Stem cells — AQA GCSE Biology

Stem cells have different roles depending on where they are found. In embryos, stem cells are pluripotent: they can differentiate into almost any cell type, which allows the embryo to develop all its tissues and organs. In adult animals, stem cells are found in tissues such as bone marrow and can replace worn-out or damaged cells, for example making new blood cells, but they are usually more limited in the cell types they can become. In plants, stem cells are found in meristems at the tips of roots and shoots; they divide to produce new cells for growth throughout the plant's life and can form any plant tissue. You should be able to compare these three locations and link each to its function, such as embryonic development, repair in adults, and unlimited growth in plants.

Human embryonic stem cells are pluripotent, so they can be grown in the laboratory and stimulated to differentiate into most human cell types, such as nerve cells, muscle cells or insulin-producing cells. Cloning here means producing many genetically identical copies of these cells by mitosis in culture, giving a large supply for research or potential medical use. Scientists control differentiation by changing the culture conditions, for example adding specific growth factors or chemicals. This makes embryonic stem cells valuable for treating conditions such as diabetes or paralysis, but their use raises ethical issues because obtaining them involves destroying early embryos. You should be able to explain both the scientific potential and the ethical concerns, and to describe how cloning and differentiation are achieved in principle.

Adult stem cells are unspecialised cells found in tissues such as bone marrow. They divide by mitosis and differentiate into a limited range of specialised cells, for example red and white blood cells (as well as forming cell fragments called platelets). This makes bone marrow important for replacing blood cells throughout life. A bone marrow transplant can introduce healthy stem cells into a patient, for example to treat leukaemia, where donated stem cells divide and differentiate to restore healthy blood cell production. The statement stresses two ideas: the source is adult bone marrow, and the resulting cells include blood cells. Adult stem cells cannot form every cell type, so their potential is described as limited rather than unlimited.

Meristem tissue is found in growing regions of plants, such as shoot tips, root tips and cambium. The cells there remain unspecialised and can divide by mitosis. Unlike most animal cells, plant meristem cells retain the ability to differentiate into any type of plant cell for the whole life of the plant. This allows plants to grow new leaves, stems, roots and flowers, and to replace damaged parts. For example, cells from a shoot tip meristem can be grown in culture and treated with plant hormones to produce roots and shoots, forming a clone of the parent plant. The statement therefore covers three linked ideas: the tissue is meristem, its cells can become any plant cell type, and this ability persists throughout the plant's life.

Stem cell therapy is an area of medicine that aims to replace damaged or missing cells. In diabetes, some cases involve cells in the pancreas that no longer produce enough insulin; stem cells might be guided to become functioning pancreas cells. In paralysis, damage to nerve cells in the spinal cord may be irreversible; stem cells might be used to replace or repair those nerve cells. The word may is important because many of these treatments are still being researched and are not yet routine. Scientists must test safety and effectiveness, and there are ethical issues around the use of embryos as a stem cell source. A balanced answer should present both the potential benefit and the uncertainty or ethical concerns.

Therapeutic cloning is a technique that produces an embryo genetically identical to a patient. The nucleus is removed from an egg cell, and a nucleus from one of the patient's body cells is inserted into the empty egg cell. The egg cell is stimulated to divide by mitosis, forming an early embryo. Stem cells are then removed from this embryo and cultured. Because the embryo's genes came from the patient, the stem cells have the same genes as the patient. This reduces the risk that the patient's immune system will reject the transplanted cells. The embryo is not allowed to develop into a baby; it is used as a source of stem cells for treatment.

In therapeutic cloning, an embryo is produced with the same genes as the patient. Stem cells from this embryo are not rejected by the patient's immune system because they contain the exact same genetic material and therefore the same surface antigens. This makes them highly valuable for medical treatments, as they can differentiate into any type of human cell to replace those lost to disease or injury, such as in diabetes or paralysis. Without therapeutic cloning, donor stem cells might be rejected unless immunosuppressant drugs are used. Understanding this mechanism highlights why these specific embryonic stem cells are so useful in regenerative medicine.

Stem cell treatments are not risk-free. Cells grown in the laboratory can carry viruses, and if those cells are transplanted into a patient, a viral infection could be transferred. There is also a risk that stem cells divide in an uncontrolled way and form tumours, although the statement highlights viral transfer. Separately, some people object on ethical or religious grounds. A common ethical objection is that obtaining embryonic stem cells involves destroying an early embryo, which some people regard as a potential human life. Religious views may also hold that life begins at conception. These objections do not mean the science is invalid; they mean that decisions about using stem cells involve values as well as evidence. Examiners expect balanced discussion of benefits and risks.

Meristems are regions of a plant where unspecialised cells divide rapidly, for example at the tips of roots and shoots. These cells are plant stem cells. Because they can divide and differentiate into any plant cell type, a small piece of meristem tissue can be grown in culture to produce many genetically identical plants, called clones. The process is quick because meristem cells divide rapidly, and it is economical because a large number of plants can be produced from a small amount of starting material in a controlled space. Growers can use this to multiply a plant with desirable features, such as disease resistance or high yield, without waiting for seeds. The statement links the source of the cells, the method of cloning, and the advantages of speed and low cost.

Cloning produces genetically identical offspring from a single parent cell or organism. In conservation, cloning a rare species can increase numbers when natural breeding is difficult, creating a genetic backup that may protect the species from extinction. In agriculture, cloning a crop plant that shows a special feature, such as disease resistance, produces many identical plants for farmers. Because the offspring are genetically identical, each should show the same useful feature, giving a reliable crop. However, identical plants share the same weaknesses, so a new disease could affect the whole crop. Cloning therefore supports conservation and food production but reduces genetic variation, which can make populations vulnerable to changing conditions.