A Level Biology Practice Exam 2025 - Free A Level Biology Practice Questions and Study Guide

Active transport is a process that requires energy to move molecules against their concentration gradient, which is often from an area of lower concentration to an area of higher concentration. This process is essential for maintaining cellular homeostasis and allowing cells to accumulate necessary substances or remove waste products. The primary source of energy for active transport in cells is ATP (adenosine triphosphate).

When ATP is hydrolyzed, it releases energy that is harnessed by transporter proteins embedded in the cell membrane. These proteins change conformation and transport specific molecules across the membrane, effectively concentrating them inside or outside the cell. This necessity for energy distinguishes active transport from passive transport mechanisms, which rely solely on concentration gradients and do not require an input of energy.

In contrast, random molecular movement is a characteristic of diffusion, concentration gradients describe the natural movement in passive transport, and chlorophyll is a pigment involved in photosynthesis but irrelevant to the process of active transport. Thus, the requirement of energy in the form of ATP is critical for the mechanisms and functions of active transport.