Absorption in the Small Intestine

• How glucose, amino acids, fructose, fatty acids, and glycerol are absorbed through the intestinal wall.
• The difference between active transport, secondary active transport, and facilitated diffusion in absorption.
• Why fats take a different route (lacteals/lymph) compared to sugars and amino acids (blood capillaries → portal vein).
• Where specific nutrients are absorbed: which region of the small intestine handles what.
• The role of the large intestine in absorption.

Level 1 — The Journey from Gut Lumen to Blood

After digestion, nutrients are in the lumen of the small intestine as: monosaccharides (glucose, fructose, galactose), amino acids, fatty acids + monoglycerides, vitamins, minerals, and water. These must cross the intestinal epithelium (enterocytes) and enter either blood capillaries or lymph capillaries (lacteals) inside the villi.

The key principle:

• Water-soluble nutrients (sugars, amino acids, water-soluble vitamins, minerals) → blood capillaries → portal vein → liver.
• Lipid-soluble nutrients (fatty acids, fat-soluble vitamins A, D, E, K, cholesterol) → lymph capillaries (lacteals) → thoracic duct → blood (bypasses liver initially).

Level 2 — Transport Mechanisms, Chylomicrons, Regional Specialisation

Glucose and galactose absorption:

• Transported by SGLT1 (sodium-glucose linked transporter 1) — secondary active transport (co-transport with Na⁺).
• Na⁺ gradient is maintained by Na⁺/K⁺-ATPase on the basolateral side of enterocytes (this uses ATP → primary active transport).
• SGLT1 is on the apical (brush border) membrane of enterocytes.
• Exit from enterocyte into blood: via GLUT2 on the basolateral membrane (facilitated diffusion).

Fructose absorption:

• Transported by GLUT5 — facilitated diffusion (no energy directly required, no Na⁺ coupling).
• Exit from enterocyte: via GLUT2 on basolateral membrane.
• Fructose is more slowly absorbed than glucose.

Amino acid absorption:

• Multiple Na⁺-dependent secondary active transporters (different ones for neutral, acidic, basic, and branched-chain amino acids) on the apical membrane.
• Some di- and tripeptides are absorbed intact via PepT1 (H⁺-coupled peptide transporter) — more efficient than single amino acid transport.
• Exit: via amino acid transporters on basolateral membrane into portal blood.

Fatty acid and monoglyceride absorption:

• Fatty acids + monoglycerides are solubilised in bile salt micelles in the intestinal lumen.
• They diffuse across the apical membrane of enterocytes (simple diffusion — being lipid-soluble).
• Inside the enterocyte: re-esterified to triglycerides by smooth ER enzymes.
• Triglycerides packaged with cholesterol, phospholipids, and apolipoproteins into chylomicrons (large lipoprotein particles) in the Golgi.
• Chylomicrons exit the cell by exocytosis on the basolateral side → enter lacteals (lymph capillaries) → travel via lymph → thoracic duct → left subclavian vein → bloodstream.
• In blood, chylomicron triglycerides are hydrolysed by lipoprotein lipase (on capillary walls of adipose tissue and muscle).

Fat-soluble vitamins (A, D, E, K): absorbed with fats via chylomicrons (same route as dietary fat).

Water-soluble vitamins (B-complex, C): primarily by passive diffusion or carrier-mediated transport directly into blood capillaries.

Special case — Vitamin B₁₂ (cobalamin):

• Can only be absorbed in the terminal ileum.
• Requires intrinsic factor (a glycoprotein secreted by gastric parietal cells) — the intrinsic factor–B₁₂ complex binds to specific receptors in the ileum.
• Absence of intrinsic factor (e.g., after gastrectomy, or autoimmune destruction of parietal cells) → pernicious anaemia.

Minerals:

• Iron (Fe²⁺): active transport; absorbed mainly in duodenum; transported in blood as transferrin.
• Calcium (Ca²⁺): active transport dependent on vitamin D (calcitriol upregulates calbindin, a calcium-binding protein in enterocytes); absorbed in duodenum and jejunum.

