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Overview of Cellular Respiration
The general equation for cellular respiration is as follows:
C6H12O6 + 6O2  6CO2 + 6H2O + 36-38ATP
Glycolysis: In the cytoplasm
The first step of cellular respiration is called glycolysis. It is when glucose is broken down into 2 pyruvic acids.
To start the reaction, 2 phosphate groups from two molecules of ATP are added during the energy investment
phase. After these are added, the glucose molecule is gradually broken down and rearranged by various enzymes.
More rearrangement of the molecule, and substrate level phosphorylation during the energy payoff phase yields
2 pyruvic acids, 4 ATP’s, 2 NADH’s and 2 molecules of water. Therefore there is a net Gain of 2 ATP’s from
glycolysis. Below is the general reaction equation:
C6H12O6 + 2ATP + 2NAD+  2 pyruvic acids + 4ATP’s (net of 2) + 2NADH’s + 2H2O
Fermentation: In the cytoplasm
If O2 is not present the pyruvic acids are then used for
fermentation. There are two general types of
fermentation:
Lactic Acid Fermentation
and
Alcoholic Fermentation
Lactic acid fermentation is used in animals and
alcoholic fermentation is used by yeast and other
fungi. The purpose of fermentation is to generate
more NAD+ so glycolysis can keep working to
produce ATP. Below is the general formula:
Pre-Krebs/Krebs Cycle: In the mitochondrion
If O2 is present, the pyruvic acids are then sent to the
mitochondria. While in the mitochondria, the 2
pyruvic acids are first converted into 2 acetylCoA’s
(this is one of the starting reactants in the Krebs
cycle) via the addition of coenzyme A. Below is the
reaction for how this is done:
2 pyruvic acid + 2 CoA + 2NAD+  2 acetylCoA +
2 CO2 + 2 NADH
The two acetylCoA produced then enter the Krebs
cycle or the Citric Acid Cycle. While in the Krebs
cycle, one Acetyl CoA first combines with another
4 carbon molecule oxaloacetate generating a six
Lactic acid fermentation:
carbon molecule called citrate. That citrate is then
2 pyruvic acid + 2 NADH  2 lactic acid + 2NADH +
gradually changed to a five carbon molecule, and
2 CO2
then to a four carbon molecule (which ultimately
becomes oxaloacetate). While this rearrangement
Alcoholic fermentation:
and breakdown is happening, 1 ATP (via 1 GTP), 3
2 pyruvic acid + 2 NADH  2 ethyl alcohol +
NADH, 1 FADH2 and 1 CO2 is produced.
2NADH + 2 CO2
Remember that for every glucose molecule, the
Krebs happens twice, because two acetylCoA’s are produced from one glucose molecule and that ATP
generation is via substrate level phosphorylation. Below is the general reaction per one glucose molecule:
2 acetylCoA + 2 oxaloacetate  2 oxaloacetate + 2ATP + 6NADH + 2FADH2 + 4CO2
Electron Transport Chain: Along the inner membrane of the mitochondria
The final step of aerobic cellular respiration is called the electron transport chain (ETC). The ETC works
with the 10 NADH’s and 2 FADH2’s produced from glycolysis and the Krebs cycle. The electrons stored by
NADH and FADH2 get passed along a line of proteins in the mitochondrial inner membrane due to increasing
affinities. While being passed along the ETC, these high energy electrons are used by the ETC proteins to
pump hydrogen ions across the inner membrane. This creates a high H+ concentration in the intermembrane
space and a low H+ concentration in the matrix. The final acceptor of the electrons is oxygen, which upon
receiving the electrons produces 4 more H2O molecules. A different protein found in the membrane called
ATP synthase then generates 32 ATP molecules via a chemiosmotic model. This entire step generates ATP
via oxidative phosphorylation because ATP production is derived from the redox reactions of the ETC.
Below is the following reaction:
10NADH + 2FADH2 + O2  32ATP + 4H2O