Digestion and Absorption of Nutrients (First part)

Carbohydrate Digestion and Absorption

• Starch Digestion:

  • Salivary Amylase:

    • Site of action: Mouth (oral cavity)

    • Action: Breaks down starch (polysaccharide) into oligosaccharides and disaccharides.

    • 50\% of starch is broken down by salivary amylase.

  • Pancreatic Amylase:

    • Site of action: Duodenum (small intestine).

    • Action: Continues to break down polysaccharides.

    • 50\% of starch is broken down by pancreatic amylase.

  • Brush Border Enzymes:

    • Site of action: Lumen of small intestine.

    • Action: Finish the breakdown of oligosaccharides and disaccharides into monosaccharides.

  • Monosaccharides Absorption:

    • Monosaccharides are absorbed into the blood.

Protein Digestion and Absorption

• Pepsinogen Activation:

  • Chief cells secrete pepsinogen in the stomach.

  • Parietal cells secrete hydrochloric acid (HCl).

  • HCl activates pepsinogen into pepsin: Pepsinogen + HCl \rightarrow Pepsin

• Pepsin Action:

  • Site of action: Stomach.

  • Action: Pepsin breaks down proteins into peptides.

• Pancreatic Enzymes:

  • Site of action: Duodenum.

  • Action: Further breakdown of peptides.

• Brush Border Enzymes:

  • Action: further break down the peptides into amino acids.

• Amino Acid Absorption:

  • Amino acids are absorbed into the blood.

Fat Digestion and Absorption

• Lingual and Gastric Lipase:

  • Site of action: Stomach.

  • Action: Start to break down fats.

• Bile:

  • Site of action: Duodenum.

  • Action: Emulsifies fats (large fat droplets into smaller fat droplets).

• Pancreatic Lipase:

  • Site of action: Duodenum.

  • Action: Breaks down fats further.

• Micelles:

  • Form in the small intestine lumen to transport fatty acids and monoglycerides to the absorptive cell.

  • Not enzymes but transport shells.

  • Micelles transport long-chain fatty acids and monoglycerides.

• Absorption Process:

  • Triglycerides are broken down into fatty acids and monoglycerides in the lumen.

  • Micelles transport fatty acids and monoglycerides to the absorptive cell surface.

  • Fatty acids and monoglycerides enter the absorptive cell.

  • Inside the absorptive cell, fatty acids and monoglycerides are repackaged into triglycerides.

  • Triglycerides are combined with cholesterol, phospholipids, and proteins to form chylomicrons.

  • Chylomicrons exit the absorptive cell via exocytosis into the interstitial fluid.

  • Chylomicrons enter the lacteals and are transported via the lymph.

• Chylomicrons:

  • Transport triglycerides, cholesterol, and phospholipids coated with proteins.

  • Move into the lymph.

Absorption Mechanisms in the Small Intestine

• Monosaccharide Absorption:

  • Apical Surface (lumen to absorptive cell):

    • Glucose and galactose: Secondary active transport with sodium.

    • Fructose: Facilitated diffusion.

  • Basolateral Surface (absorptive cell to interstitial fluid):

    • Glucose, galactose, and fructose: Facilitated diffusion into the blood capillary of the villus.

• Amino Acid Absorption:

  • Apical Surface:

    • Amino acids: Secondary active transport with sodium and facilitated diffusion.

    • Dipeptides and tripeptides: Secondary active transport with hydrogen ions.

  • Inside Absorptive Cell:

    • Dipeptides and tripeptides are broken down into amino acids.

  • Basolateral Surface:

    • Amino acids: Facilitated diffusion into the blood capillary of the villus.

• Fatty Acid Absorption:

  • Short-Chain Fatty Acids:

    • Move through simple diffusion directly into the blood.

  • Long-Chain Fatty Acids and Monoglycerides:

    • Micelles transport them to the absorptive cell surface.

    • Enter the cell via simple diffusion.

    • Repackaged into chylomicrons.

  • Chylomicrons:

    • Exit the basolateral surface via exocytosis into the lacteal (lymph).