Carrier-mediated Transport Flashcards
On a Michaelis-Menten Kinetics graph, is on the X and Y axis?
X axis: Solute concentration
Y axis: Rate of transport / enzyme activity
What does a typical Michaelis-Menten Kinetics graph look like?
Steep increase at the beginning when Transport Rate is dependent on Substrate availability / Concentration.
The Rate of Transport continues to increase but the gradient becomes shallower until it plateaus at Vmax
In a Michaelis-Menten Kinetics graph what does the Vmax reflect?
Vmax is when the transport rate is independent of substrate/solute concentration and reflects the rate of carrier reorientation
In a Michaelis-Menten Kinetics graph what does the Km point reflect?
This is the solute concentration at 1/2 Vmax and it reflects the concentration for binding and release of the solute (needs to be correct for environment so it can bind and release it on correct sides of membrane)
What are the general, and specific to carrier-mediated transport, Michaelis-Menten Kinetics equations?
E + S <> ES <> E + P
“Enzyme + Substrate <>
Enzyme-substrate complex
<> Enzyme + Product”
C + So <> CS <> C + Si
“Carrier + Solute (outside-facing) <>
Carrier-solute/substrate complex <>
Carrier + Solute (inward-facing)”
Why might plants, bacteria and fungi need multiple types of mechanisms for uptake of a nutrient?
Their environment nutrients can differ, eg; roots of a plant might grow through pockets of high and low levels of nutrient.
In low levels of nutrient high affinity uptake (low capacity) mechanisms are important, but these are costly, don’t want to use this if not needed such as when in high levels of nutrient, where its more efficient to use low affinity uptake (high capacity) mechanisms, so have both mechanisms available
How did chilling experiments prove valinomycin carries solutes by diffusing across the membrane?
Lowering temperature of artificial bilayer (membrane) causes it to stiffen, decreasing efficiency of diffusion and valinomycin transport
(Cold temperatures do not affect ion channels so much this way)
What is a practical starting point when trying to learn about a carrier’s secondary structure?
Look at its amino acids, observe how hydrophobic or hydrophilic a string of them are, create a hydropathy plot to predict where they are positioned with respect to the membrane
What carrier is the main transporter of glucose between liver and blood?
GLUT2
What carrier is the main transporter for glucose absorbance from gut?
GLUT2
What carrier forms part of the glucose sensor in pancreatic beta cells?
GLUT2
What action does insulin have on facilitators?
Recruits more GLUT2 to the plasma membrane
Dysregulation of what carrier is associated with diabetes?
GLUT2
What is the main transporter of glucose across the blood-brain barrier?
GLUT1
Deficiency in what carrier can cause epilepsy?
GLUT1
Over-expression in what carrier can indicate cancer?
GLUT1 (cancer needs energy source for uncontrolled growth)
There are two stereoisomers for glucose, what are they called?
L-glucose and D-glucose
Which stereoisomer of glucose is the biologically active form?
D-glucose
Uptake of L-glucose is due to…?
Diffusion across membrane via lipids (not facilitated diffusion)
Uptake of D-glucose is due to…?
Facilitated diffusion (with some lipid-dependent diffusion but this is significantly less of a contribution)
When a graph follows a typical Michaelis Menten Kinetics curve, what does this indicate?
Involvement of enzyme activity
What organism is Saccharomyces cerevisiae ? (S. cerevisiae)
Yeast
What is special about glucose transport in yeast?
Doesn’t have carriers, the glucose just diffuses across the membrane and this is sufficient for the cell, since yeast evolved in glucose-rich environments such as rotting fruits
How do most fungi transport / uptake glucose?
Uptake is “energised” via being coupled to H+ electrochemical gradient, glucose symported due to low-glucose environment
In the cells transporting glucose from the intestinal lumen into the blood, what is the name of the membrane facing the intestinal lumen?
Apical membrane
In the cells transporting glucose from the intestinal lumen into the blood, what is the name of the membrane facing the blood?
Basolateral membrane
What transporters do you find in the cells transporting glucose from the intestinal lumen into the blood?
2 Na+ / glucose symporter
Na+/K+ ATPase
GLUT2
How does cholera work?
Dehydration due to diarrhea
Cholera toxin targets chloride channel in apical membrane, reverses solute concentrations, efflux of chloride, net efflux of solute out of apical cells, causes osmotic gradient, water leaves cells. Solution of glucose and sodium targets this so you can reverse this while your body targets the toxin
Where would you find a Na+ _ Ca2+ antiporter?
Cardiac muscle cell membranes
What are Na+ _ Ca2+ antiporters important for?
Clearing Ca2+ after an action potential
In a cardiac muscle cell plasma membrane, why do Na+ _ Ca2+ antiporters bring in three Na+ for every Ca2+?
Ca2+ needs to be very low concentration within the cell, (nanomolar conentrations) so has to overcome a large electrochemical gradient (since inside cell is negative as well), needs the energy associated with 3 Na+ down their electrochemical gradient to pump the Ca2+ out
In cardiac muscle cells, what maintains the Na+ gradient across the membrane?
Na+/K+ ATPase maintains Na+ gradient, pumping Na+ out
This is to enable Ca2+ efflux via the Na+_Ca2+ antiporter
How did heart failure use to be treated?
oubain was used to inhibit the Na+/K+ ATPase, to elevate cytosolic Ca2+ which would increase force of heart muscle contractions
In plant roots, how are nutrients like ions taken up?
Symported in due to H+ electrochemical gradient
(how many H+ are required depends on concentration gradient of desired ion, how difficult it is to symport it)
