PLASMA MEMBRANES

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).

STRUCTURE OF A BIOLOGICAL MEMBRANE

the physical organisation and functioning of all biological membranes depend on their constituents:

lipids

proteins

carbohydrates

The lipids:

1. establish the physical integrity of the membrane

2. create an effective barrier to the rapid passage of hydrophilic materials such as water and ions

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

The Fluid Mosaic Model of membrane structure

In the fluid mosaic model of membrane structure, the protein molecules are:

noncovalently embedded in the phospholipid bilayer by their hydrophobic regions

(or domains),

their hydrophilic domains are exposed to the watery conditions

on either side of the bilayer

BUT

Side view

Surface view

Proteins can move within the plasma membrane

Membrane proteins have a number of functions, including:

Each membrane has a set of proteins suitable for the specialised function of the cell or

organelle it surrounds

receiving chemical signals from the cell’s external

environment

materials moving through the membrane

The carbohydrates associated with the membranes are:

attached either to the lipids or the protein molecules

located on the outside of the cell, where they interact with substances in the external environment

outside

inside

The carbohydrates associated with the membranes are:

crucial in recognising specific molecules, such as those on the surface of adjacent cells [this is also the function of some membrane proteins]

The fluid mosaic model does not say much about membrane composition:

some membranes have more protein than lipids

others are lipid-rich

others have significant amounts of cholesterol

some are rich in carbohydrates

Lipids form the hydrophobic core of the membrane

the lipids in biological membranes are usually phospholipids

a phospholipid molecule has:

hydrophilic (“water-loving”) regions: the phosphorus-containing “head”: is electrically charged and therefore associates with polar water

molecules

Lipids form the hydrophobic core of the membrane

the lipids in biological membranes are usually phospholipids

a phospholipid molecule has:

hydrophobic regions (“water-hating”) regions: the long, non-polar fatty acid “tails” of the phospholipid: associate with other nonpolar materials, but do not dissolve in water or associate

with hydrophilic substances

Phospholipids are amphipathic molecules

i.e. have both:

Hydrophilic region

Hydrophobic region

Let us explain how a bilayer forms

4.6

If a thin layer of phospholipid molecules is spread over the surface

of water:

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

Membranes from:

different cells or

organelles

Note:

phospholipids are highly variable

a significant proportion of the lipid content of animal cell membrane may be cholesterol

may greatly differ in their lipid composition

Phospholipids can differ in terms of:

1. fatty acid chain

length (number of carbon

atoms)

2. degree of unsaturation (double bonds) in the fatty acids

Saturated [single bonds]

Unsaturated [double bonds]

Saturated fatty acid chains allow close packing of fatty

acids in the bilayer

The kinks in the unsaturated fatty acids

result in a less dense, more fluid packing

Cholesterol is present: only in animal cell membranes

in the less-dense membranes in animal cells

Role:

acts as a plug to reduce the escape/entry of polar molecules

Cholesterol

when present, cholesterol is important for membrane integrity (keeping it whole)

Cholesterol molecules are usually situated next to an

unsaturated fatty acid

The phospholipid bilayer:

stabilises the entire membrane structure,

but leaves it flexible

The fatty acids of the phospholipids make the hydrophobic interior of the membrane

somewhat fluid - about as fluid as salad oil

This fluidity permits some molecules to move laterally within the plane of the membrane

Since spontaneous phospholipid flip-flops are rare:

the inner and outer halves of the bilayer may be quite different in the kinds of phospholipids they contain

Explain why organic solvents such as alcohol, ether and chloroform penetrate

membranes more readily than water.

Alcohol, ether & chloroform are non-polar

Water is polar: repelled by non-polar portions

TWO factors affect membrane fluidity:

1. its lipid composition

2. its temperature

The fluidity of biological membranes is described by:

the rate of movement of lipid

protein molecules within the membrane

1. lipid composition

LESS FLUID if it has:

a high percentage of long-chain, saturated fatty acids packed tightly

MORE FLUID if it has:

a high percentage of shorter-chain, unsaturated fatty acids

A membrane is:

Many unsaturated fatty acids : increase membrane fluidity

make it less likely for membrane to solidify at low temperatures

Cholesterol: is slightly polar at one end

(presence of OH group)

has a variable effect on membrane fluidity

cholesterol

consists mainly of saturated fatty acids:

cholesterol disturbs the close packing of phospholipids & keeps them more fluid

contains several unsaturated fatty acids:

cholesterol fits into the gaps caused by bending at the double bonds & thus stabilises the membrane.

If membrane :

2) 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

With an increase in temperature, a sharp transition is made from a more rigid

membrane to a more fluid one.

Membrane functions may decline under cold conditions in organisms that cannot keep

their bodies warm. Reason:

molecules move more slowly

fluidity decreases at low temperatures

Membrane fluidity is essential for many functions

To address this problem, some organisms simply change the lipid composition of

their membranes in cold conditions

1. they replace saturated with unsaturated fatty acids

2. use fatty acids with shorter tails

It’s so cold!! My cell membranes may solidify.

