CELLS build the body. There are millions of them and lots of different types that are specific to their function. They all have a general structure.
Red Blood Cells
Red blood cells do not contain a nucleus, but they do contain haemoglobin. Haemoglobin can combine with oxygen. Red blood cells have a thin membrane and are biconcave in shape. This enables quick diffusion of oxygen into and out of the cell.
Nerve Cell
The long axon allows messages in the form of electrical impulses to travel all over the body. The axon is covered with a fatty sheath called the myelin sheath. This insulates the nerve cell and speeds up the nerve impulses.
Muscle Cell
Muscle cells contain filaments of protein that slide over each other to cause muscle contraction. They also contain many well-developed mitochondria to provide energy for muscle contraction.
Sperm Cell
Sperm cells have a tail, enabling the sperm to swim. The middle piece is packed with mitochondria to release the energy for the swim. The acrosome contains enzymes so that the sperm can penetrate the egg.