Secondary active transpo or co-transpo, uses energy to transpo molecules across a membrane against a concentration/electrochemical gradient.In contrast to primary active transpo, there is no direct coupling of ATP; instead, the electrochemical potential difference created by pumping ions out of the cell is used. Primary active transpo, also called direct active transpo, directly uses energy to transpo molecules across a membrane. Facilitated diffusion is a type of passive transpo that allows substances to cross membranes with the assistance of special transpo proteins. Endocytosis is a process by which cells absorb molecules (such as proteins) by engulfing them.