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Chapter Nine
Cellular Respiration
9-1 Chemical Pathways
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9-1 Chemical Pathways
Cellular Respiration
The process that releases energy by breaking down
glucose and other food molecules in the presence
of oxygen.
The equation for cellular respiration is:
6O2 + C6H12O6 → 6CO2 + 6H2O + Energy
oxygen + glucose → carbon dioxide + water + energy
Food is the source of raw materials and energy for cells.
Both plant & animal cells carry out the final stages of
respiration in the mitochondria.
Animal Cells
Animal
Mitochondrion
Plant
Plant Cells
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Mitochondria Structure
Burning Calories?
• One gram of the sugar glucose (C6H12O6),
when burned in the presence of oxygen,
releases 3811 calories of heat energy.
• A calorie is the amount of energy needed
to raise the temperature of 1 gram of
water 1 degree Celsius.
• Cells don't “burn” glucose. Instead, they
gradually release the energy from
glucose and other food compounds.
Overview of Cellular Respiration
• The process begins with glycolysis.
• If oxygen is present, glycolysis is
followed by the Krebs cycle and the
electron transport chain.
• Glycolysis, the Krebs cycle and the
electron transport chain (ETC) make up a
process called cellular respiration.
• Each of the three stages of cellular
respiration captures some of the chemical
energy available in food molecules and
uses it to produce ATP.
9-1 Chemical Pathways
Overview of Cellular Respiration
Electrons carried in NADH
Electrons carried
in NADH and
FADH2
Pyruvic
acid
Glucose
Glycolysis
Cytoplasm
Mitochondrion
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9-1 Chemical Pathways
Overview of Cellular Respiration
Glycolysis takes place in the cytoplasm & does not
require O2.
The Krebs cycle & electron transport take place in
the mitochondria and do require O2.
Glycolysis
Cytoplasm
Mitochondrion
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Glycolysis
• The process of breaking one molecule of
glucose in half producing two molecules
of pyruvic acid, a 3-carbon compound.
• The process of glycolysis is so fast cells
can produce thousands of ATP molecules
in a few milliseconds.
• Glycolysis does not require oxygen.
Glycolysis
1. The cell uses up 2 ATP molecules to start the reaction.
2. 4 high-energy e- are removed & passed to the electron carrier
NAD+.
3. NAD+ accepts a pair of high-energy e- forming NADH.
4. NADH holds the e- until they are transferred to other molecules.
2 ATP
2 ADP
4 ADP
2NAD+
4 ATP
2
2 Pyruvic
acid
To the electron
transport chain
Fermentation
• When oxygen is not present, glycolysis is
followed by a different pathway. The
combined process of this pathway and
glycolysis is called fermentation.
• Fermentation releases energy from food
molecules by producing ATP in the
absence of oxygen – it is an anaerobic
process.
• The two main types of fermentation are
lactic acid fermentation and alcoholic
fermentation.
Fermentation
• During fermentation, cells convert
NADH to NAD+ by passing high-energy
electrons back to pyruvic acid.
• This action converts NADH back into
NAD+, and allows glycolysis to continue
producing a steady supply of ATP.
Alcoholic Fermentation
• Yeasts and a few other microorganisms
use alcoholic fermentation, forming ethyl
alcohol and carbon dioxide as wastes.
• The equation for alcoholic fermentation
after glycolysis is:
pyruvic acid + NADH → alcohol + CO2 + NAD+
Lactic Acid Fermentation
• In many cells, pyruvic acid that
accumulates as a result of glycolysis can
be converted to lactic acid.
• This type of fermentation is called lactic
acid fermentation.
• The equation for lactic acid fermentation
after glycolysis is:
pyruvic acid + NADH → lactic acid + NAD+
Fermentation
• The first part of the equation is glycolysis:
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Fermentation
• The second part of the equation shows conversion of pyruvic acid to
lactic acid.
• NADH holds the e- until they can be transferred to other
molecules and NAD+ helps pass energy to other pathways in cell.
• It regenerates NAD+ so glycolysis can continue.
9-2
The Krebs Cycle and Electron Transport
• The Krebs (Citric Acid) Cycle is the second
stage of cellular respiration.
• In the presence of oxygen, pyruvic acid
from glycolysis enters the mitochondrion
and passes to the Krebs cycle.
