Membrane bilayer.pdf

Membranes and Receptors S3 Session 1 Lecture 1 MEMBRANE BILAYER Dr. Ibrahem Al-Aadily 17-9-2023  Learning outcomes After this lecture you should be able to List the main kinds of the membrane lipids, describe the properties of amphipathic molecules...

Membranes and Receptors S3 Session 1 Lecture 1 MEMBRANE BILAYER Dr. Ibrahem Al-Aadily 17-9-2023  Learning outcomes After this lecture you should be able to List the main kinds of the membrane lipids, describe the properties of amphipathic molecules and explain the process of formation the lipid bilayer. LO1 Explain the movements in lipid bilayers and discuss the influence of unsaturated fatty acids and cholesterol on membrane fluidity. LO2  General functions of biological membranes (cells and organelles) Continuous highly selective permeability barrier. Allows control of the enclosed chemical environment Communication - control the flow of information between cells and their environment Recognition - signalling molecules, adhesion proteins, immune surveillance Signal generation in response to stimuli - electrical, chemical Biological Membranes have specialized function Examples Plasma membrane - all of the mentioned functions Mitochondrial membrane - energy conservation by oxidative phosphorylation Membrane lipids LO1 Amphipathic molecules: They contain both hydrophilic and hydrophobic moieties 1. Phospholipid 2. Glycolipids 3. Cholesterol  1. Phospholipids Phosphoglycerides: They are derived from Glycerol- 3-P. Predominant lipids are phosphatidylcholine, phosphatidylserine, Phosphatidylethanolamine (PtdEtn) and phosphatidylinositol (PtdIns). Fatty acids (predominant 16-18 C) may be saturated or unsaturated. The later are present in Cis conformation. Sphengomyelin Basic structure of phosphoglycerides Sphengomyelin The only phospholipid not based on glycerol, kind of sphingolipid The backbone in sphingomyelin is sphingosine  2. Glycolipids Sugar containing lipids May contain glycerol or sphingosine and fatty acids. They are classified as: Cerebrosides - head group sugar monomers Gangliosides - head group oligosaccharides (sugar multimers) 3. Cholesterol Distribution of different lipids in tissues depends on the function Lipid % Phostcho 45-55 PhostEth 15-25 PhostIno 10-15 PhostSer 5-10 Cholesterol 10-20 Others ?  Lipid Bilayer Amphipathic molecules form one of two structures in water, micelles and bilayers. Bilayers are the favoured structure for phospholipids and glycolipids in aqueous media. Why these structures are formed? Concentrate your attention on: Different structures may be formed  Characteristics of the bilayer Bilayer formation is spontaneous in water driven by the van der Walls attractive forces between the hydrophobic tails. The co-operative structure is stabilized by non- covalent forces; electrostatic and hydrogen bonding between hydrophilic moieties and interactions between hydrophilic groups and water. Pure lipid bilayers have a very low permeability to ions and most polar molecules.  Dynamics in lipid bilayers LO2 Membranes are fluid structures. Lipid molecules possess four permitted modes of mobility in a lipid bilayer. 1. Fast lateral diffusion within the plane of the bilayer. 2. Flip-flop - movement of lipid molecules from one half of the bilayer to the other on a one for one exchange basis. 3. Intra-chain motion - kink formation in the fatty acyl chains 4. Fast axial rotation. 1. Fast lateral diffusion within the plane of the bilayer 2. Flip-flop - movement 3. Intra-chain motion - kink formation in the fatty acyl chains 4. Fast axial rotation  What is the importance of unsaturated fatty acids and cholesterol in the cell membrane? Unsaturated double bonds in the fatty acid side chains disrupt the hexagonal packing of phospholipids and so increase membrane fluidity. Cholesterol plays an important role in stabilizing the plasma membrane. At high temperature OH group of Choles interacts with the polar groups of the phospholipids as well as the remaining of the molecule (The rigid hydrocarbon rings of Choles) with the fatty acids to stabilize the fluidity, while at low temperature the motion is low and the chains are arranged to form a crystalline structure, Choles interacts to control the fluidity. Thank you for your listening