A society of cells
How do cells organise in an organism?
The human body is a closely organised community of cells. Tissues (collections of similar cells) work together in groups to form an organ. These organs work with each other in organ systems. To keep the whole organism functioning correctly, most organs or organ systems also have to respond to the state of other organs, which may be a long way away.
The liver, for instance, is made up of 80 per cent of one kind of cell: hepatocytes, which make, store or secrete many proteins, fats and digestive enzymes. The remaining 20 per cent consists of several other specialised liver cell types, along with blood vessels, nerve cells and others. All of them have to work together for the liver to do its job.
For a smooth-running community, cells need good communication. They manage this in different ways, but all use either chemical or electrical signals. Nerve cells, for example, send electrical impulses down their long fibres, but communicate with the next cell in the chain by sending chemicals – called neurotransmitters – across a narrow gap at the synapse.
Heart muscle cells, by contrast, join to their neighbours via gap junctions. At these points, the cells are close enough together for protein complexes with tiny channels inside to span both cell membranes and allow small molecules and ions to pass between them. This allows waves of electrical activity, or depolarisation, to pass along a series of heart cells and keep the heart beating regularly.