The big idea: Cell proliferation is a rapid increase in cell number by repeated cell division. Organisms use it to grow, to replace cells and to repair damage.
The points to remember
- Plant growth: cells in meristems (at root and shoot tips) keep dividing.
- Animal growth: an early embryo divides again and again, with little growth between divisions.
- Replacement: cells at the base of the skin divide all the time to replace cells lost at the surface.
- Repair: after a wound, cells at its edges divide faster until the gap is closed.
Remember it as: Grow, replace, repair: all by dividing.
Real example: new skin. The outer layer of human skin is replaced roughly every four to six weeks, by cells dividing at its base and moving up to the surface.
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Three jobs, one process: The same process, mitosis followed by cytokinesis, does all three jobs; what differs is where and why it happens.
| Job | Where | What happens |
|---|---|---|
| Growth (plants) | Meristems at root and shoot tips | Cells divide, then grow and specialise |
| Growth (animals) | Early embryo | The zygote divides into a ball of cells |
| Replacement | Base of the skin | New cells replace those lost at the surface |
| Repair | Edges of a wound | Cells divide faster until the gap closes |
Controlled, not random: Proliferation speeds up when it is needed and stops when it is not, for example once a wound has closed. The cell cycle and its controls (4.4.13 and 4.4.15) make this possible.
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How it is asked: A short answer on why cells divide in a named tissue, or a 1A item matching an example (meristem, embryo, skin, wound) to growth, replacement or repair.
IB-style questionOutline[3 marks]
Outline three different roles of cell proliferation in multicellular organisms, with an example of each.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.