PLASMA MEMBRANES
Dr. M. Azzopardi
Syllabus requirements
3.2 Cell structure and function
3.2.2 The fluid mosaic model of cellular membranes.
Structure as revealed by freeze-etching (knowledge of other cytological techniques is not required).
The plasma membrane of a cellcan be thought of as a gatekeeper:
allowing only specific substances in or outpassing messages from the external
environment to the cell’s interior
Functions of the plasma membrane:1. separates the contents of the cells from their external
environment 2. communicate with other cells
3. regulate the exchange of substances between the cytoplasm and the external environment
4. chemical reactions sometimes occur on the membrane itself
5. act as receptor sites for recognising external stimuli, e.g. hormones from the environment or from other parts of the organism
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 membranes is know as the: fluid mosaic model
The phospholipid bilayer is like a “lake” in which a variety of proteins “float”.
Fluid refers to the phospholipid
bilayer
Mosaic refers to the
proteins
Surface view
This model is referred to as the ‘fluid mosaic model’ because the components are free to
move independently of each other.
4.6
Side view
The main components of the plasma membrane are:
Protein Phospholipid
4.6
Outer & inner membrane surfaces differ.
Let us 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
4.6
They arrange themselves into a single layer
4.6
A spherical micelle
Two layers form: a bilayer
4.6
phospholipid bilayers like this are the basic structure of plasma membranes
Phospholipids can move within the membrane
The phospholipid bilayer is flexible, and the interior is fluid, allowing lateral movement of
molecules
Side view
Surface view
Proteins can move within the Plasma Membrane
Let us have a look at PROTEINS
Freeze-etchingMembrane proteins
are revealed
1
1 2
2
3
3
Frozen tissue is fractured by a glass knife.
Fracturing causes membrane to separate.
Proteins sticking out of the fractured membrane must have been embedded in the bilayer.
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:
hydrophobic regions that completely span the hydrophobic
interior of the membrane
hydrophilic ends of the molecule
exposed to the aqueous solutions on either side of the membrane
Proteins are much larger than lipids:
move more slowly
Some proteins: seem to move in a highly
directed manner
Many 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, often to the exposed parts of integral proteins
Are the inner and outer sides of the membranes different or the same?
Extrinsic protein
Different, leading to different properties
Outer side
Inner side
Proteinscomprise about 50% of the mass of
membranesare responsible for most of the membrane's
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 phospholipids
Outer side
Inner side
Typical sugars in glycoproteins and glycolipids include glucose, galactose and mannose.
cell-cell recognition communication intracellular adhesion
The carbohydrate portion found on the surface of plasma membranes contribute to:
External to the plasma membrane, all animal cells have a fuzzy coat called the glycocalyx
Outer surface of an absorptive cell of the
small intestine
Glycocalyx: also present in bacteria
Membrane permeability
Membranes have a hydrophobic interior
determining which substances can cross
Hydrophobic molecules can
easily pass through
Polar molecules require the use of transport proteins
Which substances can/cannot pass through 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 can enter a cell. How can this be?
Move through a protein.
Three factors affecting membrane fluidity:
1. Temperature2. Percentage of unsaturated fatty acids3. Presence of cholesterol
The fluidity of biological membranes is described 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 this temperature:
tails of the phospholipids are packed tightly together movement is inhibited
the membrane solidifies like cold bacon grease
Saturated[single bonds]
Unsaturated[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 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)
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. A & B antigens on RBC membranes
Antigenspresentation of MHC (major
histocompatibility complex) proteins in relation to self and non-self recognition
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 membranes
have a specific binding site where: hormones or other chemicals
this binding then triggers other events in the cell membrane or inside the cell
can bind to form a hormone-receptor complex
Receptors are needed for sperm
to bind to the ovum
End-Of-Year SEP 2014
Receptor proteins Recognition proteins Both are usually intrinsic proteins (embedded in the plasma membrane).
1
Usually simple proteins. Usually conjugated proteins with a carbohydrate chain.
1
Have specific binding sites where hormones or other chemicalscan bind triggering particular cellular responses
Serve as identification tagswhich enable cells to recognise each other e.g. cells of the immune system recogniseinvading bacteria during an infection
3
Write brief notes to distinguish between the following: Receptor and recognition proteins in cell membranes.
Structural proteins are:
on the inside surface of cell membranesattached to the cytoskeletoninvolved in: maintaining the cell's shape
changing the cell's shape for cell motility
Cytochromesare 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
Essay title
The cell surface membrane is an effective barrier between the cell and its surrounding environment. Discuss.
