Lipid Membrane and Solute Transport

Question 1

membrane permeability

Answer 1

permeable to small polar solutes and non polar compounds
impermeable to large polar solutes and ions

Question 2

membrane structure and synthesis

Answer 2

membranes are mostly polar lipids and proteins
carbohydrates present as glycoproteins or glycolipids
synthesis of membrane lipids/proteins in ER, transported via vesicles

Question 3

fluid mosaic model

Answer 3

proteins and lipids move laterally
generally glycoproteins face extracellular side of membrane

Question 4

monotopic/polytopic

Answer 4

protein spanning 1 leaflet of lipid bilayer vs 2

Question 5

Integral proteins description

Answer 5

proteins with polypeptide chains that traverse the membrane
can be monotopic, bitopic or polytopic
contain a hydrophilic region and hydrophobic region
can be removed using detergent to coat the hydrophobic regions

Question 6

Peripheral proteins description

Answer 6

associate with the membrane through electrostatic interactions and H bonding with integral proteins or hydrophilic head groups
changes in pH, chelating agent, urea or carbonate can dissociate peripheral proteins

Question 7

amphitropic proteins description

Answer 7

associate reversibly with membrane, electrostatically or lipid anchored (GPI)
association is regulated by ligand binding or phosphorylation which reveals site that had previously been inaccessible

Question 8

asymmetry of membrane composition and examples

Answer 8

serves to differentiate different functions

plasma membrane: no cardiolipin, high cholesterol and sphinolipids

mitochondrial membrane: low cholesterol and sphingolipids but high cardiolipin and phosphatidylglycerol

phosphatidylserine is normally maintained inside cell, when outside it signals apoptosis and clotting

Question 9

phosphotidylinositols

Answer 9

minor components of most membranes
critical for signaling

Question 10

liquid-ordered state vs. disordered state

Answer 10

liquid ordered: below normal temperatures, motion is constrained and bilayer is paracrystalline and firm

liquid disordered: above normal temp. increased thermal energy increases movement of side chains
general shape and dimensions of membrane maintained
increased ability to switch leaflets depending on lipid composition

Question 11

factors that change ordered state of membrane

Answer 11

Increase fluidity: increased temp, decreased saturation of FA, short chain, sterol content (when more gel-like)
- sphingolipids and phospholipids with long tails (gel-like) then cholesterol will increase fluidity
Decrease fluidity: decreased temp, increased saturation of FA, long chains, sterol content (increase compaction of unsaturated FA)

regulation of FA content to regulate fluidity

Question 12

transbilayer diffusion types

Answer 12

lateral diffusion uncatalyzed: very fast
transbilayer diffusion uncatalyzed: very slow

flippase: ATP driven outside to inside movement, P-type ATPase
floppase: ATP driven inside to outside movement, ABC transporter
Scramblase: Ca2+ activated, down concentration gradient, either direction

Question 13

demobilization of membrane proteins
ex. erythrocyte membrane

Answer 13

some proteins are anchored to internal cytosolic structures to restrict movement
ex. in erythrocyte membranes glycophorin and chloride bicarbonate exchange proteins are anchored to spectrin (cytosolic scaffolding) inside cell thus forming fences defining regions within which lipids can move

Question 14

lipid rafts

Answer 14

lipid rafts or microdomains are clusters of glycosphingolipids (long chain saturated FA) enriched with lipid anchored proteins
inner leaflet: doubly or triply acylated proteins
outer leaflet: GPI-anchored proteins
rafts are thicker regions and are more ordered
caveolae: little caves, allow membrane to form vesicles that exocytose and can help regulate cell surface area

Question 15

membrane fusion requirements

Answer 15

triggering signal, ability to recognize each other, close apposition, local disruption of bilayer, hemi-fusion, fusion proteins (SNAREs) to bring recognition and distortion of membranes to form single continuous layer

Question 16

membrane fusion purposes

Answer 16

Golgi apparatus budding, endo/exocytosis, fusion of endosomes and lysosomes, viral infections, fertilization, fusion of vacuoles in plants, cell division membrane separation

