Specifications of active transport.

[extremcurious]

In neurons during re-polarization ATP is used to "actively transport" Na ions out of the cell and K ions into the cell. Does this mean that the ions are flowing against their concentration gradient or the word "actively transported" is just used because ATP is used in this process?

 

[hivesaeed4]

I'm not really sure regarding the direction in which the ions flow during re-polarization but you got to understand two things.

In active transport ions are transported against their concentration gradients and because of that energy (in the form of ATP) is used.

 

[extremcurious]

So it is necessary in active transport that a substance MUST move against a concentration gradient?

 

[nobahar]

A drastic change in the concentration isn't required for a change in membrane voltage like that observed during an action potential. This means that the electrochemical gradients are 'pretty much' the same as before, just the slight change has altered the membrane voltage, so moving ions back across requires ATP. The 'turning off' of the flow of ions during an action potential is due to changes in the channels, its not that the gradients have dissipated.

Hope that helps.

 

[Ygggdrasil]

So it is necessary in active transport that a substance MUST move against a concentration gradient?

I suppose an active transporter would still work and expend energy even if it is transporting a substrate along its concentration gradient.

It's also important to note that not all active transporters use ATP. Some forms of active transport (secondary active transport) move one molecule along its concentration gradient and couple that favorable reaction to the transport of a different molecule against its concentration gradient. For example, the glucose symporter will import two sodium ions into the cell (along their concentration gradient) along with one glucose molecule (against its concentration gradient).

 

[extremcurious]

Maybe i am being extremely dumb, but i am still very confused. The problem is that my teacher is saying that during re polarization, Na and K ions don't flow against the concentration gradient. But the word "ACTIVELY TRANSPORTED" is specifically written wherever i have checked about re polarization. And when i told her that, she says it's just because ATP is used that the process is called active transport! Thus my question about specifications of active transport. I would be really grateful for any help.

 

[Ygggdrasil]

Albert's Molecular Biology of the Cell textbook defines active transport as "movement of a molecule across a membrane or other barrier driven by energy other than that stored in the electrochemical gradient of the transported molecule." So, transport against a concentration gradient is not a strict requirement for active transport.

 

[extremcurious]

WOW! that exactly hit the point! thanks a million for that link as well. U really helped a lot