Molecular Biology of the Cell - Volume 1 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1994

Molecular Organization of Cells
Plasma Membrane

Cell/30.html">The Plasma Membrane surrounding each cell defines its boundaries and maintains the essential differences between the cellular contents and the environment. The membrane acts as a highly selective filter and is also responsible for Active Transport, regulating The entry of nutrients into The Cell and the exit of waste products. It also generates differences in ion concentrations between the intracellular and extracellular spaces. Another function of the membrane is to detect extracellular signals, enabling the cell to respond rapidly to environmental changes.

All Introduction/36.html">Biological Membranes, including the plasma membrane and the internal membranes of Eukaryotic Cells, share common structural features: they are assemblies of lipid and protein molecules held together by noncovalent interactions. These interactions maintain the structural integrity of membranes. However, cell membranes are dynamic, fluid structures, and most of their constituent molecules are free to move in the plane of the membrane. As shown in Fig. 6-1, lipid molecules form a continuous double layer about 5 nm thick. The Lipid Bilayer is the basic Structure OF THE membrane, providing a relatively impermeable barrier to most Water-soluble molecules. Protein molecules are, in a sense, dissolved in the lipid bilayer. They mediate the diverse Functions of the membrane. Some Membrane Proteins transport molecules into or out of the cell, while others are Enzymes that catalyze membrane-associated reactions. Another Class of proteins structurally links the plasma membrane to the Cytoskeleton on one side, and/or to the Extracellular matrix or an adjacent cell on the other. A distinct group consists of proteins that act as receptors to detect and transduce chemical signals from the environment. As might be expected, Membranes are asymmetric; their two leaflets differ in lipid and protein composition, reflecting the Functional differences between their two surfaces.

Although each type of membrane has its own characteristic lipid and protein components, the basic Structural and functional features discussed in this chapter are common to both intracellular and Plasma Membranes. First, we will examine the structure and Organization OF THE major components of all biological membranes: Lipids, proteins, and CARBOHYDRATES. We will then discuss the mechanisms used by cells to transport small molecules across the plasma membrane, as well as the ways in which cells import and export macromolecules and larger particles. Subsequent chapters will analyze some additional functions of the plasma membrane: its role in Cell Adhesion (Ch. 14) and in cell signaling (Ch. 12).

Fig. 6-1. Schematic three-dimensional diagram of a small section of a cell membrane.