My teacher has not explaned, why primary active transport is not the only way to transport molecules to the side of the membrane with help of ATP.
I do not understand why secondary active transport is neccessary when the same amount of ATP is used and it is more complicated.
There are many different ways to transport molecules over the membrane. It is just to accept. A biological cell has been made by evolution and is not constructed according to a plan.
Energy can be present in the cell as ATP but energy can also be present in other forms like a concentration gradient of ions over the membrane. This energy can be transformed into action. For example Na+ accumulated on one side of a membrane can flow through a transport protein sitting in the membrane which can make this protein able to transport another kind of molecule against its concentration gradient. The formation of the gradient of Na+ can be made possible by consumption of ATP but also through exchange of H+ which are pumped over membranes in chloroplasts and mitochondria. This H+ gradient is also used to make ATP. Thus, gradients and ATP are very much connected.
-Mats Hansson
