Final answer:
Small molecules like oxygen are essential for energy production within the cell and move across the cell membrane through simple diffusion to aid in cellular respiration and maintain homeostasis.
Step-by-step explanation:
It might be useful for small molecules like oxygen to be able to move across the cell membrane because small molecules play a role in energy production within the cell. Oxygen is crucial for cellular respiration, a process that occurs within the mitochondria and generates ATP, which is the energy currency of the cell. Since the cell membrane is selectively permeable, it allows essential small molecules like oxygen to pass through via simple diffusion, directly across the phospholipid bilayer without the need for energy or transport proteins.
Furthermore, another reason why small molecules moving freely is vital is that it contributes to maintaining homeostasis, the stable internal environment of a cell. For instance, the diffusion of carbon dioxide out of the cell helps prevent an accumulation that could disrupt the cell's pH balance. Such movement across the membrane is also necessary for cells to respond to changing environmental conditions and effectively perform their functions in the organism's body.