3.3.4 Mechanisms of Absorption

Absorption of Glucose and Amino Acids:

1. Co-Transport Mechanism:

• Glucose and amino acids are absorbed into epithelial cells using the sodium ion (Na⁺) gradient.
• Process:

1. Sodium ions are actively transported out of epithelial cells into the blood using the sodium-potassium pump.
2. This creates a low Na⁺ concentration inside the epithelial cell.
3. Sodium ions move back into the epithelial cell from the ileum via co-transport proteins, carrying either glucose or amino acids with them.
4. Glucose and amino acids then move into the bloodstream via facilitated diffusion.

Absorption of Monoglycerides and Fatty Acids:

1. Diffusion into Epithelial Cells:

• Monoglycerides and fatty acids are lipid-soluble, so they easily diffuse through the phospholipid bilayer of the epithelial cell membrane.

2. Reformation into Triglycerides:

• Inside the epithelial cells, monoglycerides and fatty acids are transported to the endoplasmic reticulum, where they are reassembled into triglycerides.

3. Formation of Chylomicrons:

• Triglycerides associate with cholesterol and proteins to form chylomicrons.
• Chylomicrons are packaged into vesicles and leave the epithelial cells via exocytosis.

4. Transport into the Lymphatic System:

• Chylomicrons enter the lacteals (lymphatic vessels in the villi), where they are transported via the lymphatic system before entering the bloodstream.

Absorption of Water and Mineral Ions:

1. Water:

• Absorbed by osmosis into epithelial cells, following the concentration gradient of dissolved solutes.

2. Mineral Ions:

• Absorbed by active transport or facilitated diffusion, depending on the ion.

Adaptations of the Ileum for Absorption:

1. Large Surface Area:

• The ileum contains villi and microvilli, increasing the surface area for absorption.

2. Thin Epithelium:

• Reduces the diffusion distance for efficient transport of molecules.

3. Rich Blood Supply:

• Maintains a steep concentration gradient by transporting absorbed nutrients away rapidly.

4. Presence of Carrier Proteins:

• Enables specific transport mechanisms like co-transport and facilitated diffusion.

5. Lacteals:

• Found in the villi to transport lipids via the lymphatic system.

Summary of Transport Mechanisms:

Nutrient	Transport Mechanism
Glucose	Co-transport with Na⁺, facilitated diffusion
Amino Acids	Co-transport with Na⁺, facilitated diffusion
Fatty Acids & Monoglycerides	Diffusion, reassembly, exocytosis (via chylomicrons)
Water	Osmosis
Mineral Ions	Active transport or facilitated diffusion