MathBench > Cellular Processes

Diffusion: Fick's First Law

Directed vs. Undirected Movement


In this module, we’re going to talk about movement of materials over very short distances. First however, we need to describe two different types of movement. Living in the macroscopic world as we do, it is natural to think only of "directed" movement. When we move somewhere, we usually move with a purpose! We are used to standing in line or driving along a road. Or we might think of blood being pumped through a circulatory system, or air being sucked in and pushed out of our lungs. Even though the paths followed by blood and air are complicated, they are still examples of materials flowing in some organised, directed way.

However, this sort of organised movement (especially within a cell or body) is a relatively recent evolutionary development. True vertebrates, insects, and plants all have it and use it, but early life forms didn’t. Single-celled organisms like bacteria don’t have circulatory systems, and after completing this entire module, you'll see why. Single-celled organisms were the earliest forms of life and still represent the lion’s share of modern biodiversity. We're going to focus our module on the type of movement that these biological pioneers relied and still rely upon.

Early life forms didn’t actively suck in, circulate, or spit out material. How then did they accomplish getting necessary gases such as oxygen to all parts of their "bodies"? The answer is that small particles, like gas molecules and ions, move around and bounce off each other constantly, in an undirected way. You can think of the movement of each individual particle as being essentially random – like balls bouncing around in a pinball machine, or people milling around at a party.

In fact, one of the most prominent modern physicists, Richard Feynman, said that if all scientific knowledge was destroyed and humans could pass on only one sentence to the next generation, it should begin with:

All things are made of atoms – little particles that move around in random motion

From your chemistry studies you might recognise this as Brownian motion. When we think about movement, we now know that there are two main types of movement -- directed and undirected -- and our focus in this module is on undirected movement