Thank you for an interesting question.
Firstly, I think you should understand cellular respiration in general.
Every living thing must extract energy from the organic food molecules
that it either manufactures by photosynthesis (in the case of most
plants) or captures from the environment. In most complex mammals,
such as in humans, the food is broken down by the digestive system.
During digestion, proteins are split into their component amino acids,
carbohydrates are digested into simple sugars and fats are split into
glycerol and fatty acids. These nutrients are then absorbed into the
blood and transported to all the cells of the body. Each cell must
then convert the energy found in the chemical bonds of the nutrients
into chemical energy stored in molecules of ATP (adenosine
triphosphate). ATP is the useable energy source of cells (Cells can?t
just use sugar for energy; they must process it into a useable form,
ATP). The process by which the energy found in nutrients is converted
into ATP is called cellular respiration. The term cellular
respiration is used to distinguish these cellular processes from
organismic respiration, the process by which oxygen and carbon dioxide
is exchanged with the environment by organisms with special organs
such as lungs, gills, etc.
Cellular respiration may be either aerobic or anaerobic. Aerobic
respiration requires molecular oxygen (O2) and anaerobic respiration
does not require oxygen. As you mentioned, humans are considered
aerobes, meaning most human cells employ aerobic respiration to derive
energy in the form of ATP from nutrient molecules.
The overall chemical equation for aerobic respiration is as follows:
C6H12O6 + 6O2 + 6H2O 6CO2 + 12H2O + Energy (36-38 ATP)
Glucose + molecular Oxygen + water ? carbon dioxide + water + energy
Often, you will see the equation written like this:
C6H12O6 + 6O2 6CO2 + 6H2O + Energy (36-38 ATP)
This is the same equation. They simply crossed off 6H2O from each side
of the equation to simplify it.
However, this overall general equation does not give the entire story.
The process of aerobic cellular respiration actually requires many
many smaller chemical reactions for glucose to be turned into ATP.
These smaller chemical equations occur in 4 stages:
2) Formation of Acetyy Coenzyme A
3) The Citric Acid Cycle
4) The Electron Transport Chain (ETC) and Chemiosmosis (this process
requires oxygen to act as an electron acceptor)
These 4 stages are very complicated (Nobel Prizes were awarded for
discovering them), and you will have to consult a textbook or an
online source to fully understand them. But the general idea is that
glucose (sugar) or some derivative of glucose goes through these 4
successive processes. At each stage, the glucose or glucose
derivative is processed, and each time, energy (usually the form of
ATP) is released and stored. Keep in mind that this is a terribly
oversimplified explanation of the process.
At the end of the 4 stages, one molecule of glucose gets converted
into a maximum of 36-38 molecules of ATP by the process of aerobic
respiration. This process is about 40% efficient. By comparison, a
steam power plant has an efficiency of 35 ? 36%, so as you can see,
aerobic respiration is generally considered a very efficient means of
converting glucose into energy.
The majority of the time human cells employ aerobic respiration.
However, what happens when human cells are in an oxygen depleted
environment? Brief periods of oxygen depletion can happen during
periods of vigorous exercise, when all the oxygen in the bloodstream
is quickly used up by the muscle cells to meet the large energy
demand. Without oxygen, human cells must find another way to convert
glucose into ATP since stage 4 of aerobic respiration (The Electron
Transport Chain) requires oxygen as a vital component. In this case,
human cells shift briefly to a form of anaerobic respiration called
The chemical equation for lactate fermention is:
Glucose ? 2 Lactate + Energy (2 ATP)
Again, the above chemical equation is simply a representation of the
overall chemical reaction which occurs. In reality, anaerobic
respiration requires numerous smaller chemical reactions for glucose
to be converted into ATP. The process of lactate fermentation uses a
modified version of glycolysis (the chemical pathway used in stage 1
of aerobic respiration) and generates only 2 molecules of ATP per
molecule of glucose.
As you can see, anaerobic respiration is much less efficient compared
to aerobic respiration, and it cannot be sustained for long periods of
time because our skeletal muscle cells simply don?t store enough
glucose (usually in the form of glycogen) to run the anaerobic process
for extended periods. Thus, the shift to anaerobic respiration is
temporary and we must eventually revert back to the sustainable
aerobic respiration. As lactate (a waste product of anaerobic
respiration) accumulates in the muscle cells, it contributes to
fatigue and muscle cramps. When oxygen levels return to normal, about
80% of the lactate is exported to the liver where it is regenerated
into glucose. The remaining 20% is metabolized in the muscle cells in
the presence of oxygen. This explains why you continue to breathe
heavily after you have stopped exercising. The additional oxygen is
needed to oxidize the lactate, thereby restoring muscle cells to their
I hope I have provided you with some insight. I would advise you to
consult a college/university level textbook to full understand the
complex processes of cellular respiration. I would be happy to
provide clarification if necessary.