Digestive System

Question 1 (12 marks)

(a) (4 marks: 2 per valid consequence + reason)

• The ileum is a major site of nutrient absorption; bypassing it reduces absorptive surface (villi/microvilli), causing malabsorption / malnutrition (1) because there is less surface area and less contact time for nutrient uptake (1).
• Specific deficiencies e.g. vitamin B12 and bile salts are absorbed in the ileum, so their deficiency results (1) — B12 needed for red blood cells; loss causes anaemia (1). (Accept fat-soluble vitamins, iron etc. with correct reasoning.)

(b) (4 marks)

• Normally the small intestine absorbs most nutrients and water (1); the large intestine reabsorbs remaining water to solidify faeces (1).
• Bypassing the ileum means food/water passes too quickly into the large intestine (1); reduced absorption + more fluid arriving than the colon can reabsorb → watery stool/diarrhoea (1).

(c) (4 marks)

• Bile salts emulsify fats and are normally reabsorbed in the ileum and recycled back to the liver (enterohepatic circulation) (1).
• With the ileum bypassed, bile salts are lost in faeces rather than recycled (1).
• The liver cannot replace them fast enough, so the bile salt pool shrinks (1); less emulsification → poorer fat digestion/absorption (steatorrhoea) even though bile is initially produced normally (1).

Question 2 (14 marks)

(a) (6 marks: 2 per sample = region + justification)

• Sample A = Mouth/saliva (1): digests starch only, neutral pH 7 — matches salivary amylase (1).
• Sample B = Stomach (1): digests protein only at acidic pH 2 — matches pepsin in gastric acid (1).
• Sample C = Small intestine (duodenum) (1): digests all three at slightly alkaline pH 8 — pancreatic enzymes work in alkaline conditions (1).

(b) (3 marks)

• Enzyme = pepsin (1).
• It requires low/acidic pH (~pH 2) because that is its optimum pH (1); the acid also activates pepsinogen to pepsin and denatures proteins making them easier to digest (1).

(c) (3 marks: organ + 3 correct enzymes)

• Organ = pancreas (1). Enzymes: starch → pancreatic amylase; protein → trypsin (or chymotrypsin); fat → lipase (all three correct = 2; two correct = 1).

(d) (2 marks)

• Uncertain because pancreatic amylase also digests starch at near-neutral/alkaline pH, so a small-intestine sample could give a similar result (1); another possible source = pancreatic secretion / small intestine (1).

Question 3 (12 marks)

(a) (5 marks — deduct for order errors) Mouth → pharynx → oesophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine (colon) → rectum → anus. (Full correct order = 5; minor omission = 4; several errors = 1–2.)

(b) (3 marks)

• Starch digestion occurs in the mouth (salivary amylase) (1) and in the small intestine (pancreatic amylase) (1).
• It pauses in the stomach because the acidic pH denatures/inactivates salivary amylase (1).

(c) (4 marks)

• Peristalsis = waves of contraction and relaxation of circular (and longitudinal) smooth muscle in the gut wall (1) that push food along in a wave-like motion (1).
• Muscle contracts behind the bolus and relaxes in front, squeezing it forward (1).
• Because peristalsis is muscular (not gravity-dependent), food is pushed to the stomach even against gravity / upside down (1).

Question 4 (12 marks)

(a) (4 marks)

• Glucose is absorbed by active transport / secondary active transport (co-transport) across the villus epithelium (1).
• Sodium ions are pumped out of the epithelial cells (Na⁺/K⁺ pump) creating a Na⁺ gradient (1); glucose is co-transported into the cell with sodium using this gradient (1); glucose then passes into the blood capillaries (facilitated diffusion) (1).

(b) (3 marks) $\text{Area} = 0.5 \times 30 \times 20 = 300 \text{ m}^2$

• Multiply flat area by villi factor (1) then by microvilli factor (1); correct answer 300 m² (1).

(c) (3 marks)

• Sodium absorption (with glucose) raises solute concentration inside the epithelial cells/blood (1); this lowers the water potential there (1); water follows by osmosis from the gut lumen into the body — so salt drives water absorption (1).

(d) (2 marks)

• Plain distilled water lacks solutes to drive co-transport/osmotic gradient (1); the sodium–glucose solution maximises water uptake, so rehydration is faster/more effective with the salt solution (1).

Question 5 (10 marks)

(a) (3 marks)

• The cow (herbivore) has a longer small intestine relative to body size (1); plant material/cellulose is harder to digest and needs longer time and surface for digestion/absorption (1); the lion's protein/fat diet is digested more easily so a shorter gut suffices (1).

(b) (4 marks)

• Mechanical digestion = physical breakdown into smaller pieces without changing chemical structure (1); mouth example = chewing/mastication by teeth (1).
• Chemical digestion = enzymes breaking chemical bonds to change molecules (1); mouth example = salivary amylase breaking starch into maltose (1).

(c) (3 marks)

• The gallbladder normally stores and releases a large bolus of bile when fatty food arrives (1); without it, only a small continuous trickle of bile is available (1); so a very fatty meal may not be fully emulsified → reduced fat digestion/absorption for large fat loads (1).

[
  {"claim":"Total effective absorptive surface area = 300 m^2 (Q4b)","code":"area=0.5*30*20; result = (area==300)"},
  {"claim":"Villi multiply base area by 30 giving 15 m^2 before microvilli","code":"result = (0.5*30==15)"},
  {"claim":"Q2 sample C digests 3 macromolecule types (starch,protein,fat)","code":"digested={'starch','protein','fat'}; result = (len(digested)==3)"},
  {"claim":"Alimentary canal has 8 named major structures mouth to anus in Q3a","code":"path=['mouth','pharynx','oesophagus','stomach','small intestine','large intestine','rectum','anus']; result = (len(path)==8)"}
]