The word ‘cell’ derives from the Latin word cella, meaning a store or larder. The term was first used in 1665 by the microscopist Robert Hooke (1635—1703) to describe the spaces he observed in thin slices of cork. Hooke was actually looking at empty spaces left by cells, but the observation was important and led others to search for, and find, a cellular structure in all living things and to discover that the whole of biology is based on the cell.
Although the cell has long been known to be the structural unit of the body, knowledge of the internal features of the cell and of the immensely complex biochemical processes going on in every living cell is comparatively recent. The last fifty years or so have seen an explosive growth of knowledge of cell structure and function which has revolutionized medicine and is likely to have a greater effect on the future of mankind than any other branch of scientific advance.
The body is composed entirely of countless millions of cells and their products, and may be considered to be a community of cells. Most of the cells are stuck together to form tissues but many, such as those comprising the blood and those concerned with the immune system, are separate and free to move around.
Body cells vary greatly in size, from less than a hundredth of a millimetre across, in the case of red blood cells, to about a metre long, in the case of some nerve cells with very long nerve fibres (axons). The largest cell bodies are those of the egg (ovum), which is about a tenth of a millimetre across.
Cells require fuel to provide them with energy, and oxygen with which to burn up the fuel. Without such supplies they soon die. All cells are bathed in tissue fluid and supplies reach them by diffusion through this fluid.
The central part of the cell which, in most stained sections under the microscope, appears much more densely coloured than the rest of the cell, is called the nucleus and contains the chromosomes — the coiled-up lengths of DNA that form the genetic blueprint for the reproduction of the cell and for the synthesis of proteins. Surrounding the nucleus, within the cell, is the fluid cytoplasm. Each cell type has a different cytoplasm, but in all cells this is mainly water, sometimes up to 97 per cent. Dissolved in the water are substances such as proteins, including enzymes, amino acids (the ‘building bricks’ of proteins), nucleic acids, sugars (carbohydrates), sodium, potassium, calcium, magnesium, iron, copper, zinc, iodine and bromine.
Cell membranes are highly flexible structures made largely of a double layer of fat molecules, mainly fats (lipids) containing phosphorus (phospholipids, see below) held together by the basic forces that attract atoms to each other. They also contain cholesterol, an essential constituent of every cell, and are penetrated by many large functional protein molecules. These membranes are far more than bags for the cell constituents. They are complex and important structures containing specialized protein sites for the receipt of information from the external environment and others for the pumping of dissolved chemical substances into and out of the cell.
Our understanding of body function has been massively extended by the discovery that all cells possess such receptors, either on their surfaces, within their cytoplasm (cytosolic receptors), or on the membranes surrounding their nuclei. The chemical messengers, such as hormones or neurotransmitters are able to bind specifically to these receptors, and, in so doing, modify the function or actions of the cells. This, for instance, is how muscle cells are caused to contract and gland cells to secrete. Adrenaline receptors of three types occur on various cells and most have receptors for insulin. Many other hormones, prostaglandins and other chemical messengers bind to surface cell receptors. Steroids and the thyroid hormones enter cells and bind to receptors on the nuclei, prompting nuclear DNA to increase the transcription of particular genes.
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