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Chapter 6
How Cells Harvest Chemical Energy
PowerPoint Lectures
Campbell Biology: Concepts & Connections, Eighth Edition
REECE • TAYLOR • SIMON • DICKEY • HOGAN
© 2015 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Introduction
• Oxygen is a reactant in cellular respiration, the
process that breaks down sugar and other food
molecules and generates ATP, the energy currency
in cells, and heat.
• Brown fat has a “short circuit” in its cellular
respiration, which generates only heat, not ATP.
• Brown fat is important for heat production in small
mammals, including humans.
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CELLULAR RESPIRATION:
AEROBIC HARVESTING OF ENERGY
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Photosynthesis and cellular respiration
provide energy for life
• Life requires energy.
• In almost all ecosystems, energy ultimately comes
from the sun.
• In photosynthesis,
• some of the energy in sunlight is captured by
chloroplasts,
• atoms of carbon dioxide and water are rearranged,
and
• sugar and oxygen are produced.
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Photosynthesis and cellular respiration
provide energy for life
• In cellular respiration,
• sugar is broken down to carbon dioxide and water
and
• the cell captures some of the released energy to
make ATP.
• Cellular respiration takes place in the mitochondria
of eukaryotic cells.
• In these energy conversions, some energy is lost
as heat.
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Figure 6.1
Sunlight
energy
ECOSYSTEM
CO2 + H2O
Photosynthesis in
chloroplasts Organic
Cellular
respiration in
mitochondria
ATP
Heat
energy
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molecules
+ O2
ATP powers most
cellular work
Breathing supplies O2 for use in cellular
respiration and removes CO2
• Respiration, as it relates to breathing, and cellular
respiration are not the same.
• Respiration, in the breathing sense, refers to an
exchange of gases. Usually an organism brings in
oxygen from the environment and releases waste
CO2.
• Cellular respiration is the aerobic (oxygen-requiring)
harvesting of energy from food molecules by cells.
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Figure 6.2-0
O2
Breathing
CO2
Lungs
O2
Transported in
bloodstream
CO2
Muscle cells carrying out
Cellular Respiration
Glucose + O2 ➞ CO2 + H2O + ATP
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Cellular respiration banks energy in ATP
molecules
• Cellular respiration is an exergonic (energyreleasing) process that transfers energy from the
bonds in glucose to form ATP.
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Cellular respiration banks energy in ATP
molecules
• Cellular respiration
• can produce up to 32 ATP molecules for each
glucose molecule,
• uses about 34% of the energy originally stored in
glucose, and
• releases the other 66% as heat.
• This energy conversion efficiency is better than
most energy conversion systems.
• Only about 25% of the energy in gasoline produces
the kinetic energy of movement.
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C6H12O6
6 O2
6 CO2
Glucose
Oxygen
Carbon
dioxide
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6 H2O
Water
ATP
Heat
The human body uses energy from ATP for
all its activities
• Your body requires a continuous supply of energy
just to stay alive—to keep your heart pumping and
you breathing.
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• A kilocalorie (kcal) is
• the quantity of heat required to raise the
temperature of 1 kilogram (kg) of water by 1C,
• the same as a food Calorie, and
• used to measure the nutritional values indicated on
food labels.
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• The average adult human needs about 2,200 kcal
of energy per day.
• About 75% of these calories is used to maintain a
healthy body.
• The remaining 25% is used to power physical
activities.
• A balance of energy intake and expenditure is
required to maintain a healthy weight.
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Activity
kcal consumed per hour
by a 67.5-kg (150-lb) person*
Running (8–9 mph)
979
Dancing (fast)
510
Bicycling (10 mph)
490
Swimming (2 mph)
408
Walking (4 mph)
341
Walking (3 mph)
245
Dancing (slow)
Driving a car
Sitting (writing)
204
61
28
*Not including kcal needed for
body maintenance
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Cells capture energy from electrons “falling”
from organic fuels to oxygen
• How do your cells extract energy from glucose?
• The answer involves the transfer of electrons
during chemical reactions.
Cells capture energy from electrons “falling”
from organic fuels to oxygen
• During cellular respiration,
• electrons are transferred from glucose to oxygen
and
• energy is released.
