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Plasma membranes
 

Plasma membranes

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    Plasma membranes Plasma membranes Presentation Transcript

    • PLASMA MEMBRANES
    • Syllabus requirements3.2 Cell 3.2.2 The fluid Structure as revealed bystructure mosaic model freeze-etchingand of cellular (knowledge of otherfunction membranes. cytological techniques is not required).
    • The plasma membrane of a cellcan be thought of as a gatekeeper: allowing only specific substances in or out passing messages from the external environment to the cell’s interior
    • Which term is better to describe a plasma membrane?semi-permeabledifferentially permeable
    • The plasma membrane isdifferentially permeable not simply semi- permeable since:  substances e.g. amino acids, glycerol, glucose and ions can diffuse slowly through  control actively what substances enter
    • The general structure of membranesis know as the: fluid mosaic model Fluid Mosaicrefers to the refers to thephospholipid proteins bilayer The phospholipid bilayer is like a “lake” in which a variety of proteins “float”.
    • This model is referred to as the ‘fluid mosaic model’ because the components are free to move independently of each other.Surface view 4.6
    • The main components of the plasma membrane are: Protein Phospholipid Side view 4.6
    • Outer & inner membrane surfaces differ.
    • Let’s explain how a bilayer forms
    • Structure of a phospholipid molecule Hydrophilic head (phosphate) Hydrophobic tail (fatty acid)What happens when a thin layer of phospholipid molecules is spread over the surface of water?
    • 4.6
    • 4.6
    • They arrange themselvesinto a single layer 4.6
    • A spherical micelle 4.6
    • Two layers form: a bilayerphospholipid bilayers like this are the basic structure of plasma membranes 4.6
    • Phospholipids can move within the membranephospholipid bilayer is flexible, and the interior is fluid, allowing lateral movement of molecules
    • Proteins can move within the Plasma Membrane Side view Surface view
    • Let’s have a look at PROTEINS
    • Freeze-etchingMembrane proteins are revealed
    • Two classes of membrane proteins1. embedded in the bilayer (intrinsic or integral)2. attached to a surface(extrinsic or peripheral)
    • Intrinsic proteins are generally transmembrane proteins with:hydrophilic ends of the molecule exposed to theaqueous solutions on either side of the membrane hydrophobic regions that completely span the hydrophobic interior of the membrane
    • Proteins are muchlarger than lipids:move more slowly
    • Some proteins:  seem to move in a highly directed mannerMany proteins: seem to be held virtually immobile by their attachment to the cytoskeleton
    • Extrinsic Proteins are not embedded in the lipid bilayer at allExtrinsic proteins are loosely bound to the surface of the membrane, Extrinsic often to the exposed protein parts of integral proteins Are the inner and outer sides of the membranes different or the same?
    • Different, leading to different properties Outer sideInner side
    • Proteinscomprise about 50% of the mass of membranesare responsible for most of the membranes properties
    • Cholesterol is a component of ONLY animal plasma membranes: Cholesterol  is a steroid, a type of lipid  is slightly polar at one end  has an irregular flat ring structure
    • Carbohydrates may be linked with proteins and phospholipidsOuter sideInner side
    • Role of glycoproteins & glycolipids: Glycolipids  are involved in cell-cell recognition  may act as receptor sites for chemical signals Glycoproteins:  are also involved in sticking the correct cells together in tissues
    • Membrane permeability
    • Membranes have a hydrophobic interior determining which substances can cross Hydrophobic Polar molecules molecules can require the use ofeasily pass through transport proteins
    • Which substances can/cannot passthrough the phospholipid bilayer? Small amounts.
    • Aquaporins allow easy flow of water because: aquaporins have a hydrophilic channel
    • Explain why organic solvents such as alcohol, ether and chloroform penetrate membranes more readily than water.Alcohol, ether & chloroform are non-polarWater is polar: repelled by non-polar portions
    • Charged molecules & ions canenter a cell. How can this be? Move through a protein.
