Why does the cell sometimes have to expend energy to move individual molecules across the cell membrane?

Normally, molecules move naturally down a concentration gradient [From an area of high concentration to an area of low concentration] by a process called diffusion. However, sometimes the concentration of the needed substance is higher in the cell than out of it, so the cell uses a different process called Active Transport. Unlike Diffusion, Active transport requires energy, as it is going against a concentration gradient.

Cells expend energy to move cells across the cell membrane as they use Active transport rather than diffusion.

a. the production of carbon dioxide through the process of photosynthesis

answer: accepted answer: aquatic animals use the oxygen dissolved in water to carry out respiration. since the amount of dissolved oxygen in water is low as compared to the amount of oxygen in the air, therefore the rate of breathing in aquatic animals is much faster than it is in terrestrial animals.

explanation: