Cell Membrane

Question 1

Membrane Functions

Answer 1

1. separate outside and inside of the cell
2. keep inside of organelles separate
3. membrane proteins generate and use ion gradients

Question 2

Phospholipid Structure

Answer 2

2 fatty acids, 1 glycerol, 1 phosphate group
One saturated packed fatty acid and one unsaturated loose fatty acid
Amphipathic molecule

Question 3

Bilayer Characteristics

Answer 3

• form spontaneously. their orientation is more ordered than the surrounding water so it increases free energy
• create hydration shell of water around the lipid heads to prevent spontaneous fusion of cells
• embedded cholesterol modifies bilayer fluidity

Question 4

Bilayer Asymmetry

Answer 4

• used in cell signalling (apoptosis) as phosphotidylserine translocates to the exterior leaflet and acts as a eat me signal to macrophages
• extremely important
• ‘flip-flop’ of molecules from one leaflet to another is extremely rare while movement within one layer is common
• conversion of extracellular to intracellular signals
• Glycolipids are found exclusively on the outer leaflet of the cell. They self-associate through H bonds and VDW forces into lipid rafts. Their function depends on localisation within the body.

Question 5

Membrane Proteins

Answer 5

• Channels, anchors, transporters, enzymes, receptors
• Transmembrane proteins use alpha helices to cross the bilayer to maximise H bonding
• B barrels form channels and pores
• Peripheral vs integral membrane proteins

Question 6

Glycosylation

Answer 6

• Addition of oligosaccharides as proteins pass through the ER and Golgi apparatus
• Used in cell protection against mechanical or chemical damage, signalling, and stabilizing through the interactions of H bond
• Keeps cells at a distance to prevent cell-cell interactions

Question 7

Cholesterol

Answer 7

• Modulates the lipid bilayer properties
• Enhances the permeability barrier properties of the lipid - bilayer
• Rigid rings partly immobilise the chains closest to the phospholipid heads, affecting the permeability of water molecules
• Prevents crystallisation of the hydrocarbon chains

Question 8

Lipid Rafts

Answer 8

Specific membrane proteins and lipids can temporarily concentrate together, facilitated by protein/protein interactions = transient formation of specialised membrane regions.
This tendency to undergo phase partitioning can help with many membrane functions.

Question 9

Lipid Droplets

Answer 9

Cells often store excess lipid droplets to be used for membrane synthesis or energy. They exist as organelles surrounded by a phospholipid monolayer.