Previously, we’ve seen that the first step of glycolysis is phosphorylation…
…in which glucose is converted into glucose phosphate…
..using two phosphates gained from the breakdown of two ATP molecules.
Next, we’re going to look at the final two steps of glycolysis.
To kick off, in the second step of glycolysis, glucose phosphate breaks down into two identical molecules, each containing three carbons and one phosphate group.
These are molecules of triose phosphate
Where ‘triose’ refers to the three carbons…
And phosphate refers to the phosphate group.
Finally, in the third step of glycolysis, both triose phosphate groups are converted into pyruvate.
However, this third step doesn’t just produce pyruvate…
The conversion of each triose phosphate molecule into pyruvate also produces two ATP molecules
But, if we take another look at our diagram…this doesn’t make much sense.
Each triose phosphate molecule only contains one phosphate group
And so, based on this, we should only be able to make one ATP molecule per triose phosphate molecule.
So, where’s the second ATP molecule coming from?
Well, as we’ve said, glycolysis is a far more complicated process than is usually shown at A level
It contains extra steps that you don’t need to know.
One of these extra steps is the addition of a phosphate group to triose phosphate…
Leaving us with a molecule that contains two phosphate groups.
As a result, when each molecule is converted into pyruvate, there are two phosphate groups available to react with ADP…
Meaning that two ATP molecules form!
Next, since you don’t need to know this step in the exam…
We’ll just represent it like this, as a reminder for why two ATP molecules are formed.
Finally, looking at the entire process of glycolysis…
We can see that for a single glucose molecule, 2 ATP molecules break down in the first step…
And 4 ATP molecules in total form in the third step.
Therefore, during glycolysis, for a single molecule of glucose, there’s an overall yield of…
For a single molecule of glucose, there’s an overall yield of 2 ATP molecules.
To sum up, in the second step of glycolysis, glucose phosphate breaks down into two molecules of…
In the second step of glycolysis, glucose phosphate breaks down into 2 molecules of triose phosphate…
And in the third step, each triose phosphate molecule is converted to pyruvate.
When this happens, the conversion from triose phosphate produces…
Before triose phosphate is converted into pyruvate, another phosphate group is added
Therefore, when a single triose phosphate molecule is converted to pyruvate, 2 ATP molecules are produced…
Meaning that, in total, 4 ATP molecules are produced in this third step of glycolysis.
Finally, for a single glucose molecule undergoing glycolysis, there is an overall yield of…
For a single glucose molecule undergoing glycolysis, there is an overall yield of 2 ATP molecules.
