3.3.1 Digestion

Key Points About Digestion:

1. Purpose of Digestion:

• Large molecules such as carbohydrates, proteins, and lipids cannot pass through cell membranes.
• These molecules must be broken down into smaller components like monosaccharides, amino acids, and fatty acids for absorption.

2. Enzymes in Digestion:

• Enzymes are biological catalysts that speed up hydrolysis reactions.
• Different enzymes are specific to the type of molecule being digested.

Stages of Digestion:

1. Carbohydrate Digestion:

• Amylase breaks down starch into maltose.

• Found in saliva and pancreatic juice.

• Maltase hydrolyses maltose into glucose.

• Found in the small intestine. Other disaccharides:

• Sucrose is broken down by sucrase into glucose and fructose.

• Lactose is broken down by lactase into glucose and galactose.

2. Protein Digestion:

• Proteins are hydrolysed into amino acids by proteases (also called peptidases):
• Endopeptidases break peptide bonds within the protein chain.
• Exopeptidases remove terminal amino acids.
• Dipeptidases break down dipeptides into individual amino acids.
• Proteases are secreted by the stomach, pancreas, and small intestine.

3. Lipid Digestion:

• Lipids are hydrolysed into glycerol, fatty acids, and monoglycerides by lipase.
• Lipase is produced in the pancreas and acts in the small intestine.
• Lipid digestion is aided by bile salts:
• Emulsification breaks large lipid droplets into smaller ones, increasing surface area for lipase action.

4. Nucleic Acid Digestion:

• DNA and RNA are broken down into nucleotides by nucleases.

Role of Bile:

• Bile is produced by the liver and stored in the gallbladder.
• It does not contain enzymes but aids digestion by:
  • Neutralising stomach acid to create an alkaline environment for enzymes in the small intestine.
  • Emulsifying fats to increase the surface area for lipase.

Digestion Locations:

Organ	Role in Digestion
Mouth	Amylase in saliva starts starch digestion; mechanical digestion via teeth.
Stomach	Proteins broken down by pepsin in acidic conditions.
Small Intestine	Main site for enzymatic digestion and absorption.
Pancreas	Produces amylase, lipase, and proteases, which are released into the small intestine.
Liver/Gallbladder	Produces bile for fat emulsification.

Key Terms:

1. Hydrolysis: The chemical breakdown of molecules by water, catalysed by enzymes.
2. Emulsification: The process of breaking down large fat droplets into smaller ones.
3. Absorption: The movement of digested molecules across cell membranes into the bloodstream.