Session 2.1 - Lecture 1 - The Membrane Bilayer: main biophysical properties Flashcards
Slides 1 -
What do I need to know about biological membranes?
• What are the functions of biological membranes? • Are all membranes the same? • What is the composition? • Which lipids are involved? – Phospholipids – Glycolipids – Cholesterol • What is meant by a fluid membrane? • How does cholesterol contribute to membrane stability
What are the functions of biological membranes?
(ILO) Describe some functions of membranes
- Continuous, highly selective permeability barrier
- Control of the enclosed chemical environment
- Communication
- Recognition
- signalling molecules
- adhesion proteins,
- immune surveillance - Signal generation in response to stimuli (electrical, chemical)
Are all membranes the same?
(ILO) Feeling for whether all membranes are the same or different
No - different for different parts of the membrane depending on location and therefore function.
What is the composition [of biological membranes]?
(ILO) Describe
Generally membranes contain approximately (dry weight) – 40 % lipid – 60 % protein – 1-10 % carbohydrate
(General rule of thumb rather than lawful description)
NB. Membranes are hydrated structures,
thus, 20 % of total weight is water
Which lipids are involved [in biological membranes]?
(ILO) Describe
– Phospholipids
– Glycolipids
– Cholesterol
What is meant by a fluid membrane?
(ILO) Discuss
Membranes are dynamic structures - not polythene static bags.
How does cholesterol contribute to membrane stability
(ILO) What cholesterol is doing within a membrane structure
45% cholesterol in our membrane reduces fluidity via reduced PL chain motion (rigid sterol ring structure) but increases fluidity via reduced PL packing.
This means at lower temperatures itreduces packing, preventing the formation of a crystalline array, and at higher temperatures reduces motion - abolishing endothermic phase transition (crystalline to fluid) and homogenising the membrane into one dynamic state.
Name 5 general functions of biological membranes (cells and organelles)
- Continuous, highly selective permeability barrier
- Control of the enclosed chemical environment
- Communication
- Recognition
- signalling molecules
- adhesion proteins,
- immune surveillance - Signal generation in response to stimuli (electrical, chemical)
_________ ________ form a continuous and highly selective permeability around _____ and around __________ within the cell
Biological membranes, cells, organelles
Why is it important that biological membranes provide control of the enclosed chemical environment?
You need to close off the chemical environment to control that environment for life to be properly expressed in genetic material, separate to variances in environment.
So DNA and gene expression is important but membranes are the most important because they are the structure that allows this to happen, and therefore for you to define life.
Why does the biological membrane function in communication?
Once we’ve enclosed our cell or organelle with membrane, next function we need is communication - we want to regulate that environment in relation to what’s happening outside so need communication mechanisms
How is signalling involved in the recognition of biological membranes?
Need to be able to recognise signals from elsewhere – need receptors for molecules, need some form of transduction mechanisms to convert signals into molecular events into cell.
How are adhesion proteins involved in the recognition of biological membranes?
Since cells are not living in isolation, but in tissues, we need to talk to each other through adhesion proteins which link cells together in our tissue
Why is immune surveillance important in the recognition of biological membranes?
Cells will also need to be recognised as self, need to be messages in membrane that tell immune system I am me and don’t need to be got rid of - need mechanisms both for immune surveillance of self and invading pathogens
What two ways can we have signal generation in biological membranes?
Need mechanisms for signal generation in response to stimuli – need receptors: convert signals into CHEMICAL events into stimulating chemical pathway; or ELECTRICAL, we use messages received by membrane to convert into electrical event which will also happen on membrane.
Are all membranes and/or regions of the membrane the same?
No - different regions of plasma membrane may have different functions
Why are not all regions of the plasma membrane the same?
Because different regions of the plasma membrane may have different functions
For example • Interaction with basement membrane • Interaction with adjacent cells • Absorption of body fluids • Secretion • Transport • Synapses – nerve junctions • Electrical signal conduction • Changing shape may change the properties of a particular region
Why might the part of membrane that interacts with the basement membrane be different?
Let’s consider a cell in a tissue: an epithelial cell sitting on the basement membrane (BM). First thing cell is going to be doing is interacting with BM, and there may be specific molecules in that part of the membrane OF THE CELL sitting on the BM specifically there for that purpose
Why might parts of the membrane that interact with adjacent cells be different?
Bc our cell on that basement membrane will be interacting with adjacent cells, so there might be different proteins that are involved in linking those 2 adjacent cells together
Give an example of how a plasma membrane can be specialised to absorb body fluids.
Bc our epithelial cell, attached to the basement membrane, it might be facing on another surface some lumen or void within the body from which it’s absorbing body fluids - so the top surface might be specialised for drinking solutes from the outside, for example.
Why is it important for parts of the membrane to be specialised for secretion?
Might be doing opposite to absorption – secreting stuff into gut or bloodstream
Why are only some of the plasma membrane regions responsible for transport?
So if it’s doing those functions [absorption and secretion] there must be transport functions. But it would be sensible for the cell to only put those transport functions in that region of the membrane where the function’s required, rather than sticking it everywhere and wasting effort making protein for no reason.
Why is the plasma membrane different for synapses (nerve junctions)?
We have specialised nervous cells that form specialised junctions, where nerve impulses are chemically transmitted from the stimulating cells to the receiving cells.
So we have nerve junctions where we’ll locate both substance
- releasing mechanisms, and
- on the receiving side, receptors to respond to those signals mechanisms.
Why does electrical signal conduction change the make up of a membrane?
