12.5. The Respiratory Balance Sheet

It is possible to make calculations of the net gain of ATP for every glucose molecule oxidised; but in reality this can remain only a theoretical exercise. These calculations can be made only on certain assumptions that:

• There is a sequential, orderly pathway functioning, with one substrate forming the next and with glycolysis, TCA cycle and ETS pathway following one after another.
• The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
• None of the intermediates in the pathway are utilised to synthesise any other compound.
• Only glucose is being respired – no other alternative substrates are entering in the pathway at any of the intermediary stages.

When scientists calculate the energy yield of respiration, they assume that the pathway of glucose metabolism (glycolysis, TCA cycle, and ETS) happens in a specific, linear order. They also assume that all NADH produced during glycolysis gets used efficiently in the mitochondria for energy production, and that no other compounds interfere in this process. This makes the theoretical maximum yield of ATP appear higher than what is usually observed in real life, where pathways are more complex and dynamic.

Think of this like following a recipe strictly step-by-step without making any adjustments. If an ingredient gets used for another meal while you cook, you can't achieve the same result as per the recipe. Just like in cooking, biological processes are flexible and adapt based on available resources.

But this kind of assumptions are not really valid in a living system; all pathways work simultaneously and do not take place one after another; substrates enter the pathways and are withdrawn from it as and when necessary; ATP is utilised as and when needed; enzymatic rates are controlled by multiple means. Yet, it is useful to do this exercise to appreciate the beauty and efficiency of the living system in extraction and storing energy.

In actual biological systems, processes are not as straightforward as in theoretical calculations. For instance, different substrates can enter the respiration pathways, and cells use ATP as needed rather than holding onto it for later use. Additionally, enzymes that help in these biochemical reactions don’t always work at the same rate. Despite these complexities, calculating potential ATP yields still helps us understand how efficiently organisms can convert energy stored in food into usable energy.

Think of a multifunctional kitchen where multiple meals are prepared at once. A chef may not stick totally to one recipe if they can adapt based on what ingredients are available or what sounds good at the moment. Similarly, a cell’s processes adjust based on what it needs at that moment.

Hence, there can be a net gain of 38 ATP molecules during aerobic respiration of one molecule of glucose.

The maximum theoretical yield is approximately 38 ATP molecules for each glucose molecule during aerobic respiration. This occurs through glycolysis, the TCA cycle, and the electron transport chain where ATP is produced through oxidative phosphorylation. It’s important to note that not all conditions yield this maximum due to various cellular influences.

Imagine a power plant that produces energy from a single source, like coal. Under ideal conditions, it can produce a certain amount of electricity (like 38 ATP). However, if the plant experiences maintenance issues or efficiency losses, it might end up generating less energy than expected.

Now let us compare fermentation and aerobic respiration:

• Fermentation accounts for only a partial breakdown of glucose whereas in aerobic respiration it is completely degraded to CO and H O.
• In fermentation there is a net gain of only two molecules of ATP for each molecule of glucose degraded to pyruvic acid whereas many more molecules of ATP are generated under aerobic conditions.
• NADH is oxidised to NAD+ rather slowly in fermentation, however the reaction is very vigorous in case of aerobic respiration.

Fermentation is a less effective way to obtain energy from glucose compared to aerobic respiration. While aerobic respiration breaks glucose down completely, allowing for significant ATP production (up to 38 molecules), fermentation only allows for a tiny fraction of that ATP (only 2 molecules). The difference in how quickly NADH is processed also highlights the efficiency of aerobic respiration, as it regenerates NAD+ much more effectively compared to fermentation.

Consider a bicycle (aerobic respiration) that can take you anywhere with speed and power, compared to walking (fermentation). Walking will get you there, but it’s much slower and you won’t cover as much distance over the same period of time.

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