• Oxygen attracts electrons very strongly.
• An electron loses potential energy when it is
transferred to oxygen.
6.5 Cells capture energy from electrons
“falling” from organic fuels to oxygen
• Energy can be released from glucose by simply
burning it.
• This electron “fall” happens very rapidly.
• This energy is dissipated as heat and light and is
not available to living organisms.
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• Cellular respiration is a more controlled descent of
electrons and like rolling down an energy hill.
• Energy is released in small amounts and can be
stored in the chemical bonds of ATP.
• The movement of electrons from one molecule to
another is an oxidation-reduction reaction, or
redox reaction. In a redox reaction,
• the loss of electrons from one substance is called
oxidation,
• the addition of electrons to another substance is
called reduction,
• a molecule is oxidized when it loses one or more
electrons, and
• a molecule is reduced when it gains one or more
electrons.
Oxidation States of Carbon
-4
+4
Highest Energy
Least Stable
Lowest Energy
Most Stable
In Respiration, Carbon Carbon is Oxidized from its highest energy to a lower one.
The energy coming out is eventually trapped and held in the cells as ATP. ATP
provides this energy to run all of life’s processes.
In Fats, most of the carbon atoms are at the -4 level. In Sugars and starches, they
are in the -2 or 0 level.
Cells capture energy from electrons “falling”
from organic fuels to oxygen
• A cellular respiration equation is helpful to show
the changes in hydrogen atom distribution.
• Glucose loses its hydrogen atoms and becomes
oxidized to CO2.
• Oxygen gains hydrogen atoms and becomes
reduced to H2O.
Loss of hydrogen atoms
(becomes oxidized)
C6H12O6 + 6 O2
(Glucose)
6 CO2 + 6 H2O + ATP + Heat
Gain of hydrogen atoms
(becomes reduced)
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• An important player in the process of oxidizing
glucose is a coenzyme called NAD+, which
• accepts electrons and
• becomes reduced to NADH.
Becomes oxidized
+2H
NAD+
+ 2H
Becomes reduced
2 H+ + 2
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NADH
(carries)
2 electrons)
H+
• NADH delivers electrons to a string of electron
carrier molecules, which moves electrons down a
hill.
• These carrier molecules constitute an electron
transport chain.
• At the bottom of the hill is oxygen (1/2 O2), which
• accepts two electrons,
• picks up two H+, and
• becomes reduced to water.
NAD+
NADH
2
Energy released
and available
for making
ATP
2
1
−
2
O2
H2O
2 H+
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STAGES OF CELLULAR
RESPIRATION
Cellular respiration occurs in three main
stages
• Cellular respiration consists of a sequence of steps
that can be divided into three stages.
• Stage 1: Glycolysis
• Stage 2: Pyruvate oxidation and the citric acid cycle
• Stage 3: Oxidative phosphorylation
Cellular respiration occurs in three main
stages
• Stage 1: Glycolysis
• occurs in the cytosol,
• begins cellular respiration, and
• breaks down glucose into two molecules of a threecarbon compound called pyruvate.
Cellular respiration occurs in three main
stages
• Stage 2: Pyruvate oxidation and the citric acid
cycle
• take place in mitochondria,
• oxidize pyruvate to a two-carbon compound, and
• supply the third stage with electrons.
• The cell makes a small amount of ATP during
glycolysis and the citric acid cycle.
Cellular respiration occurs in three main
stages
• Stage 3: Oxidative phosphorylation
• NADH and a related electron carrier, FADH2, shuttle
electrons to an electron transport chain embedded
in the inner mitochondrial membrane.
• Most ATP produced by cellular respiration is
generated by oxidative phosphorylation, which uses
the energy released by the downhill fall of electrons
from NADH and FADH2 to oxygen to phosphorylate
ADP.
• Stage 3: Oxidative phosphorylation
• As the electron transport chain passes electrons
down the energy hill, it also pumps hydrogen ions
(H+) across the inner mitochondrial membrane, into
the narrow intermembrane space, and produces a
concentration gradient of H+ across the membrane.
