Diffusion is the tendency of particles to move from an area of high concentration to lower concentration. \u00a0It happens spontaneously, meaning it does not require an output of energy.<\/p>\n
Moving from high concentration to lower concentration can be referred to as following the concentration gradient<\/strong>. \u00a0Therefore, diffusion follows the concentration gradient.\u00a0The gradient can be thought of as a “slope” that the molecules “roll down” to get to lower concentration.<\/p>\n In everyday life, diffusion is everywhere. \u00a0Consider, for instance, a pleasant walk on a calm, dark night. \u00a0You smell steaks barbecuing. \u00a0You look around, but can’t seem them. Yet, the airborne aromatic molecules have reached you from the barbecue. \u00a0That movement of the molecules from the cooking steaks to your nose is an example of diffusion. \u00a0Note that it happens by itself; it’s spontaneous.<\/p>\n The human body relies on diffusion for some means of transport. \u00a0For instance, at the cell membrane, oxygen passes in and carbon dioxide leaves by diffusion. \u00a0It’s perfect: since the cell is constantly using oxygen, its concentration is always low inside. \u00a0The concentration of oxygen in the surrounding blood is much higher. \u00a0Therefore, oxygen constantly diffuses into the cell. \u00a0Carbon dioxide, on the other hand, is constantly being produced in the cell, but is much lower in the blood. \u00a0Therefore, it diffuses out of the cell into the blood, whence it is carried away.<\/p>\n The cell can depend on diffusion for gas exchange for two reasons:<\/p>\n 1) \u00a0The cell membrane is permeable to oxygen and carbon dioxide.<\/p>\n 2) \u00a0Diffusion happens fast enough, at the cellular level, to be effective.<\/p>\n Permeable<\/strong> means that it can be passed through. \u00a0The cell membrane is permeable to oxygen and carbon dioxide, allowing them to diffuse in and out. \u00a0The cell membrane is not permeable to many molecules and\/or ions, however. \u00a0For briefing on that issue, check my post here<\/a> about the cell membrane.<\/p>\n The reason that diffusion happens fast enough, at the cellular level, for effective gas exchange is that the cell is very small. Therefore, it has high efficiency. See my post here<\/a> about cell efficiency.<\/p>\n Diffusion is only one method of transport in the body. It is spontaneous, but depends on permeability and efficiency. It is sufficient, for example, for gas exchange between the cells and the blood. However, there are many other contexts in which diffusion is not sufficient. Therefore, I’ll be discussing other transportation methods in future posts:)<\/p>\n Source: Mader, Sylvia S. Inquiry into Life<\/em>, 11th edition. New York: McGraw-Hill, 2006.<\/p>\n