1. Chapter 6 How Cells Harvest Chemical Energy
In eukaryotes, cellular respiration
harvests energy from food,
yields large amounts of ATP, and
Uses ATP to drive cellular work.
A similar process takes place in many prokaryotic organisms.
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2. CELLULAR RESPIRATION: AEROBIC HARVESTING OF ENERGY
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3. 6.1 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
glucose and oxygen are produced.
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4. 6.1 Photosynthesis and cellular respiration provide energy for life
In cellular respiration
glucose 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.
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5. ATP powers most cellular work
Figure 6.1
Sunlight energy
ECOSYSTEM
Photosynthesis in chloroplasts
Organic molecules
CO2 + H2O
+ O2
Cellular respiration in mitochondria
Figure 6.1 The connection between photosynthesis and cellular respiration
ATP powers most cellular work
ATP
Heat energy 5

6. 6.2 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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7. Muscle cells carrying out
Figure 6.2
Breathing O2
CO2
Lungs
CO2
Bloodstream O2
Figure 6.2 The connection between breathing and cellular respiration
Muscle cells carrying out
Cellular Respiration
Glucose  O2
CO2  H2O  ATP 7

8. 6.3 Cellular respiration banks energy in ATP molecules
Cellular respiration is an exergonic process that transfers energy from the bonds in glucose to form ATP.
Cellular respiration
produces up to 32 ATP molecules from each glucose molecule and
captures only about 34% of the energy originally stored in glucose.
Other foods (organic molecules) can also be used as a source of energy.
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9. Figure 6.3 Summary equation for cellular respiration
C6H12O6 6 O2 6
CO2 6
H2O
ATP
Glucose
Oxygen
Carbon dioxide
Water
 Heat
Figure 6.3 Summary equation for cellular respiration 9

10. 6.4 CONNECTION: The human body uses energy from ATP for all its activities
The average adult human needs about 2,200 kcal of energy per day.
About 75% of these calories are used to maintain a healthy body.
The remaining 25% is used to power physical activities.
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11. 6.4 CONNECTION: The human body uses energy from ATP for all its activities
A kilocalorie (kcal) is
the quantity of heat required to raise the temperature of 1 kilogram (kg) of water by 1oC,
the same as a food Calorie, and
used to measure the nutritional values indicated on food labels.
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12. Figure 6.4 Energy consumed by various activities
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)
204
Figure 6.4 Energy consumed by various activities
Driving a car 61
Sitting (writing) 28
*Not including kcal needed for body maintenance 12

13. 6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen
The energy necessary for life is contained in the arrangement of electrons in chemical bonds in organic molecules.
An important question is how do cells extract this energy?
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14. 6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen
When the carbon-hydrogen bonds of glucose are broken, electrons are transferred to oxygen.
Oxygen has a strong tendency to attract electrons.
An electron loses potential energy when it “falls” to oxygen.
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15. 6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen
Energy can be released from glucose by simply burning it.
The energy is dissipated as heat and light and is not available to living organisms.
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16. 6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen
On the other hand, cellular respiration is the controlled breakdown of organic molecules.
Energy is
gradually released in small amounts,
captured by a biological system, and
stored in ATP.
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17. 6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen
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
reduced when it gains one or more electrons.
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18. 6.5 Cells tap 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.
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19. Loss of hydrogen atoms (becomes oxidized)
Figure 6.5A Rearrangement of hydrogen atoms (with their electrons) in the redox reactions of cellular respiration
Loss of hydrogen atoms (becomes oxidized)
C6H12O O2
6 CO H2O + ATP Heat
(Glucose)
Gain of hydrogen atoms (becomes reduced)
Figure 6.5A Rearrangement of hydrogen atoms (with their electrons) in the redox reactions of cellular respiration 19

20. 6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen
Enzymes are necessary to oxidize glucose and other foods.
NAD+
is an important coenzyme in oxidizing glucose,
accepts electrons, and
becomes reduced to NADH.
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21. Figure 6.5B A pair of redox reactions occuring simultaneously
Becomes oxidized 2H
Becomes reduced
NAD 2H
NADH H
Figure 6.5B A pair of redox reactions occuring simultaneously
(carries 2 electrons) 2 H 2 21

22. 6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen
There are other electron “carrier” molecules that function like NAD+.
They form a staircase where the electrons pass from one to the next down the staircase.
These electron carriers collectively are called the electron transport chain.
As electrons are transported down the chain, ATP is generated.
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23. Energy released and available for making 2
Figure 6.5c
NAD+
NADH
Energy released and available for making 2
ATP 2 − 2 1
Figure 6.5C In cellular respiration, electrons fall down an energy staircase and finally reduce O2. O2
H2O 2 H+ 23

