HSC Exam Practice

Sample response. Absorption is the uptake of digested nutrients from the lumen of the small intestine into the bloodstream or lymphatic system. The majority of nutrient absorption occurs in the small intestine (specifically in the jejunum and ileum, with the duodenum absorbing iron, calcium, and some other nutrients).

Marking notes. 1 mark for defining absorption as uptake of nutrients from the intestinal lumen into blood or lymph. 1 mark for naming the small intestine (or jejunum/ileum) as the primary site. A response stating “stomach” or “large intestine” as the main site scores 0 for the second mark.

Sample response. A chylomicron is a large lipoprotein particle (~80–1200 nm) assembled inside enterocytes. Fatty acids and glycerol, once inside the enterocyte, are reassembled into triglycerides in the smooth endoplasmic reticulum, then packaged with cholesterol and protein into chylomicrons by the Golgi apparatus. Chylomicrons are too large to squeeze through the tight endothelial junctions of blood capillaries; instead they enter the lacteals (lymph vessels), which have looser endothelial walls with wider gaps.

Marking notes. 1 mark for identifying a chylomicron as a lipoprotein particle packaging reassembled triglycerides. 1 mark for naming smooth ER (reassembly) and Golgi (packaging) or equivalent organelle detail. 1 mark for explaining that capillary junctions are too tight for chylomicrons, must name lacteals as the alternative entry point.

Sample response. SGLT1 (sodium-glucose linked transporter 1) is located on the brush border membrane of enterocytes. It is a co-transporter that moves one glucose molecule into the cell simultaneously with two Na♠ ions, using the electrochemical gradient of Na♠ as the driving force (secondary active transport, no direct ATP use). The Na♠ gradient (low intracellular Na♠) is maintained by Na♠/K♠ ATPase pumps on the basolateral membrane, which continuously pump Na♠ out of the enterocyte into the blood using ATP.

Marking notes. 1 mark for describing SGLT1 as a sodium-glucose co-transporter on the brush border, moving glucose and Na♠ together. 1 mark for stating this is secondary active transport: Na♠ gradient (not direct ATP) drives glucose entry. 1 mark for stating Na♠/K♠ ATPase on the basolateral membrane maintains the low intracellular Na♠ gradient using ATP.

Sample response. The large intestine reabsorbs water by osmosis. Na♠ ions are actively pumped (Na♠/K♠ ATPase) from the colonic lumen into the bloodstream by epithelial cells. This lowers the water potential of the blood relative to the lumen, so water moves by osmosis from the lumen into the blood down its water potential gradient. When transit is abnormally slow (constipation), contents remain in the colon for longer, so more water is reabsorbed than normal. Faecal water content decreases below the normal ~25%, producing hard, dry faeces.

Marking notes. 1 mark for stating Na♠ is actively pumped from lumen to blood and water follows by osmosis. 1 mark for explaining the water potential gradient direction correctly. 1 mark for predicting that slow transit → excess water reabsorption → lower water content → hard faeces.

Sample response. Vitamin K: deficiency causes impaired blood clotting / increased bleeding (vitamin K is required to activate clotting factors). Vitamin B12: deficiency causes a form of anaemia (megaloblastic anaemia) and neurological damage because B12 is required for DNA synthesis and myelin sheath maintenance.

Marking notes. 1 mark per vitamin correctly named with a correct consequence of deficiency (max 2). A named vitamin without a consequence, or vice versa, scores 0 for that entry. Accept any biologically accurate consequence.

Sample response (a). The nutrient most affected by proximal resection (Group P) is iron (absorption ~28%, the lowest of the four nutrients in that group). The nutrient most affected by distal resection (Group D) is vitamin B12 (absorption ~22%, the lowest in that group).

Marking notes (a). 1 mark for correctly identifying iron as the nutrient most reduced in Group P. 1 mark for correctly identifying vitamin B12 as the nutrient most reduced in Group D. Data values not required but strengthen the answer.

