Hello,

 

Ok I researched it but I don't understand. What I understand is that when one molecule normally diffuses(high to low) another molecule from a (low to high gradient) attaches to the carrier protein. How does this molecule attach without ATP? I don't get it. Any help would be appreciated. I'm talking about membrane transport of course!!

Just to reiterate: this form of transport utilizes gradients across the membrane. For instance, if the outside of the cell is more positive than the inside, the resulting electrochemical gradient allows a passive influx (ie without ATP consumption) of positive ions into the cell (within certain limitations). I think this part is clear, yes?

The case you illustrate is that of an antiporter, in which one of the substrates follows the gradient thus facilitating the move of another against it. The binding to carriers does not usually require energy. The step requiring it is the confirmation change of the transporter that moves the substrates. Again, in the case of the antiporter the flow with the gradient powers that change.

nicely put CharonY.