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• Energy is the ability to do work
• All living thing livings (organisms) need energy to
• Directly or indirectly nearly all the energy for life
comes from the Sun
• Some organisms make their own food (autotrophs)
• While others must obtain it from other organisms
• The chemical bonds between atoms of food
molecules provide the energy used by all living
organisms to sustain life
• Respiration is the life process in which this energy is
released for use in the cell
• Once released, it becomes available to the cell in the
operation of all other life functions
• Remember that we referred to all the chemical
reactions in a cell as the cell’s metabolism.
Where Does All This Energy Come From?
• Complex carbohydrates are the major source of energy
for most organisms
• But – before the energy in those carbohydrates can be
used by cells, the carbohydrates are broken down into
simple sugars such as glucose
FYI: How much energy is in glucose?
One gram of the sugar glucose (C6H12O6), when burned
in the presence of oxygen, releases 3811 calories of heat
• The process where these glucose molecules
are broken down to release energy is
cellular respiration
• Cellular respiration is a series of enzyme
catalyzed chemical reactions which
produces ATP (adenosine triphosphate)
• Most of the processes that occur in cells use
the ATP for their energy source
• ATP is an energy-transfer compound or
• Cell can use ATP because (in part) of it’s
unique structure
• It makes ATP PERFECT for the storage and
release of energy
High Energy Bond
• The high energy bonds contain a large amount
of energy
• ATP releases energy when the bond between the
second and third phosphate groups is broken,
forming a molecule called adenosine
diphosphate (ADP)
• ATP and ADP can be interchanged by the
addition or removal of a phosphate group
• Sometimes ADP becomes adenosine
monophosphate (AMP) by losing an additional
phosphate group but there is less energy
released in this reaction
Overview of Cellular Respiration
6O2 + C6H12O6
6CO2 + 6H2O + Energy
Cellular respiration occurs in 2 main parts
• Glycolysis and aerobic respiration
• The 1st stage, glycolysis, is an anaerobic process
• Anaerobic metabolic processes do not require
• The 2nd stage – aerobic respiration – includes the
Krebs cycle and electron transport and is an
aerobic process
• Aerobic metabolic processes require oxygen.
• Glucose (6 carbon molecule) is broken down in the
cytoplasm into two 3-carbon molecules called pyruvic acid
• It takes 2 molecules of ATP to split each molecule of
• At the end of glycolysis, however, 4 molecules of ATP are
Glucose + 2 ATP
2 Pyruvic Acid + 4 ATP
C-C-C-C-C-C + 2ATP
C-C-C + C-C-C + 4ATP
Remember: This is occurring in the cytoplasm in the
absence of oxygen
In the absence of Oxygen
In some cells lacking the enzymes necessary for
aerobic respiration
In cells when oxygen is lacking,
Anaerobic respiration will go to fermentation
Lactic Acid Fermentation
Glucose is gradually broken down in a series
of enzyme controlled reaction to lactic acid
(anaerobic respiration)
2 Lactic Acid + 2 ATP
Lactic Acid Fermentation
• Lactic acid is produced in animals and is associated
with muscle fatigue when the muscle cells are
using oxygen faster than the circulatory system can
supply it.
• Lactic acid is also produced by some bacteria and is
important in the production of cheeses, buttermilk,
and yogurt
• As a result of anaerobic respiration, there is a
net gain of 2 ATP
Another End Product of
Anaerobic Respiration
Depending on the organism (like yeast and some bacteria in
the baking and brewing industries), anaerobic respiration
can also produce the end products ethyl alcohol and
carbon dioxide
2 alcohol + 2CO2 + 2 ATP
Aerobic Respiration
• Many of the enzymes involved in aerobic cellular
respiration are located in the mitochondria
• In the process with the presence of oxygen, the
chemical energy of glucose is released gradually in a
series of enzyme-controlled reactions
• Aerobic respiration is much more efficient than
anaerobic respiration
Glucose + Oxygenwater + carbon dioxide + 36ATP
C6H12O6 + 6O2
6H2O + 6CO2 + 36ATP
Krebs Cycle
• Glycolysis has a net result of 2 ATP and 2 pyruvate
(pyruvic acid)
• In the presence of oxygen, pyruvate is transported
into the mitochondrial matrix where it is converted
to carbon dioxide.
• The series of reactions that does this called the
Krebs cycle (or citric acid cycle)
• For each glucose molecule, there are two turns of
the Krebs cycle
• The net yield from the Krebs is six carbon dioxide
molecules & 2 ATP
The Electron Transport Chain
• The final step in the breakdown of glucose
• High-energy electrons and hydrogen ions produced
in the Krebs cycle are used to convert ADP to ATP
• Electrons move along the mitochondrial membrane
and are released
• H+ ions are released and diffuse into the matrix
• Oxygen is the final electron acceptor
(H+ plus O = H2O one of the by-products)
• In total 32 molecules of ATP are formed for each
molecule of glucose
Total ATP Production in Aerobic Respiration
From Glycolysis
From the Krebs Cycle
From the Electron Transport Chain
Only 2 molecules of ATP are produce in
anaerobic respiration
While 36 molecules of ATP are produced in
aerobic respiration
Prokaryotic cellular respiration
• Some prokaryotes also undergo aerobic respiration
• They do not, however, have a mitochondria
• Main difference – use of cell membrane as
electron transport site
• The movement of pyruvate to mitochondria not
necessary, saving prokaryotic cell two ATP
• Increases net total of ATP to 38
• During a race, runners rely on the energy supplied
by ATP to make it to the finish line.
• To obtain energy, the body uses ATP already in
muscles and new ATP made by lactic acid
fermentation and cellular respiration
• At the beginning of a race, the body uses all three
ATP sources, but stored ATP and lactic acid
fermentation can only supply energy for a limited
• When the gun goes off in a footrace, the muscles
of the runners contain only enough of this ATP for
a few seconds of intense activity
• Before most of the runners have passed the e50meter mark, that store of ATP is nearly gone
• At this point, their muscle cells are producing
most of their ATP by lactic acid fermentation
• These sources can usually supply enough ATP to
last about 90 seconds
• In a 200 or 300-meter sprint, this may be just
enough to reach the finish line
What happens if the race is longer?
For exercise longer than 90 seconds, cellular respiration is
the only way to generate a continuing supply of ATP
Cellular respiration release energy more slowly than
fermentation, which is why even well-conditioned athletes
have to pace themselves
Your body stores energy in muscle and other tissues in the
form of the carbohydrate glycogen
These stores of glycogen are usually enough to last for 15 or
20 minutes of activity
After that, your body begins to break down other stored
molecules, including fats, for energy
This is one reason why aerobic forms of exercise such as
running, dancing and swimming are so beneficial for weight
Comparing Photosynthesis and Cellular Respiration