[SEP, 2000]
WORKING OUT DILUTIONS
LEARN how to work out dilutions starting from a:
1. given % stock solution 2. 1M sucrose solution.
How to work out % concentration starting from 5% detergent
e.g. if you want 20 cm3 of a 0.3% detergent solution:
0.3 x 20 5
Required % Original %
X volume needed
= 1.2 cm3 of 5% detergent
Add 18.8 cm3 of distilled water
Table of dilutions
Final Concentration /
%
Volume of detergent /
cm3
Volume of distilled water /
cm3
1 20 00.8 16 40.6 12 80.4 8 120 0 20
Or more simply
Final Concentration /
%
Volume of detergent /
cm3
Volume of distilled water /
cm3
1 20 00.5 10 10
0.25 5 150.125 2.5 17.5
0 0 20
2. How much volume of a 1M sucrose solution and distilled water need to be mixed to
produce 20 cm3 of 0.6M sucrose solution?
1M sucrose solution? volume
Distilled water? volume
20 cm3 of 0.6M sucrose solution
Work out CONC x VOLUME to find volume of 1M sucrose solution needed.0.6 x 20 = 12 cm3
20 – 12 = 8 cm3
1M sucrose solution:12 cm3
Distilled water8 cm3
20 cm3 of 0.6M sucrose solution
EXPERIMENT
AIM: To show that heat affects the permeability of cell membranes.
Beetroot
70C50C15CFrozen disc
A cork borer is used to cut out cylinders of
beetroot.
Precaution: cut beetroot discs – SAME SIZE
To 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 pigments released during cutting the
beetroot cylinders. Any pigment 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
This introduces a source of error:BUT
SUBJECTIVITY
A COLORIMETER removes bias!!Colorimeter measures:
% absorbance % transmittance
OR
Colorimeter must be calibrated before use by:
Placing a cuvette filled with distilled water and making instrument read : 0% absorbance OR 100% transmittance
cuvette
How a colorimeter works
transmittance
absorbance
Absorbance %
5°C
0.04
Click the arrows to adjust the temperature
Experiment: Effect of heat on membranes
22.5°C
Absorption %
0.075
Experiment: Effect of heat on membranes
40°C
Absorption %
0.12
Experiment: Effect of heat on membranes
52°C
Absorption %
0.25
Experiment: Effect of heat on membranes
60°C
Absorption %
0.64
Experiment: Effect of heat on membranes
68°C
Absorption %
0.70
Experiment: Effect of heat on membranes
Results
Graph to show change in membrane permeability with an increase in temperature
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 10 20 30 40 50 60 70 80
Temperature/°C
Ab
sorp
tio
n /
%
4.6
Temperature (C) 5 22.5 40 52 60 68Absorption (%) 0.04 0.075 0.12 0.25 0.64 0.7
Conclusion
The increase in temperature causes the proteins in the membrane to denature and so its permeability increases, causing substances (purple dye in this case) to escape.
PAPER 4 MAY 2010 Betalains are a class of red and yellow pigments restricted to plants in the order Caryophyllales, and which are analogous to the anthocyanin pigments characteristic of other orders of plants. They are often most prominent in the petals but may also occur in other parts of the plant body including the fruits, leaves, stems, and roots. They include powerful antioxidant pigments such as those found in Beetroot (Beta vulgaris). Members of the Caryophyllales, including Beetroot (Beta vulgaris), are known to be decolourised by exposure to detergents. This decolourisation occurs as a consequence of the release of betalains from the cells of the plant. You are required to devise and implement an experimental procedure to investigate the conditions under which betalains are released from the cells of Beetroot (Beta vulgaris).
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 required
Devise and describe an experimental procedure that investigates the effect of detergent on release of betalains and consequent decolourisation of beetroot. (20)
Mix distilled water & stock detergent
Volume of detergent solution needed??
Volume of distilled water??
Method:A piece of beetroot is placed in a
plastic containerThe beetroot is totally covered with detergent
solution [e.g. 20 cm3]Apparatus is left for 30 minutesThe solution is poured into a boiling tubeThe colour of the solution is observed against a
white sheet of paper The same procedure is repeated for other
detergent solutions
The higher the detergent conc., the darker the colour became. Which parts of the
membrane were affected?
1%
00.5
Detergent damages proteins & phospholipids
in membrane = more permeable.
% transmittance when beetroot discs were exposed to varying detergent concentrations
What is the correlation shown by the graph?
NegativeWhich is the dependent
variable?
Transmittance
T (%)
Detergent concentration (%)
Ethanol affects the phospholipid bilayer – makes it more permeable
THE END
I CONTROL who
enters!!
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