Membrane protein interactions - the cytoskeleton

Question 1

Plasma membrane and salt solutions

Answer 1

RBC used to identify and purify plasma membrane
Good model studying membranes and membrane proteins
When placed different salt solutions:
- Hypotonic - swell up, can burst, once burst called RBC ghosts and now just purified plasma membrane
- Isotonic - stay the same
- Hypertonic - shrivel up

Question 2

Cytoskeleton linked to plasma membrane

Answer 2

RBC as example
Mutations in spectrin cause certain types hemolytic anaemia (hereditary elliptocytosis and hereditary spherocytosis)

Question 3

Key features biological membranes

Answer 3

Are asymmetric
Proteins always have same orientation in membrane
Lipid composition of each half bilayer different
Key features:
- Ligand binding domain
- Transmembrane binding domain
- Cytoplasmic domain
Disulphide bonds used keep shape peripheral proteins
Peripheral and integral work together link cytoskeleton

Question 4

Blood group substances

Answer 4

ABO system
Either O, A, B or AB
BG determined by structure oligosaccharides attached to sphingomyelin and protein in RBC membrane and proteins in plasma and other bodily fluids
O universal donor
AB universal acceptor

Question 5

Terminal sugars of the oligosaccharide chains of the blood group substances

Answer 5

Difference between groups is terminal sugar on end of chain
O - only fucose (Fuc) on left branch and nothing on right
A - fucose on left branch and N-acetylgalactosamine (GalNAc) on right
B - fucose on left branch and galactose (Gal) on right

Question 6

When membrane asymmetry is important

Answer 6

Coagulation (clotting):
- Phosphatidylserine on platelets and other cell membranes provides nucleation site for coagulation cascade
Cell recognition and clearance:
- Macrophage plasma membrane contains receptors, recognise amino phospholipids (phosphatidylserine or phosphatidylethanolamine) which are transferred to outer leaflet of plasma membrane apoptotic cells
- Regulation lipids control

Question 7

Membrane transport proteins of small molecules

Answer 7

Active transport
Electrochemical gradients
Carriers and channels

Question 8

Phospholipid bilayer acts barrier to diffusion many different solutes

Answer 8

Lipid bilayers highly impermeable polar molecules and ions
Is diffusion barrier caused by hydrophobic tails phospholipids
Liposomes - made of phospholipids to deliver small amounts of … to … (need to go back over)

Question 9

Membrane transport proteins

Answer 9

Each protein transports particular solute
All multi-pass integral membrane proteins
Allow solute pass through membrane so doesn’t come in contact with membranes hydrophobic core

Question 10

Active vs passive transport

Answer 10

Passive transport - solute moves down concentration gradient (channels and carriers)
Active transport - solute moves against concentration gradient, requires energy (only carriers)
Carrier protein often referred to as pump, permease or transporter

Question 11

Transport solutes affected by electrochemical gradients

Answer 11

Transport influenced by membrane potential
Voltage difference across cells because excess +ve ions on one side and -ve ions on other
Combination membrane potential and concentration gradient gives electrochemical gradient
Electro chemical gradient greater if membrane potential negative inside with positive ion inside (e.g. opposites to each other)
Electro chemical gradient lesser if membrane potential positive inside with positive ion inside (e.g. the same)
This is compared to normal electrochemical gradient with no membrane potential

Question 12

Electrochemical gradient

Answer 12

Established by ionic concentration differences on either side membrane
Difference produced through action ion channels and carriers/pumps
Drive transport processes, convey electric signals in nerves, make ATP in mitochondira, chloroplasts and bacterial membranes

Question 13

Channels transport solute more rapidly than carriers

Answer 13

Carrier proteins directly bind to solute, undergo conformational change, then release solute other side
Channel proteins only interact v weakly with solute and form aqueous pore, v selective
Channel can transport 100 million ions per second (x10^5 faster than any carrier)

Question 14

Ion channel

Answer 14

Form narrow hydrophilic pores through membrane
Are specific for different ions - e.g. potassium, sodium
Allow rapid movement ions down concentration gradient or electrochemical gradient (can be both)
Open and close rapidly
Regulated by binding of ions, changes in voltage or binding ligands (extracellular or intracellular)
Over 100 different types channel with different properties
Target of many toxins and medicines
Mutations in voltage-gated sodium channel SCN9A causes congenital insensitivity to pain

Question 15

Active transport can be mediated by

Answer 15

Reactive coupling (carrier-mediated transport)
- Uniport (only transports the wanted molecules)
- Symport (transports wanted molecule and a co-transported ion in same direction)
- Antiport (transports wanted molecule and co-transported ion in opposite directions at same time)
ATP driven pump
Light driven pump

Question 16

Carrier-mediated diffusion enhances rate of transport

Answer 16

Mammalian plasma membrane transport driven by Na+ gradients
Bacteria, yeast, intracellular membranes transport driven by H+ gradients
E.g. proton driven symport lactose permease

Question 17

Glucose uptake driven by electrochemical gradient

Answer 17

Glucose transport across epithelial cells involves combined action of three carriers:
- Glucose/sodium ion symporter at apical surface
- Sodium/Potassium pump at basal surface
- Glucose carrier at basal surface
E.g. this example as the enterocytes lining the lumen of small intestine

Question 18

Transcellular transport of glucose

Answer 18

Goes against concentration gradient (low in lumen, high in epithelium, low in extracellular fliud)
Symport has resulted in excess sodium inside cell, uses sodium potassium pump to move it out of cell and potassium into cell
Still example in lumen moving glucose to extracellular fluid

Question 19

Summary

Answer 19

Integral and peripheral membrane proteins make up the RBC cytoskeleton. Defects in this cytoskeleton cause RBC to be more fragile.
The diffusion of membrane proteins and lipids can be constrained (physical barriers and direct interactions with the cytoskeleton).
The distribution of membrane proteins and lipids is asymmetric.
Changes in lipid asymmetry underlie clotting and the identification of dead cells.
Passive transport (facilitated diffusion)
- Molecules move with the concentration gradient
- Through carriers or channels
- The movement through a channel can be regulated by the channel opening and closing - by ions, voltage changes or ligand binding – called Gating
Active transport
- Molecules move against concentration gradients through the action of carriers
- Energy dependent process. The energy can come from the electrochemical gradient, ATP or light