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Cellular Respiration and Lactic Acid Fermentation
Cellular respiration – the process by which food molecules are broken down in the presence of
oxygen to release energy for work. This process includes glycolysis, the Krebs cycle, and the Electron
Transport Chain.
Cellular Respiration Equation:
C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP
Oxidation-Reduction Reactions:
Reduction has to do with “reducing” the charge of a molecule by adding electrons. If we add a
negative charge from an electron, we must then add a corresponding positive charge from a H+ ion.
Diagram of ReDox Reaction:
ReDox reactions occur in pairs. Whenever one molecule is reduced, another is oxidized.
The Reduction of Oxygen!
O2 + 4H+ + 4e- = 2H2O
Glycolysis (occurs in the cytosol with or without the presence of Oxygen)
Glycolysis – the process by which glucose is converted to Pyruvic acid (pyruvate), and energy is released.
This process is extremely fast (thousands of ATP can be produced in milliseconds), but it is not very
efficient (There is only a net gain of 2 ATP per glucose).
Simplified Glycolysis Equation:
Glucose + 2 ATP + 2NAD+ → 4 ATP + 2 Pyruvic Acid + 2 NADH
Glycolysis splits glucose into two 3-carbon molecules of Pyruvic acid.
Two molecules of ATP are needed to begin glycolysis, but the outcome is a production of 4 ATP
molecules. Therefore the result is a net gain of only 2 ATP molecules.
In addition to producing a net gain of two ATP molecules, glycolysis reduces (energizes) two molecules
of the electron carrier NAD+ to become NADH. The energy in these two molecules of NADH is
eventually used to make ATP during the electron transport chain.
Summary: In glycolysis, glucose is broken down into 2 NADH, 2 ATP, and 2 Pyruvic acid molecules.
Only 2-5% of the energy in glucose is released during glycolysis. The two Pyruvic acid molecules will
now enter one of two different pathways, depending on the presence of oxygen.
If oxygen is present, the Pyruvic acid molecules will enter the Krebs cycle. In the absence of oxygen
they will enter Lactic Acid Fermentation.
Anaerobic Respiration
Anaerobic Respiration – chemical reactions that release energy from food molecules (breakdown
Pyruvic acid) in the absence of oxygen.
I. Lactic acid fermentation – anaerobic respiration in which Pyruvic acid is converted to lactic
acid. Occurs in animal cells.
When oxygen is present within your body, Glycolysis is followed by the Citric Acid Cycle (Krebs Cycle)
and the Electron Transport Chain. However, under large loads of physical activity, sometimes your body
does not get enough oxygen. At this time Glycolysis is followed by an anaerobic process called
Lactic acid fermentation equation:
Pyruvic acid + NADH → Lactic Acid + NAD+
A buildup of lactic acid causes fatigue and muscle soreness. Most lactic acid made in muscle cells
diffuses into the bloodstream, then to the liver, where it is converted back to Pyruvic acid. The remaining
lactic acid may remain in your cells for days leading to extreme muscle soreness. Also, when exercising at
a very fast pace, such as sprinting, your body builds up an oxygen debt. This is why you breathe really
hard even after you stop sprinting. Your lungs are repaying the debt!
Although glycolysis is not very efficient, it is very fast. Without oxygen lactic acid begins to accumulate
very quickly and fatigue sets in. Glycolysis can therefore only be used as the only source of energy for a
short time (approximately 40 seconds).
The mitochondria found within a cell are able to reproduce independent of the cell itself. When a person
begins to exercise on a regular basis, the body requires more energy and the mitochondria reproduce.
More mitochondria equals more of an opportunity to produce energy. This is recognized as an increased
metabolism. Additional affects: the lungs increase in capacity to provide for the increased need for
oxygen. This is exactly why people who sleep too much never seem to have enough energy. The more
they relax, the fewer mitochondria that are produced. This in turn provides them with less energy, and
they want to sleep even more!
Aerobic Respiration (occurs in the mitochondria when oxygen is present)
Aerobic Respiration – the breakdown of Pyruvic acid in the presence of oxygen using the Krebs cycle
and the electron transport chain (breaks down Pyruvic acid into carbon dioxide, water, and energy).
Krebs cycle (named after Hans Krebs):
Citric Acid Cycle: series of ReDox reactions that make up the second
phase of aerobic respiration
For each molecule of Pyruvic acid produced during glycolysis, the Krebs
cycle yields one molecule of ATP, 3 molecules of CO2, four molecules of
NADH, and one molecule of FADH2
NAD+ + H+ + 2 electrons = NADH
FAD + 2H+ + 2 electrons = FADH2
The ATP can be used at this time as energy for bodily processes. The NADH and FADH2 travel to the
Electron Transport Chain where they will be oxidized to form NAD+ and FAD. The electrons that they
lose will power the production of ATP.
During the process from glycolysis through the Krebs cycle, one glucose molecule yields 4 ATP, ten
NADH, and two FADH2. Each molecule of glucose provides reactants (Pyruvic acid) for two turns of the
Krebs cycle. One turn for each molecule of Pyruvic acid.
Electron Transport Chain
Electron Transport Chain – a chain of electrons found in the inner membrane of the mitochondria along
which energy is passed before being transferred to ATP.
Glucose contains a lot of energy. Some energy from glucose was released as ATP. The rest of the energy
from glucose was stored in the electron carriers NADH and FADH2.
Before the cell can use this energy to do work it must be converted to ATP. Energy is transferred from
the electron carriers (NADH and FADH2) to ATP using the electron transport chain. This process occurs
in the cristae (folds of the inner membrane of the mitochondria).
Electron transport chain process:
 High energy electrons carried by NADH and FADH2 collect on the cristae. The electrons move
from the high energy NADH and FADH2 to the low energy electron acceptor oxygen. Electrons
that are passed down the chain reduce oxygen, forming water. Meanwhile, the electrons pump H+
across the inner mitochondrial membrane. The diffusion of H+ back across the membrane is
responsible for forming ATP.
 90% of the ATP produced by a glucose molecule is formed during the electron transport chain. 32
molecules of ATP are produced during the ETC.
 Finally, the final electron acceptor (oxygen) is reduced by NADH to form water (H2O).
 NAD+ and FAD return to the Krebs cycle to pick up more electrons and begin the process again.
 When cells are completely deprived of oxygen, there is no final electron acceptor, and ATP cannot
be produced. The cell runs out of energy and then dies.
Electron Transport Chain
ATP – Adenosine Triphosphate
Adenosine is a combination of Adenine and Deoxyribose.
Where have we seen these terms before?
DNA! Deoxyribonucleic Acid!
DNA is composed of Nucleotides. Nucleotides are composed of a sugar a phosphate and a nitrogenous
Sugar = Deoxyribose
Nitrogenous Bases = A, T, C, G (A = Adenine)
So ATP is very similar to DNA in that it contains the same sugar, phosphate, and nitrogenous base. The
difference is that it contains ONLY Adenine and it also contains three phosphates.
There are other molecules that are very similar to ATP as well. They are ADP and AMP.
ADP = Adenosine Diphosphate
AMP = Adenosine Monophosphate
How do these molecules carry energy? Which one contains the most energy?
The answer is in the phosphates and its charge. Opposite charges attract – Like charges repel!
Each of the phosphates within ATP has the same charge. It takes a great deal of energy to hold them
together. This energy is released when they are separated.
When ATP is used for cellular energy one of the phosphates is removed resulting in ADP, a single
phosphate, and ENERGY!