The Cell — Structure and Methods of Observation

Abstract

The body is composed of small, membrane-bound units of protoplasm called cells. Each cell can usually carry out all of the processes, such as respiration, excretion and division, necessary for the maintenance of life. Cells vary enormously in size from 5–8 μm (red blood cells) to several hundred micrometres in diameter (egg cells), although most fall within the 5–30 μm range. Cells are also of many diverse forms, with their shape often reflecting their function. Thus, nerve cells tend to be elongated for the transmission of messages along their cytoplasmic processes. Within the body, like cells are usually grouped together to form various sorts of tissues. These tissues, however, are rarely composed of just one cell type, although one kind of cell usually predominates.