Biological Membranes Flashcards
What are the leaflets that make up the plasma membrane?
Outer extracellular leaflet
Inner cytoplasmic leaflet
What can diffuse laterally through the membrane?
Phospholipids and proteins
Why is the plasma membrane like a mosaic?
Different lipids and proteins in different membranes
Can lipids move from the extracellular leaflet to the inner cytoplasmic leaflet and vice versa?
no to both
Why are phospholipids amphipathic (two moieties)?
They have two parts, non-polar hydrophobic part and a polar hydrophilic part
What are lipids?
Insoluable fatty acid derivatives
What are phospholipids?
Lipids with a phosphate group
How many fatty acids are attached to each phosphate group?
Two
What are the four major phospholipids in the mammalian plasma membrane?
Phosphatidyl-ethanolamine (amino alcohol)
Phosphatidyl-serine (amino acid)
Phosphatidyl-choline
Sphingomyelin
Do the major phospholipids ionise at physiological PH?
yes
Which of the four phospholipids has an overall negative charge?
phosphatidyl-serine
What causes a kink in a fatty acid chain?
A double bond
What is the structure of phosphatidylcholine?
Choline bonded to a phosphate bonded to a glycerol bonded to fatty acid chains (one with a cis double bond)
How can you demonstrate that membranes are fluid?
- Fluorescently tag a proteins on the membrane
- Bleach an area of the proteins
- some of the bleached proteins will diffuse out of the area and non-bleached proteins will move in
What is more energetically favourable, a bilayer in a planar shape or a spherical shape?
Spherical because all the hydrophobic fatty acid chains are protected by the hydrophilic heads so they are not exposed to water
Why is fluidity important?
- Allows lipids and proteins to diffuse in the lateral plane and interact with one another
- Allows membranes to fuse with other membranes
- Ensures membranes are shared equally between daughter cells following cell division
- Allows cells to change shape (cell motility) so they can move
How do bacteria and yeast keep their membranes fluid in colder environments?
They synthesise:
-shorter fatty acid chains
-fatty acid chains with a degree of unsaturation (more double bonds)
This decreases the interactions between FA chains and means that membranes remain fluid
What is the role of cholesterol in the cell membrane?
Cholesterol inserts between membrane phospholipids. This tightens packing in the bilayer and decreases membrane permeability to small molecules (membrane more tightly packed)
Where are the lipid bilayers assembled?
In the ER
Describe the synthesis of phosphatidylcholine
- Fatty acids synthesised in cytosol are transported to ER by FABP (fatty acid binding protein) (highly hydrophobic so need to be transported in an aqueous environment)
- FAs embed in membrane and in a succession of steps glycerol, phosphate and choline added (this happens in the outer cytosolic)
