Section 1 of 9What are stem cells?
A stem cell is an undifferentiated cell that can divide by mitosis to produce more cells and can differentiate into one or more specialised cell types. Stem cells are found in embryos and, in smaller numbers, in some adult tissues. They are essential in growth, and in adults they replace worn-out and damaged cells.
| Type | Source | What they can become |
|---|---|---|
| Embryonic stem cells | Early embryo | Almost any type of cell |
| Adult stem cells | Tissues such as bone marrow | Limited range, e.g. blood cells |
| Meristem cells | Tips of roots and shoots in plants | Any plant cell type, throughout life |
Section 2 of 9Medical uses
Because stem cells can become specialised cells, they offer ways to treat diseases in which cells are damaged or lost.
A problem with donated tissue is rejection, because the patient's immune system sees the foreign cells as antigens and attacks them. Cloned cells with the patient's own genes avoid this.
Section 3 of 9Plant stem cells
Meristem cells in plants can be used to produce clones quickly and cheaply. Rare species can be saved from extinction and disease-resistant crops can be grown in large numbers, as all the offspring have identical genes. This is also a way to preserve plants that are threatened.
Section 4 of 9Benefits and risks
Advantages of stem cell therapy include the potential to cure diseases that currently have no cure, and a new supply of tissue without needing a donor. Risks and drawbacks include:
- Some people feel that an embryo is a human life with a right to life;
- others argue that the potential to relieve suffering justifies the research, especially because the embryos used are often unwanted ones from fertility treatment.
- Laws regulate this research in many countries, and adult stem cells and reprogrammed cells avoid the objection.
Section 5 of 9Evaluating the argument
Exam questions often ask you to evaluate. Give reasons on both sides, use the correct term (embryonic, adult, meristem), and reach a reasoned conclusion. A good answer separates scientific points (possible tumours, contamination, success rates) from ethical points (status of the embryo, consent, who should have access). You should not simply say that stem cells are 'good' or 'bad'.
This topic follows from differentiation and mitosis: stem cells divide by mitosis, but because they are not yet specialised they can then differentiate. In later topics, you will meet genetic engineering and clones, which raise related ethical questions.
Section 6 of 9Reprogrammed cells and current research
Scientists have found ways to take ordinary adult cells, such as skin cells, and reprogramme them to behave like embryonic stem cells by switching on certain genes.
Remember, however, that a technique being possible does not mean it is safe: any treatment must pass trials before it is used widely. When you weigh the evidence in an answer, mention that some treatments, such as bone marrow transplants, are well established, while others remain experimental.
Stem cells in the bone marrow are a good example from everyday medicine: they divide constantly to produce red cells, white cells and platelets. People who donate bone marrow give a small sample of these stem cells, which can restore blood production in a patient whose own marrow has been destroyed by radiotherapy.
Section 7 of 9Exam-style questions
Explain why embryonic stem cells could be used to treat a wide range of diseases. [2]2 marks
Embryonic stem cells are unspecialised and can differentiate into almost any cell type [1], so they could replace many types of damaged cells, such as nerve or muscle cells [1].
Give one risk of using stem cells to treat a patient and one ethical objection to the use of embryonic stem cells. [2]2 marks
Risk: virus transfer from cultured cells / tumour formation [1]. Ethical objection: embryos are destroyed, and some believe they have a right to life [1].
Describe how meristem cells from a plant can be used to protect a rare species. [3]3 marks
Meristem cells are taken from the rare plant [1]. They are grown on agar with nutrients and hormones, and differentiate to form many new plants [1]. All of them are clones with the same genes, so many plants can be produced quickly and the species preserved [1].
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Stem cell | Undifferentiated cell that can divide and become specialised. |
| Embryonic stem cell | Stem cell from an early embryo that can form most cell types. |
| Adult stem cell | Stem cell found in adult tissues with a limited range. |
| Meristem | Plant tissue where stem cells divide. |
| Therapeutic cloning | Making an embryo genetically identical to the patient to obtain stem cells. |
| Rejection | Immune attack on transplanted foreign tissue. |
| Bone marrow | Tissue in bones containing stem cells that make blood cells. |
Section 9 of 9Exam technique
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.