Active Transport

Active Transport — Moving Against the Gradient

Where diffusion (facilitated or not) only ever carries a substance downhill, active transport does the opposite — it pumps molecules from a region of lower concentration to a region of higher concentration, i.e., uphill against the gradient. Because this works against the natural direction of diffusion, it cannot happen for free; it requires metabolic energy, usually supplied as ATP.

Active transport depends on specific pump proteins built into the membrane, and interestingly the same class of membrane proteins can sometimes take part in both active transport and passive (facilitated) transport, depending on conditions. Like facilitated diffusion, active transport:

• Reaches a maximum rate once every pump protein is fully occupied (it saturates).
• Is highly specific about which molecule it carries — much as an enzyme is specific to its substrate.
• Is sensitive to inhibitors that interfere with the pump protein. …