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Membranes in Cells Objective: • To explain the structure and function of the plasma membrane Cells have many membranes: plasma membrane tonoplast outer mitochondrial membrane inner mitochondrial membrane outer chloroplast membrane nuclear envelope Membranes have several functions: 1. They are partially permeable, controlling what passes through them. 2. They are involved in cell signalling (communication between cells). 3. They provide attachment sites for enzymes and other molecules involved in metabolism. 4. They can allow electrical signals to pass along them. 5. They produce different compartments inside cells. 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 Membranes are mainly made of phospholipids phosphate group hydrophilic head phosphoester bond glycerol ester bond fatty acid hydrophobic tail The polar hydrophilic heads are water soluble and the hydrophobic heads are water insoluble Hydrophobic (water-hating) tail air aqueous solution Hydrophilic (water-loving) head Phospholipids form micelles when submerged in water Membranes are flexible and able to break and fuse easily Neutrophil engulfing anthrax bacteria. Fill your plate with some water. Add some drops of oil (phospholipid). The oil should form micelles, just like a membrane. Use a cocktail stick to move a drop of oil close to another one. What happens? 5 μm In 1925 Gorter and Grendel proposed that the unit membrane is formed from a phospholipid bilayer Extracellular space (aqueous) Phosphate heads face aqueous solution phospholipid bilayer Cytoplasm (aqueous) Hydrophobic tails face inwards Question: Explain why phospholipids form a bilayer in plasma membranes (4). • Phospholipids have a polar phosphate group which are hydrophilic and will face the aqueous solutions • The fatty acid tails are non-polar and will move away from an aqueous environment • As both tissue fluid and cytoplasm is aqueous phospholipids form two layers with the hydrophobic tails facing inward and Click to reveal answers phosphate groups outwards interacting with the aqueous environment Click here to hide answers Initial studies showed that the plasma membrane had layers: Scientists also found that protein were present in membranes so Davson-Danielli proposed in 1935 the following model for membrane structure: Protein Phospholipid bilayer The development and use of electron microscopes showed that the Davson-Danielli model was incorrect In the early 1970s Singer and Nicholson used techniques such as freeze-etching to confirm the lipid bilayer. They also showed that the proteins were distributed throughout the bilayer in a mosaic pattern. In addition they found that the membrane was fluid and had considerable sideways movement of molecules within it. Hence they proposed the Fluid-Mosaic Model for Plasma Membrane Structure. Activity: Watch the demonstration of the fluid mosaic model. 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 their tails. 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 Question 4: How can polar and non-polar molecules pass through the membrane (2). •Polar molecules require proteins to enable them to pass through the membrane •Non-polar molecules canClick diffuse directly through the phospholipid to reveal answer bilayer Click here to hide answers 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 peripheral protein Enzyme or signalling protein integral protein channel protein hydrophilic channel Question: Label the diagram (11marks) 4 1 5 6 Note: label the proteins based on location or structure, e.g. you do not need to identify receptors and enzymes. 3 2 7 10 9 11 8 1) carbohydrate; 2) glycoprotein; 3)integral protein; 4) peripheral protein; 5) channel protein 6) hydrophilic channel; 7)Click phosphate group; 8) fatty acid; 9) phospholipid; to reveal answers 10) glycocalyx; 11) phospholipid bilayer click to cover answers Question: Explain why the model for membrane structure is known as the fluid mosaic model (3). • The phospholipid molecules can move freely laterally and makes the membrane fluid. • The proteins are distributed throughout the membrane unevenly and in a mosaic pattern. to revealupon the answers • The agreed structureClick is based experimental and chemical evidence and so is classed as a model. Click here to hide answers Question: Describe the structure and function of the glycocalyx (3) • Consists of glycoproteins • Which are proteins with added carbohydrate chains • Used for cell recognition/receptors Click to reveal answers Click here to hide answers There are different types of carrier proteins in the membrane: ATP Carrier protein (passive) Gated-channel protein Channel protein Carrier protein (active) Membrane bound proteins allow chemical processes to occur on membranes in a sequential manner: proteins membrane Cyt c Q I III II Enzyme and transporter proteins involved in aerobic respiration in the inner mitochondrial membrane IV ATP synthase Question: Other than as carrier proteins state two functions of membrane bound proteins (2). • Receptors • Enzymes • Structural (attached to microtubules) Click to reveal answers Click here to hide answers Summary • The unit membrane consists of a phospholipid bilayer • Phospholipids consist of a polar, hydrophilic phosphate head and a nonpolar, 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.