2. Figure 9.2. The Carbon Cycle
Light
energy
Organic
molecules
O2
CO2 H2O+
Photosynthesis
in chloroplasts
Cellular respiration
in mitochondria
ECOSYSTEM
ATP powers
most cellular workATP
Heat
energy
+
Energy flows into an ecosystem as sunlight and leaves as heat
3. Catabolic Pathways and Production of ATP
• Electron transfer plays a major role
• The breakdown of organic molecules is exergonic
• Fermentation is a partial degradation of sugars that occurs
without oxygen
• Aerobic Cellular respiration consumes oxygen and organic
molecules and yields ATP
• Anaerobic respiration is similar to aerobic respiration but
consumes compounds other than O2
• Although carbohydrates, fats, and proteins are consumed
as fuel, it is helpful to trace cellular respiration with the
sugar glucose: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP + heat)
4. Redox Reactions: Oxidation and
Reduction
• The transfer of electrons during chemical reactions
releases energy stored in organic molecules
• This released energy is ultimately used to synthesize
ATP
• Chemical reactions that transfer electrons
between reactants are called oxidation-
reduction reactions, or redox reactions
5. The Principle of Redox
• Chemical reactions that transfer electrons between
reactants are called oxidation-reduction reactions, or
redox reactions
• In oxidation, a substance loses electrons, or is oxidized
• In reduction, a substance gains electrons, or is reduced
(the amount of positive charge is reduced)
Xe-
+ Y X + Ye-
becomes oxidized
(loses electron)
becomes reduced
(gains electron)
• Some redox reactions do not transfer electrons but
change the electron sharing in covalent bonds.
Remember our first lab!
7. 1. Because of differences in their initial oxidation state, the full oxidation of
a single carbon molecule can result in different free energy changes, even
though each forms just one CO2 molecule. Which choice correctly matches
the reactions shown with the proper relative energy change in attojoules (1
aJ = 10-18
J)?
1.CH4 + 2 O2 → CO2 + 2 H2O
2.CH3OH + 3/2 O2 → CO2 + 2 H2O
3.H2CO + O2 → CO2 + H2O
4.HCOOH + 1/2 O2 → CO2 + H2O
1. 1: −0.32 aJ; 2 −0.57 aJ; 3 −0.89 aJ; 4 −1.14 aJ
2. 1: −0.32 aJ; 2 −1.14 aJ; 3 −0.89 aJ; 4 −0.57 aJ
3. 1: −0.57 aJ; 2 −0.32 aJ; 3 −1.14 aJ; 4 −0.89 aJ
4. 1: −0.89 aJ; 2 −0.32 aJ; 3 −0.57 aJ; 4 −1.14 aJ
5. 1: −1.14 aJ; 2 −0.89 aJ; 3 −0.57 aJ; 4 −0.32 aJ
8. 1. Because of differences in their initial oxidation state, the full oxidation of
a single carbon molecule can result in different free energy changes, even
though each forms just one CO2 molecule. Which choice correctly matches
the reactions shown with the proper relative energy change in attojoules (1
aJ = 10-18
J)?
