Cell Division (AQA GCSE Biology Combined Science): Revision Notes
Stem cells
What are stem cells?
Stem cells are special cells that haven't become specialised yet. They are undifferentiated, which means they haven't turned into specific cell types like nerve cells or muscle cells.
Stem cells can do two important things:
- Make copies of themselves
- Turn into specialised cells through a process called differentiation
The ability of stem cells to remain undifferentiated while maintaining the potential to become any specialised cell type makes them incredibly valuable for both natural development and medical research.
Stem cells in animals
Zygotes and embryos
When a sperm cell joins with an egg cell, it forms a single cell called a zygote. This zygote divides many times to create a ball of cells called an embryo.
The embryo contains embryonic stem cells. These are very powerful because they can turn into any type of cell in the body.
Embryonic stem cells are considered totipotent or pluripotent - this means they have the remarkable ability to become literally any cell type found in a fully developed organism.
Adult stem cells
Adult animals still have some stem cells even after they're fully grown. The main place to find these is in the bone marrow inside bones.
Adult stem cells in bone marrow can make different types of blood cells, including red blood cells and white blood cells.
Bone marrow is like a stem cell factory for blood cells. It continuously produces millions of new blood cells throughout an animal's life to replace old or damaged ones.
Stem cells in plants
Plants have stem cells in special tissues called meristems. You can find meristem tissue at:
- The tips of roots
- The tips of shoots
Plant stem cells can differentiate throughout the plant's whole life to make:
- Root hair cells for absorbing water
- Xylem cells for transporting water
- Phloem cells for transporting food
- Mesophyll cells in leaves for photosynthesis
Unlike animals, plants keep their stem cells active throughout their entire lives. This is why plants can continue growing and producing new tissues even when they're very old - some trees can live for thousands of years!
Medical uses of stem cells
Scientists believe stem cells could revolutionise medicine by helping treat serious medical conditions. The potential applications include treating:
- Diabetes - by replacing damaged pancreas cells
- Paralysis - by replacing damaged nerve cells
The idea is to inject healthy stem cells into patients so they can replace the damaged or missing cells.
This field of medicine is called regenerative medicine - the goal is to regenerate or regrow damaged tissues and organs rather than just treating symptoms.
Types of stem cells for treatment
Embryonic stem cells
Advantages:
- Easy to extract from embryos
- Can turn into any type of cell
Disadvantages:
- The embryo gets destroyed when cells are removed
- Some people think this is wrong
Adult stem cells
Advantages:
- No embryo is harmed
- Can be taken from the patient being treated
- The body won't reject them
Disadvantages:
- Can only make a few types of cells
Risks of all stem cells
- Stem cells might keep dividing and cause cancer
- Risk of viral infections
Safety First: All stem cell treatments must go through extensive testing and clinical trials before they can be used on patients. The risks must be carefully weighed against the potential benefits.
Cloning and stem cells
Clones are living things that have exactly the same genes. They are genetically identical.
Uses of cloning
- Saving rare plant species from extinction
- Making large numbers of useful crop plants
Therapeutic cloning
This is a specialised technique that combines cloning technology with stem cell research to create patient-specific treatments.
Worked Example: Therapeutic Cloning Process
Step 1: Take the nucleus from a patient's cell
Step 2: Put it into an empty egg cell
Step 3: Make the egg cell divide to form an embryo
Step 4: Extract stem cells from this embryo
Result: The stem cells will have the same genes as the patient, so the body won't reject them when used for treatment.
The advantage is that these stem cells will have the same genes as the patient, so the body won't reject them.
Key Points to Remember:
- Stem cells are undifferentiated cells that can make copies of themselves and turn into specialised cells
- Animal stem cells are found in embryos and adult bone marrow
- Plant stem cells are found in meristems at root and shoot tips
- Stem cells could help treat diseases like diabetes and paralysis
- Different types of stem cells have different advantages and disadvantages for medical treatment
- Cloning can be used to save rare species and produce stem cells for treatment