Some cells will conduct an electrical signal - so will need to put channels in the membrane, maybe even the channels are different in different parts of the membrane – depends how the signal is transmitted along the cell.
How does changing shape change the properties of the membrane?
So far our cell has been sitting on our basement membrane, immobile. But of course, cells can move – as soon as they are moving they send out structures to advance the cell into the region they’re moving to, the tail of the cell is following on behind. You’ll find those two regions, the advancing and the retreating regions are different, they have different structures. So bc our cell is moving now the membrane is changing, one moment the leading edge is the leading edge but then it gets overtaken and becomes the middle of the cell and the retreating membrane, so the membrane is changing it’s a dynamic environment, not a static polythene bag around the cell.
By dry weight, what is the most abundant component of membrane bilayers?
Proteins
Note: NOT lipid/phospholipids!
What is (arguably) the most important lipid in biological membranes (membrane bilayers)?
Cholesterol
By dry weight, phospholipids are the most abundant component of membrane bilayers.
True or false?
False
We have an image of a sea of lipids in a long thin lipid bilayer with the occasional protein stuck in it - we will see that is not the case.
Proteins are the most abundant component of membrane bilayers.
What is the membrane composition by dry weight?
Generally membranes contain approximately (dry weight) – 40 % lipid – 60 % protein – 1-10 % carbohydrate
(NB: general rule of thumb rather than lawful description)
Generally membranes contain approximately (dry weight) – 40 % lipid – 60 % protein – 1-10 % carbohydrate
Why doesn’t this add up?
Amounts vary a lil bit so there will be more or less carbohydrate in any membrane. That composition will vary with diff membranes - varies with source of membrane
Generally membranes contain approximately (dry weight) – 40 % lipid – 60 % protein – 1-10 % carbohydrate
What does this not tell you?
The TOTAL weight
Membranes are hydrated structures, thus, 20% of total weight is water hydrogen-bonded to both surfaces
What and how is bonded to the biological membrane other than dry weight components?
Water is hydrogen-bonded
membranes can’t exist without water, they are hydrated structures
Define amphipathic molecules.
i.e. they contain both a
hydrophilic (water-loving) and a hydrophobic (water-hating/-fearing) moiety
Membranes lipids have a part of the structure that likes water and a second part that hates water. What is the single term to describe this?
Amphipathic (molecule)
i.e. they contain both a
hydrophilic (water-loving) and a hydrophobic (water-hating/-fearing) moiety
What is the predominant lipid in membrane bilayers?
Phospholipid
Why are phospholipids so-named?
They are lipids with phosphate in them
What does a phospholipid contain?
- Glycerol
- Phosphate - Head Group
- 2 Fatty Acids
What is the structure of glycerol?
It is a short, 3-carbon sugar
How many carbons does glycerol have?
3
What is attached to ONE of the carbons of glycerol of a phospholipids?
A head group linked through a phosphate
What is the head group linked to on phospholipids?
A phosphate
What is attached to TWO of the carbons of glycerol of a phospholipids?
Fatty acids
Fig. 8
What does this show?
Phospholipid (predominant lipid in membrane bilayer)
Draw a phospholipid
See Fig. 8
Glycerol
- Phosphate - Head Group
- Fatty Acid
- Fatty Acid
Phosphate on end and on opposite side to fatty acids.
Fig. 9
Label and caption this image
Phosphatidylcholine
- Blue: glycerol (3 carbon sugar)
Attached to one carbon:
- Phosphate linker
- Attached to phosphate linker is the choline head group
Attached to other two carbons:
- Fatty acids attached as
- —- R1
- —- R2
(would not need to draw full structure)
What is phosphatidylcholine?
Phospholipids that incorporate choline as a head group
Draw phosphatidylcholine (simple).
See Fig. 9
- Draw 3 C glycerol
- 1 phosphate (P) group attached with a choline group
- 2 fatty acids (R1 & R2)
Fig. 10 (left)
Label and caption the image.
A phospholipid molecule
Choline Phosphate Glycerol Fatty acid Fatty acid
As much as you need to know
Draw a phospholipid.
See Fig. 10 (left)
A phospholipid molecule
- Choline
- Phosphate
- Glycerol
- Fatty acid
- Fatty acid
(Glycerol and fatty acids on opposite sides)
As much as you need to know
Fig. 10 (middle)
Label and caption the image
A phospholipid molecule
- Choline
- Phosphate
- Glycerol
- Fatty acid
- Fatty acid
(Note: would not need to draw)
Fig. 10 (middle)
What can you notice about the fatty acids in the phospholipid molecule when the molecule is drawn out in full?
They are big hydrophobic regions
Note: would not need to draw
Fig. 10 (right)
Label and caption the image
A phospholipid molecule
Space-filling molecule
Fig. 10 (right)
What is significant about representing the membrane in this way rather than schematic?
This is a space-filling diagram: they remind you that membranes are filling all the space, and are not just stick diagrams.
How do the head groups of phospholipids link?
Phospholipids have head groups linked through the phosphate
How big are the phospholipid head group molecules?
They are always small molecules
What chemical property do the head groups of phospholipids have?
They have a range of polar head groups
Give examples of 4 things that can be head groups in phospholipids.
- Choline
- Amines
- Amino acids
- Sugars
What is a phospholipid with a choline head group called?
Phosphatidylcholine
Give an example of a phospholipid with an amino acid head group.
Phosphatidylserine
Give an example of a phospholipid with a sugar head group.
Phosphatidylinositol
Give 3 facts about phosphatidylinositol.
