direct vs. indirect transport

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rleung
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direct vs. indirect transport

Post by rleung » Wed Jun 21, 2006 4:10 am

Hi,

It says in my textbook that direct active transport and indirect active transport can be differentiated by their requirement for metabolic energy, but I don't quite see that. Don't they BOTH require metabolic energy (since they both are forms of ACTIVE transport)? For example, direct active transport requires the energy that is liberated when ATP is hydrolyzed to ADP, while indirect active transport requires that one solute be moving DOWN its gradient in order to move the other solute AGAINST its gradient?

The only way that I can see that metabolic energy is required for onebut not the other is if, by metabolic energy, they mean solely the free energy liberated by the hydrolysis of ATP and not free eenrgy generated from an ion going down its gradient....

Thanks.

Ryan

majik1213
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Post by majik1213 » Fri Jun 30, 2006 5:35 pm

To be honest, I don't know what indirect vs. direct transport really means. BUT, I may be able to help, as I know what direct active transport vs. secondary active transport is. Basically, your text is very fuzzy in what it says. It's true that direct active transport and indirect active transport can be differentiated by their requirement for metabolic energy, BUT that doesn't imply that direct uses ATP AND that indirect doesn't. In direct and indirect transport, ATP is used in BOTH cases, as you say (and you seem to say so in a frustrated tone, so rest assured, you are CORRECT!). The distinction lies not in what kind of energy is used, but in HOW the same type of energy is used. In direct active transport, the energy obtained by the oxidation of ATP directly is used to acquire or to expel a molecule. In secondary (and, I hope, indirect) active transport, (AND THIS IS WHERE I MAY BE INCORRECT, SO DOUBLE CHECK!!!!!!), the ATP is used to create an electrochemical gradient, and then the resulting, OTHER FORM OF ENERGY, the energy of the electrochemical gradient, is used to acquire or to expel a molecule.

In summary:
Direct->ATP used to acquire or to expel molecule directly
Secondary->ATP used to establish an electrochemical gradient that propels certain molecules through a pump; the resulting imbalance causes a homeostatic disturbance, and a restorative electrochemical gradient is established. The molecule uses not the electrochemical gradient established by the ATP but rather the restorative electrochemical gradient to enter or to exit the cell.

Hope this helps. I just don't know if indirect=secondary.

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