and non-polar parts in water.2 marks
The polar head groups are hydrophilic and face the aqueous environment, while the non-polar tails are hydrophobic and face each other to avoid contact with water.
Conclude how this arrangement forms a bilayer.1 mark
This arrangement results in a bilayer structure where the polar heads form the outer surfaces and the non-polar tails form the inner core, creating a barrier that controls the movement of substances.

Final answer: Phospholipids form a bilayer in aqueous environments because their polar head groups face the water, while their non-polar fatty acid tails face each other to avoid contact with water.

Work through every step correctly and you earn all 4 marks.

Common mistakes

Misidentifying fatty acids as saturated or unsaturated based on their structure.
Why it happens: Students may overlook the presence of double bonds in the hydrocarbon chain, leading to incorrect identification.
Fix: 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.
Why it happens: Students may confuse the formation of triglycerides with other lipids or forget the specific components involved.
Fix: 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.
Why it happens: Students may not fully grasp how the polar and non-polar parts of phospholipids interact in aqueous environments.
Fix: 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.
Why it happens: Students may not understand the relationship between the hydrocarbon chains and solubility.
Fix: 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.
Why it happens: Students may forget the specific steps or misinterpret the results of the emulsion test.
Fix: 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.
Why it happens: Students may mix up the roles of these two types of lipids, leading to incorrect explanations.
Fix: 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

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Classify Fatty Acid	Decide whether the fatty acid is saturated or unsaturated from its structural formula.	2
Explain Bilayer Formation	Explain why phospholipids arrange into a bilayer when placed in water.	4
Biochemical Food Test	Describe the method and expected results of the emulsion test used to identify lipids.	3
Explain Biological Molecule	Explain how the structure of triglycerides makes them suitable for energy storage.	4
Total across these question types		13

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.