1. Chapter 7 How Cells Release Chemical Energy

2. 7.2 Overview of Carbohydrate Breakdown Pathways
Photosynthetic organisms capture solar energy and store it in sugars
Energy stored in sugars must be converted to molecular energy (in the form of ATP) for use in energy-requiring reactions
Most eukaryotes and some bacteria breakdown carbohydrates via aerobic respiration

3. Cycle of Photosynthesis and Aerobic Respiration
energy S Y N T H O E T S O I S H P
CO2 H2O
sugar O2
Figure 7.2 The global connection between photosynthesis and aerobic respiration. note the cycling of materials, and the one-way flow of energy N A O E I R T O A B I C I R R E S P
energy

4. Overview of Aerobic Respiration
C6H12O6 (glucose) + O2 (oxygen) →
CO2 (carbon dioxide) + H2O (water)
Three stages
Glycolysis
Acetyl-CoA formation and Krebs cycle
Electron transfer phosphorylation (ATP formation)
Coenzymes NADH and FADH2 carry electrons and hydrogen

5. Aerobic Respiration vs. Anaerobic Fermentation
Aerobic respiration and fermentation both begin with glycolysis, which converts one molecule of glucose into two molecules of pyruvate
After glycolysis, the two pathways diverge
Fermentation is completed in the cytoplasm, yielding 2 ATP per glucose molecule
Aerobic respiration is completed in mitochondria, yielding 36 ATP per glucose molecule

6. Comparison of Aerobic Respiration and Fermentation
cytoplasm
ATP
GLYCOLYSIS
cytoplasm
ATP
GLYCOLYSIS
mitochondrion
KREBS CYCLE
ATP
additional reactions
in cytoplasm
ATP
Figure 7.3 Comparison of aerobic respiration and fermentation.
ATP
ATP
ELECTRON TRANSFER
PHOSPHORYLATION
B Fermentation
A Aerobic respiration.

7. 7.3 Glycolysis—Sugar Breakdown Begins
Glycolysis begins the sugar breakdown pathway in both aerobic respiration and fermentation
Occurs in cytoplasm of cells
Requires initial investment of 2 ATP molecules
Produces 4 ATP total, for a net yield of 2 ATP

8. Glycolysis – ATP-Requiring Steps
glucose
ATP-Requiring Steps
An enzyme transfers a phosphate group from ATP to glucose, forming glucose-6-phosphate. (You learned about this enzyme, hexokinase, in FIGURE 5.10 and Section 5.4.)
ADP
glucose-6-phosphate
A phosphate group from a second ATP is transferred to the glucose-6-phosphate. The resulting molecule is unstable, and it splits into two three-carbon molecules.
The molecules are interconvertible, so we will call them both PGAL (phosphoglyceraldehyde).
Two ATP have now been invested in the reactions.
ADP
fructose-1,6-bisphosphate
Glycolysis
ATP-Requiring Steps
1 An enzyme transfers a phosphate group from ATP to glucose, forming glucose-6-phosphate. (You learned about this enzyme, hexokinase, in Figure 5.10 and Section 5.4).
2 A phosphate group from a second ATP is transferred to the glucose-6-phosphate. The resulting molecule is unstable, and it splits into two three-carbon molecules. The molecules are interconvertible, so we will call them both PGAL (phosphoglyceraldehyde). Two ATP have now been invested in the reactions.
ATP-Generating Steps
3 An enzyme attaches a phosphate to each PGAL , so two PGA (phosphoglycerate) form. Two electrons and a hydrogen ion (not shown) from each PGAL are accepted by NAD+, so two NADH form.
4 An enzyme transfers a phosphate group from each PGA to ADP, forming two ATP and two intermediate molecules (PEP). The original energy investment of two ATP has now been recovered.
5 An enzyme transfers a phosphate group from each PEP to ADP, forming two more ATP and two molecules of pyruvate.
6 Summing up, glycolysis yields two NADH, two ATP (net), and two pyruvate for each glucose molecule.
Depending on the type of cell and environmental conditions, the pyruvate may enter the second stage of aerobic respiration or it may be used in other ways, such as in fermentation.
2 PGAL
ATP-Generating Steps
2 NAD+
An enzyme attaches a phosphate to each PGAL, so two PGA (phospho- glycerate) form. Two electrons and a hydrogen ion (not shown) from each PGAL are accepted by NAD+, so two NADH form.
2 coenzymes reduced

