ATP Production in Glycolysis: The Basics
Glycolysis is an essential energy producing form of cellular respiration wherein glucose, a simple 6-carbon sugar also known as blood sugar, is broken down into pyruvate and energy storing molecules (ATP) for use in the cell. Glycolysis occurs in both the presence and absence of oxygen.
The purpose of glycolysis is to use two adenosine triphosphate (ATP) molecules to break glucose down, producing pyruvate, nicotinamide adenine dinucleotide (NADH), and more ATP molecules than were used to fund the reaction. This basic reaction is used by cells to gather ATP molecules which can then be broken down into adenosine diphosphate, releasing energy that can be utilized by the cell.
Where it Takes Place:
Glycolysis occurs in the cytosol of both prokaryotic (bacteria and archaea) and eukaryotic (plant, animal, etc...) cells.
1. Initally, the enzyme hexokinase splits a phosphate from one ATP molecule and attaches it to glucose, releasing one ADP molecule and converting glucose to glucose 6-phosphate (meaning six carbons plus a phosphate group).
2. Another enzyme called phosphoglucoisomerase rearranges the glucose 6-phosphate into fructose 6-phosphate. This is an isomer of glucose 6-phosphate, which means that nothing is added or subtracted from the glucose 6-phosphate, only arranged into different positions.
3. A new enzyme phosphofructokinase (as opposed to isomerase) transfers a phosphate group from a new ATP and attaches it to fructose 6-phosphate into fructose 1,6-biphosphate). Now two ATP molecules have been spent on glycolysis.
Hint: If an enzyme ends in "kinase," that enzyme will transfer a phosphate group from one molecule to another. If an enzyme ends in "isomerase," it will alter the structure of a molecule to create an isomer of that molecule.
4. The enzyme aldolase splits the fructose 1,6 biphosphate molecule into two three carbon molecules called dihydroxyacetone phosphate and glyceraldehyde 3-phosphate respectively.
5. the enzyme isomerase (remember phosphoglucoisomerase?) rearranges dihydroxyacetone phosphate into the isome, glyceraldehyde 3-phosphate. The cell now has two glyceraldehyde 3-phosphate molecules to work with.
6. The two molecules of glyceraldehyde 3-phosphate are oxidized (given a hydrogen/proton) by the molecule triose phosphate dehydrogenase and are are also each given a phosphate group from the cytosol to produce two molecules of 1,3-biphosphoglycerate. This reaction results in the release of two molecules of NADH + H+.
7. The enzyme phosphoglycerokinase takes one free phosphate group given to each 1,3-biphosphoglycerate molecule by trios phosphate dehydrogenase and attaches each to an ADP molecule, resulting in two molecules of ATP and changing the two 1,3-biphosphoglycerate molecules into two 3-phosphoglycerate molecules.
8. The enzyme phosphoglyceromutase moves the phosphate groups on each 3-phosphoglycerate to a new position, creating two 2-phosphoglycerate molecules.
9. Another enzyme, enolase, rips a water molecule (H2O) off of each 2-phosphoglycerate, forming two molecules of phosphoenolpyruvate.
10. Finally, the enzyme kinase transfers a phosphate from each phosphoenolpyruvate to two ADP molecules, producing two ATP molecules and leaving behind two pyruvate molecules.
2 ATP molecules: these can be hydrolyzed (split by water) to release free energy that can be used to power other cycles and reactions.
2 NADH molecules: these can be used in oxidation/reduction (redox) reactions as a reducing agent to donate electrons to other molecules. NADH from glycolysis goes to the electron transport chain.
2 pyruvate molecules: these can be converted to acetyl CoA, which links glycolysis to other cellular respiration reactions, such as the Krebs/citric acid cycle. If you'd like to know more about how the products of glycolysis and cellular respiration continue, check out this great article on the citric acid cycle!
Terms to Know:
ADP: a molecule consisting of a 5-carbon pentose sugar, an adenine molecule, and two phosphate groups used to synthesise ATP and created as a result of ATP hydrolysis.
ATP: a molecule consisting of a 5-carbon pentose sugar, an adenine molecule, and three phosphate groups hydrolized to produce energy. Note that ATP consists of one more phosphate group than ADP
Cytosol: the thick, viscous liquid that cell organelles are suspended in, kind of like the air blown into a balloon, but much much thicker.
Enzyme: biological molecules that catalyze, or increase the speed of, biological reactions. Note that enzymes will not cause a reaction to take place if it wouldn't normally, it only causes the reaction to go faster.
Glucose: a simple sugar, called a monosaccharide, consisting of six carbons, twelve hydrogens, and six oxygens (C6H12O6) and used in glycolysis to produce ATP, NADH, and pyruvate.
Isomer: a molecule or compound with the same molecular fomula (ex. H2O) as another molecule or compound, but a different structure.
NADH: A reducing agent used in redox reactions to donate electrons to other molecules.
Oxidation: the loss of an electron or gain of a proton/hydrogen atom by a molecule.
Pyruvate: a result of glycolysis that transforms into acetyl CoA, linking glycolysis to the Krebs/citric acid cycle.
Redox Reaction: a reaction in which one reactant is oxidized and one is reduced.
Reduction: the gain of an electron or loss of a proton/hydrogen atom by a molecule.