According to Tortora and Derrickson (2009a), movement of substances across the cell membrane is essential to cell functioning. It is vital that glucose and oxygen are able to enter the cell to allow metabolism to occur, while carbon dioxide and substances produced by the cell must be removed. In the human body, the solvent is water and there are a variety of potential solutes such as oxygen, nutrients or ions. An important factor in the movement of solutes is the concentration
gradient.