Calculating P/O Ratio for Mitochondrial Respiration

[Cmunro]

I won't be giving the actual question itself - just one I've changed the numbers in that is similar, so that I can get an idea of the method and can do it myself.

Basically, Isolated mitochondria were initially incubated with stirring in a suitable buffer with a respiratory substrate and with phosphate. Under these initial conditions the rate of respiration (O2 consumption) was negligible. 18 µmol of ADP was then added which caused a burst of respiration which consumed 3 µmol of oxygen before the rate of respiration settled again to a negligible rate. Calculate the P/O ratio for mitochondria using this respiratory substrate.

Now this is a really simplistic way of looking at this problem but initially I'd say 18/3 =6. But I'm not really sure about this because I'm not sure whether it goes to half O2, and how the reduced coenzymes affect this result. Succinate, for example only produces 2 ATP per oxygen (because it accesses via FADH2), but how about when you are looking at the entire electron transport chain? I'm having trouble finding good examples of how to work this, so any help would be very much appreciated.

 

[Nino MMP]

The P/O ratio for the mitochondria using the given respiratory substrate would depend on the type of substrate used. Different substrates will produce different amounts of ATP per oxygen, so the P/O ratio will depend on what substrate is being used. Generally speaking, the higher the ATP yield per oxygen, the higher the P/O ratio. For example, if the substrate used is succinate, then the P/O ratio would be 2 (2 ATP produced per oxygen).

 

[Blueshift5]

Thank you for your question. Calculating the P/O ratio for mitochondrial respiration involves determining the ratio of ATP produced (P) to oxygen consumed (O). In this case, the initial conditions with negligible respiration indicate that no ATP was produced. However, with the addition of 18 µmol of ADP, a burst of respiration occurred, consuming 3 µmol of oxygen. This indicates that 3 molecules of ATP were produced from the 18 µmol of ADP. Therefore, the P/O ratio would be 3/3 or 1.

It is important to note that the P/O ratio may vary depending on the respiratory substrate used. As you mentioned, succinate produces 2 ATP per oxygen because it enters the electron transport chain at a later stage, through FADH2. However, when considering the entire electron transport chain, the overall P/O ratio is typically around 2.5-3, as some ATP is also produced through the reduction of NADH at earlier stages.

In order to accurately calculate the P/O ratio for a specific respiratory substrate, it is necessary to measure the amount of oxygen consumed and the amount of ATP produced. This can be done through various techniques such as spectrophotometry or using specific assays for ATP. From there, the P/O ratio can be calculated by dividing the ATP produced by the oxygen consumed.

I hope this helps clarify the process of calculating the P/O ratio for mitochondrial respiration. It is important to consider the specific conditions and substrates used in order to accurately determine this ratio. Further research and experimentation may be necessary for a more precise calculation.