Levels of organisation
In multicellular organisms, cells are organised into increasingly complex levels: cell → tissue → organ → organ system → organism. A tissue is a group of similar cells working together to carry out a shared function. An organ is a structure made of several tissues that perform a particular function. An organ system is a group of organs working together.
Key idea
Cell specialisation means a cell develops a shape and features suited to a particular job. The more efficiently a cell carries out its function, the more likely it will have distinctive adaptations.
Examples of specialised cells
- Root hair cell – a long extension gives a large surface area to absorb water and mineral ions.
- Red blood cell – no nucleus and a biconcave shape, full of haemoglobin to carry oxygen.
- Ciliated cell – tiny hairs (cilia) sweep mucus and trapped dirt along airways.
- Nerve cell (neurone) – long and thin to carry electrical impulses over distances.
- Palisade mesophyll cell – packed with chloroplasts near the leaf surface for photosynthesis.
Remember
- Order of organisation: cell, tissue, organ, organ system, organism.
- A red blood cell has no nucleus, giving more room for haemoglobin.
- Root hair cells increase surface area for absorption.