Cells and Organelles

The basic structural unit of life is the cell. All organisms are made of one or more cells. Cells contain all the molecules needed for life packaged in a living wrapper called the cell membrane or plasma membrane. This membrane isolates the gelatinous contents of the cell, or cytoplasm, from the outside world. The cell membrane allows some sustances to pass in or out but prevents others from doing so.

There are a number of structures within the cytop-lasm. Among the most important are membranes 

similar to the outer cell membrane. These interior memranes act as sites for such chemical processes as photosynthesis and respiration. Membranes may also devide the cell into compartments that enclose still more complex structures specialized for particular tasks. These membrane-bound structures are known as organelles. It is the presence of organelles that separates the two major types of cells.