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

This guide covers sections 3.2.2 (Digestion and absorption) and 3.2.3 (Cholera) of Unit 2, Biological systems and disease, in the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, for International A-level exams May/June 2018 onwards. The specification lists Unit 2 as content for both the International AS and the International A-level. For the course page see the OxfordAQA A-level Biology hub, and track your progress on the printable checklist.

Companion pages: revision notes, practice questions and the earlier causes of disease guide. Test yourself first with the free diagnostics.

What you must be able to do

Section	Statement in brief
3.2.2.1	Gross structure of the digestive system (oesophagus, stomach, duodenum, ileum, colon, rectum); salivary glands and pancreas; digestion, absorption, assimilation, egestion
3.2.2.2	Digestion of carbohydrates, lipids and proteins; role of bile salts
3.2.2.3	Ileum histology and absorption; diffusion, active transport and co-transport with sodium ions; micelles and chylomicrons
3.2.3.1	Cholera toxin, chloride ion secretion, water potential gradient, severe diarrhoea
3.2.3.2	Oral rehydration solutions (ORS); applications and implications of improving ORS; ethics of trialling them on humans

3.2.2.1 The human digestive system

Four terms are defined by the specification, so use them precisely.

• Digestion: large molecules are hydrolysed by enzymes into smaller molecules.
• Absorption: the products of digestion cross the gut lining into the body.
• Assimilation: absorbed substances become part of the body’s cells, for example amino acids built into proteins.
• Egestion: undigested food leaves as faeces. Do not call this excretion, which is the removal of waste made by metabolism.

Structure	Role
Salivary glands	Release saliva containing amylase into the mouth
Oesophagus	Muscular tube; waves of muscle contraction push food to the stomach
Stomach	Muscular churning; acid and an endopeptidase begin protein digestion
Pancreas	Releases pancreatic juice into the duodenum, containing amylase, lipase and peptidases
Duodenum	First part of the small intestine; receives pancreatic juice and bile (which contains bile salts)
Ileum	Longer, later part of the small intestine; the main site of absorption
Colon	Absorbs much of the remaining water
Rectum	Stores faeces before egestion

The specification limits the glands to the salivary glands and pancreas, so you do not need the structure of the liver or gall bladder. You do need to know that bile salts reach the duodenum.

3.2.2.2 Digestion

Every digestive enzyme here catalyses hydrolysis: a molecule of water breaks a bond.

Carbohydrates. Amylases (from salivary glands and pancreas) hydrolyse starch to maltose. Membrane-bound disaccharidases, fixed in the cell-surface membrane of ileum epithelial cells, finish the job: maltase splits maltose into two glucose; sucrase splits sucrose into glucose and fructose; lactase splits lactose into glucose and galactose. Because they sit on the membrane, the monosaccharides are made right beside the transport proteins that take them in.

Lipids. Lipases hydrolyse the ester bonds of triglycerides, giving fatty acids and monoglycerides. Pancreatic lipase is the main one.

Bile salts are not enzymes. They emulsify lipids, breaking large droplets into many small ones. Total surface area rises, so lipase meets more substrate and digestion is faster. Bile salts also take part in micelle formation (see 3.2.2.3).

Proteins need three kinds of enzyme working together:

Enzyme type	Where it acts	Effect
Endopeptidases	Peptide bonds inside a chain	Cut a polypeptide into shorter chains, making many more ends
Exopeptidases	Peptide bonds at the ends of a chain	Remove single amino acids (or dipeptides) from the ends
Membrane-bound dipeptidases	Dipeptides at the ileum surface	Hydrolyse each dipeptide to two amino acids

Worked example: counting hydrolysis reactions

A polypeptide has 90 amino acids. An endopeptidase cuts it at 4 sites. How many peptide bonds remain, and how many fragments are there?

1. A chain of 90 amino acids has 90 − 1 = 89 peptide bonds.
2. Four cuts hydrolyse four bonds: 89 − 4 = 85 bonds remain.
3. Four cuts in one chain give 4 + 1 = 5 fragments.
4. Complete digestion needs all 89 bonds hydrolysed, so exopeptidases and dipeptidases do the remaining 85 hydrolyses.
5. The extra ends are why endopeptidases speed things up: exopeptidases can only work at ends, and the cuts create more ends.

3.2.2.3 Absorption

Histology of the ileum

The ileum wall is built for absorption. Match each feature to its function.

Feature	How it helps absorption
Folds, villi and microvilli (brush border)	Very large surface area
Epithelium one cell thick	Short diffusion distance
Dense capillary network in each villus	Blood carries absorbed substances away, keeping concentration gradients steep
Lacteal (lymph vessel) in each villus	Takes up chylomicrons
Goblet cells	Release mucus that lubricates and protects the lining
Many mitochondria in epithelial cells	Supply ATP for active transport
Membrane-bound enzymes and carrier proteins	Final digestion beside the transport proteins; uptake of sugars and amino acids

Worked example: surface area

A villus is modelled as a cylinder of radius 0.10 mm and height 1.0 mm, standing on a flat footprint of the same radius. Microvilli are supplied as increasing membrane area 15-fold. A drawing of a villus is 42 mm long and the real villus is 0.6 mm long.