- Has a sugar head group
- Rare
- Important because used in cell signalling: used as a substrate for signalling molecules in response to messages that the cell might be receiving (can release messages into the cell)
How phosphatidylinositol used in cell signalling?
Used as a substrate for signalling molecules in response to messages that the cell might be receiving - can release messages into the cell
How long are fatty acid chains?
Vary in length between 14 and 24 carbons
What is the most common carbon length?
C16 and C18 most prevalent
Most fatty acid chains in phospholipids are about 16-18 carbons long. What does this mean for the membrane?
As lipids line up in the membrane, you pretty much have the same length of hydrophobic portion so you get the same thickness of membrane across the whole cell
How is the thickness of membrane maintained?
As lipids line up in the membrane, you pretty much have the same length of hydrophobic portion so you get the same thickness of membrane across the whole cell
Why do some fatty acid chains have a kink in them?
Some fatty acid chains have a cis double bond between carbons.
What shape does a cis double bond in phospholipid fatty acid chains produce?
The double bond produces a boat shape, introducing a kink into that fatty acid chain (like a stickman holding his leg out to one side)
What is a fatty acid kink in phospholipids?
When there is a cis double bond between carbons, which instead of making the fatty acid hang straight down into the membrane, it makes the ‘leg’ stick out to the side.
How are phospholipids named?
By their small molecule head groups (e.g. phosphatidylcholine with a choline head group)
Fig. 12
Label these head groups (don’t need to learn)
Choline Serine Ethanolamine Inositol (linear form) (would not need to draw)
Give an example of an amine head group for a phospholipid.
Ethanolamine
What is the ethanolamine phospholipid called?
Phosphatidylethanolamine
What is the most predominant lipid in the membrane?
Phospholipids
Other than phospholipids, name a lipid that is found in the membrane.
Sphingomyelin.
What is the structure of sphingomyelin like?
It looks rather like phosphatidylcholine (choline head group but no glycerol backbone)
Describe the structure of sphingomyelin
- phosphate linker
- choline head group
- hairpin of hydrophobic structure
How does sphingomyelin differ to other phospholipids?
It doesn’t have a glycerol backbone but it has a hairpin of hydrophobic structures instead
What does sphingomyelin lack compared to other phospholipids?
A glycerol backbone
What does sphingomyelin have that other phospholipids don’t?
A hairpin of hydrophobic structures (rather than a glycerol backbone)
What would you get if you replace the phosphocholine moiety with a sugar in sphingomyelin?
A glycolipid
What are glycolipids?
Sugar containing (membrane) lipid
How do you get a glycolipid from sphingomyelin?
Replace the phosphocholine moiety with a sugar.
Fig. 13
Label the image.
Sphingomyelin
Fig. 13
Label the part that would create a glycolipid.
Replace phosphocholine moiety with a sugar = glycolipid.
Describe the general structure of a glycolipid in relation to sphingomyelin?
Backbone structure of sphingomyelin but without any phosphate involved.
What do we call glycolipids with a single carbohydrate head group?
A cerebroside
What is a cerebroside?
A glycolipid with a SINGLE carbohydrate head group. It is a membrane lipid.
Give an example of a head group found on a cerebroside.
A galactose head group.
What is the chemical structure of glycolipids?
Membrane lipids with hydrophobic domains and hydrophilic sugar head groups.
What is the relationship of phosphatidylcholine to a cerebroside?
Sphingomyelin looks like a phosphatidylcholine, but has a hairpin hydrophobic structure rather than a glycerol backbone.
If you chop off the phosphocholine head group and replace it with a sugar, then you end up with a glycolipid.
Glycolipids with only one singular carbohydrate molecule are called cerebrosides.
What can glycolipids have as head groups?
Single carbohydrate head group - cerebroside
Complex oligosaccharide head group - ganglioside.
What do we call glycolipids with a complex oligosaccharide head group?
Gangliosides
What are gangliosides?
Glycolipids (a type of membrane lipid) with a complex oligosaccharide (of defined composition).
How are gangliosides derived?
Sphingomyelin looks like a phosphatidylcholine, but has a hairpin hydrophobic structure rather than a glycerol backbone.
If you chop off the phosphocholine head group and replace it with a sugar, then you end up with a glycolipid.
Glycolipids with a complex oligosaccharide head group are called cerebrosides.
What are gangliosides involved in?
- Membranes
- The nervous system
Name some types of lipids in the membrane.
- A classical phospholipid (glycerol backbone, phosphate linked and head group, two fatty acids)
- Sphingomyelin, the other phospholipid (looks like phosphatidylcholine but has a hairpin backbone rather than glycerol)
- Glycolipids (sphingomyelin without the phosphocholine) - including cerebroside (single carbohydrate head group) and gangliosides (complex oligosaccharides of defined composition head groups).
What are phospholipids, sphingomyelin and glycolipids?
Different types of lipid in the membrane.
Fig. 14 (left)
Label and caption the image.
Glycolipid
- Cerebroside (single carbohydrate group - Gal)
Gal = galactose
Fatty chain
Fatty acid tail
Draw, very simply, a cerebroside.
See Fig. 14 (left)
Fatty chain
Fatty acid tail
Carbon backbone
Galactose head group
Fig. 14 (right)
Label and caption the image.
Glycolipid
- GM1 ganglioside (complex oligosaccharide head group)
Gal = galactose GlcNac = N-acetylglucosamine NANA = sialic acid (N-acetyl-neuraminic acid)
Fatty chain
Fatty acid tail
Draw, very simply, a ganglioside.
