Question

In an experiment, a suspension of mitochondria was added to a phosphate-buffered solution containing B-hyrdroxybutyrate, which can be oxidized by mitochondria to generate NADH+H+. After an initial rapid burst, O2 consumption slowed to a background rate. When the rate of O2 consumption stabilized, ADP was added, causing a rapid increase in O2 consumption, until all ADP was converted to ATP, at which point O2 consumption slowed to background rate.

Why is there an initial rapid burst of O2 consumption? Why in the absence of ADP does the mitochondria consume O2 at a background rate?

Answer 1

At physiological condition mitochondrial electron transport chain (ETC) and ATP synthesis is interlinked. Reducing equivalent of NADH⁺ + H⁺ have been transfered to the ETC which will transfer the electron through the Complex I and complex II to complex III and then complex IV through different electron carriers and produce H⁺ gradient through pumping out against electrochemical gradient. Now, Complex IV will transfer the electron to the terminal electron acceptor O₂ molecule to produce H₂O. Whatever proton gradient had been made at the time of ETC, these protons will now flow down the electrochemical gradient through the channels and the released energy will combine ADP & Pi to produce ATP through the help of enzyme ATP synthase.

Now, in the current synthetic situation, mitochondria have been placed to buffer media at physiological pH. Initially there will be a rapid brust of O₂ consumption because the system will have free NADH⁺ + H⁺ and ADP and Pi, which will help to produce ATP.

Now if all the ADP has been used up, then O₂ consumption will come to background level because there are no free ADP to produce ATP. So, the whole system will stabilized itself with the feedback loop.