On the evidence of electron micrographs, biologists believe that it has pores through which certain small molecules pass intact. Small chambers located on the cell membrane called caveolae gather chemical signals and channel them into the cells. The caveolae may also help distribute proteins throughout the cell. Large molecules enter the cell by a process called diffusion, in which they may be dissolved by substances in the cell membrane. They can then pass through the membrane without difficulty.
Some cells take in large molecules by means of pinocytosis.
In this process, the cell membrane forms a pocket around large molecules floating against it. Finally, the vacuole wall breaks up and the molecules are released into the cytoplasm. The cytoplasm is mainly water. Its water content varies from a minimum of about 65 percent to a maximum of about 95 percent. The solids in the cytoplasm include granular proteins, carbohydrates, droplets of fat, and pigments.