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30 Cards in this Set
- Front
- Back
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1. Why do we care about carrier mediated transport?
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1. Only way for certain necessary solutes to get into cells
2. Allows regulated transport of solutes 3. Allows "uphill" transport of necessary solutes into cell and unneeded solutes out of cell |
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2. What are carrier mediated transport mechanisms?
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Those where a solute binds to a specific binding site on a carrier protein (an integral membrane protein) and then is transported to the other side of the membrane when the carrier's binding site is exposed to the opposite side of the membrane
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3. What are examples of carrier mediated transport?
What does it not included? |
1. Facilitated transport (facilitated diffusion)
**just one solute 2. Primary active transport 3. Secondary active transport Doesn't include channels or pores |
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4. What is secondary active transport?
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Transporters that use the energy available in the gradient of electrochemical potential across the membrane
Can be co-transporters (symport - same direction) or counter-transporters (exchange, antiport - opposite direction) |
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5. What happens in carrier-mediated transport?
What is the occluded state? |
1. Solute binds to a carrier's binding site on one side of the membrane
2. Something happens to carrier protein causing binding site to be exposed to other side of membrane 3. Solute unbinds from the binding and is now on the other side of the membrane When the outer gate closes and X becomes occluded, still attached to its binding site |
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6. What are some characteristics of carrier mediated transport?
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1. Specificity
**has to be a binding site for solute on integral protein 2. Saturability |
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7. What is absolute specificity?
What are other types of specificity? |
Carriers only recognize a specific molecule
Sites bind similar molecules and isomer (perhaps similar sugars) Others are fairly non-specific |
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8. For carrier mediated transport what criteria must be satisfied?
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1. Organism must have a gene for the transporter
2. Transporter must be expressed in specific cell 3. Transporter must be inserted into the plasma membrane 4. Post translational modifications might be necessary 5. Energetics must be favorable 6. Actual rates of transport are even more complicated |
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9. What is meant by saturability?
Can the transport rate be increased by changing the concentration gradient? |
There are only a finite number of carrier proteins on the cell membrane
When there is a large amount of solute present, all the carriers bind solute and are transporting it No (if mechanism was diffusion this would work) |
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10. When the solute concentration outside the cell is low how do the inward flux for diffusion and carrier mediated transport compare?
What happens when the solute concentration outside increases? |
Inward flux via carrier is higher than what it would be by diffusion
The rate by both mechanisms increases **influx by diffusion increases linearly as concentration difference across membrane increases **influx via transporter reaches a plateau and does not increase any more |
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11. How does the inward flux by diffusion and a carrier-mediated mechanism compare?
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When there is not solute inside the cell, increasing the outside concentration increases the inward flux in a linear way if the mechanism is diffusion
If the mechanism is carrier-mediated the inward flux will increase w/ increasing Cout until maximum transport (Jmax) is reached **the mechanism is saturation - all the carriers are in use so there cannot be a higher rate of transport |
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12. What is Km?
What is a transporter w/ a low Km like? In carrier mediated transport, how is the flux often regulated? |
The value of Cout that produces an influx equal to 1/2 of Jmax
It is able to transport its solute very effectively at low concentrations By the number of active carriers |
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13. What is the simple carrier (or facilitated diffusion) like?
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1. Only one solute is involved
2. Carrier protein can expose its binding site to either face of the membrane 3. Direction is "downhill" |
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14 What are co-transport and counter-transpor?
Where does the energy for the solute movements come from? |
Mechanisms where 2 or more solutes are transported by the same carrier
Co-transport means the solute are going in the same direction Counter-transport means the solute are going in opposite directions The concentration gradients and electrical voltage differences across the membrane **there is no direct involvement of chemical energy from ATP - these are not "pumps" |
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15. What is the mechanism of co-transport or "synport"?
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1. Carrier protein has binding sites for two or more solutes
2. These sites are alternately exposed to both surfaces of the membrane 3. Both solute have to be present for the process to work 4. Empty carrier can reorient to the other side (can come back empty) |
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16. What is an example of co-transport?
What does it mean that co-transport can be electrogenic or electroneutral? |
Na and glucose cross the apical surface of the kidney proximal tubule
Electrogenic = net movement of electrical charge Electroneutral = no net movement of charge |
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17. What are some characteristics of counter-transport or "anti-port"?
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1. Both solutes have to be present for transport of either one of them
2. Removing one of the solutes stops the transport of the other 3. Transport is reversible - direction is determined by electrochemical gradients for each of the solutes 4. Counter transport of ions can be electrogenic or electroneutral |
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18. What are some examples of counter-transport?
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1. Na/H exchangers in renal proximal tubule
2. Cl/HCO3 exchange in RBC 3. Na/Ca exchange in heart cells |
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19. Describe the energetics of carrier-mediated transport.
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1. Carriers are reversible
2. Direction depends on the net energetics 3. If the energy of the "reactants" on the left is greater than the energy of the "products" on the right the rxn goes from left to right (i.e. solutes enter the cell) |
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20. What dictates how the energetics terms combine?
What happens if the left side is greater than the right side? What happens if the transport mechanism is electrogenic? |
Stoichiometry of the binding sites on the transport protein
Solutes are transported inside Energetics are affected by the membrane potential **if its electroneutral than membrane potential has no effect - only the concentrations do |
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21. What is the energy source in active transport (often called a pump)?
What type of solutes are transported? |
ATP
Can be either a single solute that is transported or multiple solutes |
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22. What does the Na/K pump do?
What does this pump use? What is the important of the Na/K pump? |
Transports 3 Na out of cells and 2 K into cells for every 1 ATP hydrolyzed
Uses a major portion of the cells ATP productions Responsible for creating the high K concentration and low Na concentrations in the cytoplasm **these concentrations make the resting membrane potential possible and action potential possible |
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23. What does the low Na concentrations of the cytoplasm provide?
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Some of the energy in the electrochemical potential gradient of Na
This makes Na/Ca exchange possible helping keep the cells Ca concentration very low |
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24. How does the Na/K pump affect an action potential?
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During an AP, K leaves the cell and Na enters the cell
The pump returns these ions so the cellular concentrations are maintained |
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25. How does the Na/K pump affect the resting potential of a cell?
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There is a continuous flux of K from the cell through channels and a continuous flux of Na into the cell through channels
The pump returns these ions so the cellular concentrations are maintained |
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26. What happens of the Na/K pump stops?
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1. Cells lose K
2. Cell gains Na 3. Cells swells |
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27. How can the Na/K pump be stopped or slowed?
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1. Low sodium inside
2. Low potassium outside 3. Ouabain, digitalis and other drugs 4. No ATP 5. Cooling or anoxia limit ATP supply so they slow the pump |
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28. What is the E~P state?
Where is the binding site for digitalis or ouabain? What does digitalis do? |
The pump protein is phosphorylyated once each cycle
On the extracellular surface of the pump By slowing the pump it causes increased intracelluluar [Na] lessening the energy for Na/Ca exchange As a result, intracellular [Ca] increases |
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29. Is the pump electrogenic or electroneutral?
How does it contribute to the membrane potential? What is this "pump current"? |
Electrogenic
Pumps 3 positive ions out for every 2 it pumps in Hyperpolarizing **may change the resting potential by as much as 10-20 mV |
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30. What can stimulate pump activity?
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Increasing intracellular [Na]
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