Cell Biology 101 - Cellular Respiration

In cellular respiration, the high-energy carbon-carbon bonds in glucose are broken to release chemical energy for the cell to use. This happens through a series of chemical reactions that ultimately break down a glucose molecule into separate CO2 and H2O molecules.

Steps 1-3 are known as glycolysis. PO32- is used to split a glucose molecule into two 3-carbon chains (pyruvate). Pyruvate is then oxidized in step 4, releasing a CO2 molecule and forming a 2-carbon chain (acetyl). Acetyl is fed into the citric acid cycle (steps 5-7) where it combined with oxaloacetate to re-form citrate. Citrate is then re-broken down into oxaloacetate, releasing more CO2 and energy in the process.

Step	Reactants	Products
1	C6H12O6 (glucose) + PO32-	C6H11O9P2- (glucose-6-phosphate)
2	C6H11O9P2- (glucose-6-phosphate) + PO32-	2 C3H5O6P2- (glyceraldehyde-3-phosphate)
3	C3H5O6P2- (glyceraldehyde-3-phosphate)	C3H3O3- (pyruvate) + PO32- + 4 energy units
4	C3H3O3- (pyruvate)	C2H3O- (acetyl) + CO2 + 4 energy units
5	C2H3O- (acetyl) + C4H2O52- (oxaloacetate) + H2O	C6H5O73- (citrate)
6	C6H5O73- (citrate) + O2	C5H4O52- (ketoglutarate) + CO2 + 2 H2O + 4 energy units
7	C5H4O52- (ketoglutarate) + 2 O2	C4H2O52- (oxaloacetate) + CO2 + 2 H2O + 8 energy units

The metabolic pathway for cellular respiration in Petri Dish has been simplified, but the overall reaction does balance to give: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy. Here is a more detailed version of the actual pathway:

Step	Reactants	Products
1	glucose + ATP4-	glucose-6-phosphate2- + ADP3- + H+
2	glucose-6-phosphate2-	fructose-6-phosphate2-
3	fructose-6-phosphate2- + ATP4-	fructose-1,6-biphosphate4- + ADP3- + H+
4	fructose-1,6-biphosphate4-	dihydroxyacetone phosphate2- + glyceraldehyde-3-phosphate2-
5	dihydroxyacetone phosphate2-	glyceraldehyde-3-phosphate2-
6	glyceraldehyde-3-phosphate2- + Pi2- + NAD+	1,3-biphosphoglycerate4- + NADH + H+
7	1,3-biphosphoglycerate4- + ADP3-	3-phosphoglycerate3- + ATP4-
8	3-phosphoglycerate3-	2-phosphoglycerate3-
9	2-phosphoglycerate3-	phosphoenolpyruvate3- + H2O
10	phosphoenolpyruvate3- + ADP3- + H+	pyruvate- + ATP4-
11	pyruvate- + CoA + NAD+	acetyl-CoA + NADH + CO2
12	acetyl-CoA + oxaloacetate + H2O	citrate + CoASH
13	citrate	cis-aconitate + H2O
14	cis-aconitate + H2O	isocitrate
15	isocitrate + NAD+	oxalosuccinate + NADH + H+
16	oxalosuccinate	α-ketoglutarate + CO2
17	α-ketoglutarate + NAD+ + CoASH	succinyl-CoA + NADH + H+ + CO2
18	succinyl-CoA + GDP + Pi	succinate + CoASH + GTP
19	succinate + ubiquinone (Q)	fumarate + ubiquinol (QH2)
20	fumarate + H2O	L-malate
21	L-malate + NAD+	oxaloacetate + NADH + H+

In the actual pathway for cellular respiration, energy from breaking the high-energy carbon-carbon bonds in glucose is used to build ATP, NADH, GTP, and QH2 molecules. NADH, GTP, and QH2 are then fed into the electron transport chain where they are used to reduce O2 and re-form ATP from ADP.