Overview: Life at the Edge
The plasma membrane separates the living cell from its nonliving surroundings. This thin barrier, 8 nm thick, controls traffic into and out of the cell. Like all biological membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others. Concept 7.1 Cellular membranes are fluid mosaics of lipids and proteins
The main macromolecules in membranes are lipids and proteins, but carbohydrates are also important. The most abundant lipids are phospholipids.
Phospholipids and most other membrane constituents are amphipathic molecules. Amphipathic molecules have both hydrophobic regions and hydrophilic regions. The arrangement of phospholipids and proteins in biological membranes is described by the fluid mosaic model. Membrane models have evolved to fit new data.
Models of membranes were developed long before membranes were first seen with electron microscopes in the 1950s. In 1915, membranes isolated from red blood cells were chemically analyzed and found to be composed of lipids and proteins. In 1925, E. Gorter and F. Grendel reasoned that cell membranes must be a phospholipid bilayer two molecules thick. The molecules in the bilayer are arranged such that the hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact with water. Actual membranes adhere more strongly to water than do artificial membranes composed only of phospholipids. One suggestion was that proteins on the surface of the membrane increased adhesion. In 1935, H. Davson and J. Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins. Early images from electron microscopes seemed to support the Davson-Danielli model, and until the 1960s, it was widely accepted as the structure of the plasma membrane and internal membranes. Further investigation revealed two problems.
First, not all membranes were alike. Membranes differ in thickness, appearance when stained, and percentage of proteins. Membranes with different functions differ in chemical composition and structure. Second, measurements showed that membrane proteins are not very soluble in water. Membrane proteins are amphipathic, with hydrophobic and hydrophilic regions. If membrane proteins were at the membrane surface, their hydrophobic regions would be in contact with water. In 1972, S. J. Singer and G. Nicolson presented a revised model that proposed that the membrane proteins are dispersed and individually inserted into the phospholipid bilayer. In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in maximum contact with water, and the hydrophobic regions are in a nonaqueous environment within the membrane. A specialized preparation technique, freeze-fracture, splits a membrane along the middle of the phospholipid bilayer. When a freeze-fracture preparation is viewed with an electron microscope, protein particles are interspersed in a smooth matrix, supporting the fluid mosaic model. Membranes are fluid.
Membrane molecules are held in place by relatively weak hydrophobic interactions. Most of the lipids and some proteins drift laterally in the plane of the membrane, but rarely flip-flop from one phospholipid layer to the other. The lateral movements of phospholipids are rapid, about 2 microns per second. A phospholipid can travel the length of a typical bacterial cell in 1 second. Many larger membrane proteins drift within the phospholipid bilayer, although they move more slowly than the phospholipids. Some proteins move in a very directed manner, perhaps guided or driven by motor proteins attached to the cytoskeleton. Other proteins never move and are anchored to the cytoskeleton. Membrane fluidity is influenced by temperature. As temperatures cool, membranes switch from a fluid state to a solid state as the phospholipids pack more closely. Membrane fluidity is also influenced by its...
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