ABSTRACT
All living things are made of cells-small individually functional units which, in higher organisms, are organised into collections called tissues. A typical cell consists largely of cytoplasm, an aqueous liquid in which a wide range of biochemical processes occur. The cytoplasm is held as an intact unit by a cell membrane, which surrounds it and prevents it from mixing with its surroundings. Depending on the cell type and function, a number of other structures may be present, particularly a nucleus, in which the cell genetic information is stored in the form of DNA. Provision must be made for the supply and retention of substrates from extracellular sources and for the secretion of waste products that would otherwise accumulate in toxic amounts. The outer membrane of the cell must therefore allow penetration of some substances and not others, i.e. it must be selectively permeable. This is one of the most important features of the cell membrane.
