1. Cellular Respiration: Harvesting Chemical Energy
Chapter 9
Cellular Respiration: Harvesting Chemical Energy

2. Overview: Life Is Work Living cells Ex: The giant panda
Require transfusions of energy from outside sources to perform their many tasks
Ex: The giant panda
Obtains (chemical, potential) energy for its cells by eating plants
Figure 9.1

3. powers most cellular work
Energy
Flows into an ecosystem as sunlight and leaves as heat
Light energy
ECOSYSTEM
CO2 + H2O
Photosynthesis in chloroplasts
Cellular respiration in mitochondria
Organic molecules
+ O2
ATP
powers most cellular work
Heat energy
Figure 9.2

4. Review:
1st law- Energy cannot be created or destroyed. Energy can be converted from one form to another. The sum of the energy before the conversion is equal to the sum of the energy after the conversion.
2nd law- Some usable energy dissipates during transformations and is lost. During changes from one form of energy to another, some usable energy dissipates, usually as heat. The amount of usable energy therefore decreases.

5. Cellular Respiration Glucose + O2  energy (ATP) + CO2 + H2O
This is exergonic –DG and converts one form of chemical energy (glucose) into another (ATP), with heat dissipates
Consumes oxygen and organic molecules (such as glucose)
Glucose Oxidation is the most prevalent and efficient catabolic pathway
Yields lots of ATP
An alternate catabolic process, fermentation
Is a partial degradation of sugars that occurs without oxygen (in anaerobic conditions)

6. More on ATP ATP (adenosine triphosphate)
ATP is a nucleotide.  Nucleotides are the building blocks of nucleic acids such as DNA and RNA.  They contain a nitrogen-containing base, a 5-carbon sugar, and phosphate groups.
The energy in one glucose molecule is used to produce up to 38 ATP.  ATP has approximately the right amount of energy for most cellular reactions.

7. ATP, ADP, AMP
The bonds between the phosphate groups are high-energy bonds. Energy is required (and stored) to form the bonds and energy is released when the bonds are broken. (below)

8. Phosphorylation
ATP is produced and used continuously. The entire amount of ATP in an organism is recycled once per minute. Most cells maintain only a few seconds supply of ATP.
To keep working, cells must regenerate ATP
When a phosphate group is transferred to another molecule, it is called PHOSPHORYLATION.
ADP + Pi + energy ---->  ATP
It takes about 7.3 kcals of energy to phosphorylate ADP into ATP
Enzyme that catalyzes this reaction is ATP synthase

9. Phosphorylation
Phosphate groups from ATP can also be transferred to other molecules.
Enzymes that catalyze this reaction are called KINASES.
In these phosphorylation reactions, energy is transferred from the phosphate group in ATP to the phosphorylated compound. This newly energized compound will participate in other reactions.

10. Redox Reactions: Oxidation and Reduction
Catabolic pathways also yield energy due to the transfer of electrons
In oxidation
A substance loses electrons, or is oxidized
In reduction
A substance gains electrons, or is reduced
“Redox” reactions
Transfer electrons from one reactant to another by oxidation and reduction
Na Cl Na Cl–
becomes oxidized (loses electron)
becomes reduced (gains electron)

11. “Redox” reactions
Transfer of electrons from one reactant to another by oxidation and reduction
When the electron moves to a lower energy level, energy is released.

12. Redox cont’d Some redox reactions
Do not completely exchange electrons, instead they change the degree of electron sharing in covalent bonds
CH4 H C O
Methane (reducing agent)
Oxygen (oxidizing agent)
Carbon dioxide
Water +
2O2
CO2
Energy
2 H2O
becomes oxidized
becomes reduced
Reactants
Products
Figure 9.3

13. Oxidation of Organic Fuel Molecules During Cellular Respiration
Glucose is oxidized and oxygen is reduced
C6H12O6 + 6O CO2 + 6H2O + Energy
becomes oxidized
becomes reduced
Glucose + Oxygen ----> Carbon dioxide + water + Energy
Yields DG = -686 Kcal/mole
This energy is used to add phosphates to ADP + P  ATP
There is a need to convert glucose (sugar) which is stored chemical energy into ATP which is usable cell energy

14. Electron carriers
Some compounds can accept and donate electrons readily, and these are called electron carriers in organisms. There are a number of molecules that serve as electron carriers.
NAD (nicotinamide adenine dinucleotide) is used in respiration. Reduced to NADH.
NADP (nicotinamide adenine dinucleotide phosphate) is another used in photosynthesis. Reduced to NADPH
These molecules readily give up electrons (oxidized) and gain electrons (reduced).
FAD (flavin adenine dinucleotide) is reduced to FADH2
H often represents one electron, one proton.

