Digestion and the digestive system · GCSE Biology

The digestive system

The digestive system is an organ system that breaks down large, insoluble food molecules into small, soluble ones that can be absorbed into the blood.
Salivary glands make saliva containing amylase. The stomach churns food and makes hydrochloric acid and protease (pepsin).
The pancreas makes carbohydrase, protease and lipase, which it releases into the small intestine.
The small intestine makes more enzymes and absorbs digested food through its villi. The large intestine absorbs water.
The liver makes bile, which is stored in the gall bladder.

Carbohydrases break down carbohydrates into simple sugars. Amylase breaks down starch into sugars.
Proteases break down proteins into amino acids.
Lipases break down lipids (fats and oils) into fatty acids and glycerol.
Amylase is made in the salivary glands, pancreas and small intestine; protease in the stomach, pancreas and small intestine; lipase in the pancreas and small intestine.

Bile is alkaline, so it neutralises hydrochloric acid from the stomach. This gives the right pH for enzymes in the small intestine.
Bile emulsifies fat into small droplets, which increases the surface area for lipase to work on.
Bile is made in the liver and stored in the gall bladder before being released into the small intestine.

The small, soluble products are absorbed into the blood and used to build new carbohydrates, lipids and proteins.
Some glucose is used in respiration to release energy.

Starch: add iodine solution; it turns from orange-brown to blue-black.
Reducing sugars (such as glucose): add Benedict's solution and heat in a water bath at about 75 °C; it turns from blue to green, yellow or brick red.
Protein: add Biuret solution; it turns from blue to purple (lilac).
Lipids: add Sudan III stain and shake; a red-stained oily layer forms on top.
Grind food with water and filter it first to make a solution to test.

Digestion: Breaking down large, insoluble food molecules into small, soluble ones.
Carbohydrase: An enzyme that breaks down carbohydrates into simple sugars.
Amylase: A carbohydrase that breaks down starch into sugars.
Protease: An enzyme that breaks down proteins into amino acids.
Lipase: An enzyme that breaks down lipids into fatty acids and glycerol.
Bile: An alkaline liquid made by the liver that neutralises stomach acid and emulsifies fats.
Emulsify: To break fat into small droplets, increasing its surface area.
Villi: Finger-like projections in the small intestine that absorb digested food.
Benedict's test: A test for reducing sugars: heated with Benedict's solution, a positive result turns green, yellow or brick red.
Biuret test: A test for protein: a positive result turns the blue solution purple.