    • Three factors affecting membrane fluidity: 1. Temperature 2. Percentage of unsaturated fatty acids 3. Presence of cholesterol The fluidity of biological membranes isdescribed by the rate of movement of lipid and protein molecules within the membrane
    • 1) Temperature: affects the tight packing of molecules at a certain temperature the membrane changes from the solid (gel) phase to the liquid phase and vice-versa
    •  as temperature decreases:  a critical temperature is reached at which the membrane solidifies like cold bacon grease  at this temperature:  tails of the phospholipids are packed tightly together  movement is inhibited
    • 2) Percentage of unsaturated fatty acids: What are ‘unsaturated fatty acids’?
    • Unsaturated & saturated fatty acids Saturated Unsaturated [single bonds] [double bonds]
    • 2) Percentage of unsaturated fatty acids:the higher the percentage of phospholipids having unsaturated tails, the more fluid the membrane is
    • Reason:unsaturated tails have kinks created by double bonds and the kinks reduce compaction
    • Many unsaturated fatty acids in the membrane:increase membrane fluiditymake it less likely for membrane to solidify at low temperatures Some fish adjust the proportion of different lipids as they migrate from waters of one temperature to another.
    • 3) Presence of cholesterol:Cholesterol:  has a variable effect on membrane fluidity  acts as plugs to reduce the escape/entry of polar molecules
    • Effect of cholesterol on membrane fluidity depends on type of fatty acids present: membrane consists mainly of saturated fatty acids:  cholesterol disturbs the close packing of phospholipids & keeps them more fluid. membrane contains several unsaturated fatty acids: cholesterol fits into the gaps caused by bending at the double bonds & thus stabilises the membrane.
    • Functions of the protein molecules in membranes
    • Transport proteinsmust span the membraneare involved in the selective transport of:  polar molecules  ions across the membrane (facilitated diffusion & active transport)
    • Enzymesproteins sometimes act as enzymesenzyme proteins catalyse reactions in the:  cytoplasm
    •  outside the cell, e.g. maltase in the small intestine
    • Antigensact as cell identity markersantigen proteins:  are involved in cell recognition  are often glycoproteins e.g. the A and B antigens on RBC membranes
    • Receptor moleculesproteins have very specific shapes, making them ideal as receptor molecules for chemical signalling between cells
    • Receptor proteins must:be on the outside surface of cell membraneshave a specific binding site where:  hormones or can bind to form a hormone-  other chemicals receptor complexthis binding then triggers other events in the cell membrane or inside the cell
    • Receptors are needed for spermto bind to the ovum
    • Structural proteins are: on the inside surface of cell membranes attached to the cytoskeleton involved in:  maintaining the cells shape  changing the cells shape for cell motility
    • Cytochromes proteins which play an important role in photosynthesis & respirationtake part in the energy transfer systems that exist in the membranes of chloroplasts & mitochondria
    • Summary: Proteins in the cell membrane
    • Functions of the plasma membrane:1. separates the contents of the cells from their external environment2. communicate with other cells3. regulate the exchange of substances between the cytoplasm and the external environment4. chemical reactions sometimes occur on the membrane itself5. act as receptor sites for recognising external stimuli, e.g. hormones from the environment or from other parts of the organism
    • Essay titleThe cell surface membrane is an effectivebarrier between the cell and its surroundingenvironment. Discuss. [SEP, 2000]
    • EXPERIMENTAIM: To show that heat affects the permeability of cell membranes. BeetrootFrozen 15C 50C 70C disc
    • A cork borer is used to cut out cylinders of beetroot.
    • Precaution: cut beetroot discs – SAME SIZETo ensure discs have same amount of pigment at the start of the experiment.
    • Sources of error:Discs do not have the same amount of pigment in themDifficult to cut discs exactly to the same size
    • Why is rinsing the discs before use an important precaution? To remove pigmentsreleased during cutting the beetroot cylinders. Anypigment released would be due to temperature.
