Cell Membrane Structure: Lipids, Proteins, and Fluidity
Cell Membrane Structure
The plasma membrane, a thin layer of lipids and proteins, separates a cell from its surrounding. To maintain their internal environment, the cells of all organisms must constantly exchange molecules and ions with the fluid environment that surrounds them. Transport proteins move particular ions and molecules, including water, in a directed way across the membrane.
Lipids in Biological Membranes
Biological membranes consist of lipids. Phospholipids and sterols (such as cholesterol) are the two major types of lipids in membranes. The membrane separates the aqueous solution outside the cell from the aqueous solution inside the cell. Phospholipids have a polar (electrically charged) end containing a phosphate group, and a nonpolar (uncharged) end containing two nonpolar fatty-acid tails.
Phospholipids have dual solubility properties; the polar end is hydrophilic and the nonpolar end is hydrophobic. In an aqueous medium, phospholipid molecules assemble into a bilayer (a layer two molecules thick).
- In a bilayer, the polar ends of the phospholipid molecules are located at the surfaces, where they face the aqueous media. The nonpolar fatty-acid chains assemble in the membrane interior, in a nonpolar region that excludes water. At low temperatures, the phospholipid bilayer freezes into a semisolid, gel-like state. When a phospholipid bilayer is shaken in water, it breaks and spontaneously forms small vesicles.
Membrane Sterols
Membrane sterols also have dual solubility characteristics.
Sterols (such as cholesterol) have nonpolar carbon rings with a nonpolar side chain at one end and a single polar group (—OH) at the other end. In a phospholipid bilayer, sterols align with the nonpolar interior, with the polar end extending into the polar surface. Cholesterol, the main sterol in animal membranes, is important for keeping the membranes fluid.
Membrane Proteins
There are proteins embedded inside the plasma membrane called integral proteins, on the surface are peripheral proteins. Carbohydrates attached to proteins are called glycoproteins and carbohydrates attached to lipids are called glycolipids and are found on the outer surface of cell membranes.
- Transport proteins form channels that allow selected polar molecules and ions to pass across a membrane.
- Recognition proteins in the plasma membrane identify a cell as part of the same individual or as foreign.
- Receptor proteins recognize and bind molecules from other cells that act as chemical signals, such as hormones.
- Cell adhesion proteins bind cells together by recognizing and binding receptors or chemical groups on other cells.
The Fluid Mosaic Model
The fluid mosaic model proposes that the membrane consists of a fluid phospholipid bilayer in which proteins are embedded and float freely. “Fluid” refers to the phospholipid molecules, which constantly move and exchange places within the same layer. Unsaturated fatty acid chains in membrane phospholipids help keep membranes fluid at low temperatures. At high temperatures, the rigid rings of cholesterol help reduce membrane fluidity.
“Mosaic” refers to membrane proteins – most float individually in the lipid bilayer, some are attached to the cytoskeleton. Particular functional groups and active sites face either inside or outside the cell. Integral proteins are embedded in the phospholipid bilayer. Peripheral proteins are held to membrane surfaces by noncovalent bonds.