How does cellular respiration work?

There are two types of cell respiration: anaerobic and aerobic. If oxygen is not present (anaerobic), glycolysis is followed by either alcoholic fermentation or lactic acid fermentation. If oxygen is present (aerobic), glycolysis is followed by the Krebs cycle, the electron transport chain, and oxidative phosphorylation.

Glycolysis breaks down 1 molecule of glucose into 2 three-carbon molecules of pyruvate or pyruvic acid and released 4 molecules of ATP. However, the energy of activation to begin glycolysis is 2 ATP, so the net gain of glycolysis is 2 ATP (4 - 2 = 2). Glycolysis occurs in the cytoplasm and does not require oxygen.

Fermentation is the process by which certain cells convert pyruvate from glycolysis into a ethyl alcohol and carbon dioxide (alcohol fermentation) or into lactic acid (lactic acid fermentation), in the absence of oxygen. In the process, they oxidize NADH back to NAD+. This is important because glycolysis requires a supply of NAD+ to accept electrons. Therefore, fermentation recycles NADH in order to keep glycolysis (and ATP production) going.

The Krebs cycle is a cyclical series of enzyme-catalyzed reactions that require oxygen. It turns twice for each glucose molecule that enters glycolysis (once for each pyruvate molecule that enters the mitochondrion). The cycle generates ATP from ADP and inorganic phosphates.

Electrons extracted by the series of Krebs cycle reactions are then carried by NADH and FADH2 to the electron transport chain. The ETC couples two reactions, one exergonic and one endergonic. It uses energy released from the flow of electrons to pump protons against a gradient from the matrix of the mitochondria into the outer compartment. This results in the establishment of a proton gradient, which sets the stage for ATP production during chemiosmosis.

Chemiosmosis, the process by which 90% of ATP is produced, is where protons flow through ATP-synthase channels and generate energy to phosphorylation ADP to ATP.

You do not need to know the steps in glycolysis or Krebs cycle, or the structures of the molecules and names of the enzymes involved.