The means by which the cells of a living thing extract energy from the bonds in organic molecules depend on the type of organism being studied. Prokaryotes (the Bacteria and Archaea domains) are limited to anaerobic respiration because they cannot make use of oxygen. Eukaryotes (the domain Eukaryota, which includes animals, plants and fungi) do incorporate oxygen into their metabolic processes and as a result can obtain far more adenosine triphosphate (ATP) per fuel molecule entering the system.
All cells, however, make use of the ten-step series of reactions collectively known as glycolysis. In prokaryotes, this is usually the only means of obtaining ATP, the so-called "energy currency" of all cells. In eukaryotes, it is the first step in cellular respiration, which also includes two aerobic pathways: the Krebs cycle and the electron transport chain.
The combined end product of glycolysis is two molecules of pyruvate per molecule of glucose entering the process, plus two molecules of ATP and two of NADH, a so-called high-energy electron carrier.
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The complete net reaction of glycolysis is:
C6H12O6 + 2 NAD+ + 2 ADP + 2 P → 2 CH3(C=O)COOH + 2 ATP + 2 NADH + 2 H+
The label "net" is critical here, because in reality, Two ATP are needed in the first part of glycolysis to create the conditions needed for the second part, in which four ATP are generated to bring the overall balance sheet to a plus-two in the ATP column.
Each step in glycolysis is catalyzed by a particular enzyme, as is customary of all cellular metabolic reactions. Not only is every reaction influenced by an enzyme, but each enzyme involved is specific for the reaction in question. Hence, there is a one-to-one reactant-enzyme relationship in place.
Glycolysis is typically divided into two phases that indicate the energy flow involved.
Investment phase: The first four reactions of glycolysis include the phosphorylation of glucose after it enters the cell cytoplasm; the rearrangement of this molecule into another six-carbon sugar (fructose); the phosphorylation of this molecule at a different carbon to yield a compound with two phosphate groups; and the splitting of this molecule into a pair of three-carbon intermediates, each with its own phosphate group attached.
Payoff phase: One of the two phosphate-bearing three-carbon compounds created in the splitting of fructose-1,6-bisphosphate, dihydroxyacetone phosphate (DHAP), is converted to the other, glyceraldehyde-3-phosphate (G3P), meaning that two molecules of G3P exist at this stage for every glucose molecule entering glycolysis.
Next, these molecules are phosphorylated, and in the next several steps, the phosphates are peeled off and used to create ATP as the three-carbon molecules are rearranged into pyruvate. Along the way, two NADH are generated from NAD+, one per three-carbon molecule. Thus the net reaction above is satisfied, and you can now confidently answer the question, "At the end of glycolysis, which molecules are obtained?"
In the presence of oxygen in eukaryotic cells, the pyruvate is shuttled to the organelles called mitochondria, which are all about aerobic respiration. The pyruvate is divested of a carbon, which exit the process in the form of the waste product carbon dioxide (CO2), and left behind as actetyl coenzyme A.
Krebs cycle: In the mitochondrial matrix, the acetyl CoA combines with the four-carbon compound oxaloacetate to yield the six-carbon molecule citrate. This molecule is pared back down to oxaloacetate, with the loss of two CO2 and the gain of one ATP, three NADH and one FADH2 (another electron carrier) per turn of the cycle. This means you need to double these numbers to account for the fact that two acetyl CoA enter the Krebs cycle per molecule of glucose entering glycolysis.
Electron transport chain: In these reactions, which occur on the mitochondrial membrane, the hydrogen atoms (electrons) from the aforementioned electron carriers are stripped off their carrier molecules used to drive the synthesis of a great deal of ATP, about 32 to 34 per "upstream" glucose molecule.