Multiple Choice
1. B Fatty acids are reassembled into triglycerides and packaged into chylomicrons inside the enterocyte. Chylomicrons are too large (~80–1200nm) to squeeze through the tight junctions of blood capillary endothelium. Lacteals have looser endothelial junctions that accommodate chylomicron entry. The liver bypass is a consequence of the lymphatic route, not the reason for it.
2. D SGLT1 is secondary active transport, it doesn't directly use ATP. Instead, Na⁺/K⁺ ATPase pumps on the basolateral membrane continuously pump Na⁺ out of the enterocyte, maintaining a low intracellular Na⁺ concentration. The resulting electrochemical gradient drives Na⁺ into the cell via SGLT1, and glucose is co-transported in the same direction.
3. A Amino acids follow the same route as glucose: enterocyte → villus capillary → hepatic portal vein → liver (first pass) → hepatic vein → vena cava → systemic circulation. The hepatic portal vein is the key distinguishing feature from fat absorption.
4. C The large intestine reabsorbs approximately 1.5L water per day. Diarrhoea accelerates transit time, leaving insufficient time for water reabsorption, large volumes of water pass through in liquid faeces. Drinking cannot keep pace because water must still be absorbed from the small intestine and this too may be impaired in conditions causing diarrhoea.
5. C Fatty acids enter the enterocyte normally by simple diffusion (they are lipid-soluble and cross membranes directly without transport proteins). The problem occurs inside the enterocyte, without triglyceride assembly, chylomicrons cannot form, and without chylomicrons, fats cannot enter the lacteal. Fat would accumulate inside enterocytes or be lost.
Q6, Model Answer
At the macroscopic scale, the inner wall of the small intestine is folded into plicae circulares, large circular folds that triple the surface area compared to a smooth tube and slow chyme transit, increasing contact time between digestive contents and the absorptive surface.
At the tissue scale, each plica is covered in villi, finger-like projections approximately 0.5–1.6mm tall extending into the lumen. Each villus contains a capillary network and a lacteal. The villus structure increases surface area by approximately 10 times compared to the plica surface alone, and positions transport proteins close to the lumen where nutrients are present.
At the cellular scale, each enterocyte on the villus surface has its own surface covered in microvilli, tiny projections forming the brush border visible only by electron microscopy. The brush border increases absorptive surface area by approximately 20 times compared to a flat cell surface, and is the location of the transport proteins (SGLT1, GLUT2, amino acid transporters) responsible for moving nutrients into enterocytes.
The combined effect of all three levels of folding produces a total absorptive surface area of approximately 250m², sufficient to absorb the full range of nutrients from a typical daily diet.
Q7, Model Answer
Glucose crosses the brush border into enterocytes via SGLT1 (sodium-glucose co-transport, secondary active transport driven by the Na⁺ gradient) and exits into the villus blood capillary via GLUT2 (facilitated diffusion). Capillaries drain into the hepatic portal vein, which carries glucose directly to the liver before it enters systemic circulation, the liver gets first pass and can store glucose as glycogen or allow it to pass through depending on blood glucose levels.
In contrast, fatty acids enter the enterocyte by simple diffusion (they are lipid-soluble and cross the membrane directly without transporters). Inside the enterocyte they are reassembled into triglycerides and packaged into chylomicrons by the Golgi apparatus. Chylomicrons are too large to enter blood capillaries and instead enter the lacteal, the lymph vessel in the villus core. They travel via the lymphatic system and thoracic duct into the left subclavian vein, entering systemic circulation near the heart and bypassing the liver entirely on first pass.
Q8, Model Answer
The large intestine reabsorbs approximately 1.3–1.8 litres of water per day. The mechanism is osmotic, Na⁺ ions are actively pumped from the colon lumen into the bloodstream by Na⁺/K⁺ ATPase pumps in the colon epithelium. This lowers the water potential of the blood and raises it in the colon lumen, causing water to move by osmosis from lumen to blood down its water potential gradient. Electrolytes (Cl⁻, K⁺) are also reabsorbed, further driving osmotic water movement.
When this process is disrupted, for example in diarrhoea caused by infection or gut motility disorders, intestinal contents pass through the colon too quickly for sufficient water reabsorption to occur. Large volumes of water remain in the faeces and are eliminated. This causes dehydration because more water is lost through the gut than can be replaced by drinking, particularly in severe cases where the infection also impairs small intestinal absorption.