A large organism is built in levels, each made of the one before it.

• Cell: the basic unit, often specialised (a muscle cell, a red blood cell).
• Tissue: a group of cells with a similar structure and function (muscle tissue, glandular tissue, epithelial tissue).
• Organ: different tissues working together on one job. The stomach has muscle tissue to churn, glandular tissue to make juices and epithelial tissue to line it.
• Organ system: organs working together, such as the digestive system.

A leaf is an organ, not a tissue: it contains several tissues (epidermis, mesophyll, xylem and phloem).

Digestion breaks large insoluble molecules into small soluble ones that can pass into the blood.

• Mouth: salivary glands release amylase.
• Stomach: hydrochloric acid (pH about 2) kills bacteria; a protease called pepsin works here.
• Pancreas: makes amylase, protease and lipase and releases them into the small intestine.
• Liver makes bile; the gall bladder stores it.
• Small intestine: digestion finishes and the products are absorbed through the villi.
• Large intestine: water is absorbed.

Enzymes are proteins that act as biological catalysts: they speed up reactions and are not used up. The substrate fits the active site like a key in a lock, so each enzyme works on one substrate.

Enzyme	Breaks down	Into
amylase (a carbohydrase)	starch	sugars
protease	proteins	amino acids
lipase	lipids	fatty acids and glycerol

Bile is not an enzyme. It is alkaline, so it neutralises stomach acid, and it emulsifies fat into tiny droplets, giving lipase a much larger surface area to work on.

Enzymes are not alive, so they are never "killed". Bile does not break any bonds: it only makes the drops of fat smaller.