15. The electron transport chain
NADH, the reduced form of NAD+
Passes the electrons along an electron transport chain
Electron are passed in a series of steps instead of in one explosive reaction
If electron transfer is not stepwise
A large release of energy occurs = dangerous to living cells
Uses the energy from the electron transfer to form ATP
2 H
1/2 O2
(from food via NADH)
2 H e–
2 H+
2 e–
H2O
Controlled release of energy for synthesis of ATP
ATP
Electron transport chain
Free energy, G
(b) Cellular respiration +
Figure 9.5 B

16. ATP originates in cellular respiration
What happens is that sugar is broken down into smaller molecules and energy is released.
This energy is used to generate ATP from ADP and P.
Sugar > smaller molecules >ADP + Pi > ATP

17. The Stages of Cellular Respiration: A Preview
Respiration is a cumulative function of three metabolic stages
Glycolysis
Breaks down glucose into two molecules of pyruvate
The citric acid (Krebs) cycle
Completes the breakdown of glucose
Oxidative phosphorylation
Is driven by the electron transport chain (ETC)
Generates ATP

18. Oxidative phosphorylation: electron transport and chemiosmosis
An overview of cellular respiration
Figure 9.6
Electrons
carried
via NADH
Glycolsis
Glucose
Pyruvate
ATP
Substrate-level
phosphorylation
Electrons carried
via NADH and
FADH2
Citric acid cycle
Oxidative phosphorylation: electron transport and chemiosmosis
Oxidative
Mitochondrion
Cytosol

19. Both glycolysis and the citric acid cycle
Can generate ATP by substrate-level phosphorylation
Figure 9.7
Enzyme
ATP
ADP
Product
Substrate P +

20. Glycolysis Means “splitting of sugar”
Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate
Glycolysis
Means “splitting of sugar”
Breaks down glucose into pyruvate
Occurs in the cytoplasm of the cell

21. Energy investment phase
Glycolysis consists of two major phases
Energy investment phase
Energy payoff phase
Glycolysis
Citric acid cycle
Oxidative phosphorylation
ATP
2 ATP
4 ATP
used
formed
Glucose
2 ATP + 2 P
4 ADP + 4
2 NAD+ + 4 e- + 4 H +
2 NADH
+ 2 H+
2 Pyruvate + 2 H2O
Energy investment phase
Energy payoff phase
4 ATP formed – 2 ATP used
2 NAD+ + 4 e– + 4 H +
Figure 9.8
exergonic, energy liberating  -DG                                                    
endergonic, energy intake +DG 

22. A closer look at the energy investment phase:
[Step 1] Energy input required   
Terminal P from an ATP (-->ADP) is bonded to the C6 of a glucose
DG = (enzyme: hexokinase)
[Step 2] glucose phosphate------> fructose-6-phosphate
atoms rearranged
DG = (enzyme: phosphoglucoisomerase)
[Step 3] fructose-6-phosphate------> fructose 1,6 biphosphate
Phosphate added to C1
DG = (enzyme: phosphofructokinase)
 splits into two products
[Step 4] fructose -1, 6 - biphosphate---> dihydroxyacetone phosphate
AND glyceraldehyde phosphate*
DG = ( enzyme: aldolase)
Our 6-carbon sugar is split into TWO  3-carbon products
[Step 5]    *Ultimately, all of the dihydroxyacetone phosphate will be converted into glyceraldehyde-3-phosphate, so each step is actually x 2 from here (because each of the two molecules will proceed through Steps 5- 9 )
dihydroxyacetone phosphate > glyceraldehyde-3-phosphate
(enzyme: isomerase)
Dihydroxyacetone
phosphate
Glyceraldehyde-
3-phosphate H OH HO
CH2OH O P
CH2O
CH2 C
CHOH
ATP
ADP
Hexokinase
Glucose
Glucose-6-phosphate
Fructose-6-phosphate
Phosphoglucoisomerase
Phosphofructokinase
Fructose-
1, 6-bisphosphate
Aldolase
Isomerase
Glycolysis 1 2 3 4 5
Oxidative
phosphorylation
Citric
acid
cycle
Figure 9.9 A

