Absorption of Digested Products — Revision Notes

⚡ 30-Second Revision

Primary Site: — Small Intestine (duodenum, jejunum, ileum).
Surface Area Boosters: — Plicae circulares, Villi, Microvilli (Brush Border).
Carbohydrates (Monosaccharides):

* Glucose, Galactose: Active transport (SGLT1 with $\text{Na}^+$) into enterocytes; Facilitated diffusion (GLUT2) into blood. * Fructose: Facilitated diffusion (GLUT5) into enterocytes; Facilitated diffusion (GLUT2) into blood.

Proteins (Amino Acids, Di/Tripeptides): — Active transport into enterocytes; then into blood.
Fats (Monoglycerides, Fatty Acids):

* Form Micelles with bile salts. * Diffuse into enterocytes. * Re-esterified to Triglycerides. * Form Chylomicrons (Triglycerides + Protein). * Enter Lacteals (lymphatic capillaries), then lymphatic system, then blood.

Water: — Primarily by Osmosis (small and large intestine).
Electrolytes: — Active transport ($\text{Na}^+$), passive diffusion ($\text{Cl}^-$), regulated ($\text{Ca}^{2+}$, Fe).
Vitamins:

* Fat-soluble (A, D, E, K): With fats, via micelles and chylomicrons. * Water-soluble (B, C): Diffusion/Active transport. * Vitamin $\text{B}_{12}$: Requires Intrinsic Factor (from stomach) for absorption in terminal ileum.

2-Minute Revision

Absorption is the transfer of digested nutrients from the GI tract into the blood or lymph, predominantly occurring in the small intestine. This organ is highly specialized with plicae circulares, villi, and microvilli, vastly increasing its surface area.

Carbohydrates are absorbed as monosaccharides: glucose and galactose use $\text{Na}^+$-dependent active transport (SGLT1) into enterocytes, then facilitated diffusion (GLUT2) into blood. Fructose uses facilitated diffusion (GLUT5) into enterocytes, then GLUT2 into blood.

Proteins are absorbed as amino acids and small peptides via active transport. Fats are unique: monoglycerides and fatty acids form micelles with bile salts, diffuse into enterocytes, re-esterify into triglycerides, form chylomicrons, and then enter lacteals (lymphatic capillaries) before reaching the bloodstream.

Water is absorbed by osmosis, and electrolytes by various active and passive mechanisms. Fat-soluble vitamins follow the fat absorption pathway, while most water-soluble vitamins use diffusion or active transport.

Crucially, Vitamin $\text{B}_{12}$ requires intrinsic factor for its absorption in the terminal ileum. The large intestine primarily absorbs remaining water and electrolytes.

5-Minute Revision

The absorption of digested products is the vital process where the end products of digestion cross the intestinal lining into the circulatory or lymphatic systems. The small intestine is the primary site, structurally optimized with circular folds (plicaecirculares), finger-like villi, and microscopic microvilli (brush border) to create an immense surface area. Each villus contains a rich capillary network for blood-borne nutrients and a central lacteal for fat absorption.

Carbohydrate Absorption: Monosaccharides are the absorbable forms. Glucose and galactose are absorbed into enterocytes by secondary active transport, co-transported with $\text{Na}^+$ via the SGLT1 protein. From the enterocytes, they move into the blood capillaries by facilitated diffusion via the GLUT2 transporter. Fructose, however, is absorbed solely by facilitated diffusion, using GLUT5 to enter the enterocytes and GLUT2 to exit into the blood.

Protein Absorption: Proteins are broken down into amino acids, dipeptides, and tripeptides. Amino acids are absorbed by various active transport systems, many of which are $\text{Na}^+$-dependent. Dipeptides and tripeptides are absorbed more rapidly than free amino acids, primarily by $\text{H}^+$-dependent active transport (PEPT1) into enterocytes, where they are further hydrolyzed into amino acids before entering the bloodstream.

Fat Absorption: This is a distinct pathway. Digested fats (monoglycerides and fatty acids) are insoluble in water. Bile salts emulsify them and form tiny, water-soluble structures called micelles.

Micelles transport these lipids to the brush border, where monoglycerides and fatty acids diffuse into the enterocytes. Inside, they are re-esterified to triglycerides, which are then packaged with cholesterol, phospholipids, and proteins to form chylomicrons.

Chylomicrons are too large for blood capillaries; they are exocytosed into the lacteals (lymphatic capillaries) within the villi, eventually reaching the blood via the lymphatic system.

Water, Electrolytes, and Vitamins: Water is absorbed primarily by osmosis, following the osmotic gradient created by solute absorption, occurring in both small and large intestines. Electrolytes like $\text{Na}^+$ are actively absorbed, often coupled with glucose or amino acid transport.

