Stem Cells and Their Uses — AQA GCSE Biology

What you'll learn

By the end of this guide you should be able to describe what a stem cell is and why "undifferentiated" is the word that does all the work in that definition. You should be able to compare embryonic stem cells, adult stem cells and the stem cells found in plant meristems, and say what each can and cannot do. You should be able to explain therapeutic cloning and, in particular, why it solves the rejection problem that otherwise limits transplants of every kind. You should be able to give the medical benefits and the risks side by side, and set out the ethical arguments both for and against embryonic stem cell research without simply asserting that one side is right. Finally, you should be able to explain why plant cloning from meristems is useful to growers and to conservation, and why plant stem cells raise none of the ethical difficulties that animal ones do.

This is a topic where the biology is fairly simple and the marks are mostly lost on precision of language and on one-sided answers to evaluation questions. Both of those are fixable.

Key terms and definitions

Stem cell — an undifferentiated cell that can divide to produce more cells of the same type, and can differentiate into one or more specialised cell types.

Differentiation — the process by which a cell develops the structures and features that let it carry out a particular job. A cell that has differentiated has switched on some of its genes and switched others off; it still contains the organism's whole genome.

Undifferentiated — not yet specialised. The cell has the genetic instructions for every cell type but has not committed to any of them.

Embryonic stem cell — a stem cell from an early embryo. It can differentiate into any type of cell in the organism.

Adult stem cell — a stem cell found in certain tissues of a developed organism, most importantly bone marrow. It can form only a limited range of cell types.

Meristem — a region of a plant, at the root and shoot tips, where cells remain undifferentiated and continue dividing throughout the plant's life.

Therapeutic cloning — producing an embryo with the same genes as a patient, so that stem cells taken from it are not rejected by the patient's immune system.

Rejection — the immune system attacking transplanted cells or tissue because it recognises them as foreign.

Clone — an organism or cell genetically identical to the one it came from.

Core concepts

What makes a stem cell different

Almost every cell in your body is specialised. A red blood cell has lost its nucleus and filled itself with haemoglobin. A nerve cell has stretched into a long thin fibre. A root hair cell has pushed out a long extension to increase its surface area. Each of these is very good at one job and completely unable to do any other.

That specialisation is a one-way street in most cases. Once a cell has differentiated it stays differentiated, and if it is destroyed the body cannot simply convert a neighbouring cell into a replacement. This is the problem stem cells solve. A stem cell has not committed to anything. It can divide to make more stem cells, and it can differentiate into a specialised cell when the right signal arrives.

It is worth being clear about what does not differ between a stem cell and a specialised cell. They contain the same DNA. A nerve cell has the genes for haemoglobin sitting in its nucleus; it simply never switches them on. Differentiation is a change in which genes are expressed, not a change in which genes are present. This is why examiners mark down answers saying a specialised cell "has lost" the genes for other cell types.

The three types you need to know

Embryonic stem cells come from an embryo at a very early stage, when it is a ball of a few dozen cells. At that point none of the cells has specialised, so every one of them can become any cell type in the finished organism — nerve, muscle, blood, skin, anything. This is what makes them so valuable medically, and it is also the source of the ethical objection, because obtaining them destroys the embryo.

Adult stem cells are found in some tissues of a fully developed organism. The clearest example is bone marrow, which contains stem cells that produce the various types of blood cell throughout a person's life. Adult stem cells are more restricted than embryonic ones: bone marrow stem cells make blood cells, not nerve cells. The advantage is that taking them does not destroy an embryo, and they can often be taken from the patient's own body, which removes the rejection problem entirely.

Plant stem cells are found in meristems, at the tips of roots and shoots. Unlike animals, plants keep undifferentiated tissue available for their whole lives, which is why a plant can keep growing new organs indefinitely while an animal cannot grow a new limb. Meristem cells can differentiate into any plant cell type at any point in the plant's life.

A useful way to hold these apart is by what limits each one. Embryonic stem cells are limited by ethics, not by ability. Adult stem cells are limited by ability, not by ethics. Plant stem cells are limited by neither, which is why plant cloning is already routine while human stem cell therapy is still largely experimental.

