Which biological process requires energy to move substances across cell membranes against a gradient?

• A. Active Transport
• B. Adaptation
• C. Artificial System of Classification
• D. Autotroph

Answer: (A)

Moving substances across a membrane against their gradient requires energy because you’re pushing them from a region of low concentration to a region of higher concentration, or against the electrochemical gradient. The process that does this is active transport. It uses energy, usually from ATP, to power membrane proteins called pumps that change shape and move ions or molecules across the membrane. A classic example is the Na+/K+ pump, which maintains essential ion gradients for nerve signaling and cell function.

Other options don’t describe a transport mechanism. Adaptation is a long-term change that helps organisms survive, not a method of moving substances through membranes. An artificial system of classification isn’t a biological process. Autotroph refers to an organism capable of producing its own organic compounds, not the mechanism of moving substances across membranes.