Cells are the basic units of life. Cells can function as individual organisms or can be part of higher organizational units. The outer layer of cytoplasm is in a gel state and called ectoplasm , and the more fluid inner is called endoplasm . Single cells are small, since these organisms need lots of surface area for diffusion and other processes, but a small volume for the nucleus to control all of the cytoplasm.
Membranes are fluid and constantly changing.
This allows the controlled passage of material and to define inside from outside. Cholesterol can be found wedged in the hydrophobic portion, helping to make the membrane less permeable and more rigid. Nucleus , surrounded by the nuclear envelope , also contains the nucleolus. It is the control and information center of the cell. It directs the types of biochemical activities that go on inside the cell, and also acts as the repository of information, storing genetic material and information for the next generation.