Cells
Often thought of as the smallest unit of a living organism, a cell is made up of many even smaller parts, each with its own function. Human cells vary in size, but all are quite small. Even the largest, a fertilized egg, is too small to be seen with the naked eye.
Human cells have a membrane that holds the contents together. However, this membrane is not just a sac. It has receptors that identify the cell to other cells. The receptors also react to substances produced in the body and to drugs taken into the body, selectively allowing these substances or drugs to enter and leave the cell. Reactions that take place at the receptors often alter or control a cell's functions. An example of this is when insulin binds to receptors on the cell membrane to maintain appropriate blood sugar levels and to allow glucose to enter cells.
Within the cell membrane are two major compartments, the cytoplasm and the nucleus. The cytoplasm contains structures that consume and transform energy and perform the cell's functions. The nucleus contains the cell's genetic material and the structures that control cell division and reproduction. Inside every cell are mitochondria. Mitochondria are tiny structures that provide the cell with energy.
 See the figure Inside a Cell.
The body is composed of many different types of cells, each with its own structure and function. Some, such as white blood cells, move freely, unattached to other cells. Others, such as muscle cells, are firmly attached one to another. Some cells, such as skin cells, divide and reproduce quickly; others, such as nerve cells, do not divide or reproduce at all under usual circumstances. Some cells, especially glandular cells, have as their primary function the production of complex substances, such as a hormone or an enzyme. For example, some cells in the breast produce milk, some in the pancreas produce insulin, some in the lining of the lungs produce mucus, and some in the mouth produce saliva. Other cells have primary functions that are not related to the production of substances--for example, muscle cells contract, allowing movement. Nerve cells conduct electrical impulses, allowing communication between the central nervous system (brain and spinal cord) and the rest of the body.
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