BIOLOGY Lecture Notes - Golden Pages 2003

1. THE CELL

Structure of the Plasma Membrane

According to research, the membrane has a fluid-mosaic Structure (S. Singer and G. Nicolson, 1972).

The membrane matrix is based on a lipid bilayer structure. A large proportion of Membrane Lipids consists of Phospholipids, which are capable of forming a bilayer consisting of an inner hydrophobic region (the aliphatic part of Fatty acids or the steroid Skeleton of Cholesterol) and hydrophilic surfaces (glycerol, a phosphoric acid residue, amino alcohol, and OH-fatty acid). Therefore, fat-soluble substances can penetrate through membranes, whereas Water-soluble substances and hydrophilic ions are unable to cross the hydrophobic region; they enter The Cell via specialized protein permeability channels. The bulk of the lipids contained in Biomembranes are polar lipids (Glycerophospholipids, sphingophospholipids, glyceroglycolipids, sphingoglycolipids). Individual fragments of phospholipid molecules undergo thermal motion, which plays an important role:

1) in the interaction of lipids with Proteins; 2) in the processes of substance Transport Across the membrane; 3) in permeability.

The presence of short-chain Fatty Acids and branched chains in the lipid molecule enhances membrane "fluidity". The higher the degree of their unsaturation, the more pronounced the membrane liquefaction. Enzymatic activity and passive transport are closely linked to the Fluidity of Membrane lipids.

Proteins, predominantly represented by Glycoproteins, are divided into two types: peripheral and integral. Peripheral proteins are easily extracted from The Plasma Membrane using aqueous, surfactant-free Solvents. Integral proteins are closely associated with the membrane and can be readily isolated using Surfactants that disrupt The Lipid Bilayer.

Like polar lipids, peripheral proteins also possess one polar region and one nonpolar region (domain). In the polar region of the protein globule, which is in contact with water, ionized amino acid residues and all covalently linked carbohydrate residues are gathered. The nonpolar region lacks ionized and carbohydrate residues. This region of the protein globule is embedded in the hydrophobic interior of the membrane. Integral proteins spanning the membrane are anchored in such a way that their polar segments face inward and outward, while the region of the Hydrophobic core lies between them. At physiological Temperature, integral Membrane Proteins diffuse along the layer due to ordinary thermal motion. Saturated aliphatic chains and sterols increase membrane viscosity and restrict the lateral movement of protein molecules within the membrane plane. Membrane proteins may also have restricted mobility due to the presence of cytoskeletal structures linked to the inner surface of the membrane.

Classification of membrane proteins according to their Functions:

1. Catalytic Enzymes.

2. Transport proteins: selective filters (channels in biomembranes exhibiting selectivity toward ions that structurally correspond to the narrowest part of the channel—the filter); "gates" (in excitable membranes, when the Resting Potential drops below a threshold value, channels open and Na⁺ ions enter the cell; at rest, Na⁺ channels in the nerve fiber are closed by "gates"); pumps—transport ATPases (carry out the Transport of substances against a concentration gradient with the expenditure of energy).

3. Receptors are proteins typically composed of multiple domains. They contain a binding site specific for a natural Ligand (hormone, mediator, antibody, etc.), i.e., they "recognize" this ligand and interact with effector systems for which they also possess a "recognition" site. The information required to activate the effector system is entirely contained within the membrane receptor. The formation of the receptor-ligand complex is followed by specific reactions, such as the REGULATION OF ENZYMATIC Activity via the release of secondary chemical messengers (cAMP, cGMP)—Cyclic NUCLEOTIDES, the opening or closing of Ion Channels, methylation, phosphorylation, and increased DNA Synthesis.

4. Surface Antigens (antigenic determinants) are glycoprotein-lipid complexes (e.g., ABO and Rh Blood Group Antigens and other blood group systems localized in The erythrocyte membrane, tissue antigens) that provide immunochemical functions.

5. IMMUNOGLOBULINS (for instance, immunoglobulin molecules embedded in The cell membrane of B-lymphocytes serve as receptors for specific antigens). The main function of such proteins is participation in immunological reactions, i.e., contributing to the body's defense mechanisms through the formation of Antibodies.

6. Structural proteins (spectrin in the erythrocyte membrane) play a vital role in maintaining cell shape in conjunction with cytoskeletal proteins.

7. Contractile proteins possessing ATPase activity, i.e., The ability to cleave ATP to form ADP and phosphate, participate in The process of exocytosis.