HSC Exam Practice

Sample response. Physical digestion involves the mechanical breakdown of food into smaller pieces without changing the chemical structure of molecules, no chemical bonds are broken. Example: mastication (chewing) in the mouth. Chemical digestion involves enzyme-catalysed hydrolysis that breaks covalent bonds within large food molecules, converting polymers into absorbable monomers. Example: pepsin cleaving peptide bonds in proteins in the stomach.

Marking notes. 1 mark, physical digestion defined (no bond breaking; mechanical); 1 mark, chemical digestion defined (bond breaking; enzyme/hydrolysis); 1 mark, one correct named example of each (with location). Both examples needed for this mark.

Sample response. The enzyme is pepsin. Its pH optimum is approximately pH 2. At this pH, the tertiary structure of pepsin, the precise three-dimensional folding of the protein, creates an active site with the exact complementary shape required to bind protein substrates (polypeptides). At higher pH values, the ionic and hydrogen bonds stabilising this structure are disrupted, the active site changes shape (denatures), and pepsin can no longer bind its substrate.

Marking notes. 1 mark, pepsin named; 1 mark, pH optimum ~2 stated; 1 mark, explanation links pH to active site shape/tertiary structure and enzyme function.

Sample response. Bile salts (produced by the liver, released from the gall bladder into the duodenum) emulsify fat globules, they break large fat globules into millions of tiny droplets, greatly increasing the surface area of fat accessible to pancreatic lipase. This role is physical digestion. It is classified as physical (not chemical) because bile does not break any chemical bonds within the fat molecules themselves: the triglyceride molecules remain chemically unchanged; they are merely dispersed into smaller droplets. No hydrolysis occurs.

Marking notes. 1 mark, correctly describes bile's role (emulsification; breaks fat globules into droplets; increases surface area for lipase); 1 mark, correctly classifies as physical digestion; 1 mark, correct justification that no chemical bonds in fat are broken / no hydrolysis occurs.

Sample response. Salivary amylase continues to act briefly after swallowing because the interior of the food bolus has not yet been exposed to stomach acid, pH in the bolus core remains close to neutral initially, within the enzyme's active pH range [1]. As the stomach churns the bolus and HCl mixes throughout, the pH rapidly drops toward 1.5–2.0 [1]. At approximately pH 3–4 and below, salivary amylase activity becomes negligible (the enzyme data shows only ~10% activity at pH 4 and ~5% at pH 2) and the enzyme is effectively denatured by the strongly acidic conditions, ceasing to function [1].

Marking notes. 1 mark, states that initial bolus interior is near-neutral, allowing amylase to remain briefly active; 1 mark, identifies that stomach churning + HCl mixes to lower pH throughout; 1 mark, states activity ceases when pH drops below ~4 (denaturation or loss of active-site conformation) with reference to pH.

Sample response. (a) Starch → final absorbable product: glucose (monosaccharide). (b) Protein → final absorbable product: amino acids. (c) Triglycerides → final absorbable products: fatty acids and glycerol (and monoglycerides).

Marking notes. 1 mark per correct product, maximum 3. Accept glucose only (not maltose) for starch. Accept glycerol and fatty acids for triglycerides. Accept monoglycerides as part of fat answer.

Sample response. Pepsin pH optimum: ~pH 2. Pancreatic lipase pH optimum: ~pH 8.

Marking notes. 1 mark for both optima correctly read from graph (±0.5 pH units). Do not award for only one.

Sample response. As pH increases from 2 to 7, pepsin activity decreases sharply and consistently from its maximum of 100% at pH 2 [1]. By pH 6 activity has fallen to approximately 10%, and at pH 7 activity is essentially zero (approximately 2%) [1]. Accept equivalent description with data values read from graph (±10%).

Marking notes. 1 mark, general trend (steady decrease/sharp decline) described with direction; 1 mark, at least one specific data point (value at pH 6 or 7) quoted from the graph.

