Organisation, digestion and enzymes
From cells to organ systems
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).
The digestive system
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 and bile
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.
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