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John421 John421
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8 years ago Edited: 8 years ago, John421
My understanding of glucose transport in general is the following:

1) ATP is used to setup a concentration gradient for sodium (from a low concentration of sodium to a high concentration of sodium)
2) Glucose and sodium travel together through a transport protein (from a high concentration of sodium to a low concentration of sodium)

Now, in the kidney we have several things going on:
A) Sodium is pumped out of the nephron by active transport
B) Water travels out of the nephron by Osmosis.

Now, no sodium is secreted from the blood into the nephron (hence no concentration gradient that would benefit the transport of glucose-sodium is set up using ATP like in 1) )

However the removal of water would make the sodium more concentrated. Is it water being removed what sets up the sodium concentration gradient for the glucose/sodium transport in the nephrons - and if this is the case, I don't see how ATP was used to do it so shouldn't it be described as facilitated diffusion and not secondary active transport?
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wrote...
Educator
8 years ago
Hello, thanks for the question, and apologies for the late reply. I believe this animation will help you understand the concept thoroughly. If not, give us another shout and we will answer the question in full...

https://biology-forums.com/index.php?topic=3402.msg12169#msg12169
John421 Author
wrote...
8 years ago Edited: 8 years ago, John421
Hi,

Thanks for the reply. I did understand what is in the animation.

The major issue is that I'm not sure if it works exactly that way in the kidneys.

e.g. I think it would be strange if the body secreted sodium into the nephrons to build up a concentration gradient to power the sodium-glucose transport protein. I think this because all diagrams I have seen show sodium being reabsorbed into the blood, and not secreted into the nephrons to build such a concentration gradient. Besides, doesn't the body use energy to send sodium back into the blood (absorption), would it really also use energy to send sodium from the blood back into the nephrons (secretion) to build this concentration gradient? The problem is that secretion of sodium is not shown on any diagrams.

Diagrams such as this: https://upload.wikimedia.org/wikipedia/commons/7/72/Renal_Diuretics.gif
wrote...
8 years ago
Firstly, an Na+/K+ ATPase pump on the basolateral membrane of the proximal tubule cell uses ATP molecules to move 3 sodium ions outward into the blood, while bringing in 2 potassium ions. This action creates a downhill sodium ion gradient from the inside of the proximal tubule cell towards the outside (that is, in comparison to both the blood and the tubule itself.

The sodium-dependent glucose cotransporters (SGLT) proteins use the energy from this downhill sodium ion gradient created by the ATPase pump to transport glucose across the apical membrane, against an uphill glucose gradient. Members of the glucose transport family of glucose uniporters then transport the glucose across the basolateral membrane, and into the peritubular capillaries. Because sodium and glucose are in the same direction across the membrane, SGLT1 and SGLT2 are known as symporters.
John421 Author
wrote...
8 years ago Edited: 8 years ago, John421
I think the reason none of this made any sense to me was because tissue fluid was not addressed. It made no sense because glucose needs to travel together with a high concentration of sodium when using the sodium-glucose transport protein.

This is a complete guess, but would you say this is the way it works:

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