When you tutor biology, molecular movement and transport are topics you need to explain. Front and centre is diffusion.
Diffusion is the tendency of particles to move from an area of high concentration to lower concentration. It happens spontaneously, meaning it does not require an output of energy.
Moving from high concentration to lower concentration can be referred to as following the concentration gradient. Therefore, diffusion follows the concentration gradient. The gradient can be thought of as a “slope” that the molecules “roll down” to get to lower concentration.
In everyday life, diffusion is everywhere. Consider, for instance, a pleasant walk on a calm, dark night. You smell steaks barbecuing. You look around, but can’t seem them. Yet, the airborne aromatic molecules have reached you from the barbecue. That movement of the molecules from the cooking steaks to your nose is an example of diffusion. Note that it happens by itself; it’s spontaneous.
The human body relies on diffusion for some means of transport. For instance, at the cell membrane, oxygen passes in and carbon dioxide leaves by diffusion. It’s perfect: since the cell is constantly using oxygen, its concentration is always low inside. The concentration of oxygen in the surrounding blood is much higher. Therefore, oxygen constantly diffuses into the cell. Carbon dioxide, on the other hand, is constantly being produced in the cell, but is much lower in the blood. Therefore, it diffuses out of the cell into the blood, whence it is carried away.
The cell can depend on diffusion for gas exchange for two reasons:
1) The cell membrane is permeable to oxygen and carbon dioxide.
2) Diffusion happens fast enough, at the cellular level, to be effective.
Permeable means that it can be passed through. The cell membrane is permeable to oxygen and carbon dioxide, allowing them to diffuse in and out. The cell membrane is not permeable to many molecules and/or ions, however. For briefing on that issue, check my post here about the cell membrane.
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 about cell efficiency.
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:)
Source: Mader, Sylvia S. Inquiry into Life, 11th edition. New York: McGraw-Hill, 2006.
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.