Plasma Membrane

Question 1

What is the function of membranes?

Answer 1

• deliminatory barrier, separates cytosol from ECM
• selective permeability
• ATP synthesis
• molecule, ion, transport
• signal transduction
• electrical signal generation and conduction

Question 2

What general types of proteins can be embedded in membranes?

Answer 2

Based on location:

• integral
• peripheral

Based on function:

• receptors
• enzymes and pumps
• channels and carrier proteins

Question 3

Define the structural unit of PM (shape. composition, polarity, movement) What are their most common goups? Why do membranes organize in a specific manner?

Answer 3

Phospholipids:

• polar head (X+phosphate+glycerol)
• unpolar tail (two fatty acid chains [usually from 14-24 C long], saturated + unsaturated [has at least one kink which make them less packed and more flexible])
• amphipathic
• can have P attached = signal
• can be cleaved (e.g. polar head and fatty acid tail residue) = signal

Bilayer at physiological pH:

• phosphatidylcholine (neutral) and sphingomyeline (neutral) + glycolipids = outside
• phosphatidylserine (-), phosphatidylethanolamine (neutral) = inside
• both layers have cholesterol
• common movement: lateral difussion, rotation, flexion
• rare movement: flip-flop (apoptosis)

In water: wedge-shaped with one tail form micelles and cylinders with two tails form bilayers, they orient polar heads to the water and unpolar tails together = energically most favourable

Question 4

Describe the structure of cholesterol. What is the role of cholesterol in PM?

Answer 4

Polar head (hydroxyl group) oriented to the polar head of phospholipids is connected via ring structures with an unpolar tail. Evenly redistributed across both layers.

• PM less deformable and fluid, more rigid
• decreased permeability
• prevents hydrocarbons from coming togethher and crystalizing

Question 5

Name some types of transmembrane proteins and how can they be attached to the PM other than being integrated into the membrane itself.

Answer 5

• single and multipass (the integrated protein part is mostly unpolar)
• exposed to only one side of the bilayer (the peripheral protein part is amphipathic)
• single a-helix
• multiple a-helix
• b-sheet barrel
• fatty acid or prenyl linker
• oligosacharide linker (phosphatidylinositol)