1. Curved area = 2πrh = 2 × π × 0.10 × 1.0 = 0.63 mm².
2. Footprint = πr² = π × 0.10² = 0.031 mm².
3. Ratio = 0.628 ÷ 0.0314 = 20 times the flat area.
4. With microvilli: 0.628 × 15 = 9.4 mm² of membrane per villus.
5. Magnification = image ÷ actual = 42 ÷ 0.6 = ×70.

Moving glucose and amino acids

Monosaccharides and amino acids are absorbed by several processes at once.

• Diffusion (facilitated, through carrier proteins) works while the concentration in the lumen exceeds that in the cell. Fructose enters this way.
• Active transport with co-transport is needed when the lumen concentration is lower. In sequence:
  1. A sodium-potassium pump in the basal membrane uses ATP to move sodium ions out of the epithelial cell into the blood.
  2. The sodium ion concentration inside the cell falls below that in the lumen.
  3. Sodium ions move back in down their gradient through a co-transporter protein, carrying a glucose molecule (or amino acid) with them against its gradient.
  4. The sugar or amino acid builds up in the cell, then leaves through the basal membrane by facilitated diffusion into the blood.

Co-transport is called indirect active transport because ATP is used by the pump, not by the co-transporter itself.

Worked example: interpreting uptake data

Glucose uptake by ileum tissue was 38 units in normal saline. With sodium ions removed it fell to 5 units.

• Percentage fall = (38 − 5) ÷ 38 × 100 = 86.8%.
• Conclusion: most uptake depends on sodium ions, consistent with co-transport. The remaining 5 units could be diffusion.

Lipid absorption

Fatty acids and monoglycerides combine with bile salts to form micelles. Micelles carry the products to the epithelial surface, where they are released; being lipid-soluble, they diffuse through the membrane. Inside the cell they are reassembled into triglycerides, packaged with protein and phospholipid into chylomicrons, and leave by exocytosis. Chylomicrons enter the lacteals and travel in the lymph before reaching the blood.

3.2.3.1 Cholera and its symptoms

Cholera is a disease caused by the bacterium Vibrio cholerae, usually taken in through contaminated water or food (see the causes of disease guide for pathogens and toxins). The bacteria secrete a toxin in the small intestine, and the sequence is:

1. The toxin increases the secretion of chloride ions into the lumen from epithelial cells. It does this by keeping chloride channels open.
2. Sodium ions follow the chloride ions, so the lumen contains a high concentration of ions.
3. The water potential of the lumen becomes lower (more negative) than that of the epithelial cells and blood.
4. Water leaves the tissue by osmosis, down the water potential gradient, into the lumen.
5. The result is severe watery diarrhoea. Water and ions are lost faster than they are replaced, causing dehydration.

Worked example: scale of the loss

A 70 kg patient produces 900 mL of watery stool per hour for 8 hours.

• Volume = 0.9 × 8 = 7.2 L.
• Taking 1 L as about 1 kg, loss = 7.2 ÷ 70 × 100 = 10.3% of body mass.
• This explains the danger: blood volume falls and the circulation is affected.

3.2.3.2 Oral rehydration solutions

ORS is a solution of water, glucose and salts (including sodium ions) taken by mouth. Water alone is not enough. The cholera toxin affects chloride secretion, not the sodium-glucose co-transporter, so glucose still brings sodium ions into the cells. Water follows by osmosis, replacing what was lost. Severe dehydration may still need fluids given by intravenous drip.

Worked example: checking the osmolarity

A reduced-osmolarity ORS is supplied as: glucose 75, sodium ions 75, chloride ions 65, potassium ions 20 and citrate 10 (all mmol per litre).

• Total = 75 + 75 + 65 + 20 + 10 = 245 mmol of particles per litre (245 mOsm per litre).
• Mass of glucose per litre = 0.075 mol × 180 g per mol = 13.5 g.
• Blood plasma is around 290 mOsm per litre. A solution with fewer dissolved particles than a higher-osmolarity formulation pulls less water into the gut by osmosis.

Applications and implications of improved ORS

Improving ORS means changing its composition, for example a lower overall concentration, or using rice starch so that glucose is released gradually. Discuss both sides.

• Applications: less stool output, fewer patients needing a drip, cheap sachets that can be used in places with few hospitals, and treatment of other diarrhoeal diseases.
• Implications: clean water is still needed to make it up; ORS treats the symptom, not the infection or its source; a change of formula means retraining staff and new supplies, and costs money.

Ethics of human trials

A trial compares a new ORS with the standard one. Points to discuss:

• Patients (often children) must give informed consent, or a parent or guardian must; they can withdraw at any time.
• It is unethical to give a dehydrated patient an untreated placebo, so the control group receives the standard treatment.
• Risks and benefits must be weighed, and an independent ethics committee should approve the plan.
• Vulnerable groups, such as people in poverty or emergencies, should not be exploited, and the successful treatment should be made available afterwards.

Common errors

• Saying amylase digests “carbohydrates” without naming the substrate (starch) and product (maltose).
• Calling bile an enzyme, or saying bile salts digest fat. They emulsify it.
• Writing that exopeptidases cut in the middle of a chain.
• Saying glucose is absorbed by “active transport” without sodium ions and the pump.
• Saying the toxin makes the gut absorb less water without stating that chloride ions are secreted.
• Using “concentration” when the question wants water potential.
• Treating ORS as a cure for the infection.

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. Check the current specification at oxfordaqa.com.