Which process moves substances from lower to higher concentration and requires energy?

• A. Diffusion
• B. Active transport
• C. Facilitated diffusion
• D. Osmosis

Answer: (B)

Active transport is the process that moves substances from a region of lower concentration to a higher concentration and requires energy. This energy, usually from ATP, powers transporter proteins called pumps to change shape and shove molecules against their gradient. Some active transport is directly fueled by ATP, as with the Na+/K+ pump. Other forms are secondary active transport, where the energy stored in an existing gradient (often Na+) drives the uphill movement of another substance, such as glucose being brought into a cell with sodium. In contrast, diffusion and facilitated diffusion move substances down their concentration gradients and do not require energy, and osmosis is the passive movement of water across a membrane.