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Cellular Respiration
Cellular respiration is the process of oxidizing food molecules, like glucose, to carbon dioxide and water. The energy released is trapped in the form of ATP, which can be used as fuel for every energy-dependent activity of a cell. My discussion will focus upon the process as it occurs in plants, although it runs along similar lines in animal cells as well.

The process occurs in two general phases:
glycolysis: the breakdown of glucose to pyruvic acid
the oxidation of pyruvic acid: complete breakdown into carbon dioxide and water (with the energy converted into ATP)


The process of cellular respiration takes place exclusively within the confines of a mitochondria (and its various membranous coverings) and the surrounding cytoplasm. Thus, the conversion of glucose into ATP does NOT take place inside the chloroplasts, as many assume. Indeed, by this time, the chloroplasts are now no longer directly involved in the actions of the glucose created in photosynthesis.



In the cytoplasm, the glucose created by photosynthesis and released by the chloroplasts is converted into two pyruvates (pyruvic acid). This reaction yields 6 ATP and 2 NADH+, which will be used mostly to fuel the later chemical cycles of cellular respiration. The process is known as glycolysis (simply the "breaking down of glucose").

Glycolysis is the anaerobic catabolism of glucose.
It takes place exclusively in the cytoplasm.
It occurs on a cellular level in virtually every living organism.
C6H12O6 + 2NAD+ -> 2C3H4O3 + 2NADH + 2H+
The energy stored in the 2 molecules of pyruvic acid is less than what was contained by the glucose molecule before glycolysis.
2 molecules of ATP are created.


Pyruvate Breakdown (Krebs-Citric Acid Cycle)

The Krebs-Citric Acid Cycle
pyruvic acid is reacted to make a 2-carbon fragment of acetate
this molecule is then coupled to oxaloacetic acid
the resulting molecule of citric acid undergoes the series of enzymatic steps, each producing a new type of acid with some sort of carbon loss  
the final step produces another molecule of oxaloacetic acid, thus perpetuating the cycle
the reaction, in addition to freeing fixed carbon as carbon dioxide gas, also produces electrons stored by the energy-carrier molecules NADH+ and FADH2
the NADH and FADH2 are transferred to the electron transport chain


Electron Transport Chain/ATP Synthesis


The NADH+ is transferred through a series of rapid-fire reactions that are collectively known as the Electron Transport Chain. By transferring high-energy electrons, the various enzymes in the chain strip energy from the NADH+ and send it back into the mitochondrial membrane. Here, it passes through the ATP Synthase where it becomes ATP. In this way, each molecule of glucose can yield about 36-38 ATP.