Right: you've eaten some food, and you've got glucose in your blood - what do you do with it? Well, in order to get the energy from it, you need to break it down, which involves a series of reactions. Ultimately the idea is to break it down to produce something called pyruvate, as this can then be converted into acetyl CoA which goes into the TCA cycle. As well as the energy produced through the process of glycolysis, the TCA cycle produces energy - it's all very complicated, but it ends up with the best formula for producing energy.
First of all, once glucose has got into a cell, you need to trap it there. Adding a phosphate group does that, nice and simply, and the enzyme hexokinase does that here. In other places the enzyme is glucokinase, but that's not significant at this stage.
Now we're happy it's staying in the cell, we need to change it slightly so we can work on it a bit more. That's what this enzyme does - just jiggles the atoms around a bit.
Before we split it up, we need to give the molecule an extra phosphate group - ATP happily hands this over for us.
And so we split it, producing two slightly different chemicals. The glyceraldehyde 3-phosphate is helpful, we'll keep that. But the dihydroxyacetone phosphate (DHAP) isn't so helpful. What do we do?
We make it something we do want! By turning DHAP into glyceraldehyde 3-Phosphate, we have double the amount of the stuff we want, which means everything that happens from now on, happens twice.
At this point we swap a hydrogen atom for a phosphate group - it might seem random, but we can produce ATP with phosphate groups, which is actually quite a useful thing to be doing!
And that's exactly what we do - the phosphate group comes off to turn ADP into ATP. This is actually happening twice because we doubled up earlier, so in this reaction we've just produced two ATP molecules - a nice little store of energy.
The next thing we need to do is move the phosphate group, because it's not quite in the right place for us at the moment - therefore it is swapped with the -OH group on the middle carbon atom.
And to move us that one step closer to Pyruvate, a water molecule comes off, leaving us with just one small step to pyruvate.
Removing the last phosphate group - and forming another ATP molecule - is the last step to producing pyruvate. It's a bit of a marathon to get here, but finally the pyruvate is produced, and we've produced 4 molecules of ATP on the way (because remember, everything was doubled up after triose phosphate isomerase turned DHAP into glyceraldehyde 3-phosphate).
So, are we sorted? Well, not quite. Firstly, to suggest we've produced 4 molecules of ATP isn't completely true - we had to use two at the start. So strictly speaking we've only produced 2 - so far. If we only produced 2 molecules of ATP from each molecule of glucose, though, we'd have to eat a huge amount in order to get enough energy to simply breath! No, we get a lot more than that - by entering the TCA cycle. However, to get there, we need to turn pyruvate into acetyl CoA. So, after glycolysis, the following reaction occurs:
While a coenzyme A molecule comes in, two NAD+ molecules also come in and grab a couple of hydrogen atoms. The result? Acetyl CoA - a very helpful molecule indeed, as it can go into the TCA cycle and produce yet more ATP! While we did this, one of the carbons was lost as carbon dioxide - and this is got rid of when we breath out.
Because this reaction is irreversible, it means that fat cannot be turned into glucose. That might seem like an insignificant thing to say, but it's important. This reaction can only occur in the direction shown - on other occasions, the reactions are reversible, but here, pyruvate can only be turned into acetyl CoA. This is one reason why eating too much fat is considered a bigger problem than eating too much glucose - fat cannot be turned into glucose.