23. A closer look at the energy payoff phase
[Step 6]     2 Glyceraldehyde-3-phosphate molecules are then oxidized
2 hydrogens (with e-) are removed, and NAD+ is reduced to NADH and H+
Also, a free phosphate attaches to each of the glyceraldehyde-3-phosphates
2 Glyceraldehyde-3-phosphate > 1, 3 biphosphoglycerate (x2)
NAD+ is reduced to NADH and H+
Pi = free phosphates, not taken from another molecule such as ATP
NAD = nicotinamide adenine dinucleotide (niacin derivative)
DG = (enzyme: triose phosphate dehydrogenase)
[Step 7] a P is released from each 1,3 biphosphoglycerate molecule and is used to "recharge" 2 ADPs----> 2 ATPs
thus, converting the 1,3 biphosphoglycerate to 3-phosphoglycerate
(highly exergonic= large DG to pull preceding reactions forward)
(x2) 1,3 Diphosphoglycerate > 3-phosphoglycerate
ADP---> ATP
DG = (enzyme: phosphoglycerokinase)
(x 2) [Step 8]   The remaining P group is enzymatically transferred from the 3C to
the 2C
3- phosphoglycerate > 2 - phosphoglycerate
DG = (enzyme: phosphoglyceromutase)
(x 2) [Step 9] A molecule of H2O removed
2 - phosphoglycerate > phosphoenolpyruvate
- H2O
DG = (enzyme: enolase)
(x 2) [Step 10] Another phosphate P is transferred to ADP
( highly exergonic, pulls 2 preceding rxns!!!)
phosphoenolpyruvate > pyruvate (pyruvic acid) DG = ( enzyme: pyruvate kinase)
2 NAD+
NADH 2
+ 2 H+
Triose phosphate
dehydrogenase
P i P C
CHOH O
CH2 O–
1, 3-Bisphosphoglycerate
2 ADP
2 ATP
Phosphoglycerokinase
3-Phosphoglycerate
Phosphoglyceromutase
CH2OH H
2-Phosphoglycerate
2 H2O
Enolase
Phosphoenolpyruvate
Pyruvate kinase
CH3 6 8 7 9 10
Pyruvate

24. Mitochondria Review
The mitochondrion is surrounded by two membranes. The outer is smooth and the inner folds inwards. The inner folds are called cristae. Within the inner compartment of the mitochondrion, surrounding the cristae, there is a dense solution known as the matrix. The matrix contains enzymes, co-enzymes, water, phosphates, and other molecules needed in respiration.
The outer membrane is permeable to most small molecules, but the inner one permits the passage of only certain molecules, such as pyruvic acid and ATP.
Proteins are built into the membrane of the cristae. These proteins are involved with the Electron Transport Chain. The inner membrane is about 80% protein and 20% lipids. 95% of the ATP generated by the heterotrophic cell is produced by the mitochondrion.

25. The Fate of the Pyruvate
The pyruvate passes from the cytoplasm (where its produced thru glycolysis) and crosses the outer & inner membranes of the mitochondria
Before entering the Krebs Cycle, the 3-C pyruvate molecule is oxidized: the carbon and oxygen atoms of the carboxyl group are removed (into CO2) and a 2-C acetyl group is left (CH3CO)
In the course of this rxn, the carboxyl hydrogen reduces a molecule of NAD+ to NADH
The acetyl is momentarily accepted by a "coenzyme A" molecule (a large, complex molecule formed from pantothenic acid (vitamin B)), forming Acetyl-CoA
From here the Acetyl CoA can enter the Krebs (aka, Citric Acid) Cycle.  Acetyl CoA moves into the mitochondria and is completely dismantled by the enzymes in the mitochondria.

26. Before the citric acid cycle can begin
Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis
CYTOSOL
MITOCHONDRION
NADH
+ H+
NAD+ 2 3 1
CO2
Coenzyme A
Pyruvate
Acetyle CoA S
CoA C
CH3 O
Transport protein O–
Figure 9.10

27. An overview of the citric acid cycle
Discovered 1937 by British biochemist Sir Hans Adolf Krebs
CO2 is produced.
Acetyls are dismantled and reconfigured. The electrons are what's important.
The Krebs's cycle only gives us two molecules of ATP. Added with the two molecules of ATP made in Glycolysis, the total is now a meager four molecules of ATP.
The remainder of the ATPs come from the Electron Transport System, which takes the electrons produced in the Krebs's cycle and makes ATP.
ATP
2 CO2
3 NAD+
3 NADH
+ 3 H+
ADP + P i
FAD
FADH2
Citric acid cycle
CoA
Acetyle CoA
NADH
CO2
Pyruvate (from glycolysis, 2 molecules per glucose)
Glycolysis
Oxidative phosphorylation
Figure 9.11