9. Glycolysis – ATP-Requiring Steps
2 PGA
2 ADP
An enzyme transfers a phosphate group from each PGA to ADP, forming two ATP and two intermediate molecules (PEP).
The original energy investment of two ATP has now been recovered.
2 ATP produced by substrate-level phosphorylation
2 PEP
2 ADP
An enzyme transfers a phosphate group from each PEP to ADP, forming two more ATP and two molecules of pyruvate.
Glycolysis
ATP-Requiring Steps
1 An enzyme transfers a phosphate group from ATP to glucose, forming glucose-6-phosphate. (You learned about this enzyme, hexokinase, in Figure 5.10 and Section 5.4).
2 A phosphate group from a second ATP is transferred to the glucose-6-phosphate. The resulting molecule is unstable, and it splits into two three-carbon molecules. The molecules are interconvertible, so we will call them both PGAL (phosphoglyceraldehyde). Two ATP have now been invested in the reactions.
ATP-Generating Steps
3 An enzyme attaches a phosphate to each PGAL , so two PGA (phosphoglycerate) form. Two electrons and a hydrogen ion (not shown) from each PGAL are accepted by NAD+, so two NADH form.
4 An enzyme transfers a phosphate group from each PGA to ADP, forming two ATP and two intermediate molecules (PEP). The original energy investment of two ATP has now been recovered.
5 An enzyme transfers a phosphate group from each PEP to ADP, forming two more ATP and two molecules of pyruvate.
6 Summing up, glycolysis yields two NADH, two ATP (net), and two pyruvate for each glucose molecule.
Depending on the type of cell and environmental conditions, the pyruvate may enter the second stage of aerobic respiration or it may be used in other ways, such as in fermentation.
2 ATP produced by substrate-level phosphorylation
Summing up, glycolysis yields two NADH, two ATP (net), and two pyruvate for each glucose molecule.
Depending on the type of cell and environmental conditions, the pyruvate may enter the second stage of aerobic respiration or it may be used in other ways, such as in fermentation.
2 pyruvate
Net 2 ATP + 2 NADH
to second stage of aerobic respiration or fermentation

10. 7.4 Second Stage of Aerobic Respiration
The second stage of aerobic respiration completes the breakdown of glucose that began in glycolysis
Occurs in mitochondria
Includes two sets of reactions: acetyl-CoA formation and the Krebs cycle (each occurs twice in the breakdown of one glucose molecule)

11. Second Stage of Aerobic Respiration – In the Matrix
cytoplasm
outer membrane
inner membrane
The breakdown of 2 pyruvate to 6 CO2 yields 2 ATP and 10 reduced coenzymes (8 NADH, 2 FADH2). The coenzymes will carry their cargo of electrons and hydrogen ions to the third stage of aerobic respiration.
matrix
Figure 7.6 Animated The second stage of aerobic respiration, acetyl–CoA formation and the Krebs cycle, occurs inside mitochondria. Left, an inner membrane divides a mitochondrion’s interior into two fluid-filled compartments. Right, the second stage of aerobic respiration takes place in the mitochondrion’s innermost compartment, or matrix.

