What Differentiation Is

1. Every body cell in an organism is produced by mitosis and therefore carries the same genes.
2. Differentiation is the process that turns one of these unspecialised cells into a cell with a particular job.
3. It happens because the cell switches on only some of its genes and keeps the rest switched off.
4. Once a cell has differentiated it usually keeps that identity for the rest of its life.

How a Cell Becomes Specialised

1. The genes that are switched on are transcribed and translated, so the cell makes a particular set of proteins.
2. Those proteins determine the cell's structure, because they build the sub-cellular structures it needs.
3. A cell that switches on the genes for haemoglobin fills with that protein and loses its nucleus, becoming a red blood cell.
4. A neighbouring cell with the identical genome switches on a different set and becomes something else entirely.

Why Specialisation Matters

1. A specialised cell is far more efficient at one job than an unspecialised cell would be at everything.
2. Specialisation allows a division of labour, so different cells take on different tasks and the whole organism works better.
3. Specialised cells of the same type group together into a tissue, tissues form organs, and organs form organ systems.
4. Without differentiation an organism could only ever be a shapeless ball of identical cells, so no tissues, organs or organ systems could exist.
5. This is why differentiation is essential for the development of a fertilised egg into a complex multicellular organism.

Examples of Specialised Cells

1. Sperm cell. A tail for swimming, many mitochondria to release the energy for movement, and an acrosome containing enzymes to digest through the egg membrane.
2. Red blood cell. A biconcave shape for a large surface area, packed with haemoglobin, and no nucleus so there is more room for oxygen.
3. Root hair cell. A long extension that increases the surface area for absorbing water and mineral ions from the soil.
4. Neurone. A very long axon so an impulse can be carried a long way, and a myelin sheath that speeds transmission up.
5. Palisade mesophyll cell. Tall, tightly packed and full of chloroplasts near the top of the leaf, where the light is strongest.
6. Xylem vessel. Dead, hollow and open ended, with walls strengthened by lignin, forming a continuous tube for water.

Differentiation in Animals and Plants

1. In animals most differentiation happens in the embryo and in the young animal.
2. Adult animals keep a small number of stem cells, in places such as bone marrow, and these are used mainly to replace worn out cells.
3. In plants many cells keep the ability to differentiate throughout the plant's life.
4. This is why a small cutting taken from a stem can grow roots and become a complete plant, while a piece of animal tissue cannot.

When Differentiation Goes Wrong

1. Cancer cells are often described as poorly differentiated, because they divide instead of taking on a specialised job.
2. Under a microscope the cells of a malignant tumour look irregular and unlike the tissue they came from.
3. Losing specialisation means the tissue can no longer do its job properly, which is one reason a tumour damages the organ it grows in.