So far we have looked at diffusion and osmosis. What they both have in common is that no energy is required for these to take place. In both cases there is a "net" migration from areas of high concentration to areas of low concentration. 


There are times however when it is necessary for substances to "migrate" from areas where they are in low concentration to areas where they are already in higher concentration. This of course goes against the principle and to make this take place, energy has to be used.


For example, active transport takes place in our digestive system to transfer nutrients from the gut, into the bloodstream. Generally there will be a higher concentration of nutrients already in the bloodstream, and as such this migration from "low to high" requires energy to take place as this migration will be "against the concentration gradient".



A simple analogy is shown in the diagram above. Imagine that you are a cyclist at the top of the hill at a high elevation (concentration) and you wish to travel to a lower elevation (concentration), in other words you want to roll downhill! Does this require any energy?, Well of course there will be energy changes if you look at the physics of it, the transference of potential energy into kinetic energy as you pick up speed, friction and air resistance but we are looking purely at energy that you may have to provide to make the process happen. In reality, you would just release your grip on the brakes and you would start to roll downhill.


The reverse of course comes when you want to go back up the hill, from an area of low elevation (concentration) to an area of high elevation (concentration) and of course I think it's obvious that you will see that this is against the gradient and is going to require energy to be input by yourself, whether you cycle back up the hill or push the bike!


Rolling downhill is therefore an analogy for diffusion/osmosis, cycling or pushing the bike back up the hill (requiring energy) is therefore an analogy for active transport.