Biology • Year 11 • Module 2 • Lesson 12
Absorption and Elimination
Lock in the key vocabulary and structural features of the small intestine: villi, microvilli, capillaries, lacteals, and the two absorption routes.
1. Three levels of surface area amplification
The small intestine increases its absorptive surface area at three scales. Complete the table below using information from Cards 1 and the Copy-into-your-books section. 9 marks
| Structure | Scale | What it is / description | SA multiplier |
|---|---|---|---|
| Plicae circulares | |||
| Villi | |||
| Microvilli (brush border) | |||
| Combined total surface area: | |||
2. Term–definition match
The ten definitions below are shuffled. Write the matching term from this list: absorption, villus, microvillus, enterocyte, capillary, lacteal, hepatic portal vein, chylomicron, large intestine, elimination. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 2.1 | The expulsion of undigested material (faeces) from the body via the rectum and anus. | |
| 2.2 | A finger-like projection of the small intestine wall that increases surface area for nutrient absorption. | |
| 2.3 | A lipoprotein particle assembled inside enterocytes that packages reassembled triglycerides for transport through the lymphatic system. | |
| 2.4 | The uptake of digested nutrients from the small intestine lumen into the bloodstream or lymphatic system. | |
| 2.5 | A lymph vessel in the centre of each villus that receives chylomicrons and channels them into the lymphatic system. | |
| 2.6 | The smallest blood vessel, with a wall one cell thick, that runs through each villus to receive absorbed glucose and amino acids. | |
| 2.7 | An absorptive epithelial cell lining the small intestine that transports nutrients across its membrane into blood or lymph. | |
| 2.8 | The blood vessel that carries nutrient-rich blood directly from the small intestine to the liver for first-pass processing. | |
| 2.9 | The section of the digestive tract responsible for water reabsorption and compaction of undigested material into faeces. | |
| 2.10 | A microscopic hair-like projection on the surface of each enterocyte, forming the brush border to further increase absorptive area. |
3. True or false, with correction
Circle T or F. If false, write the corrected version. 8 marks (1 for T/F, 1 for the correction where needed)
3.1 Fatty acids and glucose both enter the bloodstream directly via villus capillaries. T / F
3.2 SGLT1 uses secondary active transport, driven by the sodium gradient, to absorb glucose at the brush border. T / F
3.3 Absorbed fatty acids pass through the liver before entering systemic circulation. T / F
3.4 The large intestine reabsorbs approximately 1.5 litres of water per day by osmosis following active sodium pumping. T / F
4. Function recall
Answer each in 1–2 sentences using precise terms from the lesson. 10 marks (2 each)
4.1 What is the function of the lacteal in each villus?
4.2 Why does the enterocyte contain many mitochondria?
4.3 What is the function of the hepatic portal vein?
4.4 What is the role of gut bacteria (microbiome) in the large intestine?
4.5 Why are chylomicrons secreted into lacteals rather than capillaries?
5. Absorption route summary
Complete the table to summarise how each nutrient type travels from the intestinal lumen to systemic circulation. 8 marks
| Nutrient | Mechanism entering enterocyte | Vessel entered (capillary or lacteal) | Passes through liver first? (Y/N) |
|---|---|---|---|
| Glucose | |||
| Amino acids | |||
| Fatty acids | |||
| Water |
Q1, Three levels of SA amplification
Plicae circulares: Scale: mm–cm; large circular folds of the small intestine wall; SA multiplier: ×3.
Villi: Scale: ~1 mm; finger-like projections of the intestinal epithelium, each containing a capillary network and a lacteal; SA multiplier: ×10 additional.
Microvilli (brush border): Scale: 1–2 μm; hair-like projections on the surface of each enterocyte, visible only by electron microscopy, packed with SGLT1/GLUT2 transport proteins; SA multiplier: ×20 additional.
Combined total surface area: ~250 m² (roughly the size of a tennis court).
Q2, Term–definition matches
2.1 elimination • 2.2 villus • 2.3 chylomicron • 2.4 absorption • 2.5 lacteal • 2.6 capillary • 2.7 enterocyte • 2.8 hepatic portal vein • 2.9 large intestine • 2.10 microvillus.
Q3, True / false with correction
3.1 False. Correction: Glucose and amino acids enter capillaries, but fatty acids (as chylomicrons) enter lacteals and travel via the lymphatic system, not blood capillaries.
3.2 True. SGLT1 co-transports one glucose and two Na♠ ions from the lumen into the enterocyte; the Na♠ gradient is maintained by Na♠/K♠ ATPase on the basolateral membrane.
3.3 False. Correction: Absorbed fatty acids (as chylomicrons) travel via the lymphatic system and thoracic duct into the left subclavian vein, bypassing the liver on first pass. Glucose and amino acids pass through the liver first via the hepatic portal vein.
3.4 True.
Q4.1, Function of the lacteal
The lacteal is a lymph vessel at the core of each villus. It receives chylomicrons secreted by enterocytes after fat absorption and channels them into the lymphatic system, which ultimately drains via the thoracic duct into the bloodstream near the heart.
Q4.2, Why enterocytes have many mitochondria
Enterocytes require ATP to power the Na♠/K♠ ATPase pumps on the basolateral membrane that actively pump sodium out of the cell. This maintains the low intracellular Na♠ gradient that drives SGLT1 co-transport of glucose at the brush border. Without abundant mitochondria the Na♠ gradient would collapse and active glucose absorption would cease.
Q4.3, Function of the hepatic portal vein
The hepatic portal vein carries nutrient-rich blood (containing glucose, amino acids, and other absorbed nutrients) directly from the capillaries of the small intestine to the liver before it enters systemic circulation, giving the liver first access to regulate blood nutrient levels.
Q4.4, Role of gut bacteria in the large intestine
The gut microbiome ferments undigested material (mainly dietary fibre), producing short-chain fatty acids (absorbed as energy), gases (methane, CO&sub2;), and vitamins K and B12 that are absorbed in the colon. They also help break down material the host cannot digest.
Q4.5, Why chylomicrons enter lacteals, not capillaries
Chylomicrons are large lipoprotein particles (~80–1200 nm). Blood capillaries have tight endothelial junctions that prevent particles this large from entering. Lacteals (lymph vessels) have looser, more permeable walls with large gaps that allow chylomicrons to enter.
Q5, Absorption route summary
Glucose: SGLT1 secondary active transport (co-transport with Na♠) at brush border; exits via GLUT2 facilitated diffusion; enters capillary; passes through liver first: Yes.
Amino acids: Active transport via amino acid co-transporters at brush border; facilitated diffusion out basolateral membrane; enters capillary; passes through liver first: Yes.
Fatty acids: Simple diffusion through phospholipid bilayer (lipid-soluble); reassembled into triglycerides and packaged as chylomicrons in the Golgi apparatus; enters lacteal; passes through liver first: No (enters subclavian vein via thoracic duct).
Water: Osmosis (follows Na♠ gradient); enters capillary; passes through liver first: Yes (via portal circulation).