The Mammalian Digestive System

Q1, Sample Band 6 response (8 marks), annotated

Mouth (physical + chemical): Mastication physically breaks protein-containing food into smaller pieces, no chemical bond breaking occurs in protein at this stage. Salivary amylase is present but acts on starch only; there is no protease in saliva. [1, correct identification that mouth = physical only for protein]

Oesophagus: Peristalsis transports the bolus to the stomach; no digestion occurs. [0.5, transport only]

Stomach (physical + chemical): Churning physically reduces the bolus to chyme. Gastric glands secrete pepsinogen and hydrochloric acid (HCl). HCl (not an enzyme) activates pepsinogen to active pepsin and maintains pH ~1.5–3.5. Pepsin cleaves peptide bonds within protein chains, converting proteins to shorter polypeptides. [2, pepsin, substrate, product, pH; HCl activation]

Small intestine (chemical): Sodium bicarbonate from the pancreas neutralises chyme (pH rises to ~7.5). Pancreatic trypsin and chymotrypsin (pH optimum ~8) continue cleaving polypeptides into shorter peptide fragments. Peptidases on the brush border of the intestinal epithelium complete digestion, cleaving short peptides to individual amino acids. [2, trypsin/chymotrypsin, peptidases, location, products, pH]

Why pepsin is inactive in the mouth: The mouth is neutral (pH ~7). Pepsin's active site is shaped by the tertiary structure of the protein, which is stable only at pH ~2. At pH 7 the active site is not in its functional conformation; pepsin (if present) would have negligible activity, the data shows 2% at pH 7 and 0% at pH 8. [1, pH-active site shape explanation for mouth]

Why pepsin is inactive in the small intestine: When chyme is neutralised by NaHCO₃, pH rises to ~7.5. The altered pH disrupts the hydrogen bonds and ionic interactions that maintain pepsin's active-site shape, the active site denatures. This denaturation is irreversible; pepsin cannot refold into its active conformation when pH returns to ~7. [1, irreversible denaturation in SI linked to neutralisation]

Final absorbable form and why: Proteins must be fully hydrolysed to individual amino acids before absorption. Only small, soluble monomers can be transported across the intestinal epithelial membrane into the bloodstream via specific carrier proteins. Large polypeptides are too large to cross the membrane. [1, amino acids as only absorbable form; membrane permeability reasoning]

Marking criteria.

• 1 markStates that no chemical digestion of protein occurs in the mouth (salivary amylase acts on starch; mastication is physical only).
• 2 marksNames pepsin, states it is activated by HCl from pepsinogen, identifies substrate (proteins) and product (polypeptides), and gives pH ~1.5–3.5 for stomach.
• 2 marksNames trypsin and/or chymotrypsin (small intestine, pH ~8) and peptidases (brush border), identifying their substrates and the final product as amino acids.
• 1 markExplains pepsin inactivity in the mouth using pH and active site shape (tertiary structure not in functional conformation at pH 7).
• 1 markExplains pepsin denaturation in the small intestine as irreversible change to active site due to pH neutralisation by NaHCO&sub3;.
• 1 markStates that amino acids are the only absorbable form and explains that only small soluble molecules can cross the intestinal membrane.

Q2, Sample Band 6 response (9 marks), annotated

Lost functions when pancreatic secretions are absent: The pancreas secretes (a) NaHCO₃required to neutralise acidic chyme and raise duodenum pH to ~7.5; (b) pancreatic amylase, starch → maltose; (c) pancreatic lipase, triglycerides → fatty acids + glycerol; (d) trypsin and chymotrypsin, polypeptides → shorter peptides. All four are lost. [2, all four functions correctly identified]

Steatorrhoea (fatty stools): Without pancreatic lipase, fat (triglycerides) cannot be chemically digested into fatty acids and glycerol. Fat is also not emulsified as effectively without the alkaline environment for bile activity (NaHCO₃ also needed). Undigested fat passes through the small intestine into faeces, producing pale, oily, foul-smelling stools. [2, lipase absent + fat undigested + fatty stools]

Amino acid deficiency: Without trypsin/chymotrypsin and the alkaline environment they require, polypeptides produced by pepsin in the stomach are not broken down to amino acids. Pepsin (stomach) is still functional, it converts proteins to polypeptides, but complete proteolysis requires the pancreatic enzymes; only partial protein digestion occurs. Some amino acid deficiency results. [1, partial protein digestion; pepsin still works; pancreatic proteases absent]

Fat-soluble vitamin deficiency (A, D, E, K): Fat-soluble vitamins are absorbed together with dietary fats via the same pathway. Since fat digestion is severely impaired (no lipase), the fat-soluble vitamins are not incorporated into micelles in the small intestine and cannot be absorbed, they are lost in faeces along with the undigested fat. [1, fat-soluble vitamins lost because fat absorption is impaired]

