Cells are microscopic, multipurpose containers that represent the smallest indivisible units of life in that they manifest reproduction, metabolism and other "lifelike" qualities. In fact, since prokaryotic organisms (members of the Bacteria and Archaea classification domains) almost always consist of a single cell, many stand-alone cells are literally alive.
Cells make use of a molecule called adenosine triphosphate, or ATP, as a source of fuel. Prokaryotes rely solely on glycolysis – the breakdown of glucose into pyruvate – as a pathway to synthesizing ATP; this process yields a total of 2 ATP per molecule of glucose.
In contrast, eukaryotes – animals, plants and fungi – are both far larger and in possession of far more complex individual cells than prokaryotes, making glycolysis alone inadequate for their energy needs. That's where cellular respiration, the complete breakdown of glucose in the presence of molecular oxygen (O2) into carbon dioxide (CO2) and water (H2O) to form ATP, comes in.
Cellular Metabolism Terminology
The cellular respiration process occurs in eukaryotes and technically spans glycolysis, the Krebs cycle and the electron transport chain (ETC). This is because all cells initially treat glucose the same way – by running it through glycolysis. Then, in prokaryotes, pyruvate can only enter fermentation, which allows glycolysis to continue "upstream" through the regeneration of an intermediate called NAD+.
Because eukaryotes can use oxygen, however, the carbon molecules of pyruvate enter the Krebs cycle as acetyl CoA and ultimately leave the ETC as carbon dioxide (CO2). The cellular respiration products of interest are the 34 to 36 ATP that are generated in the Krebs cycle and the ETC together – the two portions of cellular respiration that count as aerobic ("with oxygen") respiration.
The Reactions of Cellular Respiration
The complete, balanced reaction of the entire cellular respiration process can be represented by:
C6H12O6 + 6O2 → 6 CO2 + 6 H2O + ~38 ATP
Glycolysis alone, a form of anaerobic respiration that occurs in the cytoplasm, consists of the reaction:
C6H12O6 + 2 NAD+ + 2 ADP + 2 Pi → 2 CH3(C=O)COOH + 2 ATP + 2 NADH + 4 H+ + 2 H2O
In eukaryotes, a transition reaction in mitochondria generates acetyl coenzyme A (acetyl CoA) for the Krebs cycle:
2 CH3(C=O)COOH + 2 NAD+ + 2 coenzyme A → 2 acetyl CoA + 2 NADH + 2 H+ + 2 CO2
The CO2 then enters the Krebs cycle by joining to oxaloacetate.
Stages of Cellular Respiration
Cellular respiration starts with glycolysis, a series of 10 reactions in which a glucose molecule is phosphorylated twice (that is, it has two phosphate groups attached at different carbons) using 2 ATP, and then split into two three-carbon compounds that each yield 2 ATP en route to the formation of pyruvate. Thus glycolysis supplies 2 ATP directly per glucose molecule as well as two molecules of the electron carrier NADH, which has a strong role downstream in the ETC.
In the Krebs cycle, CO2 and the four-carbon compound oxaloacetate join to form the six-carbon molecule citrate. Citrate is gradually reduced again to oxaloacetate, spinning off a pair of CO2 molecules and also generating 2 ATP per CO2 molecule entering the cycle, or 4 ATP per glucose molecule far upstream. More importantly, a total of 6 NADH and 2 FADH2 (another electron carrier) are synthesized.
Finally, the electrons of NADH and FADH2 (that is, their hydrogen atoms) are stripped away by enzymes of the electron transport chain and used to power the attachment of phosphates to ADP, yielding lots of ATP – about 32 in all. Water is also released in this step. Thus the maximum ATP yield of cellular respiration from glycolysis, the Krebs cycle and the ETC is 2 + 4 + 32 = 38 ATP per molecule of glucose.