BASIC PRINCIPLES OF BIOMEMBRANES’ ORGANISATION

STRUCTURE AND FUNCTIONS OF MEMBRANES

The main cell membrane structures are plasma membrane, endoplasmic reticulum (and rough), Golgi apparatus, mitochondrial and nuclear membranes.

Function of membranes: barrier, control of transport of metabolites; signal reception and transmission; enzyme; contact with other cells; binding to cytoskeleton.

Each membrane has structural features and performs specific functions in the cell, but they are all built by the same type. Most membranes contain 50-75% of protein, and the rest are lipids. Carbohydrates are presented only in glycoproteins and glycolipids. Water makes up 20%, while the ratio of protein / lipid, depending on the type of membrane varies from 0.25 (the myelin sheath) to 3.0 (mitochondrial membrane).

Lipids play the major role in the formation of membranes as cellular structures: lamellar, " membrane" form and the basic physical and chemical properties of the membrane are determined precisely by lipids. Membrane lipids are represented by four major groups:

1. Phospholipids (90%) - glycerophospholipids and sphingophospholipids:

 

Phosphatidylcholine

 

2. Sphingolipids (sphingomyelins) - Lipids containing amino alcohol sphingosine.

ceramide

3. Glycolipids - carbohydrate compounds in which the carbohydrate part of the covalently bound lipid.

  galactozilseramide

A characteristic feature of the molecules of phospholipids and glycolipids is their amphipathic charactere, one end of the molecule is hydrophobic and the other is hydrophilic.

4. Steroids are represented by cholesterol:

Proteins may be completely or partially embedded in the membrane ( intrinsic, or integral proteins and semi integral proteins ) or placed on its surface ( extrinsic, or peripheral proteins ). Sites of protein, which are turned into the extracellular medium, may be glycosylated.

The protein composition of different membranes is different. Membrane proteins perform different functions: structural proteins, enzymes, proteins carrying out the transmembrane transport of metabolites, hormone receptors.

Biological membranes are composed of a lipid bilayer of the small thickness (6 - 10 nm). The membranes are asymmetric in their original structure, which provides the gradient of the curvature and the spontaneous formation of closed structures.

Typically, plasma membrane proteins are highly glycosylated from the extracellular medium. Intracellular membranes contain few glycoproteins and glycolipids and are characterized by lower microviscosity. Therefore, they can form small organelles.

Cell membranes form significant limitations to the transfer of substances. The main obstacle is the hydrophobic region of the membrane. However, membranes are not impermeable structures. One of the main functions of membranes is regulation of substances transfer.

Fig. 15. The structure of the biological membrane

 

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