The cell is the basic unit of structure and function of all living organisms, and the size and form of human cells are highly dependent on their environment and function. The structure of a cell has three main parts: the cell membrane, the cytoplasm and the nucleus. When cells are observed under an electron microscope, they can be distinguished into membrane phase structures and non-membrane phase structures. The cell membrane is a thin film on the surface of the cell, which is about 7.5 nm thick. The chemical composition of the cell membrane is mainly lipids, proteins and certain amount of sugars. Under electron microscopy, the cell membrane can be seen to be structured in three layers, with two dark inner and outer layers and a lighter middle layer. In addition, the cytoplasm is the part of the cell between the cell membrane and the nucleus, including the organelles and the stroma.