OxfordAQA International AS and A-level Biology: Digestion, absorption and cholera (9610) – Revision Notes

These revision notes condense sections 3.2.2 and 3.2.3 of Unit 2 in the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, for International A-level exams May/June 2018 onwards. Unit 2 is listed for both the International AS and the International A-level. Full explanations are in the study guide; when you are ready, try the practice questions.

Course pages: OxfordAQA A-level Biology hub, the checklist, the free diagnostics and the causes of disease revision notes.

3.2.2.1 Digestive system at a glance

Food path: mouth, oesophagus, stomach, duodenum, ileum, colon, rectum. Glands you need: salivary glands and pancreas.

Part	One-line job
Salivary glands	Saliva with amylase
Oesophagus	Moves food by muscular contraction
Stomach	Churns food; protein digestion starts
Pancreas	Pancreatic juice into the duodenum
Duodenum	Receives pancreatic juice and bile salts
Ileum	Main absorption site
Colon	Absorbs remaining water
Rectum	Stores faeces

Key terms:

Term	Meaning
Digestion	Hydrolysis of large molecules by enzymes into smaller ones
Absorption	Products cross the gut wall into the body
Assimilation	Absorbed substances become part of cells and tissues
Egestion	Undigested material leaves as faeces

3.2.2.2 Digestion

Food type	Enzyme	Substrate and products
Carbohydrate	Amylase (saliva and pancreas)	Starch to maltose
Carbohydrate	Membrane-bound disaccharidases	Maltose to glucose + glucose; sucrose to glucose + fructose; lactose to glucose + galactose
Lipid	Lipase	Triglyceride to monoglyceride + fatty acids
Protein	Endopeptidase	Cuts internal peptide bonds, giving shorter chains
Protein	Exopeptidase	Removes amino acids from the ends of chains
Protein	Membrane-bound dipeptidase	Dipeptide to two amino acids

Role of bile salts in steps:

1. Bile salts reach the duodenum.
2. They emulsify lipids into small droplets.
3. Surface area increases, so lipase works faster.
4. Later they help form micelles.

Remember: bile salts are not enzymes, and they do not chemically digest fat.

Method for peptide-bond questions:

1. Bonds in a chain = amino acids − 1.
2. Each endopeptidase cut removes one bond and adds one fragment.
3. Fragments = cuts + 1 (for one chain).
4. Remaining bonds are hydrolysed by exopeptidases and dipeptidases.

Worked reminder: a 60-amino-acid chain has 59 bonds. Three cuts leave 56 bonds and give 4 fragments.

3.2.2.3 Absorption

Ileum histology

Feature	Link to function
Villi and microvilli	Large surface area
Single layer of epithelium	Short diffusion path
Capillary network	Maintains gradients by removing products
Lacteal	Carries chylomicrons
Goblet cells	Mucus
Mitochondria	ATP for active transport

Magnification = image size ÷ actual size, with both in one unit (convert first). If a drawing is 36 mm and the real structure is 0.45 mm, magnification is 36 ÷ 0.45 = ×80.

Absorption of glucose and amino acids

Method in steps:

1. Sodium-potassium pump in the basal membrane removes sodium ions from the cell using ATP.
2. Cell sodium ion concentration is low.
3. Sodium ions enter from the lumen down their gradient through a co-transporter.
4. Glucose or an amino acid enters with them, against its own gradient.
5. Glucose or amino acid leaves into the blood by facilitated diffusion.

Process	Needs ATP?	Direction relative to gradient
Diffusion (facilitated)	No	Down
Active transport	Yes, directly	Against
Co-transport	Not directly (the pump uses it)	Solute enters against its gradient, sodium ions down theirs

Worked reminder: uptake of 50 units falls to 14 units when sodium ions are removed. Fall = (50 − 14) ÷ 50 × 100 = 72%.

Lipids

1. Fatty acids and monoglycerides join with bile salts to form micelles.
2. Micelles release the products at the epithelial surface.
3. Products diffuse across the membrane because they are lipid-soluble.
4. Inside, they are rebuilt into triglycerides and packaged with protein and phospholipid as chylomicrons.
5. Chylomicrons leave by exocytosis and enter the lacteals, then the lymph.

3.2.3.1 Cholera

Pathogen: Vibrio cholerae, usually spread through contaminated water or food. It secretes a toxin.

Sequence:

1. The toxin increases chloride ion secretion into the lumen.
2. Sodium ions follow, so lumen ion concentration rises.
3. Lumen water potential falls below that of the cells and blood.
4. Water moves out by osmosis.
5. Severe watery diarrhoea and dehydration follow.

Worked reminder: a 50 kg patient loses 500 mL of stool per hour for 10 hours. Volume = 0.5 × 10 = 5 L, or 5 ÷ 50 × 100 = 10% of body mass if 1 L is about 1 kg.

