system
Given the following post, respond as if you're commenting on Reddit:
human
How we know that how kreb cycle actually happens in the cell?

How do scientists study microscopic biochemical mechanism at cellular level like Na/K Pump, glycolysis, kreb cycle etc. In cycles products keep changing/converting so how do they keep a track of all of them? Considering they are so so minute and so so less in amount in comparison to average lab samples and experimentation.
gpt
One powerful technique is isotopic labeling - a compound that is known to be part of a process is labeled with either a stable isotope or a radioactive one, then the label is tracked through the various transformations. For instance 13C or 14C labeled pyruvate could be fed into the citric acid cycle, producing labeled citrate and subsequent constituents over time. Varying the position of the label can show which parts of molecules are being transferred through the cycle. 

In the mid-20th century radioactive labeling was particularly useful because the compounds could be detected through simple techniques like thin layer chromatography, whereas we can observe stable isotope labels in a much more fine grained fashion now using LC-MS/MS.