The cell- 36 Flashcards
Membrane proteins and membrane fluidity
What are membranes? What is their role?
Mosaic of proteins embedded and dispersed in the phospholipid bilayer.
Glycans- recognition and signalling
Anchors cell in environment
Anchors membrane to cytoskeleton
What is the importance of membrane fluidity?
-Lipids diffuse laterally,
-Proteins are not involved in anchoring, they also diffuse.
-Proteins need to transmit signals.
-Transport across by diffusion or via transporter.
-Vesicles need to bud off and fuse.
How do you measure the rate of lateral diffusion in the membrane?
Membrane with fluorophores.
Intense light bleaches fluorophores.
Rate of diffusion of fluorophores can be measured.
How fluid is the membrane?
Biological membranes have constant movement within the bilayer.
Rate of membrane lipid movement can be measured.
How fast does rotation and flexion occur? Give examples? How does this differ with proteins?
Rotation and flexion occur at high rates.
Lateral diffusion: ~3um per secons (1/10 of the length of the mammalian cell).
Transverse diffusion: flip-flop once every 3 days (rare).
Proteins are similar but generally move slower.
How does temperature effect membrane fluidity?
<Tm = solidlike
>Tm = fluidlike
Too fluid: membrane disordered, too much permeability.
Too solid: gel slows down movement too much.
What is the relationship between membrane fluidity and temperature?
Fluidity increases as temperature increases.
Lipid molecules move faster.
The membrane becomes more permeable.
Fluidity decreases as temperature decreases.
Lipid molecules move slowly (gel-like).
The membrane becomes less permeable.
How does the lipid composition effect the membrane fluidity?
Increases fluidity:
-unsaturated lipids give kinks
-short chains allow fewer interactions between lipids
-high temperature
Decreases fluidity:
-saturated chains
-long chains
-low temperature
What happens if you cannot intrinsically regulate your temperature?
You must adapt to your surroundings, organisms regulate their lipid composition.
-short unsaturated fatty acis predominate at low temperatures.
-long saturated fatty acids predominate at high temperatures.
Plants have sensors in the plasma membrane that detect changes in fluidity.
Fluidity increases, indicates temperature is increasing.
This allows the plant to prepare for heat stress.
How does cholesterol affect membrane fluidity?
Less fluid at warm temperatures (e.g. body temperature), by restraining the phospholipid movement.
More fluid at lower (cool) temperatures by preventing close packing of phospholipids.
With cholesterol the increase of fluidity and temperature is linear, whereas without cholesterol it is sigmoidal.
There is no cholesterol in plants and bacteria.
How does ethanol effect membrane fluidity?
It increases the membrane fluidity.
Chronic alcoholics compensate by increasing cholesterol content of membrane.
Are lipid bilayers symmetrical or asymmetrical? Why?
Asymmetric.
The two layers have different lipid composition.
How is the asymmetric composition of the lipid bilayers regulated?
Proteins known as phospholipid translocators (flippases) catalyse the flip-flop event to maintain phospholipids in the correct monolayer.
How is the ER critical to membrane synthesis?
The synthesis and modification of the membrane by the ER determines the asymmetric distribution of lipids, proteins and carbohydrates.
Where are the proteins destined for membranes synthesised?
On one of the membrane of the ER, not IN the ER.
Membrane proteins have clear direction, cannot flip-flop.
What are the types of membrane proteins?
-Integral membrane proteins transverse (cross) the membrane.
-Peripheral membrane proteins associate with a membrane face.
-Some proteins bind to the surface of integral proteins.
-Some proteins are covalently anchored to the membrane.
What are the types of integral membrane proteins?
-Single span hydrophobic alpha-helix. Either C- or N-terminal can be intra-cellular
-Multi-spanning containing alpha-helices. 7 transmembrane helix proteins, but can have few or more helices.
-Beta-barrel protein forming a pore.
What is an integral membrane protien?
Crosses the membrane at least once.
What is membrane topology?
Arrangement relative to the membrane.
This does not change
