Chemical digestion:
- It is the process of breaking down large, insoluble molecules into small, soluble molecules.
- This process makes use of chemicals (mainly enzymes), for the breakdown of food.
- This process produces small, soluble molecules that can be absorbed by the body.
- The salivary glands, stomach, liver, gall bladder, pancreas, and small intestine are involved in chemical digestion.

Site of secretion and action of enzymes:
- Amylase:
  - The salivary glands and pancreas secrete amylase.
  - Amylase breaks down starch into a simple reducing sugar called maltose. This happens in the mouth and the duodenum.
  - Maltose is a disaccharide. It is broken down to glucose by maltase, which is secreted by the epithelial cells of the small intestine.
  - Glucose is then small enough to be absorbed into the bloodstream.
- Protease:
  - The stomach and pancreas secrete protease, which is used to break down proteins into amino acids. This happens in the stomach and the duodenum.
- Lipase:
  - This enzyme is also secreted by the pancreas.
  - It breaks down fats and oils into fatty acids and glycerol in the duodenum.

Role of hydrochloric acid in gastric juice:
- Hydrochloric acid is present in the gastric juice which is produced in the stomach.
- This acid kills microorganisms in ingested food.
  - Low pH denatures enzymes and prevents them from working.
  - Thus, microorganisms are killed as their protein structure and enzymes are denatured.
- This acid also provides the optimum acidic pH for the protease in the stomach.
  - Pepsin is a protease present in the stomach that breaks down protein into amino acids in the acidic conditions of the stomach.

Role of bile:
- Liver produces bile, which is stored in the gall bladder.
- When the mixture of food and gastric juice enters the duodenum, bile is released.
- Bile is not an enzyme. It only neutralises the acid coming from the stomach and breaks down the fat into smaller droplets, so that lipase can easily work on it.
- Trypsin, lipase, and amylase cannot work in acidic pH. So, bile is required to provide a suitable pH for enzyme action.
- Trypsin is also a protease secreted by the pancreas that breaks down protein into amino acids in the alkaline condition of the duodenum.
- Bile also emulsifies fats to increase the surface area, allowing the lipase secreted by the pancreas to effectively break down fat into fatty acids and glycerol.
  - That means bile emulsifies fats into smaller droplets, but it does not break down fat into fatty acid and glycerol.
Illustration for the optimal pH for pepsin and trypsin:

7.04 - Chemical Digestion

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