    • Discs are immersed for ONE minute at a different temperature
    • Discs are transferred into a boiling tube with distilled water and left for 20 minutes at room temperature
    • Results can be recorded visually BUT This introduces a source of error: SUBJECTIVITY
    • A COLORIMETER removes bias!!Colorimeter measures:  % absorbance OR  % transmittance
    • Colorimeter must be calibrated before use by:Placing a cuvette filled with distilled water and making cuvette instrument read :  0% absorbance OR  100% transmittance
    • How a colorimeter works transmittance absorbance
    • Experiment: Effect of heat on membranes 5C 0.04 Absorbance % Click the arrows to adjust the temperature
    • Experiment: Effect of heat on membranes 22.5 C 0.075 Absorption %
    • Experiment: Effect of heat on membranes 40 C 0.12 Absorption %
    • Experiment: Effect of heat on membranes 52 C 0.25 Absorption %
    • Experiment: Effect of heat on membranes 60 C 0.64 Absorption %
    • Experiment: Effect of heat on membranes 68 C 0.70 Absorption %
    • ResultsTemperature (C) 5 22.5 40 52 60 68Absorption (%) 0.04 0.075 0.12 0.25 0.64 0.7 Graph to show change in membrane permeability with an increase in temperature 0.8 0.7 0.6 Absorption / % 0.5 0.4 0.3 0.2 0.1 0 0 10 20 30 40 50 60 70 80 Tem perature/°C 4.6
    • ConclusionThe increase in temperature causes the proteins inthe membrane to denature and so its permeabilityincreases, causing substances (purple dye in thiscase) to escape.
    • PAPER 4 MAY 2010Betalains are a class of red and yellow pigments restricted toplants in the order Caryophyllales, and which are analogousto the anthocyanin pigments characteristic of other orders ofplants. They are often most prominent in the petals but mayalso occur in other parts of the plant body including thefruits, leaves, stems, and roots. They include powerfulantioxidant pigments such as those found in Beetroot (Betavulgaris). Members of the Caryophyllales, including Beetroot(Beta vulgaris), are known to be decolourised by exposure todetergents. This decolourisation occurs as a consequence ofthe release of betalains from the cells of the plant.You are required to devise and implement an experimentalprocedure to investigate the conditions under whichbetalains are released from the cells of Beetroot (Betavulgaris).
    • You are provided with the following materials:  Parts of a Beetroot (Beta vulgaris)  1% stock solution of a household detergent  Distilled water  A number of plastic containers  A sheet of white paper  Other laboratory apparatus as requiredDevise and describe an experimental procedurethat investigates the effect of detergent on releaseof betalains and consequent decolourisation ofbeetroot. (20)
    • Mix distilled water & stock detergentVolume of detergent Volume of distilled solution needed?? water??
    • How to work out % concentration starting from 1% detergentE.g. if you want 20 cm3 of a 0.3% detergent solution:Divide (Required % by Original %) and multiply by volume needed0.3/1 = 0.30.3 x 20 = 6 cm3 of 1% detergentAdd 14 cm3 of distilled water
    • Table of dilutions Final Volume of Volume ofConcentration / % detergent / distilled water / cm3 cm3 1 20 0 0.8 16 4 0.6 12 8 0.4 8 12 0 0 20
    • Or more simply Final Volume of Volume ofConcentration / % detergent / distilled water / cm3 cm3 1 20 0 0.5 10 10 0.25 5 15 0.125 2.5 17.5 0 0 20
    • Method:Place a piece of beetroot in plastic container givenTotally cover the beetroot with detergent solution [e.g. 20 cm3]Leave for 30 minutesPour solution into a boiling tubeObserve colour of solution against a white sheet of paperRepeat for for other detergent solutions
    • The higher the detergent conc., the darker the colour became. Which parts of the membrane were affected? Detergent damages proteins & phospholipids in membrane = more permeable.
    • % transmittance when beetroot discs wereexposed to varying detergent concentrations What is the correlation T (%) shown by the graph? Negative Which is the dependent variable? Transmittance Detergent concentration (%)
    • Ethanol affects the phospholipidbilayer – makes it more permeable
    • THE ENDI CONTROL who enters!!