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Chapter 6
How Cells Release Energy
Snake © Gunter Ziesler/Photoshot
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Cells Use Energy in Food to Make ATP
Every organism requires a steady
food supply to survive.
All plants and animals, as well as many
microbes, use food (such as glucose) and
oxygen gas to produce ATP, an energy
carrier used to power cell activities.
Section 6.1
Bluebird: © Getty Images/Purestock RF
Cells Use Energy in Food to Make ATP
The process of using glucose and oxygen to
produce ATP is called aerobic respiration.
C6H12O6 + 6O2
6CO2 + 6H2O + 36ATP
(Glucose)
Section 6.1
Bluebird: © Getty Images/Purestock RF
Cellular Respiration Is Linked to Breathing
Inhaled oxygen is consumed
in cellular respiration.
Carbon dioxide, produced as
a byproduct, is then exhaled.
The cell uses the ATP formed
during cellular respiration to
do work, such as muscle
contraction.
Section 6.1
Mitochondrion: © Thomas Deerinck, NCMIR/Science Source
Figure 6.1
Clicker Question #1
Do plants carry out cellular respiration?
A. No, photosynthesis has the same function
in plants as respiration has in animals and
microbes.
B. No, their energy needs are too small to
require respiration.
C. Yes, they require ATP like other living
things, and respiration generates ATP.
D. Yes, they require cellular respiration as a
way to get rid of extra CO2.
Flower: © Doug Sherman/Geofile/RF
Clicker Question #1
Do plants carry out cellular respiration?
A. No, photosynthesis has the same function
in plants as respiration has in animals and
microbes.
B. No, their energy needs are too small to
require respiration.
C. Yes, they require ATP like other living
things, and respiration generates ATP.
D. Yes, they require cellular respiration as a
way to get rid of extra CO2.
Flower: © Doug Sherman/Geofile/RF
Cellular Respiration Occurs in Three Stages
ATP synthesis requires
energy input. Cellular
respiration releases energy
from glucose in several
steps.
During glycolysis, glucose is
split into two three-carbon
molecules of pyruvate.
The pyruvate molecules
then enter a mitochondrion,
where they are
disassembled into carbon
dioxide molecules during the
Krebs cycle.
Section 6.2
Figure 6.2
Cellular Respiration Occurs in Three Stages
Glycolysis and the Krebs
cycle transfer some of the
potential energy in glucose
to ATP. Meanwhile, electrons
are transferred to NADH and
FADH2.
NADH and FADH2 unload
electrons at the electron
transport chain, where the
potential energy in the
electrons is used to produce
more ATP.
Section 6.2
Figure 6.2
Clicker Question #2
What happens to glucose’s carbon atoms
during the overall process of aerobic
respiration?
A. They are donated to O2.
B. They remain in the pyruvate molecules.
C. They become part of ATP.
D. They are released as CO2.
Flower: © Doug Sherman/Geofile/RF
Clicker Question #2
What happens to glucose’s carbon atoms
during the overall process of aerobic
respiration?
A. They are donated to O2.
B. They remain in the pyruvate molecules.
C. They become part of ATP.
D. They are released as CO2.
Flower: © Doug Sherman/Geofile/RF
Mitochondria Produce Most ATP
Many of the reactions of
cellular respiration occur in
mitochondria.
Mitochondria have two
phospholipid bilayers: an
outer membrane and an
inner membrane.
Section 6.3
Figure 6.3
Mitochondria Produce Most ATP
Between the mitochondrial
membranes is an
intermembrane
compartment.
The space within the inner
membrane is the
mitochondrial matrix, which
houses the reactions of the
Krebs cycle.
Section 6.3
Figure 6.3
Clicker Question #3
Where is the mitochondrial matrix?
A. Outside the outer membrane
B. Between the inner and outer membranes
C. Inside the inner membrane
Flower: © Doug Sherman/Geofile/RF
Clicker Question #3
Where is the mitochondrial matrix?
A. Outside the outer membrane
B. Between the inner and outer membranes
C. Inside the inner membrane
Flower: © Doug Sherman/Geofile/RF
Glycolysis Splits Glucose
Glycolysis occurs outside
of the mitochondrion, in
the cytoplasm.
During glycolysis, a
glucose molecule is split
into two three-carbon
pyruvate molecules.
