Membranes in cells Membrane structure and function Lecture 21

Electron Micrograph of Bilayer Plasma Membranes

Department of Biochemistry and Molecular Biology New Jersey Medical School Dr. S. Kumar 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 IntracellularExtra-cellular Na + (10 mM)145 (mM) K + (140 mM)5 (mM) Ca ++ (0.1 mM)1-2 (mM)

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

Department of Biochemistry and Molecular Biology New Jersey Medical School Dr. S. Kumar Sphingosine CH3-(CH 2 ) 12 -CH=CH-CH-CH-CH 2 OH OH NH 2

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. Bilayers are asymmetric in the distribution of phospholipids

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

Integral and peripheral Membrane Proteins

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.

Lipid Rafts

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

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

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

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

Carrier Mediated transport

Comparison of Simple Diffusion, Facilitated Transport & Active Transport Property Simple Diffusion Facilitated Transport Active Transport Requires special membrane proteins NoYes Highly selectiveNoYes Transport saturates NoYes Can be inhibitedNoYes Hormonal regulation NoYes Uphill transportNo Yes Requires ATP energy No Yes

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