The presence of a membrane potential influences the distribution of ions. If DY is negative, going from outside to inside, then the transport of cations into the cell is favored over anions. The opposite would be true if DY were positive. This is the general case of active transport in which the cell expends energy to drive uptake of a needed compound or secretion of a waste product. Cells take up materials by either Passive transport or diffusion, which depends on the concentration gradient across the membrane.
Active transport allows cells to transport materials against a concentration gradient.
Ion pumps couple ATP hydrolysis to ion transport. The energetics are described here . This animation shows the operation of the pump. From The Virtual Cell Web Page Cotransport utilizes a favorable gradient for one compound to drive the uptake of a second compound. Sodium-glucose cotransport is one example, but many others are known, especially in the mitochondrial membrane.