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!!