See Fig. 14 (right)
Ganglioside = complex oligosaccharide head group
Fatty chain
Fatty acid tail
3 carbon backbone
Complex oligosaccharide head group
Describe a cerebroside head group.
Sugar monomer
Describe a ganglioside head group.
Oligosaccharide (sugar multimers)
What do you call sugar monomer head groups from sugar containing lipids?
Cerebroside
What do you call oligosaccharide (sugar multimer) head groups from sugar containing lipids?
Ganglioside
We find many different types of lipid into the membrane, e.g. phospholipid, sphingomyelin, glycolipids etc. Describe how we can have one membrane with different lipids.
We have a similarity of membrane lipids but they are all roughly the same shape;
- same length of hydrophobic aliphatic carbon chain
- small head group
So they all line up with each other and be equivalent to each other
What is the similarity of phospholipids?
They are all roughly the same shape:
- same length of hydrophobic aliphatic carbon chain
- small head group
What part of the membrane is hydrophobic?
The aliphatic carbon chain tails
What part of the membrane is hydrophilic?
Small head groups
Fig. 16
Label the image.
Phospholipids
- Phosphoglycerides
- – PtdSer
- – PtdEtn
- – PtdCho
- – PtdIns (labels based on head groups - AA, amine, choline, sugar)
- Sphingomyelin (backbone with phosphate)
Glycolipids
- Glucosyl-Cerebroside (backbone without phosphate)
Palmitate (straight fatty acid tail)
Oleate (kinked fatty acid tail)
Palmitate
Draw roughly the membrane lipid hierarchy.
NB: You are NOT required to learn detailed chemical structures. Describe membrane lipid structures in overview only.
Phospholipids
- Phosphoglycerides
(palmitate [straight] and oleate [kinked] tails; phosphate linker with respective heads)
— PtdSer (AA head group [-OH])
— PtdEtn (amine head group [NH3+])
— PtdCho (choline head group)
— PtdIns (sugar head group [6-carbon sugar])
- Sphingomyelin (two palmitate tails, phosphate linker, head group)
Glycolipids
- Glucosyl-Cerebroside (two palmitate tails, NO phosphate linker, single carbohydrate [e.g. galactose, 6-carbon sugar] head group)
Just need to appreciate they all have similar length hydrophobic aliphatic carbon chain and small head group so they can line up in the membrane (1 mark)
You are NOT required to learn detailed chemical structures. Describe membrane lipid structures in overview only (unless we talk specifically about one structure, e.g. later on, phosphoinositol).
What is 20% of the membrane?
20% of membranes are hydrated structures, which is hydrogen-bonded water.
How is water bound to the membrane?
Hydrogen-bonded
What do phospholipids look like?
Earwax (but slimy, not hard like a candle)
What are the physical properties of phospholipids?
- Waxy molecule
- Slimy in consistency (not hard like a candle)
[Waxy and slimy like earwax not a candle]
What experiment could you do to find out what phospholipid structure forms in water?
- Put lipid in beaker
- Put some water on
- Expose ultrasound to shake up mixture
- See what structure phospholipids form
If phospholipids are mixed with water, what structure then forms from those phospholipids?
Most lipids form MICELLES but phospholipids form BILAYERS
What two structures can lipids form?
- Micelle
- Bilayer
What are lipid micelles?
Sphere-shaped
- Hydrophobic tails face into centre of structure
- Hydropholic (water-loving) heads face out
What are sphere-shaped lipid structures called?
Lipid micelles
Give a day-to-day example of a micelle forming structure.
In washing up liquid you get micelles, the fat on your plate dissolves into the centre of the micelle – it effectively solubilises away from the plate allowing water to go down the plughole with fat dissolved
Why are phospholipids unusual lipids?
Instead of forming micelles they form bilayers.
What is the structure of a bilayer?
- Hydrophilic heads all facing out interacting with water
- Tails facing in forming vdW interactions within the plane of the membrane.
How do the hydrophobic tails in lipid bilayers interact?
They form van der Waals interactions
Why is the phospholipid structure unstable?
Because we have ends in the structure which would not like to align to the solutes in the water, so actually these structures close off and become bag-like or fully enclosed structures.
What is the stability of the lipid bilayer structure?
Unstable bc we have ends in the structure which would not like to align to the solutes in the water, so actually these structures close off and become bag-like or fully enclosed structures.
The ends of the structures of lipid bilayers do not like to align to the solutes in the water. What does this lead to?
These structures close off and become bag-like or fully enclosed structures.
Fig. 17 (top)
Label this image.
Lipid micelle
Hydrophilic head groups
Hydrophobic tails
Fig. 17 (bottom)
Label this image.
Lipid bilayer
Hydrophilic head groups
Hydrophobic tails
Draw a lipid micelle.
See Fig. 17 (top)
Lipid micelle
Hydrophilic head groups
Hydrophobic tails
Draw a lipid bilayer.
See Fig. 17 (bottom)
Lipid bilayer
Hydrophilic head groups
Hydrophobic tails
What is a liposome?
A phospholipid BILAYER that’s fully enclosed
What’s the difference between a micelle and a liposome?
Micelles are singular layers of phospholipids forming a spherical structure.
Liposomes are phospholipid bilayers that form a spherical structure.
What is the structure of a liposome?
- Spherical shaped
- Phospholipid bilayer
- Hydrophilic heads on the outside
- Hydrophobic heads on the inside
- Water on the outside and inside of the drug but not touching the inside of the bilayer.
