Digestion
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
What is Physical Digestion?
Physical digestion is the breakdown of food into smaller pieces without any chemical change to the food molecules.
Physical digestion does not change the type of molecule - a large piece of starch is broken into many smaller pieces of starch. Its purpose is to increase the surface area of food, so that digestive enzymes can act more effectively in chemical digestion.
What is Chemical Digestion?
- Chemical digestion is the breakdown of large, insoluble food molecules into small, soluble molecules, by enzymes
- Unlike physical digestion, chemical digestion changes the type of molecule - the small, soluble products can then pass through the wall of the small intestine into the blood, a process called absorption
Digestive Enzymes - Summary
| Enzyme | Substrate | Products | Where secreted | Where it acts |
|---|---|---|---|---|
| Amylase | Starch | Simple reducing sugars | Salivary glands; pancreas | Mouth; small intestine (duodenum) |
| Proteases | Proteins | Amino acids | Stomach (pepsin); pancreas (trypsin) | Stomach; small intestine (duodenum) |
| Lipase | Fats and oils | Fatty acids and glycerol | Pancreas | Small intestine (duodenum) |
Hydrochloric Acid in Gastric Juice
The stomach produces gastric juice, which contains hydrochloric acid, protease enzymes (mainly pepsin), and mucus to protect the stomach lining.
- Hydrochloric acid kills harmful microorganisms in food - the very low pH (about 2) is lethal to most microbes
- Hydrochloric acid provides an acidic pH (around pH 2) for the optimum activity of pepsin, the stomach protease
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