• In chemiosmosis, the potential energy of this
concentration gradient is used to make ATP.
Figure 6.6-1
Electrons carried by NADH
Glycolysis
Glucose
Pyruvate
CYTOSOL
ATP
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Pyruvate
Oxidation
Citric Acid
Cycle
FADH2
Oxidative
Phosphorylation
(Electron transport
and chemiosmosis)
MITOCHONDRION
Substrate-level
phosphorylation
Substrate-level
ATP
phosphorylation
ATP
Oxidative
phosphorylation
Glycolysis harvests chemical energy by
oxidizing glucose to pyruvate
• In glycolysis,
• a single molecule of glucose is enzymatically cut in
half through a series of steps,
• two molecules of pyruvate are produced,
• two molecules of NAD+ are reduced to two
molecules of NADH, and
• there is a net gain of two molecules of ATP.
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Glucose
2 ADP
2 NAD+
+2 P
2 NADH
2
ATP
2 Pyruvate
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+2 H+
Glycolysis harvests chemical energy by
oxidizing glucose to pyruvate
• ATP is formed in glycolysis by substrate-level
phosphorylation during which
• an enzyme transfers a phosphate group from a
substrate molecule to ADP and
• ATP is formed.
• The compounds that form between the initial
reactant, glucose, and the final product, pyruvate,
are known as intermediates.
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Enzyme
P
Enzyme
ADP
ATP
P
Substrate
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P
Product
• The steps of glycolysis have two main phases.
• In steps 1–4, the energy investment phase, energy
is consumed as two ATP molecules are used to
energize a glucose molecule, which is then split into
two small sugars.
• In steps 5–9, the energy payoff phase, two NADH
molecules are produced for each initial glucose
molecule and four ATP molecules are generated.
• There is a net gain of two ATP molecules for each
glucose molecule that enters glycolysis.
Glucose
ATP
Steps 1 – 3 Glucose
is energized, using ATP.
Step
ENERGY
INVESTMENT
PHASE
1
ADP
P
Glucose 6-phosphate
P
Fructose 6-phosphate
P
Fructose 1,6-bisphosphate
2
ATP
3
ADP
Step 4 A six-carbon
intermediate splits into
two three-carbon
intermediates.
P
4
P
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P
Glyceraldehyde 3-phosphate
(G3P)
P
Step 5 A redox
reaction generates
NADH.
P
NAD+
NAD+
5
P
NADH
5
P
NADH
+ H+
ENERGY PAYOFF
PHASE
+ H+
P
P
ADP
Steps 6 – 9 ATP
and pyruvate are
produced.
Glyceraldehyde 3-phosphate
(G3P)
P
P 1,3-Bisphosphoglycerate
ADP
6
6
ATP
ATP
P
P
7
3-Phosphoglycerate
7
P
P
2-Phosphoglycerate
8
H2O
P
P
ADP
Phosphoenolpyruvate
(PEP)
ADP
9
9
ATP
8
H2O
ATP
Pyruvate
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6.8 Pyruvate is oxidized in preparation for the
citric acid cycle
• The pyruvate formed in glycolysis is transported
from the cytosol into a mitochondrion where the
citric acid cycle and oxidative phosphorylation will
occur.
• Two molecules of pyruvate are produced for each
molecule of glucose that enters glycolysis.
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• Pyruvate does not enter the citric acid cycle but
undergoes some chemical grooming in which
• a carboxyl group is removed and given off as CO2,
• the two-carbon compound remaining is oxidized
while a molecule of NAD+ is reduced to NADH, and
• coenzyme A joins with the two-carbon group to form
acetyl coenzyme A, abbreviated as acetyl CoA.
• Then two molecules of acetyl CoA enter the citric
acid cycle.
NAD+
NADH + H+
2
CoA
Pyruvate
Acetyl coenzyme A
1
CO2
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3
Coenzyme A
The citric acid cycle completes the oxidation
of organic molecules, generating many NADH
and FADH2 molecules
• The citric acid cycle
• is also called the Krebs cycle (after the GermanBritish researcher Hans Krebs, who worked out
much of this pathway in the 1930s),
• completes the oxidation of organic molecules, and
• generates many NADH and FADH2 molecules.