12. Acetyl-CoA Formation
In the inner compartment of the mitochondrion, enzymes split pyruvate, forming acetyl-CoA and CO2 (which diffuses out of the cell)
NADH is formed

13. The Krebs Cycle
A sequence of enzyme-mediated reactions that break down 1 acetyl-CoA into 2 CO2
Oxaloacetate is used and regenerated
3 NADH and 1 FADH2 are formed
1 ATP is formed

14. Acetyl–CoA Formation and the Krebs Cycle
An enzyme splits a pyruvate coenzyme A NAD+ molecule into a two-carbon acetyl group and CO2. Coenzyme A binds the acetyl group (forming acetyl–CoA). NAD+ combines with released hydrogen ions and electrons,
forming NADH. 1
The Krebs cycle starts as one carbon atom is transferred from acetyl–CoA tooxaloacetate. Citrate forms, and coenzyme
A is regenerated. 2
The final steps of the Krebs cycle regenerate
oxaloacetate. 8
A carbon atom is removed from an intermediate and leaves the cell as CO2. NAD+ combines with released hydrogen ions and electrons, forming NADH. 3
NAD+ combines with hydrogen ions and electrons,
forming NADH. 7
Krebs Cycle
Figure 7.7 Acetyl–CoA and the Krebs cycle.
It takes two cycles of Krebs reactions to break down two pyruvate molecules that formed during glycolysis of one glucose molecule. After two cycles, all six carbons that entered glycolysis in one glucose molecule have left the cell, in six CO2. Electrons and hydrogen ions are released as each carbon is removed from the backbone of intermediate molecules; ten coenzymes will carry them to the third and final stage of aerobic respiration. Not all reactions are shown; Appendix V has more details.
The coenzyme FAD combines with hydrogen
ions and electrons, forming FADH2. 6
A carbon atom is removed from another intermediate and leaves the cell as CO2, and another NADH forms.
Pyruvate’s three carbon atoms have now exited the cell, in CO2. 4
One ATP forms by substrate-level phosphorylation. 5
Stepped Art

15. Acetyl–CoA and the Krebs Cycle
An enzyme splits a pyruvate molecule into a two-carbon acetyl group and CO2
binds the acetyl group, forming acetyl–CoA. NAD+ combines with released electrons and a hydrogen ion, forming NADH.
pyruvate
coenzyme A
CO2
acetyl–CoA
coenzyme A
The final steps regenerate oxaloacetate.
The Krebs cycle starts as two carbon atoms are transferred from acetyl–CoA
oxaloacetate
to oxaloacetate. Citrate forms,
citrate
and coenzyme A is regenerated.
NAD combines with hydrogen ions and electrons, forming NADH. +
One carbon atom is removed from an intermediate and
Figure 7.7 Acetyl–CoA and the Krebs cycle.
It takes two cycles of Krebs reactions to break down two pyruvate molecules that formed during glycolysis of one glucose molecule. After two cycles, all six carbons that entered glycolysis in one glucose molecule have left the cell, in six CO2. Electrons and hydrogen ions are released as each carbon is removed from the backbone of intermediate molecules; ten coenzymes will carry them to the third and final stage of aerobic respiration. Not all reactions are shown; Appendix V has more details.
KREBS CYCLE
leaves the cell as CO2.
CO2
NAD+ combines with released electrons and a hydrogen ion, forming NADH.
The coenzyme FAD combines with electrons and hydrogen ions, forming FADH2.
Another carbon atom is removed from another inter-
succinate
mediate and leaves the cell as CO2. CO2
ADP + Pi
NAD+ combines with released electrons
and a hydrogen ion, forming NADH. Pyruvate’s three carbon atoms have now exited the cell, in CO2.
One ATP forms by
substrate-level phosphorylation.