The enzymes of glycolysis
extract some of the
potential energy stored in
glucose. The process yields
two ATP molecules and
two electron-carrying
NADH molecules.
Section 6.4
Figure 6.4
Glycolysis Splits Glucose
Glycolysis requires an
input of two ATP to
“activate” glucose.
Section 6.4
Figure 6.4
Glycolysis Splits Glucose
The activated glucose is
then split into two
3-carbon molecules.
Section 6.4
Figure 6.4
Glycolysis Splits Glucose
Each of the 3-carbon
molecules proceeds to
the energy extraction
reactions of glycolysis.
Section 6.4
Figure 6.4
Glycolysis Splits Glucose
First, each 3-carbon
molecule is oxidized,
producing two NADH
molecules.
Section 6.4
Figure 6.4
Glycolysis Splits Glucose
Then, each 3-carbon
molecule donates its
phosphate groups to
ADP molecules,
producing ATP
molecules via
substrate-level
phosphorylation.
Section 6.4
Figure 6.4
Glycolysis Splits Glucose
In substrate-level phosphorylation, an enzyme transfers a
phosphate from a molecule to ADP.
Section 6.4
Figure 6.15
Glycolysis Splits Glucose
In total, four ATP are
produced. Recall that
two ATP were used to
start the reactions. The
net yield is two ATP.
Section 6.4
Figure 6.4
Glycolysis Splits Glucose
Note that these
reactions do not
require oxygen.
Glycolysis can therefore
occur in anaerobic
conditions.
Section 6.4
Figure 6.4
Glycolysis Splits Glucose
Glycolysis yields two ATP
molecules, two
electron-carrying NADH
molecules, and two
pyruvates.
Section 6.4
Figure 6.4
Glycolysis Splits Glucose
Each glycolysis molecule has a name.
Section 6.4
Figure 6.4
Clicker Question #4
If 8 glucose molecules enter glycolysis, the
net products will be ____ pyruvate
molecules and ____ ATP molecules.
A. 2 … 2
B. 4 … 4
C. 8 … 8
D. 16 … 16
Flower: © Doug Sherman/Geofile/RF
Clicker Question #4
If 8 glucose molecules enter glycolysis, the
net products will be ____ pyruvate
molecules and ____ ATP molecules.
A. 2 … 2
B. 4 … 4
C. 8 … 8
D. 16 … 16
Flower: © Doug Sherman/Geofile/RF
Aerobic Respiration Yields Many ATP
The reactions of Krebs
cycle and the electron
transport chain require
oxygen gas. These
reactions yield much
more ATP than
glycolysis.
Section 6.5
Aerobic Respiration Yields Many ATP
The two pyruvate
molecules produced in
glycolysis undergo an
oxidation reaction as they
enter the mitochondrion
(this is sometimes called
the transition step).
Section 6.5
Figure 6.5
Aerobic Respiration Yields Many ATP
A carbon atom is stripped
from each pyruvate, and
leaves the cell as a carbon
dioxide molecule. At the
same time, NAD+ is
reduced to NADH.
Through this process,
each pyruvate molecule
is converted to an
acetyl CoA molecule.
Each acetyl CoA
molecule then enters
the Krebs cycle.
Section 6.5
Figure 6.5
Aerobic Respiration Yields Many ATP
Update figure
During the Krebs cycle, the
two acetyl CoA molecules
are oxidized, yielding
4 CO2, 2 ATP, 6 NADH, and
2 FADH2.
Section 6.5
Figure 6.5
Aerobic Respiration Yields Many ATP
The Krebs cycle occurs in
several steps.
Acetyl CoA combines
with a 4-carbon
molecule, yielding
citrate.
Section 6.5
Figure 6.6
Aerobic Respiration Yields Many ATP
Citrate is then rearranged
and oxidized, yielding
3 NADH, 1 FADH2, and
1 ATP per turn. The ATP is
produced via substratelevel phosphorylation.
Section 6.5
Figure 6.6
Aerobic Respiration Yields Many ATP
The original four-carbon
molecule is re-created,
and the cycle starts
anew.
Section 6.5
Figure 6.6
Aerobic Respiration Yields Many ATP
Glycolysis
Acetyl CoA
formation
Krebs cycle
So far, aerobic respiration of one glucose molecule has yielded
only four ATP.
