Cellular respiration is a set of processes that occur in eukaryotic cells that generates ATP (adenosine triphosphate) for cell energy and involves both anaerobic and aerobic steps. In general, cellular respiration can be divided into four stages: Glycolysis, which does not require oxygen and occurs in the mitochondria of all cells, and the three stages of aerobic respiration, all of which occur in mitochondria: the bridge (or transition) reaction, the Krebs cycle and the electron transport chain reactions.
So, if you are asked to identify the stage (or stages) of cellular respiration that occurs entirely outside of the mitochondria, you can answer "glycolysis" and be done with it. But to the curious, this only invites the question: What exactly does happen inside those mitochondria? That is, what happens in the very end to a six-carbon glucose molecule that enters glycolysis in the cytoplasm?
Respiration in Prokaryotes vs. Eukaryotes
Prokaryotic cells do not have any internal membrane-bound organelles. Their DNA floats free in the cytoplasm, as do the enzyme proteins necessary to push glycolysis along. Thus the entirety of their respiration consists of glycolysis.
In eukaryotic cells, the bridge reaction, the Krebs cycle and the electron transport chain together constitute aerobic respiration, and as such are the last three steps in cellular respiration as a whole.
Which of the Four Steps of Cellular Respiration Occur in the Mitochondria?
Actually, a better question to ask, if you are in the business of knowing what processes happen and where they happen in eukaryotic cells, might be: Which of the following does not occur in mitochondria?
- The Splitting of a Sugar
- The Bridge Reaction
- The Krebs Cycle
- The Electron Transport Chain
The answer, one, is remembered by keeping in mind that all cells make use of glycolysis (the splitting of glucose into two three-carbon pyruvate molecules), but only eukaryotic cells have organelles, including mitochondria.
Also, in a way, for eukaryotes, glycolysis is almost a nuisance, serving up only two of the 36 to 38 ATP cellular respiration as a whole generates per molecule of glucose. On the basis of simple proportions, you would "expect" almost all of cellular respiration to occur somewhere in mitochondria, and this is in fact the case – three out of the four phases.
Structure and Function of Mitochondria
Mitochondria are enclosed in a double plasma membrane, like that enclosing the cell as a whole and other organelles (e.g., the Golgi apparatus). The inside of the mitochondria, a space analogous to cytoplasm if mitochondria are likened to cells, is called the matrix.
Mitochondria have their own DNA, in the cytoplasm, just where it would be found if mitochondria were still free-existing bacteria. It is passed down only through egg cells, so only through the maternal (mother's) line of ancestors and descendants.
Cellular Respiration: Phases and Sites
Glycolysis: Cytoplasm Phase. In this series of ten reactions in the cytoplasm, glucose is transformed into a pair of molecules of pyruvate. two ATP are generated, and no oxygen is required. If oxygen is present and the cell is eukaryotic, the pyruvate is passed along to the mitochondria.
Bridge Reaction: Mitochondria Phase 1. The pyruvate is converted to acetyl coenzyme A by losing a carbon atom (in the form of carbon dioxide, CO2) and gaining a coenzyme A molecule in its place. Acetyl CoA is an important metabolic intermediate in all cells.
Krebs Cycle: Mitochondria Phase 2. In the mitochondrial matrix, acetyl CoA combined with the four-carbon molecule oxaloacetate to form citrate. In a series of steps that generate two ATP (one ATP per upstream pyruvate molecule), this molecule is converted back to oxaloacetate. In the process, the electron carriers NADH and FADH2 are produced in abundance.
Electron Transport Chain: Mitochondria Phase 3. On the inner mitochondrial membrane, the electron carriers from the Krebs cycle are used to power the addition of phosphate groups to ADP (adenosine diphosphate) to make 32 to 34 ATP. In total, cellular respiration thus generates 36 to 38 ATP per molecule of glucose, 34 to 36 of them in the three mitochondrial stages.