Question

If the main purpose of cellular respiration is to make ATP, why do glycolysis and the Krebs Cycle only make 4 total ATP by the time glucose is converted to carbon dioxide?

Answer 1

The answer is that 4 ATP is all those processes are able to produce directly.

Explanation:

The chemical processes of breaking down glucose into `CO_2` and `H_2O` is only able to directly generate 4 ATP - 2 in the glycolysis chain and 2 in the Krebs cycle. ATP is generated in this process by either first attaching a phosphate to the sugar and then transferring it to ADP to make ATP (in glycolysis), or by transferring just enough energy that a phosphate can be added to ADP to make ATP (in the Krebs cycle). These transfers are mediated by enzymes, without which they cannot occur.

Simple organisms use very little energy, and are perfectly content to function in an anaerobic environment, using in fact only the 2 ATP produced from glycolysis.

However, in the presence of oxygen, much more energy can be extracted by passing the other carrier molecules - `"NADP"` and `"FADH"_2` to the electron transport chain, where additional ATP is produced.

One thing to keep in mind is that each step in the process is a drop in the chemical energy. In the absence of oxygen, there is only so far the energy can fall. The presence of oxygen lowers the "energy floor", allowing a further total energy drop which can be harnessed to make ATP.