Bacteria, plants, fungi, and animals look, from the outside, like they have almost nothing biochemically in common. At the level of how a single cell actually extracts usable energy from food, they share something remarkable: a fixed, multi-step chemical pathway, beginning with glycolysis and continuing, in organisms that use oxygen, through the Krebs cycle, that's so ancient and so deeply conserved across the tree of life that a version of it runs inside nearly every living cell on the planet.
Glycolysis breaks sugar down before oxygen even enters the picture
Glycolysis is the first stage: a sequence of ten enzyme-catalysed reactions, occurring in a cell's cytoplasm, that breaks a single molecule of glucose down into two smaller molecules of pyruvate, extracting a modest amount of usable chemical energy in the process and requiring no oxygen at all to run. Because it doesn't depend on oxygen, glycolysis functions identically whether a cell has access to oxygen or not, which is exactly why it's the one part of this energy-extraction pathway found essentially universally, even in organisms and cellular conditions that don't use oxygen-based metabolism at all.
The Krebs cycle extracts far more energy, but needs oxygen to keep running
In cells with access to oxygen, the pyruvate produced by glycolysis moves into the cell's mitochondria and enters the Krebs cycle (also called the citric acid cycle), a repeating cycle of reactions that extracts considerably more usable chemical energy than glycolysis alone, feeding high-energy electrons into a further process, oxidative phosphorylation, that ultimately produces the bulk of a cell's usable energy currency, a molecule called ATP. This combined pathway — glycolysis followed by the Krebs cycle and oxidative phosphorylation — is so fundamental and so deeply embedded in how cellular life works that its core chemical steps are conserved with striking similarity across organisms as different as bacteria and blue whales, a level of shared biochemical machinery that's often cited as strong evidence for how deep in the history of life this particular energy-extraction pathway actually goes.
What we're still unsure about
The core chemical steps of glycolysis and the Krebs cycle, and their remarkable conservation across an enormous range of living organisms, are extremely well established biochemistry, mapped out in exhaustive detail through more than a century of research. What remains a genuinely active area of research is understanding exactly how this specific pathway first arose and became so universally conserved this early in the history of life, since the very earliest steps of life's biochemical evolution left no direct fossil or molecular record researchers can examine directly — reconstructing that deep evolutionary history relies on comparative analysis across modern organisms and inference rather than being something that can be observed or verified against a preserved record of the actual events.
This sits inside Metabolic Pathways (Glycolysis, Krebs Cycle), one of seven topics in Biochemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.