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I am not sure about active transport mechanism

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I am not sure about active transport mechanism

Postby SU_reptile » Sun Nov 05, 2006 10:58 am

Hi there. Lately I had a test on osmosis, types of transport in plant cell, Fick's law etc.
I'am not sure I understood active transport approprietly. That's what I think:
it needs ATP hydrolysis for energy, that's clear for me. In my textbook there is written that active transport has two phases. First is pumping H+ outwards by H+-ATPase. Second phase is retrieval of H+ connected with co-transport of our substance into or out of the cell (symport or antiport with H+).
My question/doubt is whether that actively transported substance may be transported indyvidually (by means of ATP hydrolysis of course) or it always needs that first phase?

I hope I described my doubt quite clearly.
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Postby druid » Sun Nov 05, 2006 6:12 pm

There are two types of active transport: primary and secondary.

Primary transport directly uses energy to transport molecules across a membrane by hydrolysing ATP.
This is what you call individual trasport of a molecule.
For example, Na/K ATPase, Na/H ATPase and usual ATP Synthase from the respiration chain working in reverse direction are all examples of ATP-fueled carriers.

Secondary transport is kinda transport you've described to have two phases. It uses a preexisting elecrochemical gradient of one substance to push other substance against its gradient (symport or antiport). For example, HCO3/Cl antiport, Na/Glc symport (intestine epithelium). The preexisting gradient is generated by primary transporter ( 1st phase in your description).

MBoC4 of Alberts, MCB of Loddish, Biochemistry from Berg, Mathews or Lehninger make very clear description of primary and secondary active transport.
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Postby SU_reptile » Mon Nov 06, 2006 5:06 pm

Thanks a lot.
Now I know I understood it quite well.
It's true that for some ions, for example Na+, K+, H+, Ca2+, active transport comprises only the first phase (primary transport) but for others, for example Cl-, NO3-, disaccharide particles, primary and secondary transport is needed.

Thanks again
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