I'm struggling with something that seems really simple, and I think it's because I'm stuck on the definition of concentration gradients.
If you have a solution with a moderate concentration of molecule A crossing a membrane where the other side has no A but a high concentration of molecule B, is A moving up the concentration gradient because there's a higher ratio of solute:solvent, or down because there isn't any molecule A present in that solution?
Thank you!
The concentration gradient is typically used to describe the driving force for mass transport by diffusion. It is going to be the different concentrations of species A that drives species A to move, B can have an influence but typically it is only minor.
So typically when mentioning concentration gradients it is for a specific species/molecule and not for the total concentration of species/molecules. So in your case the species will be going from a high concentration to low concentration of A, i.e. down the gradient.
The solute does not cross the membrane in osmosis. Only the solvant (water for exemple) crosses the membrane. The solute (sugar for example) is dissolved in different concentrations in both solutions. But usually it is dissolved in only one of the two solutions. The other liquid phase is pure solvant.
The membrane has plenty of tiny holes, small enough to allow tiny molecules like water molecules to pass. But the holes are not large enough to allow big molecules of solute (like sugar) to do the same. So these big molecules are partly blocking some holes. Only water can push such big sugar molecules blocking the holes and from the other side cross the membrane. As a consequence, on the long range, water is slowly flowing from the pure solvant (or diluted solution) to the second, more concentrated solution, on the other side of the membrane. And this concentrated solution gets more and more diluted.
The difference of concentration between both sides of the membrane is called the concentration gradient. It goes slightly decreasing with the time.
