Cytoplasmic Membrane (Plasma Membrane)

The cytoplasmic membrane is a 40–80 A ˚ -thick semipermeable membrane that contains a phospholipid bilayer with proteins embedded within the bilayer (fluid mosaic model) (Fig. 1.3). The phospholipid bilayer is made of hydrophobic fatty acids oriented towards the inside of the bilayer and hydrophilic glycerol moieties oriented towards the outside of the bilayer. Cations such as
Ca2+ and Mg2+ help stabilize the membrane structure.
 

Sterols are other lipids that enter into the composition of plasma membranes of eukaryotic cells as well as some prokaryotes, such as mycoplasma (these bacteria lack a cell wall). Chemicals cross biological membranes by diffusion, active transport, and endocytosis.

Diffusion. Because of the hydrophobic nature of the plasma membrane, lipophilic compounds diffuse better through the membrane than ionized compounds. The rate of diffusion across cell membranes depends on their lipid solubility and concentration gradient across the membrane.

Active transport. Hydrophilic compounds (that is, lipid insoluble) may be transferred through the membrane by active transport. This transport involves highly specific carrier proteins, requires energy in the form of adenosine triphosphate (ATP) or phosphoenol-pyruvate (PEP) and allows cells to accumulate chemicals against a concentration gradient. There are specific active transport systems for sugars, amino acids, and ions.

Toxic chemicals gain entry into cells mainly by diffusion and some may use active transport systems similar to those used for nutrients. Endocytosis. In eukaryotic cells, substances can cross the cytoplasmic membranes by endocytosis, in addition to diffusion and active transport. Endocytosis includes phagocytosis (uptake of particles) and pynocytosis (uptake of dissolved substances).