Stem cells at Higher Level: The same statement as SL. The HL page follows the stem cells of a flatworm, which can rebuild a whole body, to show the two properties working together.
Practise this as you read
- Define and outline the two properties of stem cells.
- Explain why a specialized cell is not a stem cell.
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The big idea: A stem cell is an unspecialized cell with two special abilities.
The two abilities
- It can divide again and again to make more cells (called self-renewal).
- It can differentiate — develop into one or more specialized cell types (such as a muscle cell, a red blood cell or a nerve cell).
Potency: How many different cell types a stem cell can become is called its potency.
- Stem cell
- An unspecialized cell that can keep dividing (self-renew) and can differentiate into one or more specialized cell types.
- Self-renewal
- The ability of a stem cell to divide repeatedly to produce more stem cells, keeping a supply available in the body.
- Differentiation
- The process by which an unspecialized cell becomes a specialized cell with a particular structure and function.
- Potency
- A measure of how many different cell types a stem cell is able to differentiate into.
- Niche
- The specific location in the body where a particular type of stem cell is found (for example, blood-forming stem cells in red bone marrow).
| Defining property | What it means | Why it matters |
|---|---|---|
| Self-renewal | A stem cell can divide repeatedly to make more stem cells | Keeps a lasting supply of stem cells in the body |
| Differentiation (potency) | A stem cell can develop into one or more specialized cell types | Replaces and builds the specialized cells the body needs |
Two properties, always together: Both properties must be present for a cell to count as a stem cell:
it must be able to self-renew (keep dividing) and able to differentiate (turn into specialized cells).
A cell that can only divide, or only differentiate, is not a stem cell.
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The big idea: A stem cell can divide endlessly and its daughter cells can differentiate along different pathways: these two properties together make it a stem cell.
The points to remember
- Stem cells divide by mitosis again and again, with no limit to the number of divisions.
- After a division, one daughter cell usually stays a stem cell, so the supply never runs out.
- Other daughter cells differentiate into specialized cells.
- They can follow different pathways, so one kind of stem cell can give several cell types.
- Most specialized cells, such as neurons, have lost these abilities: they no longer divide or change type.
Remember it as: Divide without end, differentiate in different directions.
Real example: a flatworm that regrows its head: A planarian flatworm cut into small pieces regrows a whole worm from each piece. Its stem cells, called neoblasts, divide and differentiate into every missing cell type.
How it is asked: Expect a one-mark Define in Paper 2, or a Paper 1A item about why stem cells are useful: they can become the cell type that is missing.
IB-style questionOutline[2 marks]
Outline the two properties of stem cells.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.