jpixie7
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This question appeared in paper 1 (2017 AQA) and I'm unsure how co-transport is involved in the transport of Na+ and H+ and why isn't the Na+ pumped in as it talks about the carrier protein changing shape and using ATP but how would a protein involved in co-transport change shape?

Sodium ions from salt (sodium chloride) are absorbed by cells lining the gut. Some
of these cells have membranes with a carrier protein called NHE3.
NHE3 actively transports one sodium ion into the cell in exchange for one proton
(hydrogen ion) out of the cell.
Use your knowledge of transport across cell membranes to suggest how NHE3
does this.
[3 marks]

MARK SCHEME ANSWER:
1. Co-transport;
2. Uses (hydrolysis of) ATP;
3. Sodium ion and proton bind to the protein;
4. Protein changes shape (to move sodium ion
and/or proton across the membrane);

Any help would be appreciated thanks <3
Last edited by jpixie7; 1 year ago
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Jpw1097
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(Original post by jpixie7)
This question appeared in paper 1 (2017 AQA) and I'm unsure how co-transport is involved in the transport of Na+ and H+ and why isn't the Na+ pumped in as it talks about the carrier protein changing shape and using ATP but how would a protein involved in co-transport change shape?

Sodium ions from salt (sodium chloride) are absorbed by cells lining the gut. Some
of these cells have membranes with a carrier protein called NHE3.
NHE3 actively transports one sodium ion into the cell in exchange for one proton
(hydrogen ion) out of the cell.
Use your knowledge of transport across cell membranes to suggest how NHE3
does this.
[3 marks]

MARK SCHEME ANSWER:
1. Co-transport;
2. Uses (hydrolysis of) ATP;
3. Sodium ion and proton bind to the protein;
4. Protein changes shape (to move sodium ion
and/or proton across the membrane);

Any help would be appreciated thanks <3
The NHE3 transporter uses the concentration gradient of sodium to drive H+ ions out of the cell, against their concentration gradient. This process is secondary active transport, as it uses ATP, but not directly.

The sodium concentration is kept low within the cell by the sodium-potassium pump which pumps 3 sodium ions out of the cell (in exchange for two potassium ions) - this uses ATP. This low sodium concentration within the cell means that the sodium concentration gradient in the lumen of the gut is higher than within the cell - maintaining the concentration gradient. Sodium moves down its concentration gradient (from the lumen into the cell) using the NHE3 transporter and in doing so, H+ ions are pumped from the cell into the lumen. The ions are transported across the membrane as the carrier protein changes shape.
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jpixie7
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Thank you, this was really helpful! Just a few questions,

Why are the H+ ions being driven out against their concentration gradient if it's the Na+ ions that being put into the cell against their concentration gradient or are they both diffusing against the concentration gradient?

Is it the pump that changes shape for the actually carrier protein that carries both the H+ ions and Na+ ions?

(Original post by Jpw1097)
The NHE3 transporter uses the concentration gradient of sodium to drive H+ ions out of the cell, against their concentration gradient. This process is secondary active transport, as it uses ATP, but not directly.

The sodium concentration is kept low within the cell by the sodium-potassium pump which pumps 3 sodium ions out of the cell (in exchange for two potassium ions) - this uses ATP. This low sodium concentration within the cell means that the sodium concentration gradient in the lumen of the gut is higher than within the cell - maintaining the concentration gradient. Sodium moves down its concentration gradient (from the lumen into the cell) using the NHE3 transporter and in doing so, H+ ions are pumped from the cell into the lumen. The ions are transported across the membrane as the carrier protein changes shape.
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Jpw1097
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(Original post by jpixie7)
Thank you, this was really helpful! Just a few questions,

Why are the H+ ions being driven out against their concentration gradient if it's the Na+ ions that being put into the cell against their concentration gradient or are they both diffusing against the concentration gradient?

Is it the pump that changes shape for the actually carrier protein that carries both the H+ ions and Na+ ions?
Sodium ions move/diffuse into the cell ALONG their concentration gradient (i.e. from high to low concentration) and in doing so drive H+ ions out of the cell into the lumen AGAINST their concentration gradient (from low to high concentration gradient). The NHE3 transporter is a carrier protein, not a channel (I think I used the word channel in my last post). Carrier proteins transport ions across membranes by changing their shape.
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