Membranes in cells

1. Membranes in cells Membrane structure and function Lecture 21

2. Electron Micrograph of Bilayer Plasma Membranes

3. Membrane Function • Surround cells and intracellular organelles • Control the entry and exit of inorganic ions, nutrients and other biological and non-biological compounds • Maintain concentration gradients across inside the cells Intracellular Extra-cellular Na+ (10 mM) 145 (mM) K+ (140 mM) 5 (mM) Ca++ (0.1 mM) 1-2 (mM) Department of Biochemistry and Molecular Biology New Jersey Medical School Dr. S. Kumar

4. In 1925 Gorter and Grendel proposed that the unit membrane is formed from a phospholipid bilayer Phosphate heads face aqueous solution Extracellular space (aqueous) phospholipid bilayer Cytosoplasm (aqueous) Hydrophobic tails face inwards

6. Sphingosine CH3-(CH2)12-CH=CH-CH-CH-CH2OH OH NH2 Department of Biochemistry and Molecular Biology New Jersey Medical School Dr. S. Kumar

7. Bilayers are asymmetric in the distribution of phospholipids Lipid components of the plasma membrane.The outer leaflet consists predominantly of phosphatidylcholine, sphingomyelin, and glycolipids, whereas the inner leaflet contains phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol. Cholesterol is distributed in both leaflets.

8. Red Cell Membrane Outer surface Inner surface Department of Biochemistry and Molecular Biology New Jersey Medical School Dr. S. Kumar

9. Integral and peripheral Membrane Proteins

10. The fluid mosaic model of the plasma membrane: The proteins can move freely through the lipid bilayer. The ease with which they do this is dependent on the number of phospholipids with unsaturated fatty acids in the phospholipids.

11. Lipid Rafts

15. The membrane contains many types of protein: carbohydrate chain Glycocalyx: For cell recognition so cells group together to form tissues Receptor: for recognition by hormones glycoprotein Enzyme or signalling protein integral protein carrier protein peripheral protein hydrophilic channel

16. Membrane bound proteins allow chemical processes to occur on membranes in a sequential manner: Cyt c IV III I Q II proteins membrane Enzyme and transporter proteins involved in aerobic respiration in the inner mitochondrial membrane ATP synthase

17. Membranes allow cellular compartments to have different conditions pH 4.8 Contains digestive enzymes, optimum pH 4.5 - 4.8 lysosome Membrane acts as a barrier pH 7.2 cytosol

18. Fat-soluble organic molecules can diffuse through the bilayer but polar molecules require proteins Fat-soluble molecules Polar molecules Extracellular space Cytosoplasm (aqueous) hydrophilic pore

20. Carrier Mediated transport

22. Comparison of Simple Diffusion, Facilitated Transport & Active Transport

23. Summary • The unit membrane consists of a phospholipid bilayer • Phospholipids consist of a polar, hydrophilic phosphate head and a non-polar, hydrophobic tail consisting of fatty acid chains. • Proteins also occur in the membrane and float freely throughout it. • The model for membrane structure is known as the fluid mosaic model. • Peripheral proteins occur on the inner or outer face of the membrane and integral proteins extend through both lipid layers. • Membrane bound enzymes occur allowing structured metabolic pathways. • Glycoproteins form the glycocalyx and allow cell to cell recognition. • Receptor proteins can act as binding sites for hormones and other substances and can transmit the information to the interior of the cell. • A variety of carrier proteins allow for the controlled movement of substance through the membrane using both passive diffusion or active transport. • Non-polar, lipid soluble molecules diffuse through the phospholipid bilayer. • Ionic, polar molecules require carrier proteins to enable them to pass through the membrane. • Membrane structure loses integrity with high temperature or presence of organic solvents such as alcohol, thereby increasing permeability. • Lipid rafts have more sphingomyelin and cholesterol and are important in signal transduction and viral entry into the cells