Medical uses and the rejection problem

The medical promise of stem cells is replacement. In Type 1 diabetes, the cells in the pancreas that produce insulin have been destroyed by the immune system. If stem cells could be persuaded to differentiate into working insulin-producing cells and then implanted, the patient's own blood glucose control could be restored rather than managed with injections. In paralysis caused by damage to the spinal cord, the nerve cells that carried impulses past the point of injury are gone; stem cells that differentiated into nerve cells could in principle restore the connection.

Both of these run into the same obstacle that every transplant runs into. The immune system identifies cells as "self" or "foreign" and attacks anything foreign. Transplanted stem cells from a donor would be attacked in exactly the way a donated kidney is, and the patient would need immunosuppressant drugs for life — drugs that leave them vulnerable to infection.

Therapeutic cloning is the answer to this. A cell is taken from the patient, and its nucleus is used to produce an embryo that carries the patient's own genes. Stem cells taken from that embryo are genetically identical to the patient, so the immune system recognises them as self and does not reject them. No immunosuppressants are needed. This is the reason therapeutic cloning appears on the specification at all: it is not a different source of stem cells for its own sake, it is a solution to rejection.

Risks

Stem cell therapy is not risk-free, and an evaluation question expects you to know why.

Stem cells have to be grown in a laboratory before being implanted. Cell cultures can become contaminated with viruses, and a virus present in the culture would be transferred straight into a patient whose condition may already have left them vulnerable.

There is also the question of control. A stem cell's defining feature is that it divides readily, and cells that divide readily and are not properly controlled are the basis of a tumour. Making sure implanted cells differentiate as intended and then stop dividing is one of the central technical difficulties in the field.

And the techniques are new. Long-term effects, by definition, have not yet been observed over the long term.

The ethical debate

This is the part of the topic where marks are most often lost, because students write one side of the argument and stop.

The objection to embryonic stem cell research is that obtaining the cells destroys the embryo. People who regard an early embryo as a potential human life regard that as taking a life, and no medical benefit changes the nature of the act. Some hold this view on religious grounds and some on purely secular ones. It is not an objection to whether the science works.

The argument in favour rests on suffering. The embryos used are typically spare embryos from IVF that would otherwise be destroyed unused. Against that, there are people living with paralysis, with organ failure, with Type 1 diabetes, whose conditions might be curable rather than merely managed. On this view, not pursuing the research has a cost too, and that cost falls on identifiable people who are suffering now.

There is a middle position worth knowing: some object to embryonic stem cells specifically but support adult stem cell research, since no embryo is involved. That is one reason adult stem cell work is funded in places where embryonic work is restricted.

Whatever your own view, an answer that gives one side only cannot access the full marks on a "discuss" or "evaluate" question. Give both, and then, if the question asks for it, say which you find more persuasive and why.

Cloning plants from meristems

Plant cloning is the commercially established use of stem cells, and it is much simpler than anything in the medical section.

A small piece of meristem tissue is taken from a chosen plant and grown in a sterile medium containing nutrients and plant hormones. Because meristem cells are undifferentiated, they divide and develop into complete plantlets, each genetically identical to the parent.

The uses follow from that. A grower who has one plant with a desirable characteristic — a particular flower colour, disease resistance, high yield — can produce thousands of identical copies, quickly and cheaply, rather than waiting on seed that would show variation. Conservationists can build up numbers of a rare species from very little starting material. And crops can be produced free of the viruses that would otherwise be passed on, because the meristem tip is often virus-free even when the rest of the plant is infected.

The disadvantage is the disadvantage of all asexual reproduction: no genetic variation. A field of clones is uniformly susceptible to the same disease, so a new pathogen can take the whole crop.

Worked examples

Example 1: Explaining a therapy

A patient has Type 1 diabetes. Explain how stem cells could be used to treat them, and why therapeutic cloning might be used. (5 marks)

Work through it in order. Stem cells are undifferentiated and can differentiate into any cell type (1). They could be made to differentiate into insulin-producing pancreas cells (1). These would be implanted into the patient (1), restoring the ability to control blood glucose (1). Therapeutic cloning would produce an embryo with the patient's own genes, so the cells are not rejected by the immune system (1).

