1. How Cells Harvest Chemical Energy
Chapter 6
Cellular Respiration

2. Energy Flow and Chemical Cycling in the Biosphere
Fuel molecules in food
represent solar energy
traced back to the sun
Animals depend on plants:
to convert solar energy to chemical energy
In form of sugars and other organic molecules

3. Gas Exchange in the Body
Cellular respiration and breathing are closely related
Cellular respiration requires a cell to exchange gases with its surroundings
Breathing exchanges these gases between blood and outside air

4. Cellular respiration
Cellular respiration is an exergonic process that transfers energy from the bonds in glucose to ATP
produces 38 ATP molecules from each glucose molecule
Other foods (organic molecules) can be used as a source of energy as well
Respiration only retrieves 40% of the energy in a glucose molecule. The other 60% of the energy is released as heat. We use this heat to maintain a relatively steady body temperature near 37°C (98–99°F). This is about the same amount of heat generated by a 75-watt incandescent light bulb.
Organic compounds possess potential energy as a result of their arrangement of atoms.
Compounds that can participate in exergonic reactions can act as food.
Actually, cellular respiration includes both aerobic and anaerobic processes. However, it is generally used to refer to the aerobic process.
It takes about 10 million ATP molecules per second to power one active muscle cell.
Student Misconceptions and Concerns
1. Students should be cautioned against the assumption that energy is created when it is converted from one form to another. This might be a good time to review the principle of conservation of energy (the first law of thermodynamics addressed in Module 5.11).
2. Students often fail to realize that aerobic metabolism is a process generally similar to the burning of wood in a fireplace or campfire or the burning of gasoline in an automobile engine. Noting these general similarities can help students comprehend the overall reaction and heat generation associated with these processes.
Teaching Tips
1. Energy coupling at the cellular level may be new to many students, but it is a familiar concept when related to the use of money in our society. Students might be discouraged if the only benefit of work was the ability to make purchases from the employer. (We all might soon tire of a fast-food job that only paid its employees in food!) Money permits the coupling of a generation of value (a paycheck, analogous to an energy-releasing reaction) to an energy-consuming reaction (money, which allows us to make purchases in distant locations). This idea of earning and spending is a common concept we all know well.
2. During cellular respiration, our cells convert about 40% of our food energy to useful work. The other 60% of the energy is released as heat. We use this heat to maintain a relatively steady body temperature near 37°C (98–99°F). This is about the same amount of heat generated by a 75-watt incandescent lightbulb. If you choose to include a discussion of heat generation from aerobic metabolism, consider the following.
A. Ask your students why they feel warm when it is 30°C (86°F) outside, if their core body temperature is 37°C (98.6°F). Shouldn’t they feel cold? The answer is, our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37°C. Thus, we sweat and behave in ways that helps us get rid of the extra heat from cellular respiration.
B. Share this calculation with your students. Depending upon a person’s size and level of activity, a human might burn 2,000 dietary calories (kilocalories) a day. This is enough energy to raise the temperature of 20 liters of liquid water from 0 to 100°C. This is something to think about the next time you heat water on the stove! (Notes: Consider bringing a 2-liter bottle as a visual aid, or ten 2-liter bottles to make the point above. It takes 100 calories to raise 1 liter of water 100°C; it takes much more energy to melt ice or evaporate water as steam.)

5. Cellular Respiration
Release of energy from molecules accompanied by the use of this energy to synthesize ATP molecules
Metabolic pathway
Main method that chemical energy is harvested from food and converted to ATP
Aerobic
Requires oxygen and gives off carbon dioxide

6. Where Is the Energy in Food?
The process of aerobic respiration requires oxygen and carbohydrates
C6H12O6 + 6 O2  6 CO2 + 6 H2O + energy
The products are carbon dioxide, water, and energy (heat or ATP)

7. How do we get the energy??
Energy contained in the arrangement of electrons in chemical bonds in organic molecules
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

8. ATP Adenosine triphosphate (ATP)
Nucleotide with the base adenine and the sugar ribose
Main energy carrier in cells
Formed during reactions that breakdown organic compounds to CO2 and water
Requires ample oxygen
Occurs within the mitochondrion
Hydrolyzes phosphates to release energy
form adenosine diphosphate (ADP)

9. Redox Reaction (O-R)
Chemical reaction that transfers electrons from one substance to another
electrons retain their potential energy
Glucose loses its hydrogen atoms and is ultimately converted to CO2
O2 gains hydrogen atoms and is converted to H2O
Oxidation
Reduction

10. Redox Reaction
Electrons pass from atoms or molecules to one another as part of many energy reactions
Oxidation
When an atom or molecule loses an electron
Glucose is oxidized
Reduction
When an atom or molecule gains an elections
Oxygen is reduced

11. Other important players……
Enzymes are necessary to oxidize glucose and other foods
Dehydrogenase
enzyme that removes hydrogen from an organic molecule
requires a coenzyme called NAD+
(nicotinamide adenine dinucleotide)
shuttle electrons
NAD+ can become reduced when it accepts electrons and oxidized when it gives them up
Reduced to NADH

12. The Finale….
First step is transfer of electrons from organic molecule to NAD+
Other electron carrier molecules represent the electron transport chain
Undergoes series of redox reactions
Release energy to make ATP

13. NADH ATP NAD+ + 2e– Controlled release of H+ energy for synthesis
of ATP H+
Electron transport
chain
2e– H+ 1  2 O2
H2O

