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 cellcan 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-permeabledifferentially permeable
The plasma membrane isdifferentially 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?
They arrange themselvesinto a single layer 4.6
A spherical micelle 4.6
Two layers form: a bilayerphospholipid bilayers like this are the basic structure of plasma membranes 4.6
Phospholipids can move within the membranephospholipid 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-etchingMembrane 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 allExtrinsic 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
Proteinscomprise about 50% of the mass of membranesare 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
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’?
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 fluiditymake 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 proteinsmust span the membraneare involved in the selective transport of: polar molecules ions across the membrane (facilitated diffusion & active transport)
Enzymesproteins sometimes act as enzymesenzyme proteins catalyse reactions in the: cytoplasm
outside the cell, e.g. maltase in the small intestine
Antigensact as cell identity markersantigen proteins: are involved in cell recognition are often glycoproteins e.g. the A and B antigens on RBC membranes
Receptor moleculesproteins 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 membraneshave a specific binding site where: hormones or can bind to form a hormone- other chemicals receptor complexthis 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 & respirationtake 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 15C 50C 70C 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 themDifficult 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% detergentE.g. if you want 20 cm3 of a 0.3% detergent solution:Divide (Required % by Original %) and multiply by volume needed0.3/1 = 0.30.3 x 20 = 6 cm3 of 1% detergentAdd 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 givenTotally cover the beetroot with detergent solution [e.g. 20 cm3]Leave for 30 minutesPour solution into a boiling tubeObserve colour of solution against a white sheet of paperRepeat 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