Lavoisier (1743-1794) had exclaimed around 1780 that "la respiration est donc un combustion", ie, respiration is then a combustion. Which is to say that a fire is burning within all living things, a highly controlled fire. Life was beginning to look like chemical reactions in tight control.
Louis Pasteur (1822-1895), the French chemist and microbiologist, came up with a good clue when he was studying fermentation of sugar to alcohol by yeast:
"alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells".
He called his vital principle for "ferments".
The German physiologist Wilhelm Kühne (1837-1900) coined the word enzyme in 1878 and it is derived from the Greek word for "in leaven". The German Eduard Buchner at the University of Berlin in 1898 continued the yeast experiments and demonstrated that a yeast extract, that did not contain any living yeast cells, did ferment sugar for which he received the Nobel Prize in Chemistry in 1907. An intense activity in biochemistry ensued in the early years of the 20th century. Enzymes, made from protein, catalyses chemical reactions in cells.
Sir Hans Adolf Krebs (1900-1981) was such a worker, a German born British physician and biochemist, that discovered the urea cycle and the citric acid cycle, or the Kreb's Cycle. He got the Nobel Prize for this in 1953. These cycles are a series of enzymatic reactions that are often feed-back controlled of products and substrates.
The citric acid cycle produces energy from glucose, a six carbon sugar, when coupled to the so called glycolysis, or Embden-Meyerhof pathway, first discovered from Gustav Embden (1874-1933) and Otto Meyerhof (1884-1951).
Thus energy in the form of so called ATP, adenosine-tri-phosphate, is produced from sugar via its "combustion", yes oxygen is consumed, via the glycolysis, citric acid cycle and the so called oxidative phosphorylation. This is the internal fire. ATP is then used by all kinds of reactions in the cell for life support.
In the early 1940s the link between the fermentation of sugars and the formation of ATP was finally conclusively proven by the Danish physician Herman Kalckar (1908-1991). The ATP generating mechanism was solved by the so called Chemiosmotic Theory from 1961, the ATP synthase is driven by a proton gradient as suggested by Peter D. Mitchell (1920-1992). Highly controversial at first, he got the Nobel prize for it in 1978, almost 200 years after Lavoisier made his exclamation.
Data from Wikipedia