Absorption of Carbohydrates — Revision Notes

⚡ 30-Second Revision

Final Products — Glucose, Fructose, Galactose (monosaccharides).
Primary Site — Small Intestine (Jejunum).
Apical Membrane (Lumen $ ightarrow$ Enterocyte)

* Glucose & Galactose: SGLT1 (Secondary Active Transport, $Na^+$-dependent). * Fructose: GLUT5 (Facilitated Diffusion, $Na^+$-independent).

Basolateral Membrane (Enterocyte $ ightarrow$ Blood)

* All Monosaccharides: GLUT2 (Facilitated Diffusion).

Energy Source for SGLT1 — Electrochemical gradient of $Na^+$ (maintained by $Na^+/K^+$ ATPase).
Fate — Hepatic portal vein $ightarrow$ Liver (Fructose/Galactose converted to Glucose).

2-Minute Revision

Carbohydrate absorption is the crucial process of moving monosaccharides (glucose, fructose, galactose) from the small intestine into the bloodstream. This occurs primarily in the jejunum. On the apical (luminal) membrane of enterocytes, glucose and galactose are absorbed via SGLT1, a sodium-glucose linked transporter.

This is a secondary active transport mechanism, meaning it uses the energy from the sodium ion gradient, which is maintained by the Na+/K+ ATPase pump on the basolateral membrane. Fructose, however, enters the enterocyte via GLUT5, a facilitated diffusion transporter, which does not require sodium or direct ATP, relying solely on its concentration gradient.

Once inside the enterocyte, all three monosaccharides (glucose, fructose, galactose) exit the cell across the basolateral membrane into the interstitial fluid and then the capillaries via GLUT2, another facilitated diffusion transporter.

From the capillaries, they travel via the hepatic portal vein to the liver, where fructose and galactose are largely converted into glucose for systemic use or storage.

5-Minute Revision

The absorption of carbohydrates is the final step in their utilization, involving the uptake of monosaccharides—glucose, fructose, and galactose—from the small intestinal lumen into the enterocytes and subsequently into the portal circulation. This process is highly efficient due to the extensive surface area of the small intestine, provided by villi and microvilli.

Apical Membrane Transport (Lumen to Enterocyte):

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Glucose and Galactose — These are absorbed primarily by the Sodium-Glucose Linked Transporter 1 (SGLT1). This is a secondary active transport mechanism. SGLT1 co-transports one glucose (or galactose) molecule along with two sodium ions ($Na^+$). The energy for moving glucose/galactose against its concentration gradient comes from the electrochemical gradient of $Na^+$. This $Na^+$ gradient is maintained by the **$Na^+/K^+$ ATPase pump** on the basolateral membrane, which actively pumps $Na^+$ out of the cell, keeping intracellular $Na^+$ low. Thus, SGLT1 is indirectly ATP-dependent.
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Fructose — Fructose is absorbed by Glucose Transporter 5 (GLUT5). This is a facilitated diffusion process, meaning it does not require $Na^+$ ions or direct ATP hydrolysis. Fructose moves down its concentration gradient from the lumen into the enterocyte.

Basolateral Membrane Transport (Enterocyte to Blood):

Once inside the enterocyte, all three monosaccharides (glucose, fructose, and galactose) are transported out of the cell into the interstitial fluid and then into the capillaries via Glucose Transporter 2 (GLUT2). This is also a facilitated diffusion mechanism, moving sugars down their concentration gradient into the bloodstream.

Fate of Absorbed Monosaccharides:

All absorbed monosaccharides enter the hepatic portal vein and are transported directly to the liver. In the liver, most fructose and galactose are converted into glucose or glucose derivatives. Glucose is then released into the systemic circulation for energy use by various tissues or stored as glycogen. Understanding these specific transporters, their locations, and the type of transport mechanism is crucial for NEET.

Prelims Revision Notes

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Absorbable Forms — Only monosaccharides (glucose, fructose, galactose) are absorbed. Disaccharides (maltose, sucrose, lactose) and polysaccharides (starch) must be hydrolyzed first.
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Primary Site — Small intestine, mainly jejunum, due to high surface area (villi, microvilli).
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Enterocytes — Intestinal epithelial cells with distinct apical (luminal) and basolateral (blood-facing) membranes.
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Apical Membrane Transporters (Lumen $ ightarrow$ Enterocyte)

* Glucose & Galactose: Transported by SGLT1. * Mechanism: Secondary Active Transport (co-transport/symport). * Requirement: Requires $Na^+$ ions. $Na^+$ moves down its electrochemical gradient, pulling glucose/galactose against its gradient.

* Energy: Indirectly ATP-dependent, as the $Na^+$ gradient is maintained by the $Na^+/K^+$ ATPase pump. * Fructose: Transported by GLUT5. * Mechanism: Facilitated Diffusion. * Requirement: No $Na^+$ ions needed.

Moves down its concentration gradient. * Energy: No direct ATP expenditure.

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Basolateral Membrane Transporter (Enterocyte $ ightarrow$ Blood)

* All Monosaccharides (Glucose, Fructose, Galactose): Transported by GLUT2. * Mechanism: Facilitated Diffusion. * Requirement: No $Na^+$ ions needed. Moves down its concentration gradient. * Energy: No direct ATP expenditure.

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Role of $Na^+/K^+$ ATPase — Located on the basolateral membrane. It's a primary active transporter that pumps $3Na^+$ out and $2K^+$ in, consuming ATP. This maintains the low intracellular $Na^+$ concentration, creating the electrochemical gradient essential for SGLT1 activity.
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Post-Absorption Fate — Monosaccharides enter capillaries $ightarrow$ Hepatic Portal Vein $ightarrow$ Liver.

* Liver converts most fructose and galactose to glucose. * Glucose is released into systemic circulation for energy or stored as glycogen.

Vyyuha Quick Recall

For carbohydrate transporters, remember: Sweet Glucose Loves Two Na+ (SGLT1 for Glucose, Galactose, needs 2 Na+). Fruit Goes Lightly Up To 5 (Fructose uses GLUT5). Get Lost Under Two (All sugars exit via GLUT2).