14. Stages of Cellular Respiration:
Glycolysis
Citric Acid Cycle
Oxidative Phosphorylation

15. High-energy electrons
NADH
High-energy electrons
carried by NADH
Mitochondrion
NADH
FADH2
and
GLYCOLYSIS
OXIDATIVE
PHOSPHORYLATION
(Electron Transport
and Chemiosmosis)
CITRIC ACID
CYCLE
Glucose
Pyruvate
Cytoplasm
Figure 6.6 An overview of cellular respiration.
Inner
mitochondrial
membrane
CO2
CO2
ATP
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Oxidative
phosphorylation

16. 1. Glycolysis Occurs in the cytoplasm
Does not require oxygen to generate ATP
Then, enters aerobic or anaerobic reactions

17. Glycolysis
Glucose
2 ADP
6 Carbon oxidizes glucose into 2 molecules
Pyruvate
3 Carbon
breaking of the bond yields energy that is used to phosphorylate ADP to ATP
in addition, electrons and hydrogen are donated to NAD+ to form NADH 2
NAD+ + 2 P 2
NADH 2
ATP + 2 H+
2 Pyruvate

18. Enzyme
Enzyme P
ADP +
ATP P P
Substrate
Product

19. Anaerobic verse aerobic?
Absence of oxygen
Fermentation
Make lactate or ethanol
Presence of oxygen
Oxidative respiration
Pyruvate transported to mitochondria
Oxidize pyruvate to form acetyl-coA

20. When pyruvate is oxidized:
A single carbon cleaved off by the enzyme pyruvate dehydrogenase
This carbon leaves as part of a CO2 molecule
hydrogen and electrons are removed from pyruvate
donated to NAD+ to form NADH
Remaining two-carbon fragment of pyruvate is joined to a cofactor called coenzyme A (CoA)
Final compound called acetyl-CoA

21. Acetyl-CoA
The fate of acetyl-CoA depends on the availability of ATP in the cell
Insufficient ATP
The acetyl-CoA heads to the Krebs cycle
Plentiful ATP
The acetyl-CoA is diverted to fat synthesis for energy storage

22. 2. Citric Acid Cycle “Krebs Cycle” occurs within the mitochondrion
Breaks down pyruvate into carbon dioxide
electrons passed to an electron transport chain in order to power the production of ATP

23. Stages of Citric Acid Cycle
acetyl (two-carbon) compound enters the citric acid cycle
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

24. Acetyl CoA
CoA
CoA
The Krebs cycle 2
CO2
CITRIC ACID CYCLE
Note: a single glucose molecule produces two turns of the cycle, one for each of the two pyruvate molecules generated by glycolysis 3
NAD+
FADH2
FAD 3
NADH 
3 H+
ATP
ADP + P

25. by oxidative phosphorylation
Electron shuttle
across membrane
Cytoplasm
Mitochondrion 2
NADH 2
NADH
(or 2 FADH2) 2
NADH 6
NADH 2
FADH2
GLYCOLYSIS
OXIDATIVE
PHOSPHORYLATION
(Electron Transport
and Chemiosmosis) 2
Pyruvate
2 Acetyl
CoA
CITRIC ACID
CYCLE
Glucose
 2 ATP
 2 ATP
 about 34 ATP
Figure 6.12 An estimated tally of the ATP produced by substrate-level and oxidative phosphorylation in cellular respiration.
by substrate-level
phosphorylation
by substrate-level
phosphorylation
by oxidative phosphorylation
About
38 ATP
Maximum per glucose:

26. 3. Oxidative Phosphorylation
Electron transport chain
Shuttle molecules NADH and FADH take electrons to oxygen
Final acceptor
Forms H2O
Carriers bind and release electrons in redox reactions
Pass electrons down the “energy staircase”
Use energy released from the transfers to transport H+

27. OXIDATIVE PHOSPHORYLATION
Protein
complex
of electron
carriers H+ H+
Electron
carrier H+ H+
ATP
synthase
Intermembrane
space
Inner
mitochondrial
membrane
FADH2
FAD
Electron
flow
NADH
NAD+ 2  1 O2
+ 2 H+ H+
Mitochondrial
matrix H+
Figure 6.10 Oxidative phosphorylation, using electron transport and chemiosmosis in the mitochondrion.
ADP + P H+
ATP
H2O H+
Electron Transport Chain
Chemiosmosis
OXIDATIVE PHOSPHORYLATION

28. 3. Oxidative Phosphorylation
Chemiosmosis
Uses energy stored in a hydrogen ion gradient to drive ATP synthesis
H+ concentration gradient stores potential energy
ATP synthase drives hydrogen ions through
Generates ATP

29. by oxidative phosphorylation
Electron shuttle
across membrane
Cytoplasm
Mitochondrion 2
NADH 2
NADH
(or 2 FADH2) 2
NADH 6
NADH 2
FADH2
GLYCOLYSIS
OXIDATIVE
PHOSPHORYLATION
(Electron Transport
and Chemiosmosis) 2
Pyruvate
2 Acetyl
CoA
CITRIC ACID
CYCLE
Glucose
 2 ATP
 2 ATP
 about 34 ATP
Figure 6.12 An estimated tally of the ATP produced by substrate-level and oxidative phosphorylation in cellular respiration.
by substrate-level
phosphorylation
by substrate-level
phosphorylation
by oxidative phosphorylation
About
38 ATP
Maximum per glucose:

30. Fermentation Occurs when O2 is not available
Animal cells and bacteria convert pyruvate to lactate
Other organisms convert pyruvate to alcohol and CO2

31. Glucose Is Not the Only Food Molecule
Cells also get energy from foods other than sugars
The other organic building blocks undergo chemical modifications that permit them to enter cellular respiration

32. 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.
OXIDATIVE
PHOSPHORYLATION
(Electron Transport
and Chemiosmosis)
CITRIC
ACID
CYCLE
Acetyl
CoA
Glucose
G3P
Pyruvate
GLYCOLYSIS
ATP

33. Do plants perform cellular respiration??