View
42
Download
1
Category
Tags:
Preview:
Citation preview
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
Recommended