Regional specialisation of the small intestine:

Large intestine:

• Absorbs water (1–1.5 L/day) and electrolytes (Na⁺, Cl⁻, K⁺).
• Absorbs some vitamins produced by gut bacteria (vitamin K, biotin, some B vitamins via microbial fermentation).
• Does NOT significantly absorb proteins, fats, or carbohydrates.
• Converts undigested matter into faeces.

NEET-style Question:
Which of the following correctly describes the absorption of fatty acids in the small intestine?
(A) Fatty acids are absorbed by secondary active transport (Na+-coupled) and enter blood capillaries directly.
(B) Fatty acids dissolve in bile salt micelles, diffuse into enterocytes, are re-esterified to triglycerides,
    packaged as chylomicrons, and enter lacteals.
(C) Fatty acids are absorbed by GLUT5 and transported to the liver via the portal vein.
(D) Short-chain fatty acids are packaged into chylomicrons; long-chain fatty acids enter blood directly.

Step 1 — Recall the fat absorption route
  Fatty acids (long-chain) are lipid-soluble → dissolve in micelles → simple diffusion into enterocytes.
  Inside enterocytes → re-esterified to triglycerides → packaged as chylomicrons (with cholesterol,
  phospholipids, apolipoproteins) → exit via exocytosis → enter lacteals (NOT blood capillaries).
  Travel via lymph → thoracic duct → left subclavian vein → blood.

Step 2 — Evaluate options
  (A) WRONG: Na+-coupled transport is for glucose/amino acids, not fatty acids.
      Fatty acids enter blood capillaries — WRONG (they enter lacteals).
  (B) CORRECT: describes the actual pathway accurately.
  (C) WRONG: GLUT5 is for fructose. Fats do NOT go via portal vein initially.
  (D) WRONG: It is the REVERSE — short-chain fatty acids (SCFAs) go directly to blood/portal vein;
      LONG-chain fatty acids are packaged into chylomicrons and go via lymph.

Answer: B

• Name the transporter (protein) responsible for glucose absorption from the intestinal lumen into enterocytes, and classify its transport mechanism.
• Describe the pathway of a triglyceride molecule from the intestinal lumen to the bloodstream, naming all major steps.
• What are chylomicrons? State their composition and the route they take to enter the bloodstream.
• Distinguish between the absorption routes of glucose vs. fructose (transporters, energy requirement).
• In which region of the small intestine is vitamin B₁₂ absorbed, and what is required for its absorption?
• Name two minerals that require active transport for absorption, and state what regulates each.
• Why does the large intestine absorb water but not proteins or fats?

• Figure 16.5 — Absorption of nutrients across the intestinal wall showing villi structure, blood capillaries, and lacteal; arrows indicating routes of different nutrient classes (NCERT Class 11, Chapter 16).
• Diagram of chylomicron formation inside enterocytes (smooth ER → Golgi → exocytosis → lacteal).
• Diagram distinguishing portal vein route (glucose, amino acids) vs. thoracic duct route (fats via lacteals) to the heart.
• Summary table of absorption mechanisms (active, secondary active, facilitated diffusion, simple diffusion) with examples of each nutrient.

• Name the transport protein for glucose absorption at the apical (brush border) membrane of enterocytes.
• True or False: Fructose is absorbed by Na⁺-coupled secondary active transport. (Answer: False — it uses GLUT5, facilitated diffusion)
• What are chylomicrons and which vessel do they first enter after absorption?
• In which region of the small intestine does most nutrient absorption occur? (Answer: Jejunum)
• Name two fat-soluble vitamins and state how they are absorbed (which route).
• What specific condition results from inability to absorb vitamin B₁₂ due to lack of intrinsic factor?
• Stretch: A drug blocks SGLT1 transporters in the intestinal brush border. Predict the effect on (a) glucose absorption, (b) galactose absorption, (c) fructose absorption, and (d) blood glucose levels after a meal rich in all three sugars. Explain each prediction.

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