24. STAGES OF CELLULAR RESPIRATION
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25. 6.6 Overview: 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 citric acid cycle
Stage 3 – Oxidative phosphorylation
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26. 6.6 Overview: Cellular respiration occurs in three main stages
Stage 1: Glycolysis
occurs in the cytoplasm,
begins cellular respiration, and
breaks down glucose into two molecules of a three-carbon compound called pyruvate.
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27. 6.6 Overview: Cellular respiration occurs in three main stages
Stage 2: The citric acid cycle
takes place in mitochondria,
oxidizes pyruvate to a two-carbon compound, and
supplies the third stage with electrons.
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28. 6.6 Overview: Cellular respiration occurs in three main stages
Stage 3: Oxidative phosphorylation
involves electrons carried by NADH and FADH2,
shuttles these electrons to the electron transport chain embedded in the inner mitochondrial membrane,
involves chemiosmosis, and
generates ATP through oxidative phosphorylation associated with chemiosmosis.
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29. Oxidative Phosphorylation (Electron transport and chemiosmosis)
Figure 6.6
Electrons carried by
NADH
FADH2
Glycolysis
Oxidative Phosphorylation (Electron transport and chemiosmosis)
Pyruvate Oxidation
Citric Acid Cycle
Glucose
Pyruvate
Figure 6.6 An overview of cellular respiration
CYTOSOL
MITOCHONDRION
ATP
Substrate-level phosphorylation
Substrate-level phosphorylation
Oxidative phosphorylation
ATP
ATP 29

30. 6.7 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
a net of two molecules of ATP is produced.
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31. Figure 6.7A An overview of glycolysis
Glucose
2 ADP
2 NAD
2 P
2 NADH 2
ATP
2 H
Figure 6.7A An overview of glycolysis
2 Pyruvate 31

32. 6.7 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 called intermediates.
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33. Enzyme Enzyme P ADP ATP P P Substrate Product
Figure 6.7B Substrate-level phosphorylation: transfer of a phosphate group from a substrate to ADP, producing ATP
Enzyme
Enzyme P
ADP
ATP
Figure 6.7B Substrate-level phosphorylation: transfer of a phosphate group from a substrate to ADP, producing ATP P P
Substrate
Product 33

34. 6.8 Pyruvate is oxidized prior to the citric acid cycle
The pyruvate formed in glycolysis is transported from the cytoplasm into a mitochondrion where
the citric acid cycle and
oxidative phosphorylation will occur.
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35. 6.8 Pyruvate is oxidized prior to the citric acid cycle
Two molecules of pyruvate are produced for each molecule of glucose that enters glycolysis.
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,
coenzyme A joins with the two-carbon group to form acetyl coenzyme A, abbreviated as acetyl CoA, and
acetyl CoA enters the citric acid cycle.
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36. Figure 6.8 The link between glycolysis and the citric acid cycle
NAD
NADH H 2
CoA
Pyruvate 1
Acetyl coenzyme A 3
Figure 6.8 The link between glycolysis and the citric acid cycle
CO2
Coenzyme A 36

37. 6.9 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 German-British 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.
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38. Figure 6.9A An overview of the citric acid cycle
Acetyl CoA
CoA
CoA 2
CO2
Citric Acid Cycle 3
NAD
FADH2
Figure 6.9A An overview of the citric acid cycle
FAD 3
NADH
3 H
ATP
ADP P 38

39. 6.9 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 added to a four-carbon compound, forming citrate,
citrate is degraded back to the four-carbon compound,
two CO2 are released, and
1 ATP, 3 NADH, and 1 FADH2 are produced.
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40. 6.9 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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41. 6.10 Most ATP production occurs by oxidative phosphorylation
involves electron transport and chemiosmosis and
requires an adequate supply of oxygen.
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42. 6.10 Most ATP production occurs by oxidative phosphorylation
Electrons from NADH and FADH2 travel down the electron transport chain to O2.
Oxygen picks up H+ to form water.
Energy released by these redox reactions is used to pump H+ from the mitochondrial matrix into the intermembrane space.
In chemiosmosis, the H+ diffuses back across the inner membrane through ATP synthase complexes, driving the synthesis of ATP.
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43. OUTER MITOCHONDRIAL MEMBRANE
Figure 6.10a
OUTER MITOCHONDRIAL MEMBRANE H+
Mobile electron carriers H+ H+
Protein
complex of electron carriers H+ H+
Intermem- brane space H+ H+ H+
ATP synthase H+
Cyt c
III IV I Q
Inner mito- chondrial membrane II
Electron flow
FADH2
FAD
NADH 1 − O2
+ 2 H+
NAD+ 2
Mito- chondrial matrix H+
Figure 6.10 Oxidative phosphorylation: electron transport and chemiosmosis in a mitochondrion
ADP + P
ATP
H2O H+
Electron Transport Chain
Chemiosmosis
Oxidative Phosphorylation 43

