The first step when going to an old-school arcade is breaking down your dollar bills into tokens (yeah, that old shtick again). 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 into two pyruvate molecules. Sounds like a bit of a rip-off, if you ask us. That's not all, though. Glycolysis also results in two ATP molecules, and two molecules of another compound, NADH.

Glycolysis takes place in the cytoplasm and can happen aerobically or anaerobically—that is, with or without oxygen (aero– is from the Greek meaning "air," while an– means "without"). Look, ma, no oxygen! But don't get too cocky. The later steps of cellular respiration, the citric acid cycle and oxidative phosphorylation, do require oxygen.

The breaking down of glucose is one big redox reaction. Let's go step by step, starting with a glucose molecule, which has six carbons in it:

  • Step 1. Two phosphates are added to the glucose molecule. If we want to get fancy, we can say that the glucose is phosphorylated. The two phosphates come from two ATP molecules. Remember, ATP is shorthand for adenosine triphosphate, so when ATP donates a phosphate, it becomes ADP, or adenosine diphosphate. Since two ATP were used, this reaction actually takes energy to occur. No pain no gain, right?
  • Step 2. The phosphorylated glucose (or glucose diphosphate if you'd rather) molecule splits into two smaller sugar molecules, each with three carbon atoms and one phosphate. 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. The lost electron is transferred to the coenzyme NAD+ molecules patiently waiting in the wings. These two NAD+ are reduced to become NADH molecules after they each gain an electron. NADH is not exactly ATP, but it will get a makeover later to become smoking hot ATP energy.
  • Step 4. An enzyme attaches two additional phosphates to the two oxidized sugars. Recall that in Step 1 of this process the phosphates came from ATP; this time, 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—so sad, they were just getting to know each other. The two ADP join forces with the additional phosphates and become ATP. Sweet.
  • Step 6. The remaining phosphates on the three-carbon sugars are taken off to make two more ATP. What's left is pyruvate. Two pyruvates to be precise.

In pictures, glycolysis looks something like this:

The steps of glycolysis showing energy investment, energy payoff, and the net ATP generated.

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

  • 2 pyruvates
  • 2 NADH
  • 2 ATP

Wait, didn't we make four ATP? Good. Someone's paying attention. Remember, phosphates from two ATP were needed to get things moving in Step 1. We have to give those back, which means the net gain is two ATP.

The whole kit and caboodle of glycolysis looks like this:

1 Glucose + 2 NAD+ + 2 ATP → 2 Pyruvates + 2 ATP + 2 NADH + 2H+

You don't need to memorize this reaction. We just think it's awesome that the whole lengthy process can be written so neatly.

Brain Snack

You'll sometimes see the terms "pyruvate" and "pyruvic acid" used interchangeably. But don't be fooled. They're not the same thing. Can you spot the difference? Hint: one little proton can be a game changer.

Pyruvate is the end product of glycolysis, while pyruvic acid supplies energy in the citric acid cycle. Gotta watch out for those protons. They are tricky little critters.

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