Why are liposomes important clinically as a novel therapeutic?
Have we in our beaker put some drug in with water and our liposome with our drug, we would have captured some of that drug within that membrane-enclosed environment, might be possible in future to send drugs specifically within liposomes to target organ (that’s an aside, mainly talking about liposomes to get you thinking about the bilayer).
Fig. 17+ A
Caption this image and add a scale bar.
Electron microscope of a section through a liposome.
100 nm
Fig. 17+ B
Caption this image and add a scale bar.
Liposome
water water
25 nm
Draw an electron microscope of a section through a liposome, and add a scale bar.
See Fig. 17+ A
100 nm scale bar (1 mark)
Spheres (1 mark)
Draw a liposome
See Fig. 17+ B
25 nm scale bar (1 mark)
Water inside and outside liposome (1 mark)
Hydrophilic heads touching water, hydrophobic tails within (1 mark)
Phospholipid bilayer (1 mark)
Why do phospholipids form bilayers and not micelles?
This allows them to enclose a space of solute (because there are hydrophilic heads on the inside too [like a liposome], rather than hydrophobic tail sphere-shaped) , enabling you to form an organelle/plasma membrane.
Is the phospholipid bilayer static? Explain.
No, it is a DYNAMIC structure. NOT like a polythene bag around the cell rather boring, just stopping getting things in and out of the cell. But it’s much more dynamic than that
What are the 4 movements of phospholipid motion in order of increasing energy?
- Flexion
- Rotation
- Lateral diffusion
- Flip flop (rare)
(in order of increasing energy)
What is flexion of the phospholipids?
Their hydrophobic tails move slightly, and if you put more energy into it they’d start banging into their neighbour and so on. This occurs ‘at rest’ just because there’s a bit of thermodynamic motion - a structure that is not static, but a bit more dynamic.
(like kicking your legs out when standing)
What is rotation of the phospholipids?
The phospholipid can ‘turn around’ on the spot. This requires slightly more energy than flexion.
(spinning whole body around when standing)
What is lateral diffusion of the phospholipids?
Putting a bit more energy into the phospholipid, over flexion and rotation, then they can change places entirely with your neighbour - not just sitting in one place but able to move around the cell in a sort of random motion. At this level you’re constrained bc the lipids are still stuck in the lamellar of the bilayer.
https://en.wikipedia.org/wiki/Lamellar_phase
(swapping places with your neighbour in the same row of the lecture theatre)
What is ‘flip flop’ of the phospholipids?
This is the most high energy movement, where the membrane gets so much energy the head goes through the hydrophobic bilayer and comes out the other side. This motion is rare, however. Sometimes known as “transverse diffusion”
(people at the back of the lecture theatre going to the front of the lecture theatre)
Using your knowledge of phospholipid motion, what would you expect the phospholipid distribution to be?
Because the phospholipid bilayer is a dynamic structure, you would expect there to be an equilibrium distribution of all PLs, e.g. you’d expect equal phosphatidylcholine on either side of the membrane.
What is the actual phospholipid distribution?
Although you’d expect equal PL distribution on either side of the membrane, there are distinct distributions, e.g. phosphatidylcholine tends to be on the outside and phosphatidylinositol on inside
Give an example of a phospholipid that tends to be found on the outside of the membrane.
Phosphatidylcholine
Give an example of a phospholipid that tends to be found on the inside of the membrane.
Phosphatidylinositol
Where does phosphatidylcholine tend to be found on the membrane?
On the outside
Where does phosphatidylinositol tend to be found on the membrane?
On the inside
Although you’d expect equal PL distribution on either side of the membrane, there are distinct distributions, e.g. phosphatidylcholine tends to be on the outside and phosphatidylinositol on inside
Explain why this occurs.
Our membrane must be being regulated by enzyme flippases that are controlling the distribution of PLs in the bilayer.
What are flippases?
Flippases (rarely spelled flipases) are transmembrane lipid transporter proteins responsible for aiding the movement of phospholipid molecules between the two leaflets that compose a cell’s membrane (transverse diffusion, also known as a “flip-flop” transition), providing active maintenance of an asymmetric distribution of molecules in the phospholipid bilayer.
Although phospholipids diffuse rapidly in the plane of the membrane, their polar head groups cannot pass easily through the hydrophobic center of the bilayer, limiting their diffusion in this dimension. Some flippases - often instead called scramblases - are energy-independent and bidirectional, causing reversible equilibration of phospholipid between the two sides of the membrane, whereas others are energy-dependent and unidirectional, using energy from ATP hydrolysis to pump the phospholipid in a preferred direction. Flippases are described as transporters that move lipids from the exoplasmic to the cytosolic face, while floppases transport in the reverse direction.
Loss of asymmetry, in particular the appearance of the anionic phospholipid phosphatidylserine on the exoplasmic face, can serve as an early indicator of apoptosis. This effect has been observed in neurons as a response to amyloid beta peptides, thought to be a primary cause of the neurodegenerative effects of Alzheimer’s disease.
Fig. 18
Label and caption the image
Flexion
Rotation –>
Lateral diffusion
Flip flop (rare) ^|v
Draw phospholipid motion.
See Fig. 18
Phospholipid bilayer showing Flexion (single) Rotation --> (single) Lateral diffusion (one layer) Flip flop (rare) ^|v (both layers)
What is different between phospholipids with no double bonds and one with cis double bonds?
No double bonds - looks like a stickman
Cis double bond - like a stickman holding his leg out to the side.
Fig. 19 (left)
Caption and label this image.
