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.