Diffusion allows the cell to replenish chemicals needed for cellular metabolic processes. Chemicals are able to pass both in and out of the cell membrane without the cell expending any energy when diffusion occurs. Movement across the cell membrane without the expenditure of energy is known as passive transport.
In diffusion, molecules tend to spread from areas where they are more concentrated to areas where they are less concentrated until the concentrations become equivalent. The cell membrane serves as a barrier between the contents of the cell and external molecules. Molecules essential for cellular function diffuse through the membrane, while products of cellular processes pass out of the membrane to go to the rest of the body.
The cell membrane is semipermeable and allows some molecules through but not others. The concentration of the fluids around the cell is adjusted in situations where the molecules cannot pass through the membrane by a process known as osmosis.
Some molecules necessary for cellular function are too large to pass through the cell membrane without the assistance of proteins. These proteins bind to the molecules in a form of diffusion known as facilitated diffusion. Other molecules essential for cellular function require the cell to expend energy to transport them. This is form of transport is called active transport.