Final answer:
A substance that moves across a cell membrane without using the cell's energy tends to move away from the area where it is more concentrated, following the concentration gradient through diffusion, a type of passive transport.
Step-by-step explanation:
A substance that moves across a cell membrane without using the cell's energy tends to move away from the area where it is more concentrated, which is known as moving down its concentration gradient. This process is called diffusion and is a type of passive transport, which does not require energy because substances move from a high concentration area to an area of lower concentration. Examples of substances that diffuse through the lipid bilayer of the cell membrane include oxygen (O2) and carbon dioxide (CO2), which move in and out of cells respectively, due to their concentration gradients.