Lecture 15 Flashcards
Function of Biological Membranes
1) Define Cell Boundary
2) Define Enclose Compartments
3) Control Movement of Material into and out of the Cell
4) Allow Response to External Stimuli
5) Enable interactions between cells
6) Provide Scaffold for biochemical
What does a muscle cell contain?
A plasma membrane, and sarcoplasmic reticulum
Why are red blood cells useful as a model for studies of membrane structures?
They do not contain nuclei or internal membranes
What is the plasma membrane?
A trilaminar (three layer) structure made of a phospholipid bilayer
How thick is the plasma membrane?
6nm
What do lipid molecules do?
Spontaneously aggregate to bury their hydrophobic tails in the interior and expose their hydrophilic heads too water.
What is a micelle formed by?
Fatty acids with only one hydrophobic chain.
What are phospholipids?
Amphipathic, which means they have both hydrophobic (non-polar) and hydrophilic (polar) regions.
What part of the phospholipid is polar?
Hydrophilic Head
What part of the phospholipid is non polar?
Hydrophobic tail
Hydrophilic
A molecule attracted to water molecules
Hydrophobic
A molecule not attracted by water
What do phospholipids consist of?
Two fatty acyl molecules, glycerol backbone and phosphate residue
What positions are the fatty acids, glycerol and phosphate on?
Fatty Acids: sn-1 and sn-2 positions of glycerol
Head group linked by phosphate residue at the sn-3 position.
Where does phospholipids making the plasma membrane come from?
Synthesis occurs at the interface of the cytosol and outer endoplasmic reticulum membrane, which has all the enzymes for synthesis.
1st Step of Phospholipid synthesis?
In the cytosol, fatty acids are activated by the attachment of a CoA molecule
2nd Step of Phospholipid Synthesis?
The activated fatty acids bond to glycerol-phosphate and are inserted into the cytosolic leaflet of the ER membrane via acyl transferase
3rd Step of Phospholipid Synthesis
The phosphate is removed by a phosphatase enzyme
4th Step of Phospholipid Synthesis
A choline already linked to phosphate is attached via choline phosphotransferase
5th Step of Phospholipid Synthesis
Flippases transfer some of the phospholipids to the other leaflet.
What is the final step of Phospholipid Synthesis
A vesicle will come off from the ER containing phospholipids for the cytoplasmic cellular membrane on its exterior leaflet and phospholipids for the exoplasmic cellular membrane on its inner leaflet.
Fluid
Individual lipid molecules move
Mosaic
Diverse ‘particles’ like proteins, carbs, and cholesterol penetrate the lipid layer
What is the plasma membrane viewed as?
Two-dimensional liquid that restricts the diffusion of membrane components
What is embedded in the plasma membrane?
Different proteins, that can interact and are mobile
What are the dynamics of the plasma membrane?
Lipids move easily, laterally within leaflet
Lipids movement to other leaflet is difficult and slow
Membrane proteins diffuse within the bilayer:
- Movement of proteins is restricted
- Rapid movement is spatially limited
- Long range diffusion is slow
- Biochemical modification can alter protein mobility in the membrane
What is the Frye-Edidin Experiment
Just after the fusion. of the two cells, the surface proteins are segregated, but after a short period of time, the surface proteins of both cells diffuse around the unified membrane and mingle
What properties do all membrane share?
Approximately 6nm thick
Stable, flexible and capable of self assembly
What does the inner membrane of mitochondria contain?
A very high concentration of protein necessary for electron transport chain and ATP synthesis
What does myelin sheath consist of?
Layers of plasma membrane wrapped around a neurons axon.
What are the three types of proteins?
Integral, Peripheral, and lipid-anchored
Integral Proteins
Membrane proteins that span the lipid bilayer (are inside)
Peripheral Proteins
Membrane proteins associated with the surfaces of the lipid bilayer
Lipid Anchored Proteins
Proteins that attach to a lipid in the bilayer
What are the functions of integral proteins?
1) Transport of nutrients and ions
2) Cell-Cell Communication
3) Attachment
What are biological membranes?
Asymmetrical
What does the outer leaflet contain?
Glycolipids and glycoproteins
What does temperature affect?
The fluidity of biological membranes
What does warming do?
Increases fluidity
What does cooling do?
Decreases fluidity
What is membrane fluid determined by?
The nature of the lipids in membrane
What do unsaturated lipids do?
Increases fluidity
What do unsaturated lipids do?
Reduce fluidity
In responses to change in temperature, lipid composition of membranes can be changed by:
1) Desaturation of lipids
2) Exchange of lipid chains
What does balance between ordered and disordered structure allow?
- Mechanical support and flexibility
- Membrane assembly and modification
- Dynamic interactions between membrane components (Proteins can come together reversibly)
What does cholesterol modulate?
Membrane fluidity
What does cholesterol act as?
A bidirectional regulator of membrane fluidity
- High temperatures, it stabilizes the membrane and raises its melting point
- Low temperatures, it intercalates between phospholipids and prevents them from stiffening
If cholesterol is added to a liquid crystal membrane:
Fluidity will decrease
If cholesterol is added to a crystalline gel membrane:
Fluidity will increase
