Plasma Membrane

Question 1

What are common features of biological membranes?

Answer 1

membranes are sheet like structures a few molecules thick that form closed boundaries

membranes consist of lipids, proteins and carbohydrates linked to lipids or proteins

membranes are non-covalent assemblies
membranes are fluid structures
most membranes are asymmetric in composition
most membranes are electrically polarised

Question 2

What is the plasma membrane composed of?

Answer 2

membrane lipids
- phospholipids
- glycolipids
- cholesterol
membrane proteins 
- integral proteins
- peripheral proteins
- lipid anchored
membrane carbohydrates

Question 3

What are phospholipids?

Answer 3

are the most abundant membrane lipid

phosphoglycerate
- has 2 fatty acid chains and a phosphorylated alcohol
- has a glycerol backbone
phosphorylated alcohol could be serine, choline, inositol or ethanolamine

sphingomyelin
- has a sphingosine backbone

Question 4

What are glycolipids? What is cholesterol?

Answer 4

have a glycerol or sphingosine backbone
- glyceroglycolipids
- spingolipids
contain a carbohydrate component

cholesterol

• may constitute up to 50% of the membrane
• are smaller and less amphipathic

Question 5

What is an amphipathic molecule?

Answer 5

a molecule that has a polar head region and a hydrophobic tail region

Question 6

What is the most common combination of fatty acid structures found in plasma membrane phospholipids?

Answer 6

one saturated fatty acid with no cis-double bond and one unsaturated fatty acid with one cis-double bond

• most unsaturated fatty acids found in the plasma membrane phospholipids are mono-unsaturated = they have only ONE double carbon bond, and this is usually in the cis form.

Question 7

How do phospholipid domains orientate themselves within the plasma membrane?

Answer 7

the hydrophilic phosphate regions are positioned toward those environments that are aqueous (either side of the PM) so they can readily associate with water while the hydrophobic fatty acid regions face each other within the hydrophobic interior, to prevent access to water

Question 8

Why do the plasma membrane phospholipids form a bilayer instead of micelles?

Answer 8

plasma membrane phospholipids contain 2 hydrophobic fatty acid chains that give the molecule a cylindrical shape

• the cylindrical shape prevents close packing of molecules into a micelle structure
• combined with the amphipathic nature of the molecule this drives PM PLs into bilayers in aqueous environments.

bilayer is self-sealing

Question 9

Why do plasma membrane phospholipids spontaneously form bilayers in aqueous environments? What drives this process?

Answer 9

phospholipids are non-polar while water is polar. this means they cannot form hydrogen bonds

this results in the lipids forcing the water to be arranged into an ordered, ice cage-like structure that is energetically unfavourable
- reduces entropy

systems tend towards increased entropy. a disordered state is more favourable than an ordered state
- higher entropy and disorder is favoured

forcing lipids into a bilayer reduces entropy by preventing the formation of more ordered water structures around the molecules
- bilayer has higher entropy than the ordered ice-like structures

Question 10

How are the different membrane lipids arranged in the plasma membrane?

Answer 10

membrane lipids are arranged asymmetrically

outer layer

• phosphatidylcholine
• sphingomyelin

inner layer

• phosphatidylserine
• phosphatidylethanolamine

chloestrols is evenly distributed throughout

Question 11

What are integral proteins?

Answer 11

features

• are single or multipass = single protein or multiple together
• are transmembrane helix = are in a helical shape and span the entire plasma membrane, have hydrophobic and hydrophilic regions
• have non-covalent bonds

Question 12

What are peripheral proteins?

Answer 12

features

• are located inside or outside the cell
• have non-covalent bonds
• interacts with integral proteins or the polar phosphate head groups
• soluble and can readily disassociate from the membrane

Question 13

What are lipid anchored membrane proteins?

Answer 13

features
- are covalently bonded to a lipid molecule
example - glycerophosphoinositol

Question 14

What are membrane carbohydrates?

Answer 14

are associated with membrane lipids and proteins
- glycolipids and glycoproteins

are exclusively presented on the extracellular side of the plasma membrane
- asymmetry

2-5% of the membrane content, up to 8% in RBCs

Question 15

What is the fluid mosaic model?

Answer 15

describes the phospholipid membrane
fluid
- individual phospholipids and proteins can move side-to-side within the layer = mobile
mosaic
- pattern produced by the lipids, carbohydrates and scattered proteins

Question 16

What affects fluidity of the plasma membrane?

Answer 16

fatty acid composition and saturation
- saturated fatty acids have no double bonds and a straight structure = allows saturated fatty acids to densely pack together making the PM less fluid and more rigid

• un-saturated fatty acids contain a double bond and are kinked = increasing levels of un-saturated fatty acids will prevent dense packing of the lipids, increase fluidity and reduce the rigidity of the PM

cholesterol

• acts to hold PLs together at high temperatures increasing the rigidity of the PM and reducing fluidity
• at low temperatures cholesterol prevents PLs from packing tightly reducing rigidity and increasing fluidity.

Question 17

Where are glycoproteins and glycolipids located?

Answer 17

glycolipids and glycoproteins are almost exclusively expressed on the outer leaflet of the PM where they play a role in cell recognition, protection.

Question 18

What is the function of lipids in the plasma membrane?

Answer 18

establish semi-permeable barrier between external and internal aqueous environment
provide environment in which proteins can dissolve and function
Cell- Cell Recognition (Surface Identity Markers)

Question 19

What is the function of proteins in the plasma membrane?

Answer 19

Cell- Cell Recognition (Identity) = RBCs carry glycolipids that determine blood type
Signal Transduction (Receptor)
- receptor = detects signal or chemical signal (ligand) that binds to a protein receptor
- transduction = binding changes protein receptor and activates cellular pathways
- response = induces cell response
Transport
Enzymatic activity
Attachment (cytoskeleton or extracellular matrix) Intercellular junctions (Adhesion)