Sample response (b). Group P had the duodenum and jejunum removed. Iron and calcium are absorbed primarily in the duodenum via specific transport proteins (e.g. DMT1 for iron) located on enterocytes of these proximal regions; glucose and amino acids are absorbed throughout the jejunum via SGLT1 and amino acid transporters; fatty acid absorption also begins in the duodenum and jejunum. Removing these regions eliminates the high-density absorptive epithelium, so glucose, iron, and fatty acid absorption all fall substantially, with iron being worst affected because the duodenum is its near-exclusive absorption site.

Group D had the ileum removed. Vitamin B12 is absorbed exclusively in the terminal ileum via a specialised mechanism requiring intrinsic factor (a protein secreted by the stomach that binds B12 and is recognised by ileal receptors). No other part of the small intestine can absorb B12; removing the ileum therefore eliminates B12 absorption almost entirely. Glucose, iron, and fatty acid absorption in Group D are relatively preserved because the duodenum and jejunum are intact and handle the bulk of these nutrients.

Marking notes (b). 1 mark for identifying that iron/glucose absorption is highest in the duodenum/jejunum (proximal SI) and explaining why Group P loses these. 1 mark for explaining that the duodenum is the primary site for iron absorption via specific transport proteins, hence the large drop in Group P. 1 mark for identifying that vitamin B12 is absorbed exclusively in the ileum (terminal SI) via intrinsic factor. 1 mark for explaining why Group D retains glucose/iron/fatty acid absorption (jejunum and duodenum intact) while losing B12 specifically.

Sample response. The small intestine must absorb glucose efficiently from a narrow tube, a challenge governed by the surface area to volume (SA:V) principle. Without structural specialisation, the internal surface area of the small intestine would be insufficient to absorb the quantity of glucose produced during digestion of a typical meal. Three levels of structural folding together amplify absorptive surface area approximately 600-fold compared to a smooth tube, giving a total of ~250 m².

At the macroscopic scale, plicae circulares are large circular folds of the intestinal wall that triple the internal surface area and slow the passage of chyme, increasing contact time between glucose and the absorptive epithelium. At the tissue scale, villi are finger-like projections (~0.5–1.6 mm) extending from each plica into the lumen, multiplying surface area a further 10-fold. Each villus contains a core capillary network and a lacteal, and is covered in absorptive enterocytes. At the cellular scale, microvilli on the surface of each enterocyte form the brush border (1–2 μm), amplifying surface area a further 20-fold. The brush border is densely packed with transport proteins.

Glucose is transported across the brush border membrane via SGLT1, a secondary active co-transporter that moves one glucose molecule into the enterocyte together with two Na♠ ions, driven by the Na♠ electrochemical gradient. This secondary active transport is necessary because glucose accumulates in the enterocyte during active absorption; once intracellular glucose exceeds lumenal concentration, simple diffusion would reverse. The Na♠/K♠ ATPase on the basolateral membrane continuously pumps Na♠ out of the cell, maintaining the low intracellular Na♠ that drives SGLT1.

Inside the enterocyte, glucose moves to the basolateral membrane, where it exits via GLUT2 by facilitated diffusion (no ATP required; glucose moves down its concentration gradient from cell to blood). Glucose enters the villus capillary network and is drained via the hepatic portal vein directly to the liver, which receives glucose first before it enters systemic circulation for distribution to all body tissues.

Marking notes. 1 mark, states SA:V principle and links to the need for structural adaptation. 1 mark, names all three levels (plicae, villi, microvilli) with approximate multipliers or combined area (~250 m²); explains how at least two increase absorption efficiency beyond simply “increasing SA”. 1 mark, names SGLT1 on the brush border and identifies it as a sodium-glucose co-transporter (secondary active transport). 1 mark, explains why active transport is needed (potential reverse glucose gradient when enterocyte is already loaded). 1 mark, states Na♠/K♠ ATPase on basolateral membrane maintains Na♠ gradient. 1 mark, names GLUT2 and states glucose exits via facilitated diffusion on the basolateral side. 1 mark, states glucose enters villus capillaries and is transported via the hepatic portal vein to the liver.