Acetyl CoA
CoA
CoA
Citric Acid Cycle
2 CO2
3 NAD+
FADH2
3 NADH
FAD
+ 3 H+
ATP
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ADP + P
The citric acid cycle completes the oxidation
of organic molecules, generating many NADH
and FADH2 molecules
• During the citric acid cycle
• the two-carbon group of acetyl CoA is joined to a
four-carbon compound, forming citrate,
• citrate is degraded back to the four-carbon
compound,
• two CO2 are released, and
• one ATP, three NADH, and one FADH2 are
produced.
The citric acid cycle completes the oxidation
of organic molecules, generating many NADH
and FADH2 molecules
• Remember that the citric acid cycle processes two
molecules of acetyl CoA for each initial glucose.
• Thus, after two turns of the citric acid cycle, the
overall yield per glucose molecule is
• 2 ATP,
• 6 NADH, and
• 2 FADH2.
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The citric acid cycle completes the oxidation
of organic molecules, generating many NADH
and FADH2 molecules
• Thus, after glycolysis and the citric acid cycle, the
cell has gained
• 4 ATP,
• 10 NADH, and
• 2 FADH2.
• To harvest the energy banked in NADH and
FADH2, these molecules must shuttle their highenergy electrons to an electron transport chain.
CoA
Acetyl CoA
CoA
2 carbons enter cycle
Oxaloacetate
Citric Acid Cycle
Step 1
Acetyl CoA stokes
the furnace.
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1
CoA
Acetyl CoA
CoA
2 carbons enter cycle
1
Oxaloacetate
Citrate
NAD+
NADH + H+
2
Citric Acid Cycle
CO2
leaves cycle
Alpha-ketoglutarate
CO2
3
NAD+
Succinate
ADP + P
ATP
Step 1
Acetyl CoA stokes
the furnace.
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Steps 2 – 3
NADH, ATP, and CO2
are generated during
redox reactions.
NADH + H+
leaves cycle
Figure 6.9b-3
CoA
Acetyl CoA
CoA
2 carbons enter cycle
1
Oxaloacetate
Citrate
NADH + H+
NAD+
6
NAD+
NADH + H+
2
Malate
Citric Acid Cycle
CO2
H2O
leaves cycle
5
Alpha-ketoglutarate
Fumarate
FADH2
CO2
4
3
FAD
leaves cycle
NAD+
Succinate
ADP + P
NADH + H+
ATP
Step 1
Acetyl CoA stokes
the furnace.
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Steps 2 – 3
NADH, ATP, and CO2
are generated during
redox reactions.
Steps 4 – 6
Further redox reactions
generate FADH2 and
more NADH.
Most ATP production occurs by oxidative
phosphorylation
• The final stage of cellular respiration is oxidative
phosphorylation, which
• involves electron transport and chemiosmosis and
• requires an adequate supply of oxygen.
• The arrangement of electron carriers built into a
membrane makes it possible to
• create an H+ concentration gradient across the
membrane and then
• use the energy of that gradient to drive ATP
synthesis.
6.10 Most ATP production occurs by
oxidative phosphorylation
• Electrons from NADH and FADH2 travel down the
electron transport chain to O2, the final electron
acceptor.
• Oxygen picks up H+, which forms water.
• Energy released by these redox reactions is used
to pump H+ from the mitochondrial matrix into the
intermembrane space.
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• In chemiosmosis, the H+ diffuses back across the
inner membrane, through ATP synthase
complexes, driving the synthesis of ATP.
Figure 6.10a
OUTER MITOCHONDRIAL MEMBRANE
Intermembrane
space
H+
Protein
complex
of electron
carriers
+
Mobile H
electron
H+
carriers
III
H+
H+
Inner mitochondrial
membrane
H+
H+
H+
Cyt c
Q
I
H+
ATP
synthase
IV
II
Electron
flow
FADH2
NADH
Mitochondrial
matrix
FAD
1
−
O2 + 2 H+
2
NAD+
H+
H2O
Electron Transport Chain
Oxidative Phosphorylation
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ADP + P
ATP
H+
Chemiosmosis
Figure 6.10b
INTERMEMBRANE SPACE
H+
Rotor
Internal
rod
Catalytic
knob
ADP
+
P
MITOCHONDRIAL MATRIX
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ATP
Scientists have discovered heat-producing,
calorie-burning brown fat in adults
• Mitochondria in brown fat can burn fuel and
produce heat without making ATP.
