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:
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:
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.
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.