What am I to do?

Change the lipid composition of

your membranes.

These changes play a role in the survival of : plants, bacteria, hibernating animals during the winter

Some bacteria contain enzymes that can introduce double bonds into fatty acids in the

membrane when temperature is lowered.

Some fish adjust the proportion of different lipids as they migrate from waters of one

temperature to another.

Membrane proteins

typically, membranes have

1 protein molecule : 25 phospholipid molecules

this ratio varies depending on membrane function

Two classes of membrane proteins

Extrinsic or Peripheral (attached to a surface)

Intrinsic or Integral (embedded in the

bilayer)

Peripheral membrane proteins:

lack exposed hydrophobic groups

have polar or charged regions that interact with:

exposed parts of integral membrane proteins

the polar heads of phospholipid molecules

or

Integral membrane proteins have both hydrophobic and hydrophilic regions

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

Membrane proteins are asymmetrically distributed

1. peripheral membrane proteins being localised on one side of the membrane or the other

Protein asymmetry arises from:

2. certain integral proteins:

transmembrane proteins:

extend all the way through the phospholipid bilayer

protrude on both sides

may have regions with specific functions on the inner and outer sides of the membrane

transmembrane proteins

e.g. sodium-potassium pump

4.6 Surface view

Some are anchored

Movement of proteins within the membrane

Some move around relatively freely

Many proteins:

seem to be held virtually immobile by their attachment to the cytoskeleton

cytoskeleton

Membrane proteins are

revealed by the freeze-etching

technique

Freeze-etching

Specimen is rapidly frozen in liquid nitrogen.

2

Specimen is fractured with a sharp metal

blade.

The tissue fractures along planes of weakness which often run through membranes.

1

Ice sublimes [solid to gas] away leaving an etched surface.

The specimen is kept: cold in a high vacuum

3

Deep etching: the etching period is extended and a deeper layer of ice can be removed, thereby exposing structures that are located deep within the cell interior

Sublimation

[Etch: to cut into the surface]

4

A replica of this surface is made by depositing a layer of a heavy metal over it & strengthened by carbon.

Shadowing Shadowing involves spraying a thin layer of an electron-dense metal such as platinum or gold at an angle across the surface of a biological specimen.

Platinum

‘shadow’

5

The tissue is destroyed with a strong acid.

The replica is:

placed on a grid

examined using an electron microscope.

Platinum

‘shadow’

Grid

Carbon backing

Summary

Why is freeze etching especially good to detect membrane proteins?

When membrane is fractured, the proteins are either:

1. torn away:

– leave holes

2. stay with the specimen: – seen as bumps

Plasma membrane carbohydrates are recognition sites

The carbohydrates :

Location: on the outer surface of the plasma membrane

Role: serve as recognition sites for other cells and molecules

Carbohydrates may be covalently bonded to proteins and phospholipids

Outer side

Inner side

Sugars in glycoproteins and glycolipids function as signalling sites

Glycolipids: carbohydrate + lipid

work by creating a lipid/carbohydrate chain shape characteristic of tissue

play a role in tissue recognition

e.g. A, B, O blood group markers

Glycoproteins: carbohydrate + protein

carbohydrate is an oligosaccharide:

usually not exceeding 15 monosaccharide units in length

work by creating a protein/carbohydrate chain shape characteristic of individual

play a role in “self” recognition

Glycoproteins: carbohydrate + protein

e.g. major histocompatibility complex (MHC) recognised by immune system

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 chemicals can bind triggering particular cellular responses

Serve as identification tags which enable cells to recognise each other e.g. cells of the immune system recognise invading bacteria during an infection

3

Write brief notes to distinguish between the following:

Receptor and recognition proteins in cell membranes.

Proteins and protein complexes perform SIX key functions:

1. Transporters

2. Enzymes

3. Cell-surface receptors

4. Cell-identity markers

5. Cell-to-cell adhesion

6. Attachments to the cytoskeleton

1. Transport proteins membranes are very selective, allowing only

certain solutes to enter or leave the cell, either through:

channels proteins or

carriers proteins

2. Enzymes cells carry out many chemical reactions on the

interior surface of the plasma membrane, using enzymes attached to the membrane

3. Cell-surface receptors membranes are very sensitive to chemical

messages, which are detected by receptor proteins on their surfaces

4. Cell-surface identity markers

membranes carry cell-surface markers that identify them to other cells

most cell types carry their own ID tags, specific combinations of cell-surface proteins and protein complexes such as glycoproteins that are characteristic of that cell type

5. Cell-to-cell adhesion cells use specific proteins to glue themselves

to one another

some cells form:

temporary interactions

a more permanent bond

6. Attachments to the cytoskeleton

surface proteins that interact with other cells are often anchored to the cytoskeleton by linking proteins

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

Essay title

The cell surface membrane is an effective barrier between the cell and its surrounding environment. Discuss.

[SEP, 2000]

THE END