Cells are protected by an outer membrane, called the phospholipid bilayer. This bilayer is often described as a fluid mosaic model because of the variety of molecules that are found within the cell membrane. It is a mosaic of phospholipids, steroids, proteins, and glycolipids, each of which serves a different purpose and plays a different role in cellular functions. The fluid aspect of the mosaic model is the free movement of both lipid and protein components within a membrane so that the movement occurs in the lipid bilayer. In addition to offering protection, the cell membrane also helps regulate the transport of materials into and out of the cell. Solute transport within a membrane involves several types of molecules: small nonpolar molecules, uncharged polar molecules, and charged molecules. Both passive and active transport occurs, and bulk transport of large molecules occurs by means of exocytosis and endocytosis.
At A Glance
- The cell membrane is a phospholipid bilayer studded with different kinds of proteins.
Membrane phospholipids have hydrophilic heads that face outward; during membrane assembly, enzymes from the Golgi apparatus may leave the cell through vesicles that fuse with the membrane. The enzymes alter the positions of phospholipids, resulting in different arrays of phospholipids on the outer and inner sides of the membrane.
- Different kinds of cell membrane proteins perform a variety of functions; some proteins are confined to certain regions, or domains, of the cell membrane.
- The lipid bilayer is permeable to some solutes but not others.
Passive transport requires no energy expenditure other than kinetic energy, but active transport moves a solute against its concentration gradient and thus requires energy.
Membrane potential enables cells to transport materials and permits some kinds of cells to communicate with each other.
Exocytosis and endocytosis enable cells to expel or take in large molecules or larger particles.