TOPIC 3.7: CELL RESPIRATION
Assessment Statements
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3.7.1: Define Cell Respiration
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 respiration,
pyruvate can be broken down in the mitochondrion into
carbon dioxide and water with a large yield of ATP
Cell Respiration is used by all cells to
produce ATP
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Organic Molecules contain energy in their molecular
structures
Each covalent bond in a glucose, amino acid, or
fatty acid represents stored chemical energy
 Burning
wood—release stored chemical energy
 Rapid oxidation is not controlled by enzymes and
results in the breaking of many, many covalent bonds in
a very short period of time
Cell Respiration is used by all cells to
produce ATP

Cells break down (or metabolize) their organic
nutrients by way of slow oxidation
 The
ultimate goal of releasing energy in a controlled
way is to trap the released energy in the form of ATP
molecules
Glycolysis is the first step in the cell
respiration process
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Glucose enters a cell through
the plasma membrane and
floats in the cytoplasm

An enzyme modifies the
glucose slightly, then a second
enzyme modifies this molecule
even more

This is followed by an entire
series of reactions which
ultimately cleave the 6 carbon
glucose into two 3 carbon
molecules (each called pyruvate)
Glycolysis is the first step in the cell
respiration process
 Energy
2
ATPs molecules needed to
begin glycolysis
 4 ATP’s are formed during
glycolysis
 Net Gain of 2 ATP’s
 http://highered.mcgraw-
hill.com/sites/0072507470/
student_view0/chapter25/a
nimation__how_glycolysis_w
orks.html
Some cells use anaerobic respiration
for ATP production

“cell respiration” refers to a variety of biochemical
pathways that can be used to metabolize glucose
 All
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pathways begin with glycolysis
The breakdown of organic molecules for ATP
production in an anaerobic way called fermentation
 Alcoholic
Fermentation
 Lactic Acid Fermentation
Alcoholic Fermentation

All organisms use glycolysis to begin the cell
respiration sequence. Yeast uses Alcoholic
Fermentation!
 Yeast
cells take in glucose from their environment and
generate a net gain of two ATP by way of glycolysis
Alcoholic Fermentation

The organic products of glycolysis are always two
pyruvate molecules.
 Yeast
then converts both of the 3 carbon pyruvate
molecules into ethanol
 The carbon is given off in a carbon dioxide molecule
 Both
the ethanol and carbon dioxide that are produced are
waste products to the yeast and are simply given off into
the environment
Lactic Acid Fermentation
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Sometimes organisms that use aerobic respiration
sometimes find themselves with not enough oxygen
 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
Lactic Acid Fermentation

Pyruvate is converted
into a 3 carbon
molecule called lactic
acid
 No
production of
Carbon Dioxide
 The only benefit is
serves is that it allows
glycolysis to continue
with the small gain of
ATP generated.
Aerobic cell respiration is the most
efficient pathway
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Cells that have mitochondria use an aerobic
pathway for cell respiration
This pathway also begins with glycolysis and thus a
net gain of two ATPs are generated as well as two
pyruvate molecules
 The
two pyruvate molecules now enter a mitochondrion
and are further metabolized
Aerobic cell respiration is the most
efficient pathway

Each pyruvate first loses a carbon dioxide molecule
and becomes a molecule known as acetyl-CoA
 Each
acetyl-CoA enters into a series of reactions called
Krebs Cycle
 During this series of reactions, two more carbon dioxide
molecules are produced from each original pyruvate
that entered
 The Krebs cycle series of reactions is a cycle because
each time it returns to the molecule that once again
reacts with another incoming acetyl-CoA molecule
Aerobic cell respiration is the most
efficient pathway
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The Krebs cycle series of
reactions is a cycle because
each time it returns to the
molecule that once again
reacts with another incoming
acetyl-CoA molecule
Aerobic cell respiration
breaks down a glucose
molecule and the endproducts are carbon dioxide
and water
Aerobic cell respiration is the most
efficient pathway

Aerobic respiration is much
more efficient because it
completely oxidizes the
glucose and does not leave
any by products

Some ATP is directly
generated during the Krebs
cycle and some is indirectly
generated through a later
series of reactions directly
involving oxygen