Organisms need to obtain energy via cellular respiration primarily to grow and reproduce. Cellular respiration is the process by which cells convert glucose and other molecules into ATP (adenosine triphosphate), which serves as the main energy carrier in cells. This energy is essential for various biological processes, including growth, repair, and reproduction. While glycolysis is a part of cellular respiration, and breaking down ATP provides energy for cellular activities, the overarching reason for energy acquisition through cellular respiration aligns with the growth and reproduction of organisms.
Photosynthesis, on the other hand, is a process used primarily by plants and some other organisms to convert light energy into chemical energy, and it is not a direct reason for cellular respiration in all organisms.