Which process involves moving substances against their concentration gradient?

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

Answer: (B)

The process that involves moving substances against their concentration gradient is active transport. This mechanism requires energy, usually in the form of ATP, to transport molecules from an area of lower concentration to an area of higher concentration. This is crucial for maintaining cellular homeostasis and allowing cells to accumulate necessary nutrients even when they are present in lower concentrations outside the cell.

In contrast, diffusion involves the movement of substances from an area of higher concentration to an area of lower concentration without the need for energy. Facilitated diffusion similarly does not require energy; it allows larger or polar molecules to pass through the cell membrane with the help of specific transport proteins, always moving down their concentration gradient. Osmosis refers specifically to the movement of water across a semi-permeable membrane, and like the other processes mentioned, it also moves substances from areas of higher concentration to areas of lower concentration, not against the gradient. Hence, active transport is clearly distinguished by its ability to move substances in the opposite direction of their natural tendency, requiring energy input.