Sample response. The graph shows pepsin activity is approximately 10% at pH 5 and effectively 0% at pH 6–7 [1]. Raising stomach pH from 2 to 5 therefore reduces pepsin activity to approximately 10% of its maximum, a 90% reduction, meaning that protein hydrolysis in the stomach proceeds at only about one-tenth the normal rate. Proteins would largely pass to the small intestine as intact chains rather than polypeptides, placing greater demand on trypsin and chymotrypsin in the small intestine [1].

Marking notes. 1 mark, uses specific graph data to quantify the change in pepsin activity at pH 5; 1 mark, explains the functional consequence (severely reduced protein digestion in the stomach).

Sample response. Both salivary amylase and pancreatic amylase evolved to catalyse the same reaction (starch → maltose) and have similar active site structures, so their pH optima are similar (~7.0 and ~7.5 respectively) [1]. However, they are secreted by different glands (salivary glands vs pancreas) and act at different sites because the digestive system uses separate populations of the enzyme at different points along the gut to ensure continued carbohydrate digestion, salivary amylase begins starch digestion in the mouth; pancreatic amylase continues it in the small intestine after the acidic stomach has denatured the salivary enzyme [1].

Marking notes. 1 mark, similar pH optima explained by similar active site / same reaction catalysed; 1 mark, different organs because salivary amylase is denatured in the stomach and pancreatic amylase continues digestion in the small intestine (sequential function / relay digestion).

Sample response. Starch digestion begins in the mouth. Teeth physically masticate (chew) the food into smaller particles, physical digestion. Simultaneously, salivary glands secrete salivary amylase (pH optimum ~7, matching mouth pH of 6.5–7.0), which catalyses the hydrolysis of starch (a polysaccharide) to maltose (a disaccharide). Physical digestion in the mouth supports chemical digestion: mastication dramatically increases the surface area of starchy food available for salivary amylase to contact, increasing the rate of hydrolysis. In the oesophagus, peristalsis transports the bolus to the stomach; no digestion occurs. In the stomach (pH ~1.5–3.5), salivary amylase is rapidly denatured by HCl, the low pH disrupts the tertiary structure of the enzyme's active site, rendering it inactive. Stomach churning physically reduces particle size further (physical digestion) but produces no chemical digestion of starch. In the small intestine (pH ~7.0–8.5, raised by NaHCO&sub3; secreted by the pancreas), pancreatic amylase (pH optimum ~7.5) continues the hydrolysis of remaining starch to maltose. Maltase, sucrase, and lactase, embedded in the brush border of the intestinal epithelium, then complete carbohydrate digestion: maltase cleaves maltose to glucose + glucose; sucrase cleaves sucrose to glucose + fructose; lactase cleaves lactose to glucose + galactose. Only the monosaccharide form (chiefly glucose) is absorbable: the intestinal epithelial membrane contains specific carrier proteins that transport individual monosaccharides into the cell, but polysaccharides and disaccharides are too large to cross the membrane. Complete hydrolysis to monosaccharides is therefore necessary for absorption to occur.

Marking criteria.

• 1 markMouth: mastication (physical) and salivary amylase (chemical), starch → maltose, pH ~7.
• 1 markSurface area explanation: mastication increases surface area → increases rate of amylase action (with quantitative or conceptual reasoning).
• 1 markStomach: churning (physical); salivary amylase denatured by HCl (low pH disrupts active site); no chemical digestion of starch in stomach.
• 1 markSmall intestine: NaHCO&sub3; raises pH; pancreatic amylase continues starch → maltose at pH ~7.5.
• 1 markBrush-border disaccharidases: maltase (maltose → glucose + glucose), sucrase and/or lactase named and products given.
• 1 markFinal absorbable product stated as glucose (monosaccharide).
• 1 markExplanation of why monosaccharide form is required: only small soluble monomers can be transported across the intestinal epithelial membrane via carrier proteins; larger molecules (polysaccharides, disaccharides) cannot cross.