• Ion channels spanning the inner mitochondrial
membrane
• allow H + to flow freely across the membrane and
• dissipate the H+ gradient that the electron transport
chain produced, which does not allow ATP synthase
to make ATP.
• Scientific studies of humans indicate that
• brown fat may be present in most people and
• when activated by cold environments, the brown fat
of lean individuals is more active.
Review: Each molecule of glucose yields
many molecules of ATP
• Recall that the energy payoff of cellular respiration
involves
1.
2.
3.
4.
glycolysis,
alteration of pyruvate,
the citric acid cycle, and
oxidative phosphorylation.
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Review: Each molecule of glucose yields
many molecules of ATP
• The total yield is about 32 ATP molecules per glucose
molecule.
• The number of ATP molecules cannot be stated exactly for
several reasons.
• The NADH produced in glycolysis passes its electrons across
the mitochondrial membrane to either NAD+ or FAD. Because
FADH2 adds its electrons farther along the electron transport
chain, it contributes less to the H+ gradient and thus
generates less ATP.
• Some of the energy of the H+ gradient may be used for work
other than ATP production, such as the active transport of
pyruvate into the mitochondrion.
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CYTOSOL
MITOCHONDRION
2 NADH
Glycolysis
2
Pyruvate
Glucose
6 NADH + 2 FADH2
2 NADH
Pyruvate
Oxidation
2 Acetyl
CoA
Oxidative
Phosphorylation
(electron transport
and chemiosmosis)
Citric Acid
Cycle
O2
Maximum
per glucose:
H2O
+2
ATP
by substrate-level
phosphorylation
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CO2
+2
ATP
by substrate-level
phosphorylation
+ about
28 ATP
About
32 ATP
by oxidative
phosphorylation
FERMENTATION: ANAEROBIC
HARVESTING OF ENERGY
Fermentation enables cells to produce ATP
without oxygen
• Fermentation is a way of harvesting chemical
energy that does not require oxygen. Fermentation
• uses glycolysis,
• produces two ATP molecules per glucose, and
• reduces NAD+ to NADH.
• Fermentation also provides an anaerobic path for
recycling NADH back to NAD+.
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Fermentation enables cells to produce ATP
without oxygen
• Your muscle cells and certain bacteria can
regenerate NAD+ through lactic acid
fermentation, in which
• NADH is oxidized back to NAD+ and
• pyruvate is reduced to lactate.
2 ADP
+2 P
2
ATP
Glycolysis
Glucose
2 NAD+
2 NADH
2 Pyruvate
2 NADH
2 NAD+
2 Lactate
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Fermentation enables cells to produce ATP
without oxygen
• Lactate is carried by the blood to the liver, where it
is converted back to pyruvate and oxidized in the
mitochondria of liver cells.
• The dairy industry uses lactic acid fermentation by
bacteria to make cheese and yogurt.
• Other types of microbial fermentation turn
soybeans into soy sauce and cabbage into
sauerkraut.
• The baking and winemaking industries have used
alcohol fermentation for thousands of years.
• In this process, yeast (single-celled fungi)
• oxidize NADH back to NAD+ and
• convert pyruvate to CO2 and ethanol.
2 ADP
+2 P
2
ATP
Glycolysis
Glucose
2 NAD+
2 NADH
2 Pyruvate
2 NADH
2 CO2
2 NAD+
2 Ethanol
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• Obligate anaerobes
• require anaerobic conditions,
• are poisoned by oxygen, and
• live in stagnant ponds and deep soils.
• Facultative anaerobes
• can make ATP by fermentation or oxidative
phosphorylation and
• include yeasts and many bacteria.
Figure 6.13c-1
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Glycolysis evolved early in the history of life
on Earth
• Glycolysis is the universal energy-harvesting
process of life.
