SECTION 4. BIOLOGICAL MEMBRANES

Main subjects:

4.1. Structure and composition of membranes

4.2. Transport of substances across membranes

4.3. Role of membranes in cell-cell interactions

4.4. Transmembrane signaling

All cells and their organelles are enclosed by membranes which play animportant role in the structural organization and functioning of cells and organelles.

Membranes:

• separate a cell from its environment and divide it into compartments (chambers);

• regulate the transport of substances into the cell and organelles or in the reverse direction;

• provide the specificity of cell-cell communication;

• receive signals from the external environment.

Coordinated functioning of membrane systems (receptors, enzymes, and transport mechanisms) helps to maintain the cellular homeostasis and at the same time rapidly react to fluctuations in the external environment.

The main principles of structural organization of all membranes are identical. However plasma membrane,

endoplasmic reticulum, mitochondrial, and nuclear membranes have substantial structural differences, they are unique in their composition and functions.

4.1. SCTRUCTURE AND COMPOSITION OF MEMBRANES

Biological membranes are composed of lipids and proteins bound to each other by non-covalent interactions (Fig. 4.1).

Fig. 4.1. The cross-section of plasma membrane

The basis of the membrane is a lipid bilayer composed of phospholipids, glycolipids, and cholesterol. Lipid bilayer is formed by two layers of amphiphilic molecules, the hydrophobic radicals of which are hidden inside, and hydrophilic groups are oriented to the exterior and contact with the aqueous medium. It looks as if protein molecules were dissolved in the lipid layer.

Membrane lipids