4BI1
Cell Structure
Structures and Functions in Living Organisms · 6 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 18% of your exam marks.
stable
Very High
Stable18%
Cell structure and organelle function appear on nearly every paper; one of the highest-frequency topics.
What a stem cell is
A stem cell is an undifferentiated that can divide by mitosis to produce more stem cells, and that can also differentiate into specialised cells
Two key abilities make stem cells special:
- Self-renewal: they can keep dividing indefinitely to produce more stem cells
- Differentiation: when given the right chemical signals, they can develop into specialised cell types
Two types: embryonic and adult
| Embryonic stem cells | Adult stem cells | |
|---|---|---|
| Source | Very early embryos (a few days old) | Certain tissues in the adult body (bone marrow, gut lining, skin) |
| What they can become | Any cell type in the body. They are pluripotent | A limited range of cell types, usually closely related to the tissue they came from. Bone marrow stem cells, for example, can only become blood cells |
| Availability | Limited (need an embryo) | More accessible (can be taken from a living donor or the patient themselves) |
| Ethical issues | Major: many people view the destruction of an embryo as the destruction of a potential human life | Few ethical issues |
Uses of stem cells in medicine
Stem cells offer the possibility of replacing damaged cells in diseases where the body's own cells have stopped working properly. This is called therapy. Real and proposed applications include:
- Bone marrow transplants for treating leukaemia (a cancer of the blood). Healthy bone marrow stem cells from a donor are transplanted into the patient and produce new, healthy blood cells.
- Using stem cells to grow new skin for severe burns victims
- Treating Parkinson's disease by replacing damaged nerve cells in the brain
- Repairing spinal cord injuries by growing new nerve tissue
- Replacing damaged retinal cells in some forms of blindness
- Growing new heart muscle after a heart attack
Advantages of stem cell therapy
- Can treat conditions that have no other cure (paralysis, certain blindness, some cancers)
- Can replace cells exactly, restoring full function instead of just managing symptoms
- If the patient's own stem cells are used, there is no risk of rejection because the new cells carry the same antigens as the patient
Disadvantages and risks
- Ethical concerns about embryonic stem cells, since their use involves destroying a human embryo
- Risk of rejection if donor stem cells are used. The recipient is put on immunosuppressant drugs that stop their immune system from attacking the transplanted cells, but this leaves them more open to infections
- Risk of cancer: stem cells divide rapidly, and if they continue to divide uncontrollably after transplantation they can form a tumour
- Risk of viral infection carried over from the donor cells
- The technology is still expensive and experimental for most conditions