1. Chapter 7 Lecture Slides
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2. 7.1 Where Is the Energy in Food?
The energy for living is obtained by breaking down the organic molecules originally produced in plants
the energy invested in building the organic molecules is retrieved by stripping away electrons and using them to make ATP
this process is called cellular respiration

3. 7.1 Where Is the Energy in Food?
Eukaryotes produce the majority of their ATP by harvesting electrons from the food molecule glucose:
C6H12O6 + 6 O2  6 CO2 + 6 H2O + energy
The reactants are glucose and oxygen, and the products are carbon dioxide, water, and energy (heat or ATP)

4. 7.1 Where Is the Energy in Food?
Electrons pass from atoms or molecules in many chemical reactions, such as those involved in cellular respiration
oxidation is when an atom or molecule loses an electron
reduction is when an atom or molecule gains an elections
these reactions always occur together
called oxidation-reduction (redox) reactions

5. 7.1 Where Is the Energy in Food?
In many redox reactions, oxygen is a common electron acceptor
in cellular respiration, oxygen is the final electron acceptor
Energy follows the electrons, such that the reduced form of an organic molecule has a higher level of energy than the oxidized form
Figure 7.2 Redox reactions

6. 7.1 Where Is the Energy in Food?
Cellular respiration takes place in two stages
Glycolysis
occurs in the cytoplasm
does not require oxygen to generate ATP
a very ancient pathway
Krebs cycle
occurs within the mitochondrion
harvests energy-rich electrons through a cycle of oxidation reactions
the electrons are passed to an electron transport chain in order to power the production of ATP

7. Figure 7.3 An overview of cellular respiration

8. 7.2 Respiration Without Oxygen: Glycolysis
Glycolysis is a sequence of reactions that form a biochemical pathway
in ten enzyme-catalyzed reactions, the six-carbon sugar glucose is broken into two three-carbon pyruvate molecules
the breaking of the bonds yields energy that is used to phosphorylate ADP to ATP
this process is called substrate-level phosphorylation
in addition, electrons and hydrogens are donated to NAD+ to form NADH
this is the only way organisms can derive energy from food in the absence of oxygen

9. Essential Biological Process 7A: Glycolysis

10. Animation: How Glycolysis Works
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11. 7.3 Respiration With Oxygen: the Krebs Cycle
When oxygen is available, the cell harvests pyruvate’s energy in two steps:
the first step is to oxidize pyruvate to form acetyl-CoA
the second step is to oxidize acetyl-CoA in the Krebs cycle

12. 7.3 Respiration With Oxygen: the Krebs Cycle
When pyruvate is oxidized, a single of its three carbons is cleaved off by the enzyme pyruvate dehydrogenase
this carbon leaves as part of a CO2 molecule
in addition, a hydrogen and electrons are removed from pyruvate and donated to NAD+ to form NADH
the remaining two-carbon fragment of pyruvate is joined to a cofactor called coenzyme A (CoA)
the final compound is called acetyl-CoA

13. Figure 7.4 Producing acetyl-CoA

14. Essential Biological Process 7B: Transferring Hydrogen Atoms

15. Animation: How NAD+ Works
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16. 7.3 Respiration With Oxygen: the Krebs Cycle
The fate of acetyl-CoA depends on the availability of ATP in the cell
if there is insufficient ATP, then the acetyl-CoA heads to the Krebs cycle
if there is plentiful ATP, then the acetyl-CoA is diverted to fat synthesis for energy storage

17. 7.3 Respiration With Oxygen: the Krebs Cycle
The Krebs cycle occurs in mitochondria and is a series of nine reactions that can be broken down into three stages of oxidative respiration
Acetyl-CoA enters the cycle and binds to a four-carbon molecule, forming a six-carbon molecule
Two carbons are removed as CO2 and their electrons donated to NAD+. In addition, an ATP is produced.
The four-carbon molecule is recycled and more electrons are extracted, forming NADH and FADH2.

18. Essential Biological Process 7C: The Krebs Cycle

19. Animation: How the Krebs Cycle Works
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20. 7.3 Respiration With Oxygen: the Krebs Cycle
In the process of cellular respiration, the glucose is entirely consumed
the energy from its chemical bonds has been transformed into
four ATP molecules
10 NADH electron carriers
2 FADH2 electron carriers

21. 7.4 Using the Electrons to Make ATP
NADH and FADH2 transfer their electrons to a series of molecules embedded in the mitochondrial membrane
This series of membrane-associated molecules is called the electron transport chain
many of the proteins in the electron transport chain operate as proton pumps, pumping protons into the intermembrane space of the mitochondrion
the last transport protein donates the electrons to hydrogen and oxygen in order to form water

22. 7.4 Using the Electrons to Make ATP
Mitochondria use chemiosmosis to make ATP
as the concentration of protons builds up in the intermembrane space, the protons re-enter the matrix via ATP synthase channels
their passage powers the production of ATP from ADP

23. Figure 7.5 The electron transport chain

24. Animation: Electron Transport System and ATP Synthesis
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25. Animation: Electron Transport System and Formation of ATP
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26. 7.5 Cells Can Metabolize Food Without Oxygen
In the absence of oxygen, fermentation, rather than oxidation, is the pathway that occurs after glycolysis
in fermentation, an organic molecule accepts hydrogen atoms from the NADH generated by glycolysis, thus recycling NAD+ and allowing glycolysis to continue.
bacteria carryout more than a dozen kinds of fermentation
eukaryotic cells are capable of only a few types of fermentation

27. 7.5 Cells Can Metabolize Food Without Oxygen
In ethanol fermentation, which occurs in yeasts, NAD+ and ethanol are produced; this is the source of ethanol for wine and beer
In lactic acid fermentation, carried out in the muscle cells of animals, NAD+ and lactate (lactic acid) are produced

28. Figure 7.7 Fermentation

29. 7.6 Glucose Is Not the Only Food Molecule
Cells also get energy from foods other than sugars
proteins are first broken down into their individual amino acids
deamination reactions convert the amino acids into molecules that can take part in the Krebs cycle
fats are first broken down into fatty acids
a process called -oxidation then converts the fatty acid tails into acetyl groups that can be combined with coenzyme A to form acetyl-CoA, which feeds into the Krebs cycle

30. Figure 7.8 How cells obtain energy from foods

31. Inquiry & Analysis
About how much of the total lactic acid is released after exercise stops?
Is this result consistent with the hypothesis that fish maintain blood pH levels by delaying the release of lactic acid from muscles?
How Do Swimming Fish Avoid Low Blood pH?