28. A closer look at the citric acid cycle
In general:
2-C acetyl combines with 4-C oxaloacetic acid to form a 6-C citric acid.
Two carbons (per cycle) are oxidized to CO2 which regenerates a molecule of oxaloacetic acid
Each turn of the cycle uses up one acetyl group and regenerates a oxaloacetic acid, then begins the cycle again.
Energy is released by breaking C-H and C-C bonds and is stored by transforming ADP to ATP (1 molecule per turn of the cycle) and to convert NAD+ to NADH and H+ (3x per cycle)
Also, FAD (flavin adenine dinucleotide) is converted to FADH2 (one molecule per cycle)
1. No oxygen is required in Krebs Cycle
2. All e- and p+ are accepted by either NAD+ or FAD
There are 8 steps in the cycle and the aim is to totally dismantle the Acetyl CoA, using only its electrons.
We have totally taken apart the glucose molecule.
Only four ATPs have resulted, 2 from glycolysis and 2 from the CAC.
But we still have a lot of ELECTRONS captured in the form of NADH and FADH2 = potential energy
Acetyl CoA
NADH
Oxaloacetate
Citrate
Malate
Fumarate
Succinate
Succinyl
CoA
a-Ketoglutarate
Isocitrate
Citric
acid
cycle S SH
FADH2
FAD
GTP
GDP
NAD+
ADP
P i
CO2
H2O
+ H+ C
CH3 O
COO–
CH2 HO HC CH 1 2 3 4 5 6 7 8
Glycolysis
Oxidative
phosphorylation
ATP
Figure 9.12
Figure 9.12

29. Oxidative phosphorylation
Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
NADH and FADH2
Donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

30. The Pathway of Electron Transport
In the electron transport chain
Electrons from NADH and FADH2 lose energy in several steps
At the end of the chain
Electrons are passed to oxygen, forming water
Transport Chain carriers are called CYTOCHROMES (consist of protein and a heme group = atom of iron enclosed in a porphyrin ring (note the similarity to hemoglobin!)
electrons are "passed down" a "staircase" of cytochromes and the energy released in lowering the energy level is stored in additional ATP molecules...called Oxidative Phosphorylation
remember, the energy from lowering electrons can do this: ADP + P = ATP
---> for every  e- passed down the chain from NADH, 3 ATPs are formed
---> for every 2e- from FADH2, 2 ATP's are formed
H2O O2
NADH
FADH2
FMN
Fe•S O
FAD
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
2 H + + 12 I II
III IV
Multiprotein complexes 10 20 30 40 50
Free energy (G) relative to O2 (kcl/mol)
Figure 9.13

31. Chemiosmosis: The Energy-Coupling Mechanism
ATP synthase
Is the enzyme that actually makes ATP
INTERMEMBRANE SPACE H+
P i +
ADP
ATP
A rotor within the membrane spins clockwise when H+ flows past it down the H+ gradient.
A stator anchored in the membrane holds the knob stationary.
A rod (for “stalk”) extending into the knob also spins, activating catalytic sites in the knob.
Three catalytic sites in the stationary knob join inorganic
Phosphate to ADP to make ATP.
MITOCHONDRIAL MATRIX
Figure 9.14

32. Chemiosmosis
Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work
At certain steps along the electron transport chain, electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space
The resulting H+ gradient
Stores energy
Drives chemiosmosis in ATP synthase
Is referred to as a “proton-motive force”

33. Electron transport chain Oxidative phosphorylation
Chemiosmosis and the electron transport chain
Oxidative
phosphorylation.
electron transport
and chemiosmosis
Glycolysis
ATP
Inner
Mitochondrial
membrane H+
P i
Protein complex
of electron
carners
Cyt c I II
III IV
(Carrying electrons
from, food)
NADH+
FADH2
NAD+
FAD+
2 H+ + 1/2 O2
H2O
ADP +
Electron transport chain
Electron transport and pumping of protons (H+),
which create an H+ gradient across the membrane
Chemiosmosis
ATP synthesis powered by the flow
Of H+ back across the membrane
synthase Q
Oxidative phosphorylation
Intermembrane
space
mitochondrial
matrix
Figure 9.15

34. There are three main processes in this metabolic enterprise
Summary
There are three main processes in this metabolic enterprise
Electron shuttles
span membrane
CYTOSOL
2 NADH
2 FADH2
6 NADH
Glycolysis
Glucose 2
Pyruvate
Acetyl
CoA
Citric
acid
cycle
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
MITOCHONDRION
by substrate-level
phosphorylation
by oxidative phosphorylation, depending
on which shuttle transports electrons
from NADH in cytosol
Maximum per glucose:
About
36 or 38 ATP
+ 2 ATP
+ about 32 or 34 ATP or
Figure 9.16

