Question

what is active transport? the movement of substances across a cell membrane against the concentration gradient, which requires energy the movement of substances across a cell membrane with the concentration gradient the movement of cells within their environment the movement of any substance across a cell membrane

Explanation:

Active transport is defined as the movement of substances across a cell membrane against the concentration gradient, which requires energy. This is in contrast to passive transport (like diffusion or facilitated diffusion) which moves substances with the concentration gradient and does not require energy input. The option "The movement of substances across a cell membrane with the concentration gradient" describes passive transport. "The movement of cells within their environment" is not related to the definition of active transport. "The movement of any substance across a cell membrane" is too broad and does not specifically highlight the key features (against concentration gradient and energy - requiring) of active transport.

Answer:

The movement of substances across a cell membrane against the concentration gradient, which requires energy.