Specialised animal cells: ciliated, nerve and red blood cells

1One cell, then trillions

You began life as a single cell. Today your body contains roughly 30 million million cells, of about two hundred different kinds, and almost every one carries the same DNA as that first cell. Yet a nerve cell in your leg can be a metre long, while a red blood cell is less than a hundredth of a millimetre across and has no nucleus at all. How can cells with the same instructions end up so different? Each has used only part of its instructions, and grown into the shape that suits one job.

2Observation: three cells, three shapes

Compare the three photographs in this lesson: a smear of blood, a nerve cell from the spinal cord, and the lining of a tube in the body. Before you read on, write down one feature of each cell's shape and guess what it might be for.

31. The ciliated cell: keeping the airways clean

Where: lining the windpipe (trachea) and the bronchi, and also the oviducts, the tubes that carry an egg from the ovary.
Job: to move mucus, or an egg, along the tube.

Figure 1: Ciliated cells and a goblet cell lining the windpipe.

• Cilia: hundreds of tiny hair-like structures on the free surface. They beat together in waves, like a field of grass in the wind, and sweep mucus towards the throat, where it is swallowed.
• Many mitochondria near the cilia release the energy needed to keep them beating.
• Goblet cells between them make the sticky mucus that traps dust and bacteria.

42. The nerve cell: sending messages fast

Where: the brain, the spinal cord and the nerves.
Job: to carry electrical impulses from one part of the body to another.

Figure 2: A motor nerve cell, which carries impulses to a muscle.

• A long axon, up to a metre from the spinal cord to the foot, carries impulses over long distances in one cell.
• Dendrites, many short branches on the cell body, connect with many other nerve cells to receive messages.
• A myelin sheath, a fatty layer wrapped round the axon, insulates it and makes impulses travel much faster.
• Nerve endings pass the message on to a muscle or to another nerve cell.

53. The red blood cell: carrying oxygen

Where: in the blood, in every blood vessel.
Job: to carry oxygen from the lungs to every cell of the body.

Figure 3: The adaptations of a red blood cell.

• Haemoglobin: the cytoplasm is packed with this red, iron-containing protein, which picks up oxygen in the lungs and releases it in the tissues.
• No nucleus when mature, which leaves more room for haemoglobin.
• Biconcave disc shape: thin in the centre, so it has a large surface through which oxygen diffuses in and out quickly, and no point inside is far from the surface.
• Small and flexible: about 7.5 µm across, so it can bend and squeeze through capillaries narrower than itself.

6Red blood cells, blood donors and malaria

A red blood cell has no nucleus, so it cannot divide or repair itself; it wears out after about 120 days. Your bone marrow replaces them all the time, at about two million new cells every second. This is why a healthy adult can give blood to Rwanda's national blood transfusion service every few months without harm, and why a shortage of iron, needed for haemoglobin, quickly causes anaemia (unit 7). The malaria parasite attacks exactly these cells: it lives and multiplies inside red blood cells and bursts them.