Question

what is the function of active transport in moving small molecules and ions across cell membranes?

Answer 1

Active transport allows a cell to move molecules against the direction of the natural diffusion gradient. i.e. if something naturally moves from an area of higher concentration to an area of lower concentration, active transport allows this direction to be reversed. This concept is critical for many things. One example is in nerve cells, where active transport allows for the creation of a charge gradient (like a battery) so that nerve impulses can happen.