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Glycolysis is the catabolic process responsible for oxidizing one molecule of 6-carbon glucose into two molecules of 3-carbon pyruvate, in itself generating a net two ATP (four total subtracting the two ATP required at the beginning of the pathway) as well as reducing two of electron carrier NAD+ into NADH. In aerobic conditions, the two molecules of pyruvate continue by conversion into acetyl-CoA (producing an additional NADH each) to the citric acid cycle, and the reduced molecules of NADH go on to donate their hydrogens to the electron transport chain (contributing to the production of additional ATP and recreating the oxidized NAD+ required for glycolysis to continue).
The net reaction for glycolysis is:
glucose + 2 Pi + 2 ADP + 2 NAD+ → 2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O
The pathway occurs by 10 enzyme catalyzed steps. A molecule each of ATP is used in phosphoryl transfers of steps 1 and 3 (catalyzed by hexokinase and phosphofructokinase, respectively). The six-carbon molecule is cleaved into two three-carbon molecules in step 4, one molecule of which goes through the step 5 isomerization to match the second molecule. Both three-carbon molecules then proceed through steps 6-10 producing 1 NADH (step 6), 2 ATP (steps 7 and 10), and 1 H2O (step 9).
Glucose can be polymerized for storage as polysaccharides glycogen in animal cells or starch in plant cells, both characterized by α linkages.
Glycogen is produced during glucose surplus by glycogenesis. When glucose levels are low, the removal of glucose monomers from glycogen through glycogenolysis produces (after a conversion step) a phosphorylated glucose-6-phosphate ready to enter glycolysis at step 2, a process that will not require ATP hydrolysis by using available Pi instead.
Amylase enzymes are capable of breaking down starch polysaccharides during digestion for the purpose of glucose absorption.
In anaerobic circumstances, when oxygen is not available to act as the final electron acceptor of the electron transport chain, glycolysis can only continue by the regeneration of the required electron acceptor NAD+ (from NADH from glycolysis) through the process of fermentation, transforming each pyruvate into one molecule of lactate in most eukaryotes or ethanol in some yeast and bacteria. The process of fermentation foregoes any additional ATP production that would have taken place by oxidative phosphorylation but allows for the continuation of glycolysis (with its 2 net ATP production).
Gluconeogenesis, which occurs mostly in the liver, is an anabolic, energy consuming process that allows for the production of glucose from non-carbohydrates to maintain adequate glucose levels. It follows a path in reverse of glycolysis with alternatives for glycolysis's irreversible steps 1, 3, and 10, catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase, respectively.
The net reaction for gluconeogenesis is:
2 pyruvic acid + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 6 H2O → glucose + 4 ADP + 2 GDP + 2 NAD+ + 6 HPO42- + 6 H+
Precursors for gluconeogenesis include lactate converted back to pyruvate, glycerol (from fat metabolism) converted into DHAP (an intermediate), and amino acids including alanine converted to pyruvate.
The pentose phosphate pathway is an alternative path for the oxidation of glucose. In lieu of generating ATP, the pentose phosphate pathway produces the reducing (antioxidant) molecule NADPH (useful in fat synthesis), ribose-5-phosphate for nucleotide / nucleic acid synthesis, and two intermediaries of glycolysis (fructose-6-phosphate [enters step 3] and glyceraldehyde-3-phosphate [enters step 6]).
Glycolysis produces 2 net ATP and 2 NADH per glucose. Conversion of pyruvate to acetyl-CoA produces 1 NADH (2 NADH per glucose). Electron carriers contribute to the ATP count in total aerobic respiration by donating electrons (hydrogen) to the electron transport chain that drive production of 2-3 ATP per NADH and 2 ATP per FADH2.
Each turn of the citric acid cycle produces 1 ATP, 3 NADH, and 1 FADH2 (2 ATP, 6 NADH, and 2 FADH2 per glucose, two turns of citric acid cycle).
Together, glycolysis, the conversion of pyruvate, and the citric acid cycle produce 4 ATP, 10 NADH, and 2 FADH2 per molecule of glucose. Completing oxidative phosphorylation through the electron transport chain, these produce 4 + 30 + 4 = 38 ATP per glucose. Transport of NADH produced from glycolysis in the cytosol into the mitochondria in eukaryotes requires 1 ATP each, so the net ATP production becomes 36 ATP.
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