44. 6.11 CONNECTION: Interrupting cellular respiration can have both harmful and beneficial effects
Three categories of cellular poisons obstruct the process of oxidative phosphorylation. These poisons
block the electron transport chain (for example, rotenone, cyanide, and carbon monoxide),
inhibit ATP synthase (for example, the antibiotic oligomycin), or
make the membrane leaky to hydrogen ions (called uncouplers, examples include dinitrophenol (weight loss pill that caused fatalities)).
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45. 6.12 Review: Each molecule of glucose yields many molecules of ATP
Recall that the energy payoff of cellular respiration involves
glycolysis,
alteration of pyruvate,
the citric acid cycle, and
oxidative phosphorylation.
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46. 6.12 Review: Each molecule of glucose yields many molecules of ATP
The total yield is about 32 ATP molecules per glucose molecule.
This is about 34% of the potential energy of a glucose molecule.
In addition, water and CO2 are produced.
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47. Pyruvate Oxidation 2 Acetyl CoA
Figure 6.12
CYTOSOL
MITOCHONDRION 2
NADH 2
NADH 6
NADH
+ 2
FADH2
Glycolysis
Pyruvate Oxidation 2 Acetyl CoA
Oxidative Phosphorylation (electron transport and chemiosmosis)
2 Pyruvate
Citric Acid Cycle
Glucose O2
Maximum per glucose:
Figure 6.12 An estimated tally of the ATP produced by substrate-level and oxidative phosphorylation in cellular respiration
H2O
CO2
+ 2
ATP
+ 2
ATP
+ about 28 ATP
About 32 ATP
by substrate-level phosphorylation
by substrate-level phosphorylation
by oxidative phosphorylation 47

48. FERMENTATION: ANAEROBIC HARVESTING OF ENERGY
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49. 6.13 Fermentation enables cells to produce ATP without oxygen
Fermentation is a way of harvesting chemical energy that does not require oxygen. Fermentation
takes advantage of glycolysis,
produces two ATP molecules per glucose, and
reduces NAD+ to NADH.
Fermentation provides an anaerobic path for recycling NADH back to NAD+.
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50. 6.13 Fermentation enables cells to produce ATP without oxygen
Your muscle cells and certain bacteria can oxidize NADH through lactic acid fermentation, in which
NADH is oxidized to NAD+ and
pyruvate is reduced to lactate.
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51. 6.13 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.
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52. Glucose 2 ADP 2 NAD 2 P Glycolysis 2 ATP 2 NADH 2 Pyruvate 2 NADH
Figure 6.13A Lactic acid fermentation: NAD+ is regenerated as pyruvate is reduced to lactate.
Glucose
2 ADP
2 NAD
2 P
Glycolysis
2 ATP
2 NADH
2 Pyruvate
2 NADH
Figure 6.13A Lactic acid fermentation: NAD+ is regenerated as pyruvate is reduced to lactate.
2 NAD
2 Lactate 52

53. 6.13 Fermentation enables cells to produce ATP without oxygen
The baking and winemaking industries have used alcohol fermentation for thousands of years.
In this process yeasts (single-celled fungi)
oxidize NADH back to NAD+ and
convert pyruvate to CO2 and ethanol.
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54. Glucose 2 ADP 2 NAD Glycolysis 2 P 2 ATP 2 NADH 2 Pyruvate 2 NADH
Figure 6.13B Alchohol fermentation: NAD is regenerated as pyruvate is broken down to CO2 and ethanol.
Glucose
2 ADP
2 NAD
2 P
Glycolysis
2 ATP
2 NADH
2 Pyruvate
Figure 6.13B Alchohol fermentation: NAD is regenerated as pyruvate is broken down to CO2 and ethanol.
2 NADH
2 CO2
2 NAD
2 Ethanol 54

55. 6.13 Fermentation enables cells to produce ATP without oxygen
Obligate anaerobes
are poisoned by oxygen, requiring anaerobic conditions, and
live in stagnant ponds and deep soils.
Facultative anaerobes
include yeasts and many bacteria, and
can make ATP by fermentation or oxidative phosphorylation.
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56. 6.14 EVOLUTION CONNECTION: 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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57. 6.14 EVOLUTION CONNECTION: 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-bounded organelles.
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58. CONNECTIONS BETWEEN METABOLIC PATHWAYS
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59. 6.15 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.
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60. 6.15 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 than a gram of carbohydrate or protein.
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61. Pyruvate Oxidation Acetyl CoA Oxidative Phosphorylation
Figure 6.15
Food, such as peanuts
Carbohydrates
Fats
Proteins
Sugars
Glycerol
Fatty acids
Amino acids
Amino groups
Figure 6.15 Pathways that break down various food molecules
Citric Acid Cycle
Pyruvate Oxidation Acetyl CoA
Oxidative Phosphorylation
Glucose
G3P
Pyruvate
Glycolysis
ATP 61

62. 6.16 Food molecules provide raw materials for biosynthesis
Cells use intermediates from cellular respiration for the biosynthesis of other organic molecules.
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63. Cells, tissues, organisms Oxidative Phosphorylation
ATP needed to drive biosynthesis
ATP
Citric Acid Cycle
Acetyl CoA
Glucose Synthesis
Pyruvate G3P Glucose
Amino groups
Amino acids
Fatty acids
Glycerol
Sugars
Proteins
Fats
Carbohydrates
Figure 6.15
Biosynthesis of
large organic
molecules from
intermediates
of cellular respiration
Figure 6.16 Biosynthesis of large organic molecules from intermediates of cellular respiration
Cells, tissues, organisms
Citric Acid Cycle
Glucose G3P Pyruvate
Oxidative Phosphorylation
Acetyl CoA
Glycolysis 63