Glycolysis is the first step used by all living cells to extract energy from a nutrient molecule (in this case, glucose, a six-carbon sugar). In some cells, notably those of prokaryotes, it is also the last step, as these cells are not equipped to carry out cellular respiration (glycolysis plus the aerobic reactions that follow in eukaryotes) in its entirety.
Glycolysis takes place in the cytoplasm of cells and results in a net gain of two ATP (adenosine triphoshate, the nucleotide used by cells for its energy needs).
There are 10 glycolysis steps in all, but you don't need to memorize all 10 and their associated enzymes to have a firm understanding of the pathway as a whole. More important than knowing the series of reactions verbatim is being aware of the reactants, the products and the conditions under which glycolysis unfolds.
Glycolysis vs. Cellular Respiration
Question: Which of the following are products of cellular respiration?
A. Glucose; B. Pyruvate; C. Carbon dioxide; D. Acetyl CoA
The answer is C, carbon dioxide only. Glucose is a reactant of cellular respiration (and of glycolysis, the first step), while the others are intermediates along the way from deriving a total of 36 to 38 ATP from glucose so long as oxygen is present. Pyruvate is a product of glycolysis; Acetyl CoA is made from pyruvate in the mitochondria, where it then enters the Krebs cycle.
Reactants of Glycolysis
Glucose, with the formula C6H12O6, has a six-atom hexagonal ring in its center that includes five carbons and an oxygen atom. At the onset of glycolysis, it is the only reactant in the mix. Along the way, however, phosphate groups are needed for the phosphorylation steps (i.e., the addition of phosphate groups to glucose derivatives.
In addition, the reactions require an input of two molecules of NAD+, which is converted to its hydrogenated (reduced) form during glycolysis.
The Initial Steps of Glycolysis: Investment Phase
Glucose is phosphorylated when it enters a cell by diffusion through the plasma membrane. It is then rearranged to a fructose derivative and then phosphorylated a second time to yield fructose-1,6-biphosphate. These two phosphorylation reactions require the input of two ATP, which is hydrolyzed to ADP (adenosine diphosphate) to allow this to occur.
At the end of this phase, the six-carbon molecule is split into a pair of three-carbon molecules. Thus the reactants and products in every step listed from this point on need to be doubled in order to maintain a proper accounting of glycolysis as a whole.
The Final Steps of Glycolysis: Return Phase
With the second part of glycolysis underway, two three-carbon molecules of glyceraldehyde-3-phosphate are transformed into pyruvate (C3H4O3) in a series of steps. These all involve rearrangements, and one of them involves yet another phosphorylation step.
Also in the return phase, two molecules of NAD+ (nicotinamide adenine dinucleotide, an electron carrier needed later in the reactions of aerobic respiration) are transformed into two NADH and two H+ (a hydrogen ion).
In the end, the two phosphate groups on each of the two three-carbon molecules are used to make ATP, meaning that four ATP are generated in this phase. Subtracting the two ATP needed in the investment phase, it is clear that a total of two ATP are derived from one molecule of glucose during glycolysis.
Products of Glycolysis
The complete (net) reaction of glycolysis is listed differently in different sources, but these differences are a matter of the author's decision whether to include certain intermediates as part of the net reaction. One accurate representation is
C6H12O6 + 2 ADP + 2 Pi + 2 NAD → 2 C3H4O6 + 2 ATP + 2 H+ + 2 NADH
Here, Pi is inorganic phosphate, derived from the aforementioned hydrolysis of ATP.
Where Do Glycolysis Products Go?
The pyruvate then enters the mitochondria, where it is converted to acetyl CoA. This molecule enters the Krebs cycle of aerobic respiration, and ultimately, after the reactions of the electron transport chain, 36 to 38 ATP are generated from a molecule of glucose in the process of cellular respiration, including the two ATP from glycolysis.