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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the enzyme involved.
- S2
Amylases are enzymes that break down starch into maltose.
- S3
Step 2: Describe where these enzymes are found.
- S4
Amylases are found in saliva and the small intestine.
- S5
Step 3: Explain the process of hydrolysis.
- S6
During digestion, amylases add water to the glycosidic bonds in starch, breaking it down into maltose.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the enzyme involved.
- S2
Amylases are enzymes that break down starch into maltose.
- S3
Step 2: Describe where these enzymes are found.
- S4
Amylases are found in saliva and the small intestine.
- S5
Step 3: Explain the process of hydrolysis.
- S6
During 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.
Common mistakes
- Confusing amylases with disaccharidases. — 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. — 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. — 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. — 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. — 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. — 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
- Full worked solution (all marking points)3 marks