3.2.3.2 Oral rehydration solutions

Contents: water, glucose and salts including sodium ions.

Why it works: glucose and sodium ions are still absorbed by co-transport, and water follows by osmosis. Why not just water: water alone supplies no glucose to drive sodium ion uptake, so less water follows by osmosis.

Worked reminder: 1.5 litres of an ORS with 75 mmol of glucose per litre needs 112.5 mmol of glucose, which is 20.3 g using 180.16 g per mol.

Discussion points:

Theme	Points
Applications	Cheap, can be used outside hospital, less stool output when concentration is reduced, useful for other diarrhoeal diseases
Implications	Needs clean water; treats the symptom, not the infection; costs of change; severe cases still need intravenous fluid
Ethics of trials	Informed consent (parents for children); right to withdraw; no untreated placebo; ethics committee approval; avoid exploiting vulnerable people; fair access afterwards

Where each enzyme acts

Enzyme or secretion	Source and site
Salivary amylase	Salivary glands; mouth
Endopeptidase (acid conditions)	Stomach
Pancreatic amylase, lipase and peptidases	Pancreas; duodenum
Bile salts	Enter the duodenum in bile
Disaccharidases and dipeptidases	Cell-surface membrane of ileum epithelial cells

Link each location to the pH or conditions: stomach contents are acidic, while pancreatic juice makes the duodenum less acidic so that pancreatic enzymes can work. Salivary amylase does not continue to work for long once food meets stomach acid.

Evaluating trial data

Use this order when a question gives you results from a trial of a new ORS:

1. Compare the two means with units, and calculate the difference.
2. Express the difference as a percentage of the standard treatment.
3. Comment on sample size and spread, because a difference between two means may be down to chance.
4. Check that the groups were similar and that only the ORS differed.
5. Give a conclusion that does not go beyond the data.

Worked reminder: standard ORS group, mean duration of diarrhoea 70 hours; new ORS group, 56 hours. Difference = 70 − 56 = 14 hours. Percentage shorter = 14 ÷ 70 × 100 = 20%. You cannot say the new ORS is better without knowing sample size and variation.

Command words for this unit

Command	What to do
State	Short fact, no reasons
Describe	Give the sequence or pattern in the data
Explain	Give the biological reason for each step
Suggest	Apply what you know to an unfamiliar case
Discuss	Give both sides, applications and implications, with a reason for each

Must-know distinctions

• Digestion (breakdown) versus absorption (crossing the wall) versus assimilation (use in the body).
• Egestion (undigested food) versus excretion (metabolic waste).
• Endopeptidase (inside) versus exopeptidase (ends) versus dipeptidase (dipeptides, on the membrane).
• Micelle (carries products to the surface) versus chylomicron (carries triglycerides out of the cell).
• Active transport versus co-transport.

Quick self-test

1. Name the two glands whose secretions contain amylase.
2. State the substrate and the product of amylase.
3. Give the products of lactose digestion.
4. Explain why bile salts speed up lipid digestion.
5. A 75-amino-acid chain is cut by an endopeptidase at 4 places. How many bonds remain and how many fragments form?
6. Where in the epithelial cell is the sodium-potassium pump found?
7. Describe how a lacteal contributes to absorption.
8. A micrograph shows a villus 52 mm long. The real villus is 0.65 mm. Calculate the magnification.
9. Explain why cholera toxin causes water to enter the lumen.
10. Suggest why an ORS contains glucose.
11. State two ethical points about trialling a new ORS in children.

Answers

1. Salivary glands and pancreas.
2. Starch; maltose.
3. Glucose and galactose.
4. They emulsify lipid into small droplets, increasing surface area for lipase.
5. 74 bonds in the chain; 70 remain; 5 fragments.
6. In the basal membrane (the side next to the blood).
7. It takes up chylomicrons into the lymph.
8. 52 ÷ 0.65 = ×80.
9. The toxin raises chloride ion secretion, which lowers lumen water potential, so water moves out by osmosis.
10. Glucose is co-transported with sodium ions, so sodium ions and water are absorbed.
11. For example: informed consent from a parent or guardian; no untreated placebo for dehydrated patients. Ethics committee approval and the right to withdraw are also acceptable.

Where marks are usually lost

• Writing “enzymes break down food” without naming the substrate and the product.
• Describing bile salts as enzymes.
• Giving “increases surface area” for the ileum without saying which structure (villi, microvilli).
• Leaving out the pump when explaining co-transport.
• Confusing a micelle with a chylomicron.
• Saying cholera toxin “stops water absorption” instead of explaining chloride ion secretion and the water potential gradient.
• Giving the units wrongly in magnification: convert mm and micrometres before dividing.
• One-sided ethics answers with no reason attached to each point.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, for International A-level exams May/June 2018 onwards, published by OxfordAQA. Sections 3.2.2.1 to 3.2.3.2. The current specification is on oxfordaqa.com.