A-Level Biology: Digestion and Absorption

Overview

• Digestion: Conversion of large insoluble molecules into smaller soluble molecules for absorption.
• Applies to carbohydrates, lipids, and proteins.
• Important to use the correct keywords; details found in A-level notes and flashcards.

Digestion of Carbohydrates

• Key Enzymes: Amylases and membrane-bound disaccharidases.
• Amylases:
  • Produced in the pancreas and salivary glands.
  • Start hydrolyzing carbohydrates in the mouth, breaking down polysaccharides like starch into disaccharides such as maltose.
• Membrane-bound Disaccharidases:
  • Located in the duodenum and ileum.
  • Convert disaccharides into monosaccharides.
  • Examples include sucrase (sucrose to fructose and glucose) and lactase (lactose to galactose and glucose).

Digestion of Proteins

• Enzyme Types:
  • Endopeptidases: Hydrolyze peptide bonds within the protein chain.
  • Exopeptidases: Hydrolyze peptide bonds at the ends of the protein chain.
  • Membrane-bound Dipeptidases: Hydrolyze bonds in dipeptides.
• Process:
  • Begins in the stomach and continues in the duodenum and ileum.

Digestion of Lipids

• Involves both enzymes and physical processes.
• Lipase:
  • Produced by the pancreas, secreted into the duodenum and ileum.
  • Hydrolyzes ester bonds in triglycerides, resulting in fatty acids and glycerol.
• Physical Digestion:
  • Bile salts from the liver (stored in the gallbladder) emulsify lipids, increasing surface area for faster lipase action.
  • Emulsification results in micelles which assist in delivering fatty acids and monoglycerides to epithelial cells.

Absorption

• Occurs in the ileum of the small intestine.
• Structure:
  • Villi and microvilli increase surface area for rapid absorption.
  • Capillary networks maintain concentration gradients and short diffusion distances.

Absorption of Digested Molecules

• Monosaccharides and Amino Acids:
  • Absorbed via co-transport (active transport) and facilitated diffusion.
  • Necessary for absorbing sugars/amino acids when concentration gradients are unfavorable.
• Lipids:
  • Micelles transport fatty acids and monoglycerides to epithelial cells for diffusion.
  • Inside cells, reassembled into triglycerides (sometimes forming chylomicrons when combined with proteins).
  • Chylomicrons are transported via lacteals and lymphatic vessels, eventually draining into the capillary system.

Key Points

• Understand the role of enzymes in breaking down macromolecules.
• Physical and chemical processes are both necessary for effective digestion and absorption.
• The structure of the ileum is specialized for maximizing absorption efficiency.
• Co-transport systems are essential for absorption under certain conditions.

Further Study

• For detailed processes like co-transport, refer to specific videos and resources provided.