3.7 Cell Respiration
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Topic 3: Chemistry of Life
3.7 Cell Respiration
3.7.1 Define cell respiration  cell respiration is the controlled release of
energy from organic compounds in cells to form ATP.
3.7.2 State that, in cell respiration, glucose in the cytoplasm is broken down by
glycolysis into pyruvate, with a small yield of ATP.
3.7.3 Explain that, during anaerobic cell respiration, pyruvate can be converted
in the cytoplasm into lactate, or ethanol and carbon dioxide, with no further
yield of ATP.
3.7.4 Explain that, during aerobic cell respiration, pyruvate can be broken down
in the mitochondrion into carbon dioxide and water with a large yield of ATP.
3.7.1 Respiration
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3.7.1 Define cell respiration  cell respiration is the controlled release of
energy from organic compounds in cells to form ATP.
Orange book  pg. 83
Green book  pg. 47
To do:
Watch the animation: “The Big Picture”
http://www.sumanasinc.com/webcontent/animations/content/cellularrespiration.
html
Copy both the word equation and balanced formula equation for respiration into
your greenbooks.
Summarise in you own words the purpose of respiration and why enzymes are
essential to the process.
Overview of Respiration
The equation for cellular respiration is usually simplified to:
glucose + oxygen  carbon dioxide + water (+ energy)
C6H12O6 +
6O2 
6CO2
+ 6H2O (+ energy)
Organic molecules contain energy in their molecular structures. Each covalent
bond in a glucose, amino acid or fatty acid represents stored chemical energy.
The function of respiration is not to produce carbon dioxide and water but to
transfer this energy in glucose to ATP.
If the reactions were carried out in a single step it would generate a short,
intense burst of heat – like burning paper, which has the same chemical
equation! Not only would this be useless to the body’s metabolism, it would kill
the cells.
In the body, the process is carried out in a series of small steps, each
controlled by a separate enzyme. This slows oxidation. Energy is released in
small manageable amounts and as much as possible is transferred to ATP.
If a cell does not have glucose available, other organic molecules may be
substituted, such as fatty acids or amino acids.
Linking respiration and photosynthesis
Photosynthesis and respiration are the reverse of each other, and you couldn’t
have one without the other. The net result of all the photosynthesis and
respiration by living organisms is the conversion of light energy to heat energy.
The energy released within cells is needed for active transport, movement and
the maintenance of body temperature. Biosynthesis is the making of biological
molecules e.g. the synthesis of proteins from amino acids.
3.7.2 Glycolysis
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3.7.2 State that, in cell respiration, glucose in the cytoplasm is broken down by
glycolysis into pyruvate, with a small yield of ATP.
Orange book  84
Green book  47
To do:
Watch the animation: “Glycolysis in 3 Steps”
(at this stage do not worry about names)
http://www.science.smith.edu/departments/Biology/Bio231/glycolysis.html
Read the section below “The Basics”. In your green book, using two bullet points
only, give a basic description of what occurs during glycolysis.
Watch the animation: “Glycolysis”
http://www.sumanasinc.com/webcontent/animations/content/cellularrespiration.
html
Go through the powerpoint presentation: “Glycolysis” (as a class).
From what you have studied, draw a flow chart to show the process of glycolysis
in your greenbooks to include:
 glucose (6C)


ATP
Pyruvate (3C)
The Basics
Assuming that glucose is the organic nutrient being metabolised, all cells begin
the process of cell respiration in the same way. Glucose enters a cell through
the plasma membrane and floats in the cytoplasm. The process of glycolysis
occurs here in the cytoplasm.
During glycolysis, chemical reactions controlled by enzymes split a six-carbon
molecule of glucose (hexose) into two three-carbon molecules of pyruvate.
Even though glycolysis consumes two ATP molecules, it produces four ATP
molecules, for a net gain of two ATP molecules.
Study the diagram below and you should recognise the following key points:
 Glucose (6C) is the starting point
 2 x ATP are needed to start the reaction
 2 x pyruvate (3C) are the products

4 x ATP are produced (which is a net gain of 2 x ATP, due the two that were
needed initially).
3.7.3 Anaerobic Respiration
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3.7.3 Explain that, during anaerobic cell respiration, pyruvate can be converted
in the cytoplasm into lactate, or ethanol and carbon dioxide, with no further
yield of ATP.
Orange book  pg. 84
Green book  pg. 47
To do:
Read the ‘Introduction’ below and in your greenbooks write a summary to
include:


