7.4) Chemical digestion
- Breaking down large molecules to small molecules.
- The large molecules are usually not soluble in water while the smaller ones are.
- The small molecules can be absorbed through the epithelium of the alimentary canal, through the walls of the blood vessels and into the blood.
- Amylase breaks down starch to simpler sugars.
- Protease breaks down protein to amino acids.
- Lipase breaks down fats to fatty acids and glycerol.
Where digestion happens:
- Proteases catalyse the breakdown of proteins into amino acids in the stomach and small intestine.
- Lipases catalyse the breakdown of fats and oils into fatty acids and glycerol in the small intestine.
- Amylase catalyses the breakdown of starch into maltose in the mouth and small intestine.
- Maltase catalyses the breakdown of maltose into glucose in the small intestine.
Digestion of protein:
- Several proteases which break down proteins.
- Pepsin is secreted in the stomach.
- Pepsin acts on proteins and breaks them down into soluble compounds called peptides.
- Trypsin is secreted by the pancreas in an inactive form, which is changed to an active enzyme in the duodenum.
- Breaking down proteins to peptides.
- The stomach produces hydrochloric acid.
- It kills many harmful microorganisms (bacteria) that might have been swallowed along with the food.
- The enzymes in the stomach work best in acidic conditions – at a low pH.
Functions of HCL in gastric juice:
- Creates a very acid pH of 2.
- This pH is important because it denatures enzymes in harmful organisms in food, such as bacteria
- It provides the optimum pH for the protein-digesting enzyme pepsin to work.
After it has been in the stomach, food travels to the small intestine. The enzymes in the small intestine work best in alkaline conditions – but the food is acidic after being in the stomach. Bile is a substance produced by the liver and stored in the gall bladder.
- The bile emulsify the fat – they break them up into small droplets with a large surface area, which are more efficiently digested by lipase.
- Bile is slightly alkaline and has the function of neutralising the acidic mixture of food and gastric juices as it enters the duodenum.
- This is important because enzymes secreted into the duodenum need alkaline conditions to work at their optimum rate.