Question 17

SNARE proteins

Answer 17

tSNARE - assembly on target membrane
vSNARE - assemble on vesicle membrane
qSNARE (SNAP-25) - Ca2+ induced regulatory proteins
NSF - N-ethylmaleimide sensitive fusion factors disassemble SNARE complex

Question 18

SNARE process

Answer 18

v and t SNARE zip up each other, SNAP-25 on target protein helps regulate process
zipping causes lateral tension favoring hemifusion between outer layers
complete fusion creates a pore
pore widens and vesicle contents are released into target

Question 19

genes related to transport of solutes

Answer 19

2000 genes in human genome
transporters/carriers increase rate of solute movement
passive, active, primary and secondary

Question 20

facilitated diffusion

Answer 20

non-ATP driven, using a channel protein
down electrochemical gradient
polar molecules require a carrier protein for transport across

Question 21

primary active transport

Answer 21

against electrochemical gradient
ATP-driven or coupling with favorable reaction, conformational change

Question 22

secondary active transport
symport and antiport examples

Answer 22

against electrochemical gradient
driven by actively transporting a molecule against its gradient with ATP, then driven by ions moving down their electrochemical gradient

ex. symport: Na+/Glucose and Na+/amino acid in intestinal epithelium
antiport: Na+/K+ ATPase and Na+/H+ pumps in kidney

Question 23

ion channel

Answer 23

down electrochemical gradient
may be gated by ligand or ion

Question 24

ionophore mediated ion transport

Answer 24

down electrochemical gradient
small molecules mask the charge of ions and allow them to diffuse through the plasma membrane

Question 25

types of transport systems

Answer 25

uniport - molecules move one at a time in one direction

cotransport: symport and antiport
symport: 2 or more molecules move together in same direction
antiport: 2 or more molecules move through in opposite directions

Question 26

GLUT transporters

Answer 26

glucose, uniport transporters, 12 types in total in humans
glucose is ligand that catalyzes conformational change
increase rate of transport by 50,000x

GLUT1 - erythrocytes and most tissues at low level (basal glucose uptake)
GLUT2 - liver, pancreatic islets, intestine, kidney (basal glucose uptake, high turnover)
GLUT4 - muscle, fat, and heart (activity increased by insulin)

Question 27

insulin dependent glucose transporter pathway

Answer 27

1. GLUT receptors stored in vesicles
2. when insulin binds to its receptors, PIP3 signals fusion of vesicles with plasma membrane to add GLUT transporters
3. when glucose levels drop, GLUT receptors are endocytized again
4. GLUT vesicles fuse to form larger endosomes
   poor signal transduction due to overstimulation of signal leads to poor signal transduction in diabetes mellitus

Question 28

electroneutral antiport transport of anions in respiration

result

Answer 28

1. CO2 from catabolism enters erythrocyte
2. CO2 is not very soluble in blood so carbonic anhydrase converts CO2 + H2O –> Bicarbonate + H+
3. Cl- enters erythrocytes as bicarbonate exits (antiport)
4. bicarbonate enters lung tissue and Cl- is expelled (antiport)
5. carbonic anhydrase converts bicarbonate –> CO2 + H2O and it is exhaled

pH doesn’t decrease and allows conformational change of hemoglobin to bind and release O2
increases rate of transport >1 millionx

Question 29

P-type ATPase example

Answer 29

Na/K pump to establish resting membrane potential
1. 3 Na+ P-type pump
2. ATPase activity of P-type pump phosphorylates enzyme causing conformational change releasing 3 Na+ outside the cell
3. 2 K+ bind enzyme, dephosphorylation causes return conformational change and 2K+ is released into cell
creates membrane charge differential, outside is more positive than inside (negative)

Question 30

F type ATPase example

Answer 30

F type (energy coupling factors) ATPase uses ATP to pump protons across the membrane against gradient
F type catalyzes in both directions
large proton gradient supplies energy for ATP production (ATP synthase)