Organisation, digestion and enzymes

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.

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.