Why blood glucose is normal: Carbohydrate digestion has two pathways: salivary amylase (mouth, still functional) begins starch → maltose; and maltase, sucrase, and lactase (intestinal brush border, not pancreatic) complete monosaccharide release. Pancreatic amylase is lost but salivary amylase + intestinal enzymes together can still produce sufficient glucose for absorption. Blood glucose therefore remains normal. [2, salivary amylase still active; intestinal disaccharidases not pancreatic; glucose normal]

Distinction from coeliac patient: In the coeliac patient (lesson Card 1), digestion itself is intact, all enzymes function normally and food is broken down to absorbable monomers. The failure is in absorption: flattened villi reduce the absorptive surface area so nutrients cannot enter the bloodstream. In this EPI patient, the failure is in digestion: food is not broken down to absorbable units (especially fat), so the products of chemical digestion are never available to be absorbed regardless of villus integrity. [1, digestion failure (EPI) vs absorption failure (coeliac) correctly distinguished]

Marking criteria.

• 2 marksCorrectly identifies at least three of the four pancreatic functions lost: NaHCO₃ (pH neutralisation), pancreatic lipase, trypsin/chymotrypsin, pancreatic amylase.
• 2 marksAccounts for steatorrhoea: no pancreatic lipase → triglycerides undigested → fat in faeces (and/or no NaHCO₃ → suboptimal lipase environment; emulsification affected).
• 1 markAccounts for amino acid deficiency: trypsin/chymotrypsin absent → polypeptides from pepsin not fully hydrolysed to amino acids (pepsin still active).
• 1 markAccounts for fat-soluble vitamin deficiency: vitamins A, D, E, K require fat absorption pathway; fat not digested → vitamins lost in faeces.
• 2 marksExplains normal blood glucose: salivary amylase (still active, begins starch → maltose) + intestinal brush-border enzymes (maltase, sucrase, lactase, NOT pancreatic) complete carbohydrate digestion to monosaccharides.
• 1 markCorrectly distinguishes EPI (failure in chemical digestion, products never formed) from coeliac disease (failure in absorption, products formed but cannot enter bloodstream due to villous atrophy).

Q3, Sample Band 6 response (7 marks)

The claim is largely incorrect. [1, judgement]

What is partially defensible: It is technically true that enzymes would eventually digest a large food piece if given unlimited time, they can work on the outer surface. However, the word "eventually" obscures the biological reality: without physical digestion, the rate of chemical digestion would be so slow that the intestinal transit time would be far too short for complete digestion, and nutrients would pass through unabsorbed. [1, acknowledges the partially correct element (enzymes do work) but explains the rate issue]

What is incorrect, surface area: Enzymes can only act on substrate molecules at the surface of food particles. A 1 cm cube of food has 6 cm² of surface area; chewing it into eight 0.5 cm cubes doubles the surface area to 12 cm². At microscopic particle sizes the increase is orders of magnitude greater. Without mastication, the enormous interior volume of food is inaccessible to enzyme molecules. [2, surface area principle with example; enzyme access to interior]

What is incorrect, bile emulsification: Fat in large globules cannot be efficiently digested by lipase because the hydrophobic interior is inaccessible. Bile emulsification, a physical process, is not redundant; it is biologically essential for fat digestion to occur at any practical rate. [1, emulsification as non-redundant physical process]

What is incorrect, "evolutionary leftovers": Physical digestion processes (chewing, churning, emulsification) are present in virtually all vertebrate digestive systems and are highly conserved, this is strong evidence that they provide a functional advantage that selection has maintained. They are not vestigial. [1, evolutionary argument refuted]

Accurate reformulation: "Physical digestion is essential to chemical digestion: by breaking food into smaller particles and emulsifying fats, physical digestion dramatically increases the surface area of food accessible to digestive enzymes. Without physical digestion, enzyme-catalysed hydrolysis would proceed at a rate too slow for complete digestion within normal gut transit time, and nutrients would not be fully absorbed." [1, defensible reformulation linking physical to chemical digestion via surface area and rate]

Marking criteria.

• 1 markStates an overall evaluative judgement (e.g. "the claim is largely incorrect / biologically unsound").
• 1 markAcknowledges the partial truth (enzymes can act) but correctly identifies the critical limitation: rate of digestion, not eventual possibility.
• 2 marksCorrectly explains the surface area argument using a specific example (lesson's 1 cm cube example or equivalent) and links it to enzyme accessibility to substrate.
• 1 markCorrectly identifies bile emulsification as a physically essential process for fat digestion that cannot be replaced by chemical digestion alone.
• 1 markCorrectly refutes the "evolutionary leftovers" assertion using functional or evolutionary conservation reasoning.
• 1 markProvides a scientifically accurate reformulation of the claim that explicitly links physical digestion to chemical digestion via surface area and rate of hydrolysis.