A phospholipid molecule
- Choline
- Phosphate
- Glycerol
- Fatty acid
- FATTY ACID
Fig. 19 (middle)
Caption and label this image.
A phospholipid molecule
- Choline
- Phosphate
- Glycerol (3- carbon sugar)
- Fatty acid (hydrophobic aliphatic hydrocarbon straight chain)
- FATTY ACID (hydrophobic aliphatic hydrocarbon with a double bond)
Fig. 19 (right)
Caption and label this image.
A phospholipid molecule - space-filling molecule
- Choline
- Phosphate
- Glycerol
- Fatty acid
- FATTY ACID
Phosphorus - green
Oxygen - red
Nitrogen - blue
White - hydrogen(?) black - carbon/space
Draw a simple phospholipid molecule with a double bond.
See Fig. 19 (left)
A phospholipid molecule
- Choline
- Phosphate
- Glycerol
- Fatty acid
- FATTY ACID (kinked)
Draw a simple phospholipid molecule with a double bond. Label the double bond.
See Fig. 19 (middle)
Do not need to draw chemical detail!
A phospholipid molecule
- Choline
- Phosphate
- Glycerol (3- carbon sugar)
- Fatty acid (hydrophobic aliphatic hydrocarbon straight chain)
- FATTY ACID (hydrophobic aliphatic hydrocarbon with a double bond with kink)
Draw a space-filling model of a phospholipid with a double bond.
See Fig. 19 (right)
A phospholipid molecule - space-filling molecule
- Choline
- Phosphate
- Glycerol
- Fatty acid
- FATTY ACID
Phosphorus - green
Oxygen - red
Nitrogen - blue
White - hydrogen(?) black - carbon/space
What would having a ‘kink’ in the chain due to a double bond do to the membrane structure?
It would ‘push’ the neighbour away, increasing possibility of movement or fluidity within the membrane.
What is a saturated fatty acid?
No double bonds (carbons are saturated with hydrogen)
What is an unsaturated fatty acid?
1 or more double bonds (carbons are not fully saturated with hydrogen).
How can we increase possibility of movement or fluidity within the membrane?
Introducing double bonds into the phospholipid ‘tails’.
Why do we need polyunsaturated fat?
Bc you need these lipids to make phospholipids so you can keep membranes in dynamic state
Why do we need polyunsaturated fat specifically in our diet?
You need them in diet bc there are some double bonds we are unable to put into those fatty acid, so we need to get these FAs from our diet to be able to make PLs and to keep membranes in a dynamic form.
What do fatty acids in our diet do?
- Make phospholipids
- Keep membranes in dynamic form (unsaturated fats i.e. double bond)
Where do we get double-bonded phospholipids from?
Our diet - (poly)unsaturated fats.
What is the influence of cis double bonds in bilayer structure?
Saturated ‘straight’ hydrocarbon chains become unsaturated hydrocarbon chains with cis double bonds - reduce phospholipid packing
Fig. 20 (left)
Label and caption this image.
Saturated ‘straight’ hydrocarbon chains
Fig. 20 (right)
Label and caption this image.
Unsaturated hydrocarbon chains with cis double bonds - reduce phospholipid packing
What do unsaturated hydrocarbon chains do?
Reduce phospholipid packing and hence increase fluidity in the membrane.
Draw a saturated hydrocarbon chain.
See Fig. 20 (left)
Influence of cis double bonds in bilayer structure
Saturated ‘straight’ hydrocarbon chains
(hydrophilic heads outside
hydrophobic tails inside
‘staggered’ arrangement depicting instability)
Draw an unsaturated hydrocarbon chain.
See Fig. 20 (right)
Influence of cis double bonds in bilayer structure
Unsaturated hydrocarbon chains with cis double bonds - reduce phospholipid packing
(hydrophilic heads outside
hydrophobic tails inside; some have kinks
‘staggered’ arrangement depicting instability
increase in space between phospholipids where there are kinks)
Give a brief overview of the membrane.
- Made up of phospholipids, glycolipids & sphingomyelin
- These are similar in structure w/ small head group and hydrophobic legs.
- Dynamic environment (PLs can move around)
What would you expect to happen if you cooled down a bilayer in a lab (2 marks)?
You would imagine PLs would coalesce into a sort of margarine-like consistency (1 mark) of stacked PLs (1 mark)
What happens when you cool down a bilayer in a lab?
You would imagine PLs would coalesce into a sort of margarine-like consistency of stacked PLs.
Now that does happen but unfortunately for us what happens is they don’t all freeze at the same temp. So there’s a possibility as we cool our membrane down that islands of waxy solid PL will from within what is still a sea of other PLs, creating edges around islands, and possibility of leak of stuff in and out/across the membrane, so it’s not desirable to have that
Why do membranes form waxy solid phospholipids from within what is still a sea of other PLs?
Because they don’t all freeze at the same temperature (different structures/lengths/boiling points).
What is a consequence of PLs not all freezing at the same temperature?
Membranes form waxy solid phospholipids from within what is still a sea of other PLs
Why is it important biologically to know PLs don’t all freeze at the same temperature?
They freeze at different temperatures, leading to formation waxy solid phospholipids from within what is still a sea of other PLs, creating edges around islands, and possibility of leak of stuff in and out/across the membrane, so it’s not desirable to have that
Phospholipids don’t all freeze at the same temperature, hence leading to ‘leakage’ of the membrane. How do membranes combat that biologically?
Use cholesterol in the membrane to homogenise (make uniform) the proximity of the membrane, so they melt at the smae time.
