So now that we've covered primary active transport, in this video, we're going to focus on secondary active transport. Recall from our previous lesson videos that secondary active transport is not directly driven by ATP hydrolysis—that is primary active transport. Instead, secondary active transport is directly driven by another molecule's concentration gradient, and it's powered by this gradient instead of being powered by ATP hydrolysis like primary active transport. However, that being said, secondary active transport, although it may not be directly driven by ATP hydrolysis, is indirectly driven by primary active transport and ATP hydrolysis. The concentration gradient that directly drives secondary active transport is actually built using primary active transport or PAT, which we've abbreviated here for our lesson.

In order to better understand secondary active transport, we're going to take a look at a classic example: the sodium-glucose secondary active transporter. Below we have an image of this sodium-glucose transporter, and notice that the image has these numbers 1, 2, 3, and 4. The numbers that you see in the image correspond with the ones we've mentioned above in the text. This shows the four steps of sodium-glucose secondary active transport. In the very first step, you'll notice that sodium ions can be transported against their concentration gradient using primary active transport. Notice that the sodium ion, shown in step number 1, is being pumped across the membrane towards the area of higher sodium concentration, while there's a lower sodium concentration inside the cell. Since sodium is being pumped from low to high concentration, it requires energy. ATP is directly linked to this process of pumping sodium across the membrane; thus, it is a form of primary active transport.

In step number 2, this generates a higher concentration of sodium ions on the outside of the cell. You can see there are way more sodium ions outside, indicating a higher concentration than inside the cell. This concentration gradient of sodium is generated by primary active transport. Step number 3 involves another molecule—glucose, shown in green. Notice the glucose molecules have a higher concentration inside the cell, which is opposite to that of the sodium. Here, secondary active transport comes into play because sodium is going to be transported down its concentration gradient from an area of high concentration to an area of low concentration without requiring energy. In fact, it releases energy, which can power the transportation of glucose against its concentration gradient from an area of low concentration of glucose to an area of high concentration of glucose inside the cell.

To better understand this, let's review the example. Sodium is transported down its concentration gradient, which does not require energy for molecules to move down their gradients. Instead, it releases energy, and that released energy is used to power the movement of glucose against its concentration gradient. This is a type of active transport since the molecule is being transported against its concentration. Although it's not driven directly by ATP, it's driven by the concentration gradient of sodium being transported down. This makes it a classic example of secondary active transport. Secondary active transport is driven by the concentration gradient of another molecule instead of ATP hydrolysis. Notice that there's no ATP in this vicinity, just the concentration gradient of sodium that powers the secondary active transport of glucose being transported against its concentration gradient.

This concludes our introduction to secondary active transport, and we'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.