Why can't a cell solely rely on simple diffusion to transport substances across its membrane?

Diffusion is a form of passive transport, which means that it does not require the use of energy. Diffusion is defined as the movement of particles from an area of higher concentration to an area of lower concentration. In other words, particles experiencing diffusion move “down their concentration gradient."  Cells move directly across the cell membrane during simple diffusion.

However, some particles are too large to pass through the cell membrane and cannot undergo simple diffusion. In this scenario, facilitated diffusion is utilized. During facilitated diffusion, particles still move from high concentration to low concentration. Thus, facilitated diffusion is also passive and does not require the use of energy. Unlike simple diffusion, the large particles pass through a protein channel that is embedded within the cell membrane during facilitated diffusion.

Other times, particles move against their concentration gradient. This means that the particles move from areas of low concentration to areas of high concentration. This process requires the use of energy in the form of ATP (adenosine triphosphate). Therefore, this process is known as active transport. Like facilitated diffusion, active transport utilizes a protein carrier to transport substances across the cell membrane.