What does the addition of cholesterol do to the membrane?
Phospholipids don’t all freeze at the same temperature, hence leading to ‘leakage’ of the membrane. So what the cell does to guard against that is use cholesterol in the membrane to homogenise (make uniform) the proximity of the membrane.
What sort of molecule is cholesterol?
An amphipathic lipid
What is the head group on cholesterol?
Polar head group of hydroxyl (-OH) group
Which part of cholesterol is hydrophilic?
Polar head group containing hydroxyl (-OH) group
What does the bulk of cholesterol contain?
Rigid ring structure of hexagonal or pentagonal rings
What is the rigid part of cholesterol?
Ring structure of hexagonal or pentagonal rings
What do the hexagonal or pentagonal rings supply to the structure of cholesterol?
They make it rigid (difficult to twist)
What is found at the opposite end of cholesterol to the polar head group?
Flexible fatty acid chain
What is the more flexible part of cholesterol?
Fatty acid chain at the tail
Name 3 properties which make up cholesterol.
- Polar head group
- Rigid planar steroid ring structure
- Non-polar hydrocarbon tail
Fig. 21 (left)
Label and caption the image.
Cholesterol
- Polar head group (red)
- Rigid planar steroid ring structure (blue hexagons)
- Non-polar hydrocarbon tail (blue line)
Fig. 21 (middle)
Label and caption the image.
Cholesterol
- Polar head group (-OH)
- Rigid planar steroid ring structure (hexagons)
- Non-polar hydrocarbon tail (aliphatic hydrocarbon chain)
Fig. 21 (right)
Label and caption the image.
Cholesterol
Space-filling model of cholesterol
- Polar head group (-OH)
- Rigid planar steroid ring structure (bulk)
- Non-polar hydrocarbon tail (‘thinner’ tail)
Red - oxygen
White - carbon/hydrogen
Draw the basic structural outline of cholesterol.
See Fig. 21 (left)
- Polar head group (red circle)
- Rigid planar steroid ring structure (4 blue hexagons)
- Non-polar hydrocarbon tail (blue line)
Draw cholesterol.
See Fig. 21 (middle)
Cholesterol
- Polar head group (-OH)
- Rigid planar steroid ring structure (hexagons)
- Non-polar hydrocarbon tail (aliphatic hydrocarbon chain [CH2 chain])
Would not need to draw full structure
Draw a space-filling model of cholesterol.
See Fig. 21 (right)
Cholesterol
Space-filling model of cholesterol
- Polar head group (-OH)
- Rigid planar steroid ring structure (bulk)
- Non-polar hydrocarbon tail (‘thinner’ tail)
Red - oxygen
White - carbon/hydrogen
Fig. 22
Describe the experiment this graph is showing.
So let’s do an experiment, now let’s take pure phosphatidylcholine (Pure DPPC) and we’ll put it in a bilayer and we’ll gradually raise the temp from quite cold to quite warm (x-axis), and we’ll look at what happens to the amount of energy that’s going into the system (y-axis).
Fig. 22
What happens to pure PL as you heat it up?
What you see is as you warm up the PL at a given temp the PL melts (peak) – effectively goes from a semi-crystalline array within the membrane to a more dynamic fluid environment (this is an endothermic phase transition).
What happens to pure PL as you heat it up?
It melts (Fig. 22) - goes from a semi-crystalline array within the membrane to a more dynamic fluid environment.
Now what happens to that pure PL membrane then if you start adding 5% cholesterol, what happens to the amount of heat it requires?
5% chol the peak is sort of spread perhaps a little bit
Fig. 22
What happens to that pure PL membrane then if you start adding 12% cholesterol, what happens to the amount of heat it requires?
12% chol the height of the peak is coming down – we seem to be needing less energy to get from frozen to fluid state. (Fig. 22)
What happens to that pure PL membrane then if you start adding 20% cholesterol, what happens to the amount of heat it requires?
Now 20% cholesterol – amount of energy going in is much less
What happens to that pure PL membrane then if you start adding 32% cholesterol, what happens to the amount of heat it requires?
By 32% mol chol the transition is almost gone – cholesterol seems to have prevented PL from forming a crystalline array at lower temp, but equally prob. reducing amount of motion there is at higher temp – sort of homogenising property of the membrane so there is a standard dynamic property
What happens to that pure PL membrane then if you start adding 50% cholesterol, what happens to the amount of heat it requires?
So by 50% mol can’t see any transition
What does cholesterol do to the membrane?
Cholesterol abolishes endothermic phase transition of phospholipid bilayers.
How does cholesterol abolish endothermic phase transition of phospholipid bilayers?
Cholesterol seems to have prevented PL from forming a crystalline array at lower temp, but equally prob. reducing amount of motion there is at higher temp – sort of homogenising property of the membrane so there is a standard dynamic property.
What does cholesterol do to the phospholipid bilayer at lower temperatures?
Prevents it forming a crystalline array
What does cholesterol do to the phospholipid bilayer at higher temperatures?
Reduces amount of motion there is
Cholesterol affects the phospholipid bilayer at both higher and lower temperatures. Why?
By affecting both the higher and lower temperatures, it homogenises the property of the membrane - it has homogenised phase transition from crystalline to fluid - so there is a standard dynamic property; it preserves a constantly fluid environment.
How much cholesterol is there in your membrane?
