# Organisation: Tissues, Organs and Organ Systems
In multicellular organisms, cells are organised into tissues, organs, and organ systems to carry out specialised functions efficiently.
1. Levels of Organisation
- Cell: basic unit of life
- Tissue: group of similar cells working together
- Organ: group of different tissues working together
- Organ system: group of organs working together
2. Animal Tissues
| Tissue | Function | Example |
|---|---|---|
| Epithelial | Covers and lines surfaces | Gut lining |
| Muscular | Contracts to cause movement | Stomach wall |
| Glandular | Produces and secretes substances | Salivary glands |
| Connective | Supports and connects | Blood, bone |
3. The Digestive System
| Organ | Function |
|---|---|
| Mouth | Mechanical digestion (teeth); amylase breaks down starch |
| Oesophagus | Moves food to stomach by peristalsis |
| Stomach | Protease (pepsin) breaks down protein; HCl kills bacteria, pH 2 |
| Liver | Produces bile (stored in gall bladder) |
| Pancreas | Produces amylase, protease, lipase |
| Small intestine | Digestion completed; absorption of nutrients (villi) |
| Large intestine | Absorbs water |
| Rectum | Stores faeces |
Enzymes
- Amylase: starch → sugars (mouth, pancreas)
- Protease: proteins → amino acids (stomach, pancreas)
- Lipase: lipids → fatty acids + glycerol (pancreas)
Bile
- Made in liver, stored in gall bladder
- Emulsifies fats (increases surface area)
- Neutralises stomach acid (alkaline)
4. Plant Organ System
- Root: absorbs water and minerals
- Stem: transports substances; support
- Leaf: photosynthesis; gas exchange
Plant tissues: epidermal, palisade mesophyll, spongy mesophyll, xylem, phloem, meristem
5. Practice Questions
- Put in order: organ, tissue, cell, organ system, organism.
- Name three types of enzyme and their substrates.
- What is the role of bile in digestion?
- Explain why the small intestine has villi.
- Give an example of a tissue, an organ, and an organ system.
Want to check your answers and get step-by-step solutions?
Summary
- Cells → tissues → organs → organ systems → organism
- Digestive enzymes: amylase, protease, lipase
- Bile emulsifies fats and neutralises acid
- Villi increase surface area for absorption
- Plants: roots, stems, and leaves form an organ system