2 Objectives of unit:Understand the structure and properties of the plasma membraneInvestigate the properties of plasma membranes practicallyExplain passive transport mechanisms of diffusion and facilitative diffusion, including the role of transporter and carrier proteinsDefine the process of osmosisExplain the process of active transport and the role of proteins and ATPExplain the processes of endocytosis and exocytosisDescribe the properties of gas exchange surfaces in living organismsExplain how the structure of the mammalian lung is adapted for rapid gaseous exchange
3 Cells have many membranes: plasma membranetonoplastouter mitochondrial membraneinner mitochondrial membraneouter chloroplast membranenuclear envelope
4 Membranes are flexible and able to break and fuse easily Neutrophil engulfing anthrax bacteria.Cover credit:Micrograph by Volker Brinkmann, PLoS Pathogens Vol. 1(3) Nov5 μm
5 Membranes allow cellular compartments to have different conditions pH 4.8Contains digestive enzymes, optimum pHlysosomeMembrane acts asa barrierpH 7.2cytosol
6 Membranes are mainly made of phospholipids phosphate grouphydrophilicheadphosphoester bondglycerolester bondfatty acidhydrophobictail
7 The polar hydrophilic heads are water soluble and the hydrophobic heads are water insoluble Hydrophobic (water-hating) tailairaqueous solutionHydrophilic (water-loving) headPhospholipids form micelles when submerged in water
8 In 1925 Gorter and Grendel proposed that the unit membrane is formed from a phospholipid bilayer Phosphate heads face aqueous solutionExtracellular space (aqueous)phospholipidbilayerCytosoplasm (aqueous)Hydrophobic tailsface inwards
9 Click to reveal answers Question: Explain why phospholipids form a bilayer in plasma membranes (4).Click to reveal answersPhospholipids have a polar phosphate group which are hydrophilic and will face the aqueous solutionsThe fatty acid tails are non-polar and will move away from an aqueous environmentAs both tissue fluid and cytoplasm is aqueousphospholipids form two layers with the hydrophobic tails facing inwardand phosphate groups outwards interacting with the aqueous environmentClick here to hide answers
10 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:PhospholipidbilayerProtein
11 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 protein 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.
12 Activity: Read pages 100 – 103 of your textbook Answer questions 1 – 3 on page 103
13 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.
14 Fat-soluble organic molecules can diffuse through the bilayer but polar molecules require proteins Fat-soluble moleculesPolar moleculesExtracellularspaceCytosoplasm(aqueous)hydrophilic pore
15 Question 4: How can polar and non-polar molecules pass through the membrane (2). Click to reveal answerPolar molecules require proteins to enable them to pass through the membraneNon-polar molecules can diffuse directly through the phospholipid bilayer Click here to hide answers
16 The membrane contains many types of protein: carbohydrate chainGlycocalyx: For cell recognition so cells group together to form tissuesReceptor: for recognition by hormonesglycoproteinEnzyme or signalling proteinintegral proteincarrier proteinperipheral proteinhydrophilic channel
17 Question: Label the diagram (11marks) Note: label the proteins based on location or structure, e.g. you do not need to identify receptors and enzymes.1456372109118Click to reveal answers1) carbohydrate; 2) glycoprotein; 3)integral protein; 4) peripheral protein; 5) carrier protein 6) hydrophilic channel; 7) phosphate group; 8) fatty acid; 9) phospholipid;10) glycocalyx; 11) phospholipid bilayer click to cover answers
18 Click to reveal the answers Question: Explain why the model for membrane structure is known as the fluid mosaic model (3).Click to reveal the answersThe phospholipid molecules can move freely laterally and makes the membrane fluid.The proteins are distributed throughout the membrane un evenly and in a mosaic pattern.The agreed structure is based upon experimental and chemical evidence and so is classed as a model.Click here to hide answers
19 Question: Describe the structure and function of the glycocalyx (3) Click to reveal answersConsists of glycoproteinsWhich are proteins with added carbohydrate chainsUsed for cell recognition/receptorsClick here to hide answers
20 There are different types of carrier proteins in the membrane: ATPCarrier protein(passive)Gated-channel proteinChannel proteinCarrier protein(active)
21 Membrane bound proteins allow chemical processes to occur on membranes in a sequential manner: IIIIIIIVCyt cEnzyme and transporter proteins involved in aerobic respiration in the inner mitochondrial membraneATP synthase
22 Click to reveal answers Question: Other than as carrier proteins state two functions of membrane bound proteins (2).Click to reveal answersReceptorsEnzymesStructural (attached to microtubules)Click here to hide answers
23 Practical Activity: Factors affecting membrane permeability See practical sheets
24 Click to reveal answers Question 3: Describe an experiment by which you could test to see whether alcohol concentration affected membrane permeability (5).Click to reveal answersSame volume discs of beetrootSame volume of alcoholSame temperatureSame time in alcoholRange of alcohol concentrationsUse colourimeter to read amount of pigment in solutionGraph of colour intensity (% absorbance etc.) over alcohol concentrationClick here to hide answers
25 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.
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