In this video, we're going to introduce the phases of glycolysis. It turns out that glycolysis consists of a series of a total of 10 chemical reactions. These 10 chemical reactions that make up glycolysis can actually be grouped into 2 phases. Now in this video, we're not really going to focus on the 10 reactions so much, but we are going to focus on how these 10 reactions are grouped into 2 phases, and we're really going to be focusing on the 2 phases of glycolysis. The first phase of glycolysis is called the energy investment phase, and the second phase of glycolysis is called the energy harvest phase.

Notice that the energy investment phase up above is color coordinated to the energy investment phase down below, and the energy harvest phase is color coordinated to the energy harvest phase down below. In this image down below, what we're showing you is glycolysis. Notice that it starts with a single glucose molecule that has 6 carbon atoms and breaks down that glucose into 2 pyruvate molecules each with 3 carbon atoms. This whole green box represents glycolysis, but once again glycolysis occurs in 2 phases: the energy investment phase and the energy harvest phase. The energy investment phase, as its name implies, requires an investment of energy or an input of energy by using 2 ATP molecules.

This is a bit ironic because if you recall from our previous lesson videos, the whole point of aerobic cellular respiration is to make a lot of ATP. But the very first stage of aerobic cellular respiration, glycolysis, the very first thing that happens is that it does not make ATP, but it uses ATP. However, it turns out that this is just an investment of ATP because by inputting 2 ATP molecules, the cell is able to transition into the second phase of glycolysis, which is the energy harvesting phase.

As its name implies, the energy harvesting phase is going to harvest energy or produce energy, specifically by forming 2 NADH molecules and 4 ATP molecules. If we take a look at our image down below at glycolysis, notice that the first phase, the energy investment phase, uses 2 ATPs and that is using some of the cell's energy. However, this is just an energy investment because after the cell utilizes those 2 ATPs it can transition into the next phase here, which is the energy harvest phase. The energy harvest phase produces 2 electron carriers specifically 2 NADH molecules, and it also produces a total of 4 ATP molecules. Now, if we're looking at the net products from 1 single glucose molecule, what we'll see is not only are 2 pyruvates formed and 2 NADH are formed, but there's also a net of 2 ATP molecules that are formed.

But wait a second, Jason, I thought you said that there were 4 ATP molecules that were made, not 2 ATP molecules. What we need to remember is that 2 ATP molecules were being used up in the energy investment phase. So, because 2 ATP molecules were used up, of the 4 ATP molecules that are being made, 2 of them are just making up for the 2 that were invested. And so really, there's just a net of 2 ATP molecules made.

Notice if we take a look at this part of our image down below right here, we're reminding you that the energy harvesting phase does produce a total of 4 ATPs as we indicated right here. However, we can't forget that the energy investment phase burns or uses 2 ATP. So we have to subtract off those 2 ATPs from the total. And so that gives us a total net of 2 ATP molecules produced during glycolysis, 2 net ATPs made. An easy way to be able to remember that there are 2 net ATP made during glycolysis is to remember that the net products from 1 glucose molecule are just double.

You get not only 2 pyruvates but also 2 NADH and also 2 net ATP molecules. Now these 2 pyruvates that are formed at the end of glycolysis end up getting transported to the mitochondrial matrix, basically getting transported to the mitochondria so that the next step of cellular respiration can take place. Notice that here glycolysis occurs once again on the outside of the mitochondria and ends up producing 2 NADH, a net of 2 ATPs, and 2 pyruvates. These 2 pyruvates are going to make their way to the mitochondria and into the mitochondrial matrix. This here concludes our introduction to the phases of glycolysis, and we'll be able to get some practice applying these concepts as we move forward in our course.

So I'll see you all in our next video.