Lec 18 Membranes

Question 1

Membrane Functions:

Answer 1

1) barrier between cell and environment
- for eukaryotes separate the cell into different compartments
2) contain selective pumps to regulate transport of ions and biological molecules
3) Generates and maintains proton (pH) gradient
4) site of enzyme and reaction pathways
5) recognizes extracellular signals and communicates with cell exterior

Question 2

What are the 6 membrane features?

Answer 2

1) Bilayer structure
2) Non-covalent assemblies
3) Asymmetry of the layers
4) Selectively permeable
5) Fluidity
6) Self-annealing ability

Question 3

Bilayer structure

Answer 3

• made of two lipid layers
• hydrophobic tails interior
• polar head groups exposed to the hydrophilic exterior

Question 4

Non-covalent assemblies

Answer 4

membrane is assembled using non-covalent polar interactions specifically hydrophobic interactions and van der waals interactions

Question 5

Asymmetry of the layers

Answer 5

specific proteins are exposed to one surface or another to carry out their specific functions

Question 6

selectively permeable

Answer 6

• the membrane provides a impenetrable barrier to ionic and polar molecules
• therefore specific membrane proteins transport these molecules across the membrane into or out of the cell

Question 7

Fluidity

Answer 7

• lipids and proteins of the membranes can move within the membrane and the function of the protein is dependent on this movement
• the degree of fluidity is determined by the proportion of saturated, unsaturated fatty acid tails, and cholesterol found in the membrane

Question 8

Self-annealing ability

Answer 8

if the membrane is disrupted they can spontaneously reseal immediately.

• the resealing is essential for cell viability
• also crucial during membrane fusion for process such end and exocytosis

Question 9

ampipathic

Answer 9

contain both polar and hydrophobic regions

Question 10

What drives the formation of lipid monolayers and bilayers?

Answer 10

-the association of hydrophobic regions to maximize their interaction which releases water

Question 11

What factors are used to determine the stability of the lipid layer formation

Answer 11

• van der waals interactions between closely packed acyl tails
• hydrophobic effect is the dominant force

Question 12

Thermodynamically what happens when the lipid layer is formed?

Answer 12

entropy is increased

Question 13

What are two examples of vesicles?

Answer 13

• micelles

- liposomes

Question 14

Micelle

Answer 14

contain a hydrophobic interior

Question 15

What molecules form the micelles

Answer 15

the molecules that forms the micelles have short and single acyl chains on a polar head because the interior of the vesicle does not have space to accommodate longer or a larger number of acyl chains

Question 16

What is the lipid concentration what which micelles are formed called?

Answer 16

critical micelle concentration (CMC)

Question 17

What is the importance of the fluid nature of the membrane bilayer? why?

Answer 17

it is critical for the life of the cell because:

• acts as a 2 dimensional solvent for membrane proteins (tansporters and signaling molecules)
• analogous to aqueous cytosol acting as a 3 dimensional solvent for soluble proteins

Question 18

lipid raft

Answer 18

• thicker than other parts of the brain
• function: is to accommodate and concentrate certain types of proteins to enable them to function together. (Ex: proteins converting extracellular signals to intracellular signals–transmembrane signaling)

Question 19

What are the three types of protein associated with membranes?

Answer 19

1. Integral membrane (transmembrane) proteins
2. Lipid anchored membrane proteins
3. Peripheral membrane protein

Question 20

integral membrane (transmembrane) proteins

Answer 20

• proteins that completely span a bilayer thus have surfaces exposed to both the interior and exterior of the cells
• have hydrophobic bundles of a helices or Beta sheets which make favorable hydrophobic interactions with the interior of the lipid–this is what causes the protein to be inserted into the membrane

Question 21

lipid anchored membrane proteins

Answer 21

• proteins with a hydrophobic lipid tail
• inserts into the membrane to hold the protein in place
• hydrophobic interactions important

Question 22

Peripheral membrane protein

Answer 22

-proteins associated with the membrane through contacts with the polar head groups through charge-charge interactions and H-bonding

Question 23

What two structural motifs do integral membrane proteins display?

Answer 23

• Membrane spanning alpha-helical bundles

- beta barrels

Question 24

what are the three types of anchors used for lipid-anchored proteins?

Answer 24

• protein joined to a fatty acid for a tail
• protein joined to a prenyl group for a tail
• protein joined to a phosphotidylinositol through a glycan for a tail

Question 25

Lateral movement within a membrane

Answer 25

“Fluid Mosaic Model”

• membrane is a dynamic structure in which both lipids and proteins can move rapidly within the membranes bilayers
• this occurs because there is low cost in energy in lateral movement

Question 26

Transverse movement within a membrane

Answer 26

• movement from one side of the membrane to another
• polar head group would have to travel through two layers of hydrophobic tails
• great energy barrier for this process

Question 27

Membrane Fluidity

Answer 27

-controlled by fatty acid composition and cholesterol content
-fluidity is dependent on temperature
rigid at low temp
fluid at high temp

Question 28

What does the phase-transition temp depend on?

Answer 28

• fatty acid chain length (shorter chains have lower Tm)
• fatty acid saturation level (fewer double bonds have higher Tm)
• How much cholesterol is present