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Most cells in your body have already differentiated — a muscle cell will always be a muscle cell. But a small number of cells stay unspecialised, keeping the ability to divide and to turn into other cell types. These are stem cells, and they are one of the most important and most debated ideas in modern biology. This lesson, part of Topic B2 of OCR Gateway Science A, explains what stem cells are, the difference between embryonic and adult stem cells, the role of plant meristems, the medical uses of stem cells, and the risks and ethical questions they raise. Throughout, you are expected to weigh arguments even-handedly rather than simply take a side.
By the end of this lesson you should be able to define a stem cell, compare embryonic and adult stem cells, describe how plant meristems are used, outline medical uses of stem cells, and discuss the risks and ethical issues involved.
A stem cell is an unspecialised cell that can:
Because they can become other cells, stem cells are the source of new cells for growth, repair and replacement. The two ideas — unspecialised and able to become many cell types — are the heart of every definition question on this topic.
Exam Tip: The mark-scheme definition is "an unspecialised cell that can divide and differentiate into different (specialised) cell types". Saying only "a cell that can turn into other cells" usually misses the "unspecialised" and "divide" marks.
In animals there are two main sources of stem cells, and OCR expects you to compare them.
| Embryonic stem cells | Adult stem cells | |
|---|---|---|
| Found in | Early embryos (a few days old) | Certain adult tissues, especially bone marrow |
| What they can become | Almost any type of cell in the body | A more limited range of cell types |
| Ethical concern | High — the embryo is destroyed | Lower — taken from a consenting adult |
Exam Tip: The key difference to remember is how many cell types each can become: embryonic stem cells can become almost any cell type, whereas adult stem cells can become only a limited range. This difference is also why embryonic stem cells are at the centre of the ethical debate.
Plants keep stem cells throughout their lives, in regions called meristems. Meristems are found at the tips of roots and shoots (and in other growing regions). The cells there stay unspecialised and can divide and differentiate for as long as the plant lives, which is why plants can keep growing taller and produce new roots, leaves and flowers.
This has a very useful practical consequence: because meristem cells can become a whole plant, growers can take small pieces of a plant and grow them into many identical new plants — clones. Producing clones from meristems lets growers:
Exam Tip: A favourite question is why growers clone plants from meristems. Good answers mention producing many identical plants quickly and keeping a useful characteristic (because the clones are genetically identical). Plant cloning is also far easier than animal cloning because plant cells keep the ability to differentiate.
The exciting promise of stem cells is that they could be used to replace cells that have been damaged or lost, because they can differentiate into whatever cell type is needed. The general principle is the same in each case: stem cells are encouraged to differentiate into the required specialised cells, which are then used to repair the body.
| Condition | How stem cells could help |
|---|---|
| Type 1 diabetes | Make new insulin-producing cells to replace the ones the body has lost |
| Paralysis (spinal injury) | Make new nerve cells to reconnect damaged parts of the spinal cord |
| Blood disorders / leukaemia | Bone marrow transplants already use adult stem cells to make healthy blood cells |
A particularly important idea is therapeutic cloning. Here, stem cells are produced that have the same genes as the patient. Because the new cells are genetically identical to the patient, they are much less likely to be rejected by the patient's immune system when they are transplanted. This could allow damaged tissues to be repaired using a patient's own genetic make-up.
It is worth being honest about the state of this science: bone marrow (adult stem cell) transplants are already an established treatment for blood disorders, but most other stem cell therapies are still being researched and developed rather than in routine use. You should present them as potential treatments, not as cures that are already available.
Exam Tip: When asked how stem cells could treat a condition, name the specialised cell that is needed (e.g. nerve cells for paralysis, insulin-producing cells for diabetes) and say the stem cells differentiate into those cells to replace the damaged ones. Mention therapeutic cloning if the question is about avoiding rejection.
Using stem cells in medicine is not without difficulty. The main risks OCR expects you to know are:
These practical risks are part of why stem cell treatments are still being carefully researched before being used widely.
Stem cell research, especially using embryonic stem cells, raises ethical questions on which people hold strongly differing views. A good GCSE answer presents both sides fairly rather than simply asserting one opinion. Common arguments include:
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