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How can a molecule be transported across a membrane against a concentration gradient? Cellular membranes function to keep the internal environment of the cell distinct from the external environment. Concentrations of many molecules differ across cellular membranes. Another function of membranes is to transport materials in and out of the cell. However, because of the difference in environment across the membrane, it is often necessary to transport a molecule from a region of low concentration to a region of higher concentration. Movement up a concentration gradient cannot happen passively, it requires a mechanism to drive the process. The mechanism can be powered directly by the chemical breakdown of ATP to ADP and phosphate or it can be powered by other molecules moving down a concentration gradient.