$\text{Ca}^{2+}$ absorption is regulated by Vitamin D. Fat-soluble vitamins (A, D, E, K) are absorbed with dietary fats via micelles and chylomicrons. Most water-soluble vitamins (B, C) are absorbed by diffusion or active transport.

A crucial exception is Vitamin $\text{B}_{12}$, which requires intrinsic factor (secreted by gastric parietal cells) for its absorption in the terminal ileum. The large intestine's main role in absorption is for residual water, electrolytes, and some bacterially produced vitamins.

Prelims Revision Notes

Absorption of Digested Products: NEET Quick Recall

I. Sites of Absorption:

Small Intestine: — Primary site for almost all digested nutrients (carbohydrates, proteins, fats, most water, electrolytes, vitamins).

* Duodenum: Some iron, calcium, fats. * Jejunum: Bulk of nutrient absorption (carbohydrates, proteins, fats, water-soluble vitamins). * Ileum: Bile salts, Vitamin $\text{B}_{12}$, remaining nutrients.

Stomach: — Minimal absorption (alcohol, some drugs like aspirin, small amounts of water).
Large Intestine: — Water, electrolytes ($\text{Na}^+$, $\text{Cl}^-$), Vitamin K, Biotin, Folic acid (produced by gut bacteria).

II. Structural Adaptations of Small Intestine:

Plicae Circulares (Valves of Kerckring): — Large circular folds of mucosa and submucosa (3x increase in surface area).
Villi: — Finger-like projections of mucosa (10x increase).

* Contain: Capillary network (for blood-borne nutrients), Lacteal (for fat absorption).

Microvilli (Brush Border): — Microscopic projections on enterocytes (20x increase).

* Contain: Brush border enzymes for final digestion and transporters for absorption.

Total surface area: ~200-300 $\text{m}^2$.

III. Mechanisms of Absorption:

Passive Diffusion: — Down concentration gradient, no energy. (e.g., small fatty acids, alcohol, some drugs, water).
Facilitated Diffusion: — Down concentration gradient, carrier protein, no energy. (e.g., Fructose via GLUT5).
Active Transport: — Against concentration gradient, requires ATP, carrier protein. (e.g., Glucose via SGLT1, Amino acids, $\text{Na}^+$, $\text{Ca}^{2+}$, Fe).

* Primary Active Transport: Directly uses ATP (e.g., $\text{Na}^+$/$\text{K}^+$ pump). * Secondary Active Transport: Uses energy from an ion gradient (e.g., SGLT1 for glucose, co-transported with $\text{Na}^+$).

Osmosis: — Passive movement of water across a semi-permeable membrane.
Endocytosis (Pinocytosis): — Engulfment by cell membrane (e.g., Vitamin $\text{B}_{12}$-Intrinsic Factor complex).

IV. Absorption of Specific Nutrients:

Carbohydrates: — Absorbed as monosaccharides.

* Glucose, Galactose: SGLT1 (active, $\text{Na}^+$-dependent) into enterocyte; GLUT2 (facilitated) into blood. * Fructose: GLUT5 (facilitated) into enterocyte; GLUT2 (facilitated) into blood.

Proteins: — Absorbed as amino acids, dipeptides, tripeptides.

* Amino acids: Various active transport systems (some $\text{Na}^+$-dependent). * Di/Tripeptides: PEPT1 (active, $\text{H}^+$-dependent) into enterocyte, then hydrolyzed to amino acids.

Fats (Lipids):

* Digested to monoglycerides & fatty acids. * Form Micelles with bile salts (transport to brush border). * Diffuse into enterocytes. * Re-esterified to Triglycerides. * Packaged into Chylomicrons (Triglycerides + protein). * Released into Lacteals (lymphatic capillaries) -> Lymphatic system -> Bloodstream.

Water: — 80-90% in small intestine, rest in large intestine, by osmosis.
Electrolytes: — $\text{Na}^+$ (active), $\text{Cl}^-$ (passive/active), $\text{K}^+$ (passive), $\text{Ca}^{2+}$ (regulated by Vitamin D), Fe (regulated).
Vitamins:

* Fat-soluble (A, D, E, K): Absorbed with fats via micelles and chylomicrons. * Water-soluble (B, C): Simple/facilitated diffusion or active transport. * **Vitamin $\text{B}_{12}$: Requires Intrinsic Factor** (from gastric parietal cells) for absorption in terminal ileum.

Vyyuha Quick Recall

To remember the fat absorption pathway: My Dear Really Cute Little Lamb.

Micelles: Formed with bile salts.
Diffuse: Into enterocytes.
Re-esterified: To triglycerides.
Chylomicrons: Formed with protein coating.
Lacteals: Enter here.
Lymphatic: Travel through lymphatic system.
Liver (eventually): Reach systemic circulation, then liver.