Transporter proteins - binds to ion or molecule being transported may be up or down conc. Inside of cell is negative with respect to the outside of the cell. This can drive the transport of positively charged molecules. Ouabain and digoxin bind to extracellular face and inhibits transport. It is found in inner membrane of the mitochondria and the thylakoid membrane of chloroplasts and the plasma membrane of bacteria.
Water follows across cell membranes by osmosis .