The Membrane Bilayer Flashcards
What are the general functions of biological membranes?
Continuous, highly selective permeability barrier, Control of enclosed chemical environment, Communication, Recognition, Signal generation in response to stimuli
What do biological membranes capture?
An area of solute
Why are biological membranes needed for communication?
Needed to control flow of information between cells and their environment
Between where do biological membranes control communication?
Can be outside the cell, or outside of an organelle
What do biological membranes need for communication?
Mechanisms to communicate with outside environment
What do membranes recognise?
Signalling molecules
Adhesion proteins
Immune cells
What do adhesion proteins allow?
Cells to recognise each other
What stimuli do membrane generate signals in response to?
Electrical
Chemical
What do different regions of the plasma membrane have?
May have different functions
What functions can different regions of the plasma membrane have?
Interaction with basement membrane Interaction with adjacent cells Absorption of body fluids Secretion Transport Synapses Electrical signal conduction
Why is absorption of body fluids important?
Provides nutrients for cell growth
Why is it important that cells are able to transport themselves?
Can allow to look for nutrients to bring into the cell
What are the mechanisms in a synapse membrane specialised to do?
Release neurotransmitter
Have receptors to recognise neurotransmitters
What is needed for electrical signal conduction?
Proteins allow an action potential to be conducted along axon
What have a change in shape of membrane result in?
A change in properties of a different region
Why is the plasma membrane ever changing?
So it’s suited to the needs of the function that it’s doing in any one part of the cell
What do different membranes have?
Specialised function
Give an example of a membrane with a specialised function
The mitochondrial membrane- specialised for energy conservation by oxidative phosphorylation
What does membrane composition vary depends upon?
The source of the membrane
Generally, what is the membrane composition when dry?
40% lipid
60% protein
1-10% carbohydrate
What % of the membranes total weight is water?
20%
Why is water needed in membranes?
To make hydrophilic interactions
Why are hydrophilic interactions in membranes important?
To keep the bilayer organised
What kind of molecules are membrane lipids?
Amphipathic
What is meant by amphipathic?
Contain both hydrophilic and hydrophobic moiety
What does the distribution of membrane lipids vary depending on?
Cell type- it is tissue specific
What is the distribution of membrane lipids related to?
Function
What are the predominant lipids in membranes?
Phospholipids
What are phospholipids made up of?
Glycerol backbone
Two fatty acid chains
Are the two fatty acid chains in phospholipids the same?
They can be different fatty acids, with different numbers of C’s
How many C’s do the fatty acids in phospholipids have?
Beween C14 and C24
C16 and C18 most prevalent
What is the importance of the C16 and C18 prevalence?
It means that the thickness of the membrane is always about the same, as the fatty acid chains always about the same length
What does a cis double bond in a fatty acid chain introduce?
A kink
Where is sphingomyelin found?
In membranes
Why is sphingomyelin unique?
It is the only phospholipid based on glycerol
What are glycolipids?
When the phosphate head group is replaced with a sugar
What is a cerebroside?
A glycolipid with a single sugar residue
What is a ganglioside?
A glycolipid with an oligosaccharide is attached
In what ways are all membrane lipids similar?
Have a long aliphatic chains and a small head group
What is the importance of all membrane lipids being similar?
Keeps membrane thickness about the same
When is it important that membrane thickness is kept about the same?
When proteins are added to the membrane
What is a lipid micelle?
A sphere formed when hydrophobic tails clump together on the inside, and hydrophilic heads face outwards forming hydrogen bonds with water
What is a lipid bilayer?
Double sheet
How are lipid bilayers form?
When hydrophilic heads face outwards on either side, bringing the hydrophobic tails together
Do phospholipids more naturally form bilayers or micelles?
Bilayers
What are bilayers able to do?
Enclose a space
How can lipid bilayers be clinically useful?
In drug delivery
How can lipid bilayers be used in drug delivery?
The drug can be enclosed in the space formed by the bilayer, and proteins can be added into the membrane that targets the drug to the tissues
What is the lipid bilayer the favoured structure for?
Glycolipids in aqueous media
When is bilayer formation spontaneous?
In water
What is bilayer formation in water driven by?
The van der Waals attractive forces between the hydrophobic tails
What is the bilayer structure stabilised by?
Non-covalent forces
What non-covalent forces stabilise the bilayer structure?
Electrostatic
Hydrogen bonding
Interactions between hydrophilic groups and water
What is the permeability of pure lipid bilayers?
Very low to ions and most polar molecules
What are the methods of phospholipid motion?
Flexion
Fast axial rotation
Fast lateral diffusion
Flip-flop
What is flexion?
Intra-chain motion
What causes flexion?
Kink formation in fatty acyl chains
Where does fast lateral diffusion occur?
Within the plane of the bilayer
How common is flip-flop?
Rare
What is flip-flop?
Movement of lipid molecules from one half of the bilayer to another on a one-for-one exchange basis
Why is flip-flop rare?
Because its thermodynamically unfavourable
Why is flip-flop thermodynamically unfavourable?
Because have to take a hydrophilic group through a hydrophobic domain
What kind of bonds influence the bilayer structure?
Cis double bonds
What effect do unsaturated hydrocarbon chains with cis double bonds have?
Reduces phospholipid packing
What does the reduction in phospholipid packing lead to?
Disruption of hexagonal packing of phospholipids
What is the result of the disruption of phospholipid packing?
Membrane is more dynamic/flui
Why do we need polyunsaturated fats in the diet?
Because the body can’t produce them, but they are needed to keep membranes dynamic
What is cholesterol?
A plasma membrane lipid
What % of total membrane lipid does cholesterol make up?
45%
What kind of head group does cholesterol have?
Polar, hydrophilic
What structure does cholesterol have?
Rigid, planar, steroid ring structure
What kind of C-C bonds are present in cholesterol?
Largely single
What is the result of cholesterols structure?
It is very rigid
What does cholesterol abolish?
The endothermic phase transition of phospholipid membrane
What causes the endothermic phase transition?
Usually, a large amount of energy is needed for a phospholipid to change from semi-crystalline arrangement to fluid membrane
How does cholesterol remove/reduce the need for the endothermic phase transition?
It means the membrane doesn’t suddenly become fluid, so there is a gradual change
What brings cholesterol into the phospholipid bilayer?
The ß-OH group hydrogen bonds with the C=O group on the phospholipid
Does does bringing in of cholesterol bring into the membrane?
The rigid structure, which grafts onto phospholipid
What is the effect of the grafting of the rigid structure of cholesterol onto phospholipid?
Motion restricted in phospholipid adjacent to the rigid steroid ring of cholesterol.
Motion unaffected in phospholipid adjacent to flexible tail of cholesterol
What is it said of the effects of cholesterol on the phospholipid bilayer?
They are paradoxical
Why are the effects of cholesterol in the lipid bilayer paradoxical?
Reduced phospholipid chain motion leads to reduced fluidity
Reduced phospholipid packing leads to increased fluidity
What is the result of the paradoxical effects of cholesterol?
As soon as the membrane starts to move away from the standard properties in either direction, cholesterol buffers it
What functions of membranes do proteins carry out?
Enzymes Transporters Pumps Ion channels Receptors Energy transducers
What is the protein content of membranes?
From 18% to 75%