45 mol % cholesterol (for every 2 PLs there’s 1 chol in there, rly important PL)
Fig. 22
Caption and label this image
x-axis: Average temperature (oK) 290 320 350 (17 47 77oC)
y-axis: Rate of heat flow (Endothermic up)
DPPC = dipalmitoyl-phosphatidylcholine
Pure DPPC
\+ 5 mol % cholesterol \+ 12.5 mol % cholesterol \+ 20 mol % cholesterol \+ 32 mol % cholesterol \+ 50 mol % cholesterol
Draw a graph showing what addition of cholesterol to the membrane does.
See Fig. 22
Cholesterol abolishes endothermic phase transition of phospholipid bilayers
x-axis: Average temperature (oK) 290 320 350 (17 47 77oC)
y-axis: Rate of heat flow (Endothermic up)
DPPC = dipalmitoyl-phosphatidylcholine
Pure DPPC
\+ 5 mol % cholesterol \+ 12.5 mol % cholesterol \+ 20 mol % cholesterol \+ 32 mol % cholesterol \+ 50 mol % cholesterol
Explain why the phospholipid bilayer membrane needs cholesterol?
Without cholesterol, endothermic reactions occur meaning that at some point, the phospholipid bilayer melts and goes from solid to liquid (endothermic phase transition). As more cholesterol is added, the endothermic heat change becomes less and less, until eventually, this transition abolishes.
This is because at lower temperatures - cholesterol prevents the phospholipid bilayer from forming a crystalline array. Equally, at higher temperatures, it reduces the amount of motion there is. This therefore homogenises (makes uniform) the membrane by abolishing the endothermic phase transition so there is a standard dynamic property, thus preserving a constantly fluid environment.
By about 32% mol cholesterol the endothermic phase transition is almost completely abolishes, and by 50% mol cholesterol it is completely abolishes and the rate of heat flow is much less. This is why our membranes have around 45% mol cholesterol in them.
(Fig. 22)
Fig. 22
Use this graph to suggest how much % cholesterol our membranes have.
45% - so that the endothermic phase transition is completely abolished and it is at the lowest optimal rate of heat flow.
How does the membrane bilayer remain fluid?
It does it by intercalating chol molecule alongside PL molecules
How does the membrane insert cholesterol into the phospholipid bilayer?
Cholesterol forms H-bonds with the carbonyl oxygen of FAs, locking it into plce
What does the rigid sterol ring of cholesterol do?
Remember we’ve got a rigid sterol ring, and what that does is if we put some planks in between rows in lecture theatre and stops you from flexing quite so much, so rigid rings of cholesterol reduces motion of PL and therefore reduces fluidity of the membrane.
Fig. 23
Caption and label this image.
Insertion of cholesterol into the phospholipid bilayer
Headgroup
Red …HO b-OH group
Motion restricted - blue - rigid sterol ring
Motion unaffected - blue - flexible tail
Black - phospholipid, Blue - cholesterol
How frequently does cholesterol appear in our membrane?
Chol comes along and sits linked alongside every other PL (45% - for every 2 PL there is 1 chol).
How is motion restricted in the bilayer?
Via the rigid sterol ring structure of cholesterol, preventing flexion of the phospholipid tail
Where is motion unaffected in the bilayer?
At the part where there is a flexible tail from cholesterol next to the phospholipid
What does restricting motion do to the fluidity of the membrane?
Reduces fluidity
Draw a cholesterol molecule next to a phospholipid molecule. Label its properties.
See Fig. 23
Insertion of cholesterol into the phospholipid bilayer
Phospholipid: Headgroup Phosphate 3 Carbon sugar Two fatty acid tails (zig zag)
Cholesterol
Red …HO b-OH group
Rigid sterol ring (hexagon)
Flexible tail (zig zag)
Hydrogen bond between OH of cholesterol and C=O fatty acid
Motion restricted - (top) blue - rigid sterol ring
Motion unaffected - (bottom) blue - flexible tail
Black - phospholipid, Blue - cholesterol
What does the ring structure of cholesterol do to the membrane?
It reduces fluidity
How is fluidity reduced in the membrane?
Via the rigid sterol ring structure of cholesterol
What does adding cholesterol between the phospholipids do to the structure (as well as reduce fluidity)?
Paradoxically, also increases fluidity.
How is fluidity increased in the membrane?
By packing cholesterol in between the phospholipids.
How does packing cholesterol in between PLs prevent packing?
It stops PLs forming a hexagonal crystalline array
What does cholesterol do to the membrane at low temperatures?
Prevents packing
What does cholesterol do to the membrane at high temperatures?
Reduces motion
Do we need cholesterol in our diet?
Yes, but in moderation - we need to for our membranes but it is important to not overconsume it.
Does cholesterol increase or reduce fluidity of the membrane?
Both, there is a paradoxical effect of cholesterol in phospholipid bilayers
Fig. 24
Caption and label this image.
Headgroup Phosphate 3 carbon glycerol Fatty acid tails Blue = cholesterol
Paradoxical effects of cholesterol in phospholipid bilayers
- Reduced phospholipid chain motion, reduced fluidity
- Reduced phospholipid packing, increased fluidity.
Draw the paradoxical effects of cholesterol in phospholipid bilayers.
See Fig. 24
Headgroup x4 Phosphate x4 3 carbon glycerol x4 Fatty acid tails x4 sets (i.e. 4x2) Red = OH of cholesterol hydrogen-bonded to C=O of fatty acid Blue = cholesterol (x2)
Paradoxical effects of cholesterol in phospholipid bilayers
- Reduced phospholipid chain motion, reduced fluidity (at rigid sterol ring of cholesterol)
- Reduced phospholipid packing, increased fluidity (across the board)