35. The energy tally sheet Glycolysis: 2 ATP =2 ATP
2 NADH ets  6 ATP =6 ATP
Oxidation: pyruvate  acetyl CoA  1 NADH  ets  3 ATP (x2) = 6 ATP
CAC/Krebs 1 ATP x2 turns  2 ATP
3 NADH  ets  9 ATP x2 turns  18 ATP
1 FADH2  ets  2 ATP x2 turns  4 ATP = 24 ATP
TOTAL = 38 ATP

36. efficiency
Less than 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making approximately 38 ATP
DG =
ADP + Pi --> ATP
= about 7 kcal/mole
X 38 = 266 kcal/mole (captured in P bonds)
DG = -686 kcal/mole (free energy change during glycolysis and oxidation for glucose)
= about 39% efficiency

37. Oxidative cellular respiration
FERMENTATION
Concept 9.5: Fermentation enables some cells to produce ATP without the use of oxygen
Oxidative cellular respiration
Relies on oxygen to produce ATP
Involves Krebs cycle and ETS
In the absence of oxygen
Cells can still produce a little bit of ATP through fermentation

38. Without oxygen: Glycolysis
Can produce ATP with or without oxygen, in either aerobic or anaerobic conditions
Pyruvate is produced, some ATP is made
In anaerobic respiration (fermentation) pyruvate must be processed in the absence of oxygen
The breakdown of the sugar still takes place through a series of chemical reactions, but with a different outcome.

39. The Evolutionary Significance of Glycolysis
Occurs in the cytoplasm of nearly all organisms
Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere

40. Pyruvate is a key juncture in catabolism
Glucose
CYTOSOL
Pyruvate
No O2 present
Fermentation
O2 present
Cellular respiration
Ethanol or
lactate
Acetyl CoA
MITOCHONDRION
Citric
acid
cycle
Figure 9.18

41. Fermentation
Living organisms have developed numerous and different fermentation pathways; however, most organisms use the following Embden-Meyerhoff pathway, named for the two discoverers.
The pyruvate can take two pathways in anaerobic respiration:
a. Pyruvate will be converted to ethyl alcohol (ethanol) and carbon dioxide. This is called alcoholic fermentation and is the basis of our wine, beer and liquor industry.
b. The pyruvate will be converted to lactic acid. This is called lactic acid fermentation. Lactic acid is what makes your muscles burn during prolonged exercise, this process is also used to make yogurt.

42. Fermentation consists of
Types of Fermentation
Fermentation consists of
Glycolysis plus reactions that regenerate NAD+, which can be reused by glyocolysis
In alcohol fermentation
Pyruvate is converted to ethanol in two steps, one of which releases CO2
During lactic acid fermentation
Pyruvate is reduced directly to NADH to form lactate as a waste product

43. (a) Alcohol fermentation (b) Lactic acid fermentation
2 ADP + 2 P1
2 ATP
Glycolysis
Glucose
2 NAD+
2 NADH
2 Pyruvate
2 Acetaldehyde
2 Ethanol
(a) Alcohol fermentation
2 Lactate
(b) Lactic acid fermentation H OH
CH3 C
O – O O–
CO2 2
Figure 9.17

44. Fermentation and Cellular Respiration Compared
Both fermentation and cellular respiration
Use glycolysis to oxidize glucose and other organic fuels to pyruvate
Fermentation and cellular respiration
Differ in their final electron acceptor
Oxidative cellular respiration
Produces more ATP

45. The Versatility of Catabolism
Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways
Catabolic pathways
Funnel electrons from many kinds of organic molecules into cellular respiration

46. The catabolism of various molecules from food
Amino
acids
Sugars
Glycerol
Fatty
Glycolysis
Glucose
Glyceraldehyde-3- P
Pyruvate
Acetyl CoA
NH3
Citric
acid
cycle
Oxidative
phosphorylation
Fats
Proteins
Carbohydrates
Figure 9.19

47. Regulation of Cellular Respiration via Feedback Mechanisms
Is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle
Enzyme 1
Enzyme 2
Enzyme 3 A B C D
Reaction 1
Reaction 2
Reaction 3
Starting molecule
Product

48. Feedback Control Regulates Biological Processes
The control of cellular respiration
Glucose
Glycolysis
Fructose-6-phosphate
Phosphofructokinase
Fructose-1,6-bisphosphate
Inhibits
Pyruvate
ATP
Acetyl CoA
Citric
acid
cycle
Citrate
Oxidative
phosphorylation
Stimulates
AMP + –
Figure 9.20