Cells, tissues, organs and organ systems
In multicellular organisms, similar cells team up. The hierarchy is small to large:
cell → tissue → organ → organ system → organism
Tissue — a group of similar specialised cells
A tissue is a collection of cells with similar structure and function working together. Examples:
- Muscular tissue contracts to move parts of the body.
- Glandular tissue produces and releases substances (e.g. enzymes, hormones).
- Epithelial tissue covers and lines surfaces (skin, gut wall).
Organ — several tissues working together
An organ is built from two or more different tissues carrying out a specific function. The stomach is a textbook example:
- Muscular tissue churns the food.
- Glandular tissue makes hydrochloric acid and protease (pepsin).
- Epithelial tissue lines the inside and outside (with mucus to protect from acid).
Organ system — several organs working together
An organ system is several organs cooperating on a major life process. The digestive system is the GCSE flagship for this:
- Mouth — chew, mix with saliva (amylase).
- Oesophagus — peristalsis pushes food down.
- Stomach — protein digestion in acid.
- Small intestine — bile, pancreatic enzymes, absorption (villi).
- Pancreas — secretes enzymes and bicarbonate.
- Liver — produces bile.
- Large intestine — water reabsorption.
Why this matters
Understanding the organisation hierarchy explains how function emerges. A single muscle cell can contract, but only when arranged into tissue and tied into an organ (heart, biceps) does it produce something biologically useful.
⚠Common mistakes— Common mistakes / exam traps
- Mixing up "tissue" and "cell" — tissue is a group of cells, never a single cell.
- Listing organs as tissues — heart and lung are organs, not tissues.
- Missing accessory organs of the digestive system (pancreas, liver).
- Forgetting the order in size/complexity — exam might give a list and ask you to order it.
Links
Connects forward to B2.2 (animal organ systems in detail), B2.3 (plant organ systems), and to all of homeostasis (B5) which depends on coordination of organ systems.
AI-generated · claude-opus-4-7 · v3-deep-biology