• During the Krebs cycle, pyruvic acid is
broken down into CO2 in a series of energyextracting reactions.
• Because O2 is required it is aerobic.
9-2 The Krebs Cycle and
Electron Transport
The Krebs Cycle
The Krebs cycle
begins when pyruvic
acid produced by
glycolysis enters the
mitochondrion.
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9-2 The Krebs Cycle and
Electron Transport
The Krebs Cycle
One carbon molecule
is removed, forming
CO2, and electrons
are removed,
changing NAD+ to
NADH.
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9-2 The Krebs Cycle and
Electron Transport
The Krebs Cycle
Coenzyme A joins
the 2-carbon
molecule, forming
acetyl-CoA.
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9-2 The Krebs Cycle and
Electron Transport
The Krebs Cycle
Acetyl-CoA then
adds the 2-carbon
acetyl group to a 4carbon compound,
forming citric acid.
Citric acid
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9-2 The Krebs Cycle and
Electron Transport
The Krebs Cycle
Citric acid is broken down into a 5-carbon
compound, then into a 4-carbon compound.
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9-2 The Krebs Cycle and
Electron Transport
The Krebs Cycle
Two more molecules of CO2 are released and
electrons join NAD+ and FAD, forming NADH and
FADH2.
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9-2 The Krebs Cycle and
Electron Transport
The Krebs Cycle
In addition, one molecule of ATP is generated.
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The Krebs Cycle
• The energy tally from 1 molecule of
pyruvic acid is
– 4 NADH
– 1 FADH2
– 1 ATP
• These molecules carry high-energy e-.
• In the presence of oxygen, these e- can
be used to generate huge amounts of ATP
along the electron transport chain (ETC).
Electron Transport in Respiration
1. The ETC uses the high-energy e- from
the Krebs cycle to convert ADP into ATP.
9-2 The Krebs Cycle and
Electron Transport
Electron Transport
2. High-energy e- from NADH and FADH2 are
passed along the ETC from one carrier protein to
the next.
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9-2 The Krebs Cycle and
Electron Transport
Electron Transport
3. At the end of the chain, an enzyme combines
these e- with H+ ions and O2 to form water.
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9-2 The Krebs Cycle and
Electron Transport
Electron Transport
4. As the final e- acceptor of the ETC, oxygen gets
rid of the low-energy e- and H+ ions.
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9-2 The Krebs Cycle and
Electron Transport
Electron Transport
5. When 2 high-energy e- move down the ETC,
their energy is used to move H+ ions across the
membrane.
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9-2 The Krebs Cycle and
Electron Transport
Electron Transport
6. H+ ions build up in the intermembrane space, so
it is positively charged.
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9-2 The Krebs Cycle and
Electron Transport
Electron Transport
7. The other side of the membrane is now
negatively charged.
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9-2 The Krebs Cycle and
Electron Transport
Electron Transport
8. The inner membranes of the mitochondria contain
proteins called ATP synthases.
ATP
synthase
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9-2 The Krebs Cycle and
Electron Transport
ElectronTransport
9. As H+ ions escape through protein channels, the
ATP synthase spins.
Channel
ATP
synthase
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9-2 The Krebs Cycle and
Electron Transport
Electron Transport
10. As it rotates, the enzyme grabs ADP and
attaches a phosphate, forming high-energy ATP.
Channel
ATP
synthase
ATP
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11. On average, each pair of high-energy
electrons that moves down the ETC
provides enough energy to produce
3 ATP molecules.
The Totals
• Glycolysis produces just 2 ATP per
glucose molecule.
• The complete breakdown of glucose
through cellular respiration, including
glycolysis, results in the production of
36 molecules of ATP.
9-2 The Krebs Cycle and
Electron Transport
The Totals
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9-2 The Krebs Cycle and
Electron Transport
Comparing Photosynthesis and
Cellular Respiration
Comparing Photosynthesis and Cellular
Respiration
The energy flows in photosynthesis and cellular
respiration take place in opposite directions.
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Comparing Photosynthesis and
Cellular Respiration
• On a global level, photosynthesis and
cellular respiration are also opposites.
– Photosynthesis removes CO2 from the
atmosphere and cellular respiration puts it
back.
– Photosynthesis releases O2 into the
atmosphere and cellular respiration uses
that oxygen to release energy from food.