16. 7.5 Aerobic Respiration’s Big Energy Payoff
Many ATP are formed during the third and final stage of aerobic respiration
Electron transfer phosphorylation
Occurs in mitochondria
Results in attachment of phosphate to ADP to form ATP

17. Electron Transfer Phosphorylation
Coenzymes NADH and FADH2 donate electrons and H+ to electron transfer chains
Active transport forms a H+ concentration gradient in the outer mitochondrial compartment
H+ follows its gradient through ATP synthase, which attaches a phosphate to ADP
Finally, oxygen accepts electrons and combines with H+, forming water

18. Electron Transfer Phosphorylation
GLYCOLYSIS
NADH and FADH2 deliver electrons to electron transfer chains in the inner mitochondrial membrane.
Electron flow through the chains causes hydrogen ions (H+) to be pumped from the matrix to the intermembrane space. The activity of the electron transfer chains causes a hydrogen ion gradient to form across the inner mitochondrial membrane.
Hydrogen ion flow back to the matrix through ATP synthases drives the formation of ATP from ADP and phosphate (Pi).
Oxygen combines with electrons and hydrogen ions at the end of the electron transfer chains, so water forms.
KREBS CYCLE
ELECTRON TRANSFER PHOSPHORYLATION
ELEC TRON TRANSFER PHOSPHORYLATION
electron transfer
chain
Figure 7.8 In eukaryotes, the third and final stage of aerobic respiration, electron transfer phosphorylation,
occurs at the inner mitochondrial membrane.
1 NADH and FADH2 deliver electrons to electron transfer chains in the inner mitochondrial
membrane.
2 Electron flow through the chains causes hydrogen ions (H+) to be pumped from
the matrix to the intermembrane space.
3 The activity of the electron transfer chains causes a hydrogen ion gradient to form
across the inner mitochondrial membrane.
4 Hydrogen ion flow back to the matrix through ATP synthases drives the formation
of ATP from ADP and phosphate (Pi ).
5 Oxygen combines with electrons and hydrogen ions at the end of the electron
transfer chains, so water forms.
matrix
ADP + Pi
Inner membrane O2
2H2O
intermembrane space
outer membrane
cytoplasm

19. Summary: The Energy Harvest
Typically, the breakdown of one glucose molecule yields 36 ATP
Glycolysis: 2 ATP
Acetyl-CoA formation and Krebs cycle: 2 ATP
Electron transfer phosphorylation: 32 ATP

20. Aerobic Respiration 2 ATP
glucose
STAGE
Glycolysis splits a glucose molecule into 2 pyruvate; 2 NADH and 4 ATP also form. An investment of 2 ATP began the reactions, so the net yield is 2 ATP.
2 ATP
GLYCOLYSIS
4 ATP
(2 net)
2 NADH
2 pyruvate
STAGE
Acetyl–CoA formation and the Krebs cycle break down the pyruvate to CO2, which
2 NADH
leaves the cell. Ten additional coenzymes are reduced, and two ATP form.
2 NADH
2 CO2
2 acetyl–CoA
4 CO2
KREBS CYCLE
STAGE
In electron transfer phosphorylation, the reduced coenzymes give up electrons and hydrogen ions to electron transfer chains. Energy lost by the electrons as
6 NADH
2 FADH2 2
ATP
Figure 7.9 Summary of the three stages of aerobic respiration, pictured in a mitochondrion.
ATP
they move through the chains is used to move H+ across
ATP
ATP
the membrane. The resulting gradient causes H+ to flow through ATP synthases, which drives ATP synthesis.
ATP 32
ELECTRON TRANSFER PHOSPHORYLATION
oxygen H2O

21. 7.6 Fermentation Glycolysis is the first stage of fermentation
Forms 2 pyruvate, 2 NADH, and 2 ATP
Pyruvate is converted to other molecules, but is not fully broken down to CO2 and water
Regenerates NAD+ but does not produce ATP
Provides enough energy for some single-celled anaerobic species

22. Two Fermentation Pathways – Alcoholic Fermentation
Pyruvate is split into acetaldehyde and CO2
Acetaldehyde receives electrons and hydrogen from NADH, forming NAD+ and ethanol
Carried out by single-celled organisms such as fungi
Used to make beer, wine, and bread

23. Alcoholic Fermentation
GLYCOLYSIS
glucose 2
ATP
2 NAD+
e– e– e– e–
4 ATP
2 NADH
pyruvate
Figure 7.10 Alcoholic fermentation
A Alcoholic fermentation begins with glycolysis, and the final steps regenerate NAD+. The reactions yield two ATP per molecule of glucose (from glycolysis).
ALCOHOLIC
2 CO2
FERM ENTATION
acetaldehyde
2 NADH
2 NAD+
ethanol