But 10 NADH molecules have been produced, as well as two FADH2.
Section 6.5
Aerobic Respiration Yields Many ATP
NADH and FADH2 donate their electrons to the
electron transport chain, where energy from the electrons is
used to produce many ATP.
Section 6.5
Figure 6.7
Aerobic Respiration Yields Many ATP
As electrons travel through the transport chain, carrier molecules
use the potential energy of the electrons to transport hydrogen
ions into the intermembrane compartment.
Section 6.5
Figure 6.7
Aerobic Respiration Yields Many ATP
At the end of the transport chain, electrons are donated to an
oxygen atom, which combines with hydrogens to form water.
Section 6.5
Figure 6.7
Aerobic Respiration Yields Many ATP
The hydrogen ions move down their concentration gradient from
the intermembrane compartment into the matrix through
ATP synthase.
Section 6.5
Figure 6.7
Aerobic Respiration Yields Many ATP
ATP synthase produces ATP via chemiosmotic phosphorylation.
Section 6.5
Figure 6.7
Aerobic Respiration Yields Many ATP
The electron transport chain produces 34 ATP.
Section 6.5
Figure 6.7
Cellular Respiration of One Glucose
Yields 36 ATP
Glycolysis
Acetyl CoA
formation
Krebs cycle
Electron
transport
34
Section 6.5
Cellular Respiration of One Glucose
Yields 36 ATP
Glycolysis and Krebs cycle each
produce 2 ATP, and the electron
transport chain produces 34 ATP.
Transporting NADH into the
mitochondrion requires 2 ATP,
making the total production of
ATP equal to 36.
Section 6.6
Figure 6.8
Other Food Molecules Enter the
Energy-Extracting Pathways
Proteins and fats are also used as
energy sources for the cell.
These molecules enter the
energy-extracting pathways and
produce ATP.
Section 6.7
Avocado: © Digital Vision/Getty Images RF
Figure 6.9
Fermentation Generates ATP Only
in Glycolysis
Organisms produce ATP in the
absence of oxygen, as well.
Glycolysis produces ATP and
does not require oxygen.
However, glycolysis does require
NAD+, which is re-created in the
electron transport chain of cells
undergoing respiration.
Section 6.8
Figure 6.10
Fermentation Generates ATP Only
in Glycolysis
In the absence of oxygen, a cell
can re-create NAD+ other
pathways, called anaerobic
respiration and fermentation.
In anaerobic respiration, NADH
donates is oxidized at an
electron transport chain that
uses electron acceptor
molecules other than O2.
Fermentation uses pyruvate to
oxidize NADH, producing alcohol,
lactic acid, or other byproducts.
Section 6.8
Figure 6.10
Fermentation Generates ATP Only
in Glycolysis
In alcoholic fermentation, NADH
reduces pyruvate to ethanol.
NAD+ is re-created.
Section 6.8
Beer: © Adam Woolfitt/Corbis; Yogurt: © Scimat/Science Source
In lactic acid fermentation,
NADH reduces pyruvate to lactic
acid. NAD+ is re-created.
Figure 6.11
Fermentation Generates ATP Only
in Glycolysis
During fermentation,
oxidation of a glucose
molecule yields only 2 ATP.
Section 6.8
Beer: © Adam Woolfitt/Corbis; Yogurt: © Scimat/Science Source
Figure 6.11
Clicker Question #5
What is the main advantage of fermentation
over aerobic cellular respiration?
A. Fermentation generates ATP even if O2 is
not present.
B. Fermentation generates more ATP per
glucose than aerobic cellular respiration.
C. Fermentation does not generate toxic
byproducts such as CO2.
D. Fermentation gets rid of pyruvate, which
would otherwise accumulate in the cell.
Flower: © Doug Sherman/Geofile/RF
Clicker Question #5
What is the main advantage of fermentation
over aerobic cellular respiration?
A. Fermentation generates ATP even if O2 is
not present.
B. Fermentation generates more ATP per
glucose than aerobic cellular respiration.
C. Fermentation does not generate toxic
byproducts such as CO2.
D. Fermentation gets rid of pyruvate, which
would otherwise accumulate in the cell.
Flower: © Doug Sherman/Geofile/RF
Photosynthesis and Respiration
Are Related
Section 6.9