Cell: Membranes

Question 1

What is the function of the plasma membrane?

Answer 1

provides cell boundary and prevents movement of materials in and out of the cell

Question 2

What are some of the fundamental properties of membranes?

Answer 2

Barrier
Flexible (e.g red blood cell in capillary)
Self-repairing
Continuous
Selectively permeable
Only certain molecules can pass in and out of the cell

Question 3

Describe phospholipid movement in the bilayer

Answer 3

→ can move about in the bilayer
- Can rotate or exchange in the lateral plane of the membrane
- But normally move very slowly from one leaflet to the other (‘flip-flop’)

Question 4

What does the fluidity of the lipid bilayer depend on?

Answer 4

Its composition

Question 5

How does the composition of the lipid bilayer depend on its composition?

Answer 5

Number of double bonds and number of C atoms in the fatty acid chains of the phospholipids determine fluidity
- Higher no. of double bonds and shorter acyl chains (unsaturated)
= less tightly packed molecules and more fluidity

Question 6

‘Phospholipids are amphipathic’

What does this mean?

Answer 6

→ hydrophilic polar headgroup
→ hydrophobic tail

Question 7

What charge is phosphatidyl serine?

Answer 7

Negativley charged
- important in other processes e.g apoptosis

Question 8

Describe cholesterol and its function in the bilayer

Answer 8

• Amphipathic
• Smaller than other phospholipids
• Packs between phospholipids and makes membrane more rigid locally
• At high conc. It stops the membrane from becoming crystalline

Question 9

What structures do lipids form in aqueous solutions?

Answer 9

micelles or bilayers (depending on lipid shape)

Question 10

Why do lipid bilayers prefer to form sealed compartments?

Answer 10

Because it is energetically favorable
- the planaer phosphplipid bilayer with edges exposed to water = unfavourable

Question 11

What are microdomains on the lipid bilayer?

Answer 11

Clusters of membrane proteins
- often have specific functions e.g signalling

Question 12

What is the topology of a protein in relation to the lipid bilayer?

Answer 12

specific orientation of a protein in the membrane
= important for function

Question 13

What does FRAP illustrate about membrane proteins, and how does it do this?

Answer 13

Fluorescence recovery after photobleaching (FRAP) illustrates the dynamic movement of membrane proteins
→ rate that the beaches area is filled demonstrates how mobile the protein in the membrane is
= provides insight into protein function

Question 14

Describe integral membrane proteins

Answer 14

Imbedded in membrane
Strong - hard to dissociate them

Question 15

Describe peripheral membrane proteins

Answer 15

Outside or inside the membrane
Relatively easy to dissociate
Can be associated with the membrane (lipid anchor)
Can associate with integral membrane proteins
e.g Ras, mutated in many cancers

Question 16

Which molecules form microdomains called rafts?

Answer 16

Cholesterol and sphingolipids
- membrane is slightly thicker here

Question 17

Define apical surface

Answer 17

Outside
- facing the lumen or the external environ

Question 18

define basal surface

Answer 18

Inside
- the bottom edge of the epithelial tissue next to the basement membrane

Question 19

Give an example of a cell where the cytoskeleton is linked to the plasma membrane.
How does this affect function?

Answer 19

Red blood cells
- Allows for flexibility of RBCs

Question 20

What is the role of the extracellular domain important for?

Answer 20

Recognising its ligand

Question 21

Why is membrane asymmetry important in the ABO blood group system?

Answer 21

• Each blood group has a different terminal sugar on their oligosaccharide chain
• These bind to antigens and don’t elicit an immune response if the same blood group

Question 22

Why is membrane asymmetry important in coagulation?

Answer 22

(clot formation)
Phosphatidylserine on platelets and other membranes provides the nucleation site for the coagulation cascade

Question 23

Why is membrane asymmetry important for cell recognition & apoptosis?

Answer 23

Macrophage plasma membrane contains receptors, which recognise aminophospholipids
- Phosphatidylserine or phosphatidylethanolamine (transferred to the outer leaflet of the plasma membrane of apoptotic cells

Question 24

What type of molecules is the phospholipid bilayer permeable to?

Answer 24

highly impermeable to polar molecules and ions
- therefore it has evolved membrane transport proteins

Question 25

What type of protein are all membrane transport proteins?

Answer 25

Integral

Question 26

Describe passive transport, which proteins are used?

Answer 26

Passive= solute moves down conc. Gradient (channels and carriers)

Question 27

Describe active transport, which proteins are used?

Answer 27

Active = solute moves against conc. Gradient so requires energy
(only carriers)

Question 28

Describe how membrane transport is affected by electrochemical gradients

Answer 28

Influenced by membrane potential
Combination of membrane potential and conc. gradient = electrochemical gradient
Need ion pumps and channels to develop voltage difference

Question 29

Describe channel proteins

Answer 29

Transport more rapidly than carriers
Interact weakly with the solute
Can only facilitate passive transport

Question 30

What are ion channels?

Answer 30

narrow hydrophilic pores through membrane
that are specific to ions e.g potassium channel

Question 31

Describe the function of ion channels

Answer 31

• Allow rapid movement of ions down the conc./ electrochemical gradient
• Open and close rapidly
• Regulated by binding of ions / changes in voltage / binding of ligands
• The target of many toxins and medicines (e.g scorpion venom)

Question 32

Describe carrier proteins

Answer 32

• Binds directly to solute
• Can engage in active or passive transport
• Carrier mediated diffusion increases rate of transport
• Active transport: allows cells to optimise uptake where solute is less abundant

Question 33

What does uniporter mean?

Answer 33

Carrier protein with 1 solute

Question 34

What does symporter mean?

Answer 34

Moves solutes in same direction

Question 35

What does antiporter mean?

Answer 35

Moves solutes in opposite direction

Question 36

In mammalian membranes, what ion drives transport gradients?

Answer 36

Na+ gradients

Question 37

In bacteria, yeast & intracellular membranes, what ion drives transport gradients?

Answer 37

H+