24. Two Fermentation Pathways – Lactate Fermentation
Pyruvate receives electrons and hydrogen directly from NADH, forming NAD+ and lactate
Carried out be beneficial bacteria
Used to make yogurt
Also carried out by animal white skeletal muscle cells
Useful for quick, strenuous activities
Does not support prolonged activity

25. Lactate Fermentation e– e– e– e– ATP NADH NADH GLYCOLYSIS
glucose 2
ATP
2 NAD+
e– e– e– e–
4 ATP 2
NADH
pyruvate
Figure 7.11 Lactate fermentation
A Lactate fermentation begins with glycolysis, and the final steps regenerate NAD+. The reactions yield two ATP per molecule of glucose (from glycolysis).
LACTATE
FERM ENTATION 2
NADH
2 NAD+
lactate

26. 7.7 Alternative Energy Sources in Food
Aerobic respiration can produce ATP from the breakdown of complex carbohydrates, fats, and proteins
As in glucose metabolism, many coenzymes are reduced
The energy of the electrons the coenzymes carry ultimately drives the synthesis of ATP in electron transfer phosphorylation

27. Energy From Complex Carbohydrates
Enzymes break starch and other complex carbohydrates down to monosaccharide subunits
Monosaccharides are taken up by cells and converted to glucose-6-phosphate, which continues in glycolysis
A high concentration of ATP causes glucose-6-phosphate to be diverted from glycolysis and into a pathway that forms glycogen

28. Complex Carbohydrates are Broken Down into Monosaccharides
starch (a complex carbohydrate) glucose (a simple sugar)
CH2OH HO O
Figure 7.12 A variety of organic compounds from food can enter the reactions of aerobic respiration.
A Complex carbohydrates are broken down to their monosaccharide subunits, which can enter glycolysis.

29. Energy From Fats Enzymes cleave fats into glycerol and fatty acids
Glycerol products enter glycolysis
Fatty acids are converted to acetyl-CoA and enter the Krebs cycle
Fatty acid breakdown yields more ATP per carbon atom than carbohydrates
When blood glucose level is high, acetyl-CoA is diverted from the Krebs cycle and into a pathway that makes fatty acids

30. Fats are Broken Down by Separating the Glycerol Head and Fatty Acid Tails
a triglyceride (fat)
glycero l head
fatty acid tails
Figure 7.12 A variety of organic compounds from food can enter the reactions of aerobic respiration.
B Fats are broken down by separating the glycerol head from the fatty acid tails. The fatty acids are converted to acetyl-CoA, and the glycerol is converted to PGAL.

31. Energy from Proteins
Enzymes split dietary proteins into amino acid subunits, which are used to build proteins or other molecules
The amino group is removed and converted into ammonia (NH3), a waste product eliminated in urine
Acetyl–CoA, pyruvate, or an intermediate of the Krebs cycle forms, depending on the amino acid

32. Amino Acids Are Converted
Figure 7.12 A variety of organic compounds from food can enter the reactions of aerobic respiration.
C Amino acids are converted to acetyl-CoA, pyruvate, or an intermediate of the Krebs cycle.
alanine (an amino acid) pyruvate

33. Alternate Energy Sources in Foods
Fats
Complex Carbohydrates
Proteins
fatty acids
glycerol
glucose, other sugars
amino acids 2
acetyl– CoA 3
PGA L 1 4
acetyl–CoA
GLYCOLYSIS
NADH
pyruvate
intermediate of Krebs cycle
Figure 7.12 A variety of organic compounds from food can enter the reactions of aerobic respiration.
KREBS CYCLE
NADH, FADH2
ATP
ATP
ATP
ATP
ELECTRON TRANSFER PHOSPHORYLATION