A membrane transport question

Wed Oct 5 12:15:51 EST 1994

I would like to know whether coupled transport (e.g. symports and antiports) 
are strictly used in primary and secondary ACTIVE TRANSPORT or whether there 
are known examples of coupled transport where both substances travel down 
their concentration gradient (i.e. passive transport).

Is Band 3 (the chloride ion/bicarbonate ion exchange protein in red blood 
cells) an example of an antiporter involved in PASSIVE TRANSPORT or is a 
gradient of chloride ion (or ATP hydrolysis) utilized to get bicarbonate ion 
across the membrane?

I am finding some inconsistencies in various textbooks.  The general biology 
text by Purves suggests that coupled transport can be passive, but I think it 
may be wrong about that.  Your input would be much appreciated.

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