In this video, we're going to do a review of the different types of small molecule transport that we already talked about in our previous lesson videos. And so because this video is a review, there's not really going to be any new information provided in this video that we didn't already cover. So if you're feeling pretty good about this already, then you can feel free to skip this video if you'd like. However, if you're looking for some additional support, then stick around for this review.
Notice that down below in this image, we have some of these different types of transport that we covered in our previous lesson videos. Notice that on the far left, we have simple diffusion, which is a type of passive transport, meaning that no energy is required and the molecules will be transported down their concentration gradient from an area of high concentration to an area of low concentration. Simple diffusion, as its name implies, is pretty simple because the molecules are able to simply diffuse right through the membrane without any facilitation whatsoever from an area of high concentration to an area of low concentration across the membrane. No protein facilitator is needed, and these molecules are able to simply diffuse right between the phospholipids. They tend to be tiny molecules that are non-polar. Those are the ones that have the best chance of being able to diffuse across the membrane without any facilitation.
Then over here, what we have is facilitated diffusion, which is another type of passive transport. Once again, passive transport means that there's no energy input needed, and the molecules are going to be transported from an area of high concentration to an area of low concentration across the membrane. Now notice that these molecules here are ions. They are charged. They have these little plus symbols inside of them representing that they're positively charged. Ions have a difficult time crossing the membrane without any facilitation. So, these ions cannot diffuse via simple diffusion. Instead, they require facilitation from a membrane protein, like a channel, for example. This channel will allow this ion to diffuse across the membrane, from an area of high concentration to an area of low concentration, but no energy input is needed.
Now, active transport, on the other hand, does require the input of energy to transport molecules against their concentration gradient from an area of low concentration to an area of high concentration across the membrane. Primary active transport recall is going to utilize ATP directly to power the movement of a molecule across the membrane, once again, against its concentration gradient from an area of low concentration to an area of high concentration. Because ATP is being utilized directly, that makes it primary active transport.
Secondary active transport does not directly use ATP. Instead, it indirectly uses ATP, and it directly utilizes the concentration gradient of another molecule to power active transport. As this pink molecule is being transported down its concentration gradient from high to low, it will release energy in an exergonic process to power this other molecule in green against its concentration gradient from low to high. This transport from low to high here is the active transport part because it's going against its concentration gradient and it's being powered by this pink molecule going from high to low.
Last but not least, what we have over here is group translocation, which is mainly specific to bacteria and this will chemically modify the entering molecule, as it enters into the cell. The example that we talked about previously is glucose being chemically modified to glucose-6-phosphate as it enters into E. coli bacteria. It does require a high-energy molecule as well to chemically modify the entering molecule. Sometimes, you'll see group translocation grouped as some alternative type of active transport.
But here, this concludes our review of these different types of small molecule transport, and we'll be able to get some more practice as we move forward. So I'll see you all in our next video.