Notice that the last mark is the one most often missed, and it is the one the question specifically asked for.

Example 2: Evaluating the ethics

Discuss the arguments for and against the use of embryonic stem cells in medical research. (4 marks)

Two marks are available on each side, so plan two points each.

For: embryonic stem cells can differentiate into any cell type, so they could treat currently incurable conditions such as paralysis and diabetes; the embryos used are often spare IVF embryos that would be destroyed anyway.

Against: obtaining the cells destroys an embryo, which some people consider to be a potential human life; there are risks, including viral contamination of cultured cells.

An answer that lists only the benefits, however well expressed, is capped at half the marks.

Example 3: Comparing sources

Give one advantage and one disadvantage of using adult stem cells rather than embryonic stem cells. (2 marks)

Advantage: no embryo is destroyed, so the ethical objection does not apply — and if the cells are taken from the patient, they will not be rejected (1).

Disadvantage: adult stem cells can differentiate into a limited range of cell types only, so they cannot treat as wide a range of conditions (1).

Keep the comparison explicit. "Adult stem cells are good" earns nothing; "adult stem cells can form only a limited range of cell types, whereas embryonic ones can form any" earns the mark.

Common mistakes and how to avoid them

Saying specialised cells have lost their other genes. They have not. Every cell carries the full genome; differentiation switches genes on and off. Write "the genes are not expressed", not "the genes are gone".

Confusing therapeutic cloning with reproductive cloning. Therapeutic cloning produces stem cells to treat a patient. It is not an attempt to produce a cloned person, and saying so in an exam suggests you have not understood the point.

Treating adult stem cells as a straight substitute for embryonic ones. They avoid the ethical problem but they are more restricted in what they can become. Both facts matter.

Giving one side of the ethical debate. The command word is usually "discuss" or "evaluate", and both require both sides.

Vague statements about stem cells "curing everything". Name a condition and name the cell type that would be produced. Specificity is what earns marks.

Forgetting plants. Meristem cloning is the part of this topic that is actually in routine use, and it appears regularly on papers. Do not revise only the human material.

Exam technique for "Stem Cells and Their Uses"

Start by reading the command word. "Describe" wants the process; "explain" wants the reason; "evaluate" and "discuss" want both sides and, often, a judgement. This topic is unusual on the specification in how often it uses the last two.

Where a question gives a scenario — a particular patient, a particular condition — use it. Name the cell type that would need to be produced. A generic answer about stem cells being useful will not score against a scenario question.

Watch the mark allocation for balance. A four-mark "discuss" almost always means two points on each side. If you have written four points for and none against, you have written a two-mark answer.

Use the technical vocabulary precisely: undifferentiated, differentiate, rejection, therapeutic cloning, meristem. These are the words the mark scheme is looking for, and paraphrases often fail to score.

Finally, if a question asks for your own view, you are allowed to have one, but it must be supported by a reason. "I think it is wrong" is not an answer; "I think the potential to relieve serious suffering outweighs the objection, because the embryos used would otherwise be destroyed" is.

Quick revision summary

A stem cell is undifferentiated: it can divide to make more of itself and can differentiate into specialised cells. Differentiation switches genes on and off; it does not remove them.

Embryonic stem cells come from early embryos and can become any cell type — maximum potential, maximum ethical objection. Adult stem cells, chiefly in bone marrow, can become a limited range only, but destroy no embryo and may be taken from the patient. Plant stem cells, in meristems, stay undifferentiated for the plant's whole life.

Medical uses centre on replacing cells that have been lost: insulin-producing cells in Type 1 diabetes, nerve cells in paralysis. Therapeutic cloning produces an embryo with the patient's own genes so that the stem cells are not rejected.

Risks: viral contamination of cultured cells, uncontrolled division, and unknown long-term effects.

Ethics: destroying an embryo versus relieving serious suffering. Know both, and know that supporting adult stem cell research is a coherent middle position.

Plants: meristem tissue grown in sterile nutrient medium produces large numbers of identical, often virus-free plants, cheaply and quickly — valuable to growers and to conservation, but with no genetic variation, so the whole population shares the same susceptibilities.