Cellular respiration is the set of metabolic
reactions and processes that take place in the
cells of organisms to convert biochemical
energy from nutrients into adenosine
triphosphate (ATP), and then release waste

Once the energy that was in sunlight is
transformed into chemical energy, often by
photosynthesis , the organism has to now convert
the chemical energy into a usable form. It may
seem a bit odd for there still to be more steps.
After all, when you eat a candy bar isn’t the sugar
in the candy bar “burnt” by the body to provide
energy? Well the answer is yes and no. First of all
when we burn something normally in the air we
combine that substance with oxygen releasing
energy from the substance. Indeed, an analogous
process does happen in our bodies.
What goes on in living things is not really like
burning because the molecules from which we
harvest energy give up their energy in a controlled
fashion rather than all at once as what happens in
a fire. Think of your car. All the energy in the gas
tank when you get in your car is not released all at
once but rather in small bursts which allow you to
control the car’s movement. In the same way cells
take the energy from the “food” and package that
energy into manageable bursts that provide just
the right amount of energy for the organism’s
activities be those activities driving a car or
flashing a light to attract a mate.


The point of cellular respiration is to harvest
electrons from organic compounds such as
glucose and use that energy to make a molecule
called ATP. ATP in turn is used to provide energy
for most of the immediate work that the cell does.
ATP can be thought of as being like a small
package of energy that has just the right amount of
energy that can be used in a controlled manner.
ATP: Adenosine tri-phosphate,ATP is a nucleotide
with three phosphate groups instead of one
phosphate group. The point of cellular respiration
is to harvest chemical energy from food and store
it in the chemical bonds of ATP.


Types of cellular respiration: There are two basic types of cellular respiration aerobic cellular respiration and anaerobic cellular respiration. Aerobic respiration requires the use of oxygen and anaerobic respiration which does not use oxygen. There are several types of anaerobic respiration, most familiar is a process called fermentation.

Aerobic Respiration.

Aerobic respiration is the process by which ATP is produced by cells by the complete oxidation of organic compounds using oxygen . In aerobic respiration oxygen serves as the final electron acceptor, accepting electrons that ultimately come from the energy rich organic compounds we consume. We will use glucose as an illustration of an organic molecule used in cellular respiration since glucose is a common energy source for cells. In this figure the energy rich molecules (and heat) are in red, energy poor molecules(relatively speaking) are in black.

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Stages in Aerobic Respiration:

Aerobic Respiration takes place in three stages which are summarized here starting with the original glucose molecule.

Glycolysis. Glycolysis is the first step in cellular respiration and all cells regardless of the type of cellular respiration they do are able to carry out glycolysis. Because of this we believe that glycolysis probably arose very early in the evolution of life on the planet. In glycolysis glucose is partially oxidized and broken down into two 3 carbon molecules called pyruvate or pyruvic acid. In the process, glycolysis produced 4 ATP for a net gain of two ATP and two molecules of NADH. Each NADH is carrying two energy rich electrons away from the glucose and these electrons can be used by the cell to do work.

After glycolysis the pyruvate is processed to harvest 2 more NADH molecules and remove one carbon per pyruvate. The carbon and two oxygens is removed since it no longer has any useful energy. So it is waste. This little step is the source of some of the carbon dioxide we produce.


Role of oxygen in Aerobic Respiration. As the energy rich electrons from food are used to make ATP by electron transport phosphorylation they loose energy and once they are no longer useful they have to be removed. Oxygen is a great electron acceptor and so the electrons are combined with hydrogen ions and oxygen to make water. This prevents electrons from building up in the electron transport system.


Anaerobic respiration is a form of respiration using
electron acceptors other than oxygen. Although
oxygen is not used as the final electron acceptor,
the process still uses a respiratory electron
transport chain ; it is respiration without oxygen. In
order for the electron transport chain to function,
an exogenous final electron acceptor must be
present to allow electrons to pass through the
system. In aerobic organisms, this final electron
acceptor is oxygen. Molecular oxygen is a highly
oxidizing agent and, therefore, is an excellent
acceptor. In anaerobes, other less-oxidizing
substances such as sulfate (SO 4 2−), nitrate (NO 3−)
, sulphur (S), or fumarate are used. These terminal
electron acceptors have smaller reduction
potentials than O 2, meaning that less energy is
released per oxidized molecule. Anaerobic
respiration is, therefore, in general energetically
less efficient than aerobic respiration.

Anaerobic respiration is used mainly by
prokaryotes that live in environments devoid of
oxygen. Many anaerobic organisms are obligate
anaerobes meaning that they can respire only
using anaerobic compounds and will die in the
presence of oxygen.


Cellular respiration (both aerobic and anaerobic)
utilizes highly reduced species such as NADH and
FADH2 (for example produced during glycolysis
and the citric acid cycle ) to establish an
electrochemical gradient (often a proton gradient)
across a membrane, resulting in an electrical
potential or ion concentration difference across the
membrane. The reduced species are oxidized by a
series of respiratory integral membrane proteins
with sequentially increasing reduction potentials
with the final electron acceptor being oxygen (in
aerobic respiration ) or another species (in
anaerobic respiration). The membrane in question
is the inner mitochondrial membrane in eukaryotes
and the cell membrane in prokaryotes . A proton
motive force or pmf drives protons down the
gradient (across the membrane) through the
proton channel of ATP synthase . The resulting
current drives ATP synthesis from ADP and
inorganic phosphate.
Fermentation , in contrast, does not utilize an
electrochemical gradient. Fermentation instead
only uses substrate-level phosphorylation to
produce ATP. The electron acceptor NAD+ is
regenerated from NADH formed in oxidative steps
of the fermentation pathway by the reduction of
oxidized compounds. These oxidized compounds
are often formed during the fermentation pathway
itself, but may also be external. For example, in
homofermentative lactic acid bacteria, NADH
formed during the oxidation of glyceraldehyde-3-
phosphate is oxidized back to NAD+ by the
reduction of pyruvate to lactic acid at a later stage
in the pathway. In yeast , acetaldehyde is reduced
to ethanol to regenerate NAD+.