• The role of glycolysis in fermentation and
respiration dates back to life long before oxygen
was present, when only prokaryotes inhabited the
Earth, about 3.5 billion years ago.
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Glycolysis evolved early in the history of life
on Earth
• The ancient history of glycolysis is supported by its
• occurrence in all the domains of life and
• location within the cell, using pathways that do not
involve any membrane-enclosed organelles of the
eukaryotic cell.
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CONNECTIONS BETWEEN
METABOLIC PATHWAYS
Cells use many kinds of organic molecules
as fuel for cellular respiration
• Although glucose is considered to be the primary
source of sugar for respiration and fermentation,
ATP is generated using
• carbohydrates,
• fats, and
• proteins.
Cells use many kinds of organic molecules
as fuel for cellular respiration
• Fats make excellent cellular fuel because they
• contain many hydrogen atoms and thus many
energy-rich electrons and
• yield more than twice as much ATP per gram as a
gram of carbohydrate.
• Proteins can also be used for fuel, although your
body preferentially burns sugars and fats first.
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Food, such as
peanuts
Fats
Carbohydrates
Sugars
Proteins
Glycerol Fatty acids
Amino acids
Amino
groups
Glucose
G3P
Glycolysis
Pyruvate
Acetyl
CoA
ATP
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Citric
Acid
Cycle
Oxidative
Phosphorylation
Organic molecules from food provide raw
materials for biosynthesis
• A cell must be able to make its own molecules to
• build its structures and
• perform its functions.
• Food provides the raw materials your cells use for
biosynthesis, the production of organic molecules,
using energy-requiring metabolic pathways.
Figure 6.16-1
ATP needed
to drive
biosynthesis
Citric
Acid
Cycle
ATP
Acetyl
CoA
Glucose Synthesis
Pyruvate
G3P
Glucose
Amino
groups
Amino acids
Proteins
Fatty Glycerol
acids
Fats
Cells, tissues,
organisms
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Sugars
Carbohydrates
Organic molecules from food provide raw
materials for biosynthesis
• Metabolic pathways are often regulated by
feedback inhibition in which an accumulation of
product suppresses the process that produces the
product.
REVIEW
You should now be able to
1. Compare the processes and locations of cellular
respiration and photosynthesis.
2. Explain how breathing and cellular respiration
are related.
3. Provide the overall chemical equation for cellular
respiration.
4. Explain how the human body uses its daily
supply of ATP.
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You should now be able to
5. Explain how the energy in a glucose molecule is
released during cellular respiration.
6. Explain how redox reactions are used in cellular
respiration.
7. Describe the general roles of dehydrogenase,
NADH, and the electron transport chain in
cellular respiration.
8. Compare the reactants, products, and energy
yield of the three stages of cellular respiration.
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You should now be able to
9. Describe the special function of brown fat.
10. Compare the reactants, products, and energy
yield of alcohol and lactic acid fermentation.
11. Distinguish between strict anaerobes and
facultative anaerobes.
12. Explain how carbohydrates, fats, and proteins
are used as fuel for cellular respiration.
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Figure 6.UN01
C6H12O6
6 O2
6 CO2
Glucose
Oxygen
Carbon
dioxide
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6 H2O
Water
ATP
+ Heat
Figure 6.UN02
Electrons carried by NADH
Glycolysis
Glucose
Pyruvate
Pyruvate
Oxidation
Citric Acid
Cycle
FADH2
Oxidative
Phosphorylation
(Electron transport
and chemiosmosis)
CYTOSOL MITOCHONDRION
ATP
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Substrate-level
phosphorylation
ATP
Substrate-level
phosphorylation
ATP
Oxidative
phosphorylation
Figure 6.UN03
Cellular
respiration
generates
has three stages
oxidizes
uses
ATP
energy for
produce
some
produces
many
glucose and
organic fuels
(a)
C6H12O6
(b)
(d)
to pull
electrons down
(c)
cellular work
by a process called
chemiosmosis
(f)
uses
H+ diffuse
through
(e)
ATP synthase
uses
pumps H+ to create
H+ gradient
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(g)
to