Cotransport of H+ and Na+

[MedPR]

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This is from TBR Bio Chapter 3 Passage 8 Question 51.

"The reason sodium can continue to fall down its concentration gradient is because of a sodium/potassium ATPase pump which uses the energy of ATP to pump sodium out of the cell."

Sodium concentration is higher extracellularly than intracellularly, so why does it need a pump to move down its gradient (from outside the cell to inside the cell)? I thought the ATPase was there to pump Na out of the cell, against its concentration gradient?

So if you have cotransport of Na+ and H+, where Na+ is moving into the cell and H+ is moving out of the cell, why is that secondary active transport and not facilitated diffusion? The sodium is moving down its concentration gradient, thus providing the energy for the diffusion of H+.

Edit: Wait, are all cotransporters secondary active transport? Crap.

 

[SLC]

Na+/K++ ATPase uses the energy from hydrolyzed ATP to pump sodium against it's gradient. To answer your question, sodium can flow down it's concentration gradient because the Na/K ATPase pump maintains a gradient for it to flow down.

Na/H antiporters are secondary active transport because you have two ions moving in opposite directions. Na+ in and H+ out. H+ is using the concentration gradient, and movement of sodium to leave the cell. No ATP is being used.

If you have an ion that "piggyback's" with another ion that's moving along it's concentration gradient, both traveling in the same direction, that's facilitated diffusion. Two ions in, one of them using the concentration gradient of the other to "sneak" into the cell.