A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Digestion and Absorption in Mammals — mark scheme explained
The short answer
Digestion is a crucial process that allows mammals to break down large, complex biological molecules into smaller, absorbable units.
The question
Explain the role of amylases in carbohydrate digestion.
[Paraphrased for study — not reproduced from any exam paper.]
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 explanation questions, marks are typically awarded for providing a detailed and accurate description of the process or mechanism. For identification questions, marks are given for correctly naming specific components or enzymes. For comparison questions, marks are awarded for highlighting both similarities and differences clearly.
What the command words demand
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Describe
- Give a clear and concise account of the process or mechanism.
- Identify
- Recognize and name specific components or enzymes involved in digestion and absorption.
- Compare
- Highlight similarities and differences between different processes or mechanisms.
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 a detailed response, and check your work.
- Identify the enzyme involved.1 markAmylases are enzymes that break down starch into maltose.
- Describe where these enzymes are found.1 markAmylases are found in saliva and the small intestine.
- Explain the process of hydrolysis.1 markDuring digestion, amylases add water to the glycosidic bonds in starch, breaking it down into maltose.
Final answer: Amylases are enzymes that break down starch into maltose. They are found in saliva and the small intestine. During digestion, they hydrolyze the glycosidic bonds in starch by adding water, resulting in the formation of maltose.
Work through every step correctly and you earn all 3 marks.
Another worked example
Describe the role of bile salts in lipid digestion.
- Identify the function of bile salts.1 markBile salts emulsify lipids, increasing their surface area for more efficient enzymatic action by lipase.
- Explain how this process aids digestion.2 marksBy breaking down large lipid droplets into smaller ones, bile salts increase the surface area available for lipase to act on. This enhances the efficiency of lipid digestion.
Final answer: Bile salts emulsify lipids, increasing their surface area for more efficient enzymatic action by lipase. By breaking down large lipid droplets into smaller ones, they enhance the efficiency of lipid digestion.
Work through every step correctly and you earn all 3 marks.
Common mistakes
Confusing amylases with disaccharidases.
Why it happens: Students may mix up the roles of these enzymes, leading to incorrect descriptions of carbohydrate digestion.
Fix: Remember that amylases break down starch into maltose, while disaccharidases further break down disaccharides into monosaccharides. Amylases are found in saliva and the small intestine, whereas disaccharidases are membrane-bound enzymes.
Forgetting the role of bile salts in lipid digestion.
Why it happens: Students may overlook the importance of bile salts in emulsifying lipids, which is crucial for efficient enzymatic action by lipase.
Fix: Always include the role of bile salts in your explanations of lipid digestion. Bile salts emulsify lipids, increasing their surface area and making them more accessible to lipase.
Confusing endopeptidases with exopeptidases.
Why it happens: Students may mix up the roles of these enzymes in protein digestion, leading to incorrect descriptions of how proteins are broken down into amino acids.
Fix: Remember that endopeptidases break down proteins into smaller peptides, while exopeptidases cleave amino acids from the ends of peptides. Both types of peptidases work together to fully digest proteins.
Failing to explain co-transport mechanisms clearly.
Why it happens: Students may struggle to articulate how co-transport with sodium ions facilitates the absorption of amino acids and monosaccharides, leading to vague or incorrect explanations.
Fix: Practice explaining co-transport step-by-step. Co-transport involves the simultaneous transport of a molecule (like an amino acid or glucose) with Na + . This process is driven by the concentration gradient of Na + , which is maintained by the sodium-potassium pump.
Incorrectly describing the formation and role of micelles in lipid absorption.
Why it happens: Students may not fully understand how micelles are formed and their role in facilitating lipid transport to enterocytes, leading to incorrect explanations.
Fix: Practice explaining the formation and role of micelles. Bile salts form micelles that solubilize fatty acids and monoglycerides, increasing their availability for absorption at the surface of enterocytes. Once there, these lipids are re-esterified into triglycerides within the enterocyte.
Forgetting to mention chylomicrons in lipid transport.
Why it happens: Students may overlook the role of chylomicrons in packaging and transporting triglycerides from enterocytes to the bloodstream, leading to incomplete explanations.
Fix: Always include the formation and role of chylomicrons in your descriptions of lipid absorption. Triglycerides are packaged into chylomicrons within enterocytes and released into the lymphatic system for transport to the bloodstream.
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 |
|---|---|---|
| Explain Enzyme Role | Explain how amylase enzymes break down starch during carbohydrate digestion. | 3 |
| Describe Bile Salt Role | Explain how bile salts emulsify lipids and why this speeds up lipid digestion. | 3 |
| Explain Membrane Transport | Describe how amino acids cross ileum cell membranes, including co-transport and active mechanisms. | 4 |
| Describe Absorption Process | Explain how micelles transport lipid components to and across the ileum epithelial cells. | 5 |
| Total across these question types | 15 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.