how all respiration pathways start
using an example, explain under which circumstances anaerobic respiration
will occur.
Read the paragraph on “Lactate Fermentation” and in your green books draw and
annotate a flow diagram to show lactate fermentation.
Read the paragraph on “Alcoholic Fermentation” and in your green books draw
and annoate a flow diagram to show alcoholic fermentation.
Introduction
The term ‘cell respiration’ refers to a variety of biochemical pathways that can
be used to metabolize glucose. All of the pathways start with glycolysis. In
other words, glycolysis is the metabolic pathway that is common to all
organisms. There are two types of cellular respiration: aerobic (in the presence
of oxygen) and anaerobic or fermentation (in the absence of oxygen).
If oxygen is present, the products of glycolysis will enter the aerobic pathway.
Aerobic respiration produces a much larger amount of ATP, (up to 20 time more
ATP).
There are two types of anaerobic respiration or fermentation: lactic acid
fermentation and alcoholic fermentation.
Lactate Fermentation
Organisms that use an aerobic cell respiration pathway sometimes find
themselves in a metabolic situation where they cannot supply enough oxygen to
their cells. A good example of this is a person pushing beyond their normal
exercise pattern. In this situation, the person’s pulmonary and cardiovascular
systems supply as much oxygen to their cells as is physically possible. If the
person’s exercise rate exceeds their capability of supplying oxygen, then at
least some of the glucose entering into cell respiration will follow the anaerobic
pathway called lactic acid fermentation.
Once again, remember that glycolysis is used by all cells to begin the cell
respiration sequence. Also remember that glycolysis:
 Takes place in the cytoplasm
 Results in the net gain of 2 x ATP per glucose
 Results in 2 x pyruvate
Cells that are aerobic normally take the two pyruvate molecules and further
metabolize them in an aerobic series of reactions (covered in HL). In a cell that
is not receiving sufficient oxygen for the aerobic pathway, it will instead enter
the anaerobic pathway. In low-oxygen situations, excess pyruvate molecules are
converted to lactate. Like pyruvate, lactic acid molecules are 3C so there is NO
production of CO2.
What benefit does this serve?
It allows glycolysis to continue with the small gain of ATP generated.
The conversion of pyruvate to lactic acid is reversible and so the lactate can be
converted back to pyruvate to enter the aerobic pathway when oxygen is
available.
Alcoholic Fermentation
Yeast is a common single-celled fungus that uses alcoholic fermentation for ATP
generation. Remember that all organisms use glycolysis to begin the cell
respiration sequence. Yeast cells take glucose from their environment and
generate a net gain of 2 x ATP by way of glycolysis. The product of glycolysis is
2 x pyruvate molecules. Yeast then converts both of the 3C pyruvates to
molecules of ethanol. Ethanol is 2C, so a carbon atom in ‘lost’ in this conversion.
The ‘lost’ carbon atom is given off in a carbon dioxide molecule. Both the ethanol
and carbon dioxide that are produced are waste produces to the yeast and are
simply given off into the environment.
Bakers; yeast is added to bread products for baking as the generation of carbon
dioxide helps the dough to rise. It is also common to use yeast in the production
of ethanol as drinking alcohol.
3.7.4 Aerobic Respiration
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3.7.4 Explain that, during aerobic cell respiration, pyruvate can be broken down
in the mitochondrion into carbon dioxide and water with a large yield of ATP.
Orange book  pg. 86
Green book  pg. 47
To do:
Answer the following question to make a summary in you greenbooks”
1. Where is the pyruvate from glycolysis further oxidized?
2. What are the end products of aerobic respiration?
3. Explain why aerobic respiration releases more energy than anaerobic.
4. Find out the total number of ATP produced in Aerobic respiration compared
to Anaerobic.
Draw a simple flow diagram to show what happens to pyruvate in the Link
Reaction and the Krebs cycle.
Aerobic cell respiration is the most efficient pathway
Cells that have mitochondria use an aerobic pathway for cell respiration.
The diagram below is a high-resolution, false-colour SEM showing a single
mitochondrion. Any cell containing mitochondria uses aerobic cell respiration as
its primary cell respiration pathway when oxygen is available.
Aerobic respiration starts with glycolysis and thus a net gain of 2 x ATP is
generated as well as 2 x pyruvate.
The 2 x pyruvate molecules now enter a mitochondria and are further
metabolized.
Link Reaction
 Each pyruvate first loses a carbon dioxide molecule and becomes known as
acetyl (2C). It is picked up by Co-Enzyme A so the resulting molecule is
called acetyl-CoA.

Each acetyl-CoA carries the acetyl to the mitochondria and the acetyl (2C)
enters the Krebs cycle.
Krebs Cycle

The acetyl (2C) is picked up by a 4C molecule to form a 6C it then loses CO2
to form a 5C and loses an addition CO2 to form the 4C again.

Hence two more CO2 molecules are produced from each original pyruvate.

It is a cycle as the 4C molecule in the Kreb cycle goes round in a circle
undergoing changes and eventually picking up the acetyl (2C) molecule again
as it enters the cycle.

Some ATP is directly generated during the Krebs cycle.
Outcome
Aerobic respiration completely oxidizes (breaks down) a glucose molecule and
the end-products are carbon dioxide and water.
The reason aerobic respiration is so much more efficient than anaerobic
respiration is that in anaerobic respiration is due to glucose not being
completely broken down.