4.1. General Description of Membranes: Composition and Structure
Membranes embraces all cells and cell organelles, playing an important role in maintaining their structure and functions. All membranes have general structural characteristics. However, the plasma membrane, as well as membranes of the endoplasmic reticulum, Golgi apparatus, mitochondria and the nuclei have prominent compositional specificity; they all have unique components and, therefore, unique functions.
Main functions of membranes are:
► separation of a cell or organelle from the environment, creating the specific inner content;
► regulation of transport into the cells and organelles and back;
► determination of the specificity of the intercellular contacts;
► perception of the signals like hormones or other signaling molecules from the extracellular environment.
The coordinated functioning of membrane systems, including receptors, enzymes, transporters ensures the cell homeostasis and quick response to environmental changes by intracellular metabolism regulation.
Biological membranes consist of lipids and proteins, connected via non-covalent bonds. Lipids form lipid bilayer and protein molecules embedded into it (Fig. 4.1). Lipid bilayer is formed by two layers of amphiphilic molecules, mainly phospholipids and cholesterol, which interact with each other by their hydrophobic parts, forming inner hydrophobic layer of membrane, and hydrophilic groups - «polar heads» - turned outwards and contact with water medium.
Lipids of the membranes. Three main types of lipids form the membrane: glycerophospholipids, glycolipids, and cholesterol (Ch. 7.1, Table 7.1). The most commonly found are glycerophospholipids, derivatives of phosphatidic acid (Ch. 7.9, Fig. 7.31). The main glycerophospholipid in most of the membrane types is phosphatidylcholine (Fig. 4.2).