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Topics in Depth

The Theme of Glycolysis in Glycolysis and Cellular Respiration

The first step when going to an old-school arcade is breaking your dollar bills down into tokens; something similar can be said for cellular respiration, where the glucose (C6H12O6) molecule needs to be broken down into a substance called pyruvate (CH3COCOO). Instead of four tokens to the dollar, one glucose molecule is converted to two pyruvate molecules. Sounds like a bit of a ripoff, if you ask us. That's not all, though. Glycolysis also results in two ATP molecules, and two molecules of another compound, NADH. Note: You won't get a better deal if you move to Canada and try glycolysis up there.

Normally, a bill changer in an arcade wouldn't give you tickets (ATP and NADH) for just converting your dollars to tokens. That's one way glycolysis is a little different than a machine at an arcade. To better understand glycolysis, we must get up close and personal. Ready to dive into the inner workings of the machine?

Glycolysis takes place in the cytoplasm. If we rewind our brains a bit back to the Cells unit, we remember that the cytoplasm is the material inside the cell but not in any particular organelle—like a change machine in the lobby of an arcade. From the Overview section of this unit, we know that the conversion of one compound to another in the steps of cellular respiration happens due to reduction and oxidation, or redox reactions. Watch out for these guys.

One more important fact about glycolysis: it can happen with or without oxygen.

What?! That's right, ladies and gentlemen, no oxygen needed. But, don't let that fool you—later steps of cellular respiration (the citric acid cycle and oxidative phosphorylation) do require oxygen. So demanding, those two.

We start with a glucose molecule, which has six carbons in it.

  • Step 1. Two phosphates are added to the glucose molecule. In other words, the glucose is phosphorylated, which is a fancy way of saying it had phosphate added to it. The two phosphates come from two ATP molecules, which each had three phosphates—that's the tri in adenosine triphosphate—and now only have two. They are now called adenosine diphosphate, or ADP. Not very creative, these science-y naming people. Since two ATP were used, this reaction actually takes energy to occur. But hey, no pain no gain, right?
  • Step 2. The glucose-phosphate molecule splits into two sugar molecules, each with three carbons and one phosphate. Eventually, these molecules will be converted into pyruvate molecules. This splitting, or lysis, of glucose is where glycolysis gets its name.
  • Step 3. The three-carbon phosphate sugars each lose an electron, or are oxidized, which goes to the NAD+ molecules waiting in the wings. These two NAD+ become NADH molecules after they each gain an electron, which counter-intuitively, is called reduction. While NADH is not exactly ATP, it can be converted into ATP later. This part would be analogous to the arcade machine giving you euros instead of tickets, but you could convert the euros into tickets by playing another arcade game.
  • Step 4. An enzyme attaches phosphates to the two oxidized sugars. In step 1, phosphates came from ATP, but in this case, the phosphates come from a supply in the cytoplasm.
  • Step 5. The two phosphates that were just added to our sugars are abruptly taken away and added to two ADP…sad, because they were just getting to know each other. The two ADP of course, with the additional phosphates, become ATP. Sweet.
  • Step 6. The remaining phosphates on the three-carbon sugars are taken off to make two more ATP. The two remaining molecules are pyruvate. Pyruvate molecules are like our game tokens that we get to put into the pinball machine to win tickets.

In pictures, glycolysis looks something like this:

Now that we got through the inner workings of glycolysis, let's see what we ended up with. We have

  • 2 pyruvate
  • 2 NADH
  • 2 ATP

Wait, didn't we make four ATP? Yes, but remember, in step 1, two ATP were used for their phosphates, so those two even out two of the four that were made, and the net gain is two ATP.

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