As a cell differentiates it becomes specialised: it gains the structures that suit one job.

• Sperm cell: a tail to swim, many mitochondria to release energy for it, an acrosome of enzymes to get into the egg.
• Nerve cell: a long axon carries impulses a long way; branched endings connect to many cells.
• Muscle cell: protein fibres slide to shorten the cell; many mitochondria.
• Root hair cell: a long projection gives a big surface area for water and mineral ions.
• Xylem: dead cells with no end walls make hollow tubes, strengthened by lignin.
• Phloem: sieve plates let dissolved sugars flow; companion cells supply energy.

Most animal cells differentiate early in development. Afterwards, cell division is mainly for repair and replacement. Plants are different: meristems at the tips of shoots and roots keep making new cells that can become any kind of plant cell, all through the plant's life.

A stem cell is an undifferentiated cell that can divide to make more cells, which can differentiate.

• Embryonic stem cells can become any human cell.
• Adult stem cells (bone marrow) form a limited range, such as blood cells.
• Meristem cells clone plants quickly and cheaply: rare species, or crops with disease resistance.

Stem cells might one day treat diabetes or paralysis by replacing damaged cells. In therapeutic cloning an embryo is made with the patient's own genes, so the cells it gives are not rejected.

To weigh it up (a classic 6-marker), give points on both sides and finish with a judgement:

• for: could cure, not just treat; spare IVF embryos would otherwise be destroyed; no rejection with therapeutic cloning
• against: an embryo could become a person; a risk of passing on viruses; cells could divide out of control; cost

"Evaluate" needs a conclusion. A list of pros and cons alone stops at 4 marks.