Cell differentiation: In mature animals, cell division is mainly restricted to repair and replacement. As a cell differentiates it acquires different sub-cellular structures to enable it to carry out a certain function. It has become a specialised cell.

The idea

In mature animals, most cells have already become specialised, so mitosis is mainly limited to repairing damaged tissues and replacing worn-out cells, for example in skin, blood and the lining of the gut. Differentiation is the process by which a cell develops the sub-cellular structures needed for a particular job. A sperm cell gains many mitochondria and a tail for swimming; a nerve cell grows a long fibre to carry electrical impulses; a muscle cell develops protein filaments that can shorten. These changes make the cell specialised. Unlike many plant cells, mature animal cells usually cannot differentiate into a wide range of cell types, although some stem cells remain in tissues such as bone marrow.

What earns the marks

• In mature animals, cell division is mainly for repair and replacement rather than growth of new body parts.
• Examples of repair and replacement include healing skin wounds and replacing blood cells.
• Differentiation involves a cell acquiring different sub-cellular structures suited to a particular function.
• Specialised animal cells include sperm cells with many mitochondria and a tail, nerve cells with a long fibre, and muscle cells with contractile filaments.
• A specialised cell has a structure that enables it to carry out a certain function efficiently.
• Mature animal cells generally have a more limited capacity to differentiate than many plant cells, although some stem cells remain in tissues such as bone marrow.

Common mistakes and how to put them right

• Saying that mature animals grow mainly by cell division throughout the body.

  Put it right: In mature animals division is mainly for repair and replacement, while growth by cell division is more characteristic of young animals and plants.

• Thinking that differentiation only changes a cell's shape.

  Put it right: Differentiation also changes internal sub-cellular structures such as the number of mitochondria or the presence of a tail or long fibre.

• Confusing specialised cells with stem cells.

  Put it right: Stem cells are unspecialised and can differentiate, whereas specialised cells have already acquired structures for a particular function.

Exam tips

• Name a specific animal cell and link one sub-cellular structure to its function, for example many mitochondria in a sperm cell to release energy for swimming.
• Use the phrase 'repair and replacement' when describing division in mature animals, and give a named example such as skin or blood.
• Avoid saying that animal cells 'grow new organs'; focus on repair of damaged tissue and replacement of worn-out cells.
• When explaining differentiation, state that the cell acquires different sub-cellular structures, not just that it changes shape.