Students should be able to evaluate the use of stem cells in treating…

The idea

Evaluate means weigh both sides and reach a judgement. In favour: stem cells replace cell types the body cannot regenerate, offering treatment for conditions lacking a cure, like macular degeneration or spinal cord injury, and can also treat leukaemia. Adult stem cells can be taken from the patient, and induced pluripotent stem (iPS) cells are produced by reprogramming the patient's somatic cells. These carry the patient's own antigens, avoiding rejection. Also, IVF embryos used would otherwise be discarded. Against: using embryonic stem cells destroys an embryo, which some regard as a potential life; donated cells carry foreign antigens and may be rejected; iPS and embryonic cells divide indefinitely, risking tumour formation; and much evidence comes from mice, so long-term human effects are unknown.

What earns the marks

• At least one point supporting the use of stem cells, with a named disorder such as macular degeneration.
• At least one point against, such as destruction of an embryo or the risk of tumour formation.
• A point about immune rejection, either as an advantage of the patient's own cells (adult or iPS) or a disadvantage of donated cells.
• A valid limitation of the evidence, such as work done on mice rather than humans, a small sample, or unknown long-term effects.
• A justified conclusion that refers back to the data or arguments given.

Common mistakes and how to put them right

• Writing only the benefits and omitting the opposing side, which weakens an evaluation.

  Put it right: Include points for and against.

• Writing it is unethical without stating what the ethical objection actually is. Correction: specify that using embryonic stem cells destroys an embryo.

• Claiming all stem cell treatments destroy embryos, when adult and iPS sources do not.

  Put it right: Distinguish between embryonic, adult, and iPS cells.

• Stating that results obtained in mice apply directly to humans.

  Put it right: Acknowledge that animal models may not perfectly predict human responses.

• Ending without a judgement when the command word is evaluate.

  Put it right: Provide a concluding sentence weighing the evidence.

Exam tips

• Split the answer into supporting and against, and check you have at least one point under each before you run out of time.
• Use the source material: quote a named finding or a figure from the stem rather than relying on general knowledge alone.
• Finish with one sentence saying which way the evidence points and why.