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Section Outline
Section 9-2
90% of the chemical energy that was available in glucose is still
locked in the electrons of pyruvic acid
9–2 The Krebs Cycle and Electron Transport
A.The Krebs Cycle (citric acid cycle)
During the Krebs cycle, pyruvic acid is broken down into carbon
dioxide in a series of energy-extracting reactions.
Step 1 pyruvic acid enters the mitochondrion
one carbon atom becomes part of a molecule of CO2
the two remaining carbons are joined to coenzyme A to
form acetyl-CoA
Acetyl-CoA adds the 2 carbon acetyl group to a 4 carbon
molecule producing citric acid
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Step 2 Citric acid broken down into a 4-carbon molecule,
more carbon dioxide is released and electrons are
transferred to energy carriers.
For each pyruvic acid an ATP is produced, 5 carrier
molecules including 4 NADH molecules are
produced,
and one FADH2 is produced
Figure 9–6 The Krebs Cycle
Section 9-2
Citric Acid
Production
Mitochondrion
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Figure 9–6 The Krebs Cycle
Section 9-2
Citric Acid
Production
Mitochondrion
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B. Electron Transport
The electrons are passed from NADH and FADH2 to the electron
transport chain.
The electron transport chain uses the high energy electrons from
the Kreb’s cycle to convert ADP into ATP.
Step 1 High-energy electrons from NADH and FADH2 are passed
into and along the electron transport chain
The electron transport chain is composed of a series of carrier
proteins that is located in the inner membrane of the
mitochondrion
Oxygen is the final electron acceptor. Oxygen is essential for
getting rid of low-energy electrons and hydrogen ions.
Step 2: Every time 2 high-energy electrons transport down the
electron transport chain, their energy is used to transport
hydrogen ions across the membrane
Hydrogen ions build up in the intermembrane space
Step 3: The inner membranes of the mitochondria contain protein
spheres called ATP synthases
As Hydrogen ions escape through channels into these
proteins, the ATP synthases spin
Each time it rotates the enzyme attaches an ADP and a
phosphate forming high-energy ATP
Note* Each pair of high energy electrons that moves down the electron
transport chain provides enough energy to convert 3 ADP to 3 ATP
molecules.
Figure 9–7 Electron Transport
Chain
Section 9-2
Electron Transport
Hydrogen Ion Movement
Channel
Mitochondrion
Intermembrane
Space
ATP synthase
Inner
Membrane
Matrix
ATP Production
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C. The Totals
Glycolysis produces just 2ATP per glucose
Krebs Cycle and Electron Transport Chain produce 34 ATP
These 36 ATP molecules represent about 38% of the total
energy of glucose.The rest is released as heat, which is one
of the reasons you body feels warmer after exercise.
Flowchart
Section 9-2
Cellular Respiration
Glucose
(C6H1206)
+
Oxygen
(02)
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Glycolysis
Krebs
Cycle
Electron
Transport
Chain
Carbon
Dioxide
(CO2)
+
Water
(H2O)
D. Energy and Exercise
1.
Quick Energy
Cells normally contain small amounts of ATP
produced during glycolysis and cellular respiration
The muscles of the runners contain only enough of this ATP
for a few seconds of intense activity
The store of ATP is used up quickly and their muscle cells
are producing most of their ATP by lactic acid
fermentation.
To get rid of lactic acid in the muscle a chemical pathway
that requires extra oxygen is used.
2.
Long-Term Energy
Cellular respiration is the only way to generate a
continuing supply of ATP because it releases energy
more slowly than fermentation.
The body stores energy in muscle and other tissues in
the form of the carbohydrate glycogen.
Enough to last for 15 to 20 minutes of activity.
The body will then begin to break down other stored
molecules including fats, for energy.