Stem cells: Students should be able to describe the function of stem cells in embryos, in adult animals and in the meristems in plants.

The idea

Stem cells have different roles depending on where they are found. In an embryo, stem cells are pluripotent: they can differentiate into almost any cell type, so they build all the tissues and organs of the developing organism. In adult animals, stem cells are found in tissues such as bone marrow and are used to replace cells that are worn out or damaged, for example making new blood cells throughout life. In plants, stem cells sit in meristems at growing tips of shoots and roots and in cambium; they divide throughout life to produce new plant tissues, allowing roots and shoots to grow and damaged parts to be replaced. A useful comparison is that embryonic stem cells build the body once, adult stem cells maintain and repair it, and meristem cells keep a plant growing for its whole life.

What earns the marks

• Describes embryonic stem cells as able to differentiate into almost all cell types, forming the tissues and organs of the developing embryo.
• Describes adult animal stem cells as replacing worn-out or damaged cells in specific tissues, such as bone marrow stem cells forming new blood cells.
• Describes meristem stem cells in plants as dividing to produce new cells for growth at shoot and root tips and for repair.
• Compares the three sources, for example noting that embryonic stem cells are more versatile than adult stem cells, while meristem cells allow continued growth throughout a plant's life.
• Links each function to a named location, such as bone marrow in adult animals or meristems at shoot tips in plants.
• Explains that plant meristems allow growth and replacement of damaged tissues because plants grow continuously.

Common mistakes and how to put them right

• Saying adult stem cells can become any cell type.

  Put it right: Adult stem cells are usually limited to the cell types of the tissue where they are found, such as blood cells from bone marrow.

• Confusing meristems with all plant cells.

  Put it right: Meristems are specific regions of unspecialised, dividing cells at growing points, not every cell in a plant.

• Stating that embryonic stem cells only exist in adults or that adult stem cells build the whole embryo.

  Put it right: Embryonic stem cells form the embryo's tissues, while adult stem cells maintain and repair tissues after development.

Exam tips

• Answer in three clear parts: embryos, adult animals, plants; this matches the wording of the statement and keeps each function distinct.
• Name a location for each type, such as bone marrow for adult animals and shoot or root tips for plant meristems, to make the description precise.
• Use comparative language such as 'more versatile than' or 'throughout life' when contrasting embryonic, adult and meristem stem cells.