Thursday, January 8, 2009

Counting ATPs

Students usually ask me how to count the number of ATPs formed during cell respiration. So I have prepared the following table that summarizes how and where ATPs are formed.

Before we go to that however, I just wish to point out that there are two ways by which ATP is formed. One is through substrate level phosphorylation and the other is through oxidative phosphorylation. During substrate level phosphorylation, 1 molecule of ATP is formed during each process. During oxidative phosphorylation however, 2 or 3 molecules of ATP are formed per process depending on the first hydrogen acceptor. If FAD is the first hydrogen acceptor, then 2 ATPs are formed while 3 ATPs are formed if NAD is the first hydrogen acceptor.

Another thing, we have to multiply by 2 all the ATPs formed because there are 2 molecules of pyruvic acid formed after glycolysis.

Reaction Type of Phosphorylation ATPs formed
Glucose to Pyruvic acid Substrate level 2 (net)*
Pyruvic acid to Acetyl CoA Oxidative 3 x 2
Krebs cycle and Ox-Phos Oxidative (NAD) 9 x 2
Krebs cycle and Ox-Phos Oxidative (FAD) 2 x 2
Krebs cycle Substrate level 1 x 2
G3P to 1,3DPGA in glycolysis* Oxidative via GP shuttle 2 x 2

* 4ATPs are actually formed. However, 2ATPs are used to prime glucose for the process.

**one step during glycolysis converts glyceraldehyde3phosphate(G3P) to 1,3diphosphoglycerate(1,3DPGA). This releases hydrogen ions and electrons that are accepted by NAD and brought to the mitochondrion through a glycero-phosphate shuttle (GP). So only 2 ATPs (instead of 3) are formed here (even if NAD is the acceptor) because 1 ATP is used to "pay" the GP shuttle.

1 comment:

Salvacion P. Angtuaco said...

Oops! Sorry, that was supposed to be a table but it did not come out like one. I guess I should have placed a comma rather than a space.

Anyway,I think you can still follow the flow. The first term is the Reaction, the 2nd is the Type of Phosphorylation and the 3rd the number of ATPs formed.

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