1.CH4 + 2 O2 → CO2 + 2 H2O
2.CH3OH + 3/2 O2 → CO2 + 2 H2O
3.H2CO + O2 → CO2 + H2O
4.HCOOH + 1/2 O2 → CO2 + H2O
1. 1: −0.32 aJ; 2 −0.57 aJ; 3 −0.89 aJ; 4 −1.14 aJ
2. 1: −0.32 aJ; 2 −1.14 aJ; 3 −0.89 aJ; 4 −0.57 aJ
3. 1: −0.57 aJ; 2 −0.32 aJ; 3 −1.14 aJ; 4 −0.89 aJ
4. 1: −0.89 aJ; 2 −0.32 aJ; 3 −0.57 aJ; 4 −1.14 aJ
5. 1: −1.14 aJ; 2 −0.89 aJ; 3 −0.57 aJ; 4 −0.32 aJ
11. • NADH passes the electrons to the electron transport
chain
• Unlike an uncontrolled reaction, the electron transport
chain passes electrons in a series of steps instead of
one explosive reaction
• Oxygen pulls electrons down the chain in an energy-
yielding tumble
• The energy yielded is used to regenerate ATP
12. H2 +½ O2 2 H + ½ O2
2 H+
2 e−
2 e−
2 H+
+
H2O
½ O2
Controlled
release of
energy
Electrontransport
chain
ATP
ATP
ATP
Explosive
release
Cellular respirationUncontrolled reaction(a) (b)
Freeenergy,G
Freeenergy,G
H2O
13. The Stages of Cellular Respiration: A Preview
• Cellular respiration has three stages:
– Glycolysis (breaks down glucose into two molecules of
pyruvate)
– The citric acid cycle (completes the breakdown of glucose)
– Oxidative phosphorylation (accounts for most of the ATP
synthesis)
• The process that generates most of the ATP is called oxidative
phosphorylation because it is powered by redox reactions
1.
2.
3.
GLYCOLYSIS (color-coded blue throughout the chapter)
PYRUVATE OXIDATION and the CITRIC ACID CYCLE
(color-coded orange)
OXIDATIVE PHOSPHORYLATION: Electron transport and
chemiosmosis (color-coded purple)
15. Electrons
via NADH
Electrons
via NADH
and FADH2
ATP ATP
CYTOSOL MITOCHONDRION
Substrate-level Substrate-level
GLYCOLYSIS PYRUVATE
OXIDATION CITRIC
ACID
CYCLEAcetyl CoAGlucose Pyruvate
Citric acid = Kreb’s = TCA = tri-carboxylic acid
16. Electrons
via NADH
Electrons
via NADH
and FADH2
ATP ATP ATP
CYTOSOL MITOCHONDRION
Substrate-level Substrate-level Oxidative
GLYCOLYSIS PYRUVATE
OXIDATION CITRIC
ACID
CYCLE
OXIDATIVE
PHOSPHORYLATION
(Electron transport
and chemiosmosis)
Acetyl CoAGlucose Pyruvate
17. • Oxidative phosphorylation accounts for almost 90% of
the ATP generated by cellular respiration
• A small amount of ATP is formed in glycolysis and the
citric acid cycle by substrate-level phosphorylation
VIDEO: Cellular Respiration
For each molecule of glucose degraded to CO2and
water by respiration, the cell makes up to 32
molecules of ATP
18. Glycolysis harvests energy by oxidizing
glucose to pyruvate
• Glycolysis (“splitting of sugar”) breaks down glucose
into two molecules of pyruvate
• Glycolysis occurs in the cytoplasm and has two major
phases:
– Energy investment phase
– Energy payoff phase
23. The citric acid cycle completes the
energy-yielding oxidation of organic
molecules
• Before the citric acid cycle can begin, pyruvate must be
converted to acetyl CoA, which links the cycle to
glycolysis
25. • The citric acid cycle, also called the Krebs cycle, takes
place within the mitochondrial matrix
• The cycle oxidizes organic fuel derived from pyruvate,
generating one ATP, 3 NADH, and 1 FADH2 per turn
26. Figure 9.11
PYRUVATE OXIDATION
Pyruvate (from glycolysis,
2 molecules per glucose)
NADH
NAD+
CO2
CoA
CoA
+ H+
CoA
CoA
CO2
CITRIC
ACID
CYCLE
FADH2
FAD
ATP
ADP + P
i
NAD+
+ 3 H+
NADH3
3
2
Acetyl CoA
27. • The citric acid cycle has eight steps, each catalyzed by a
specific enzyme
• The acetyl group of acetyl CoA joins the cycle by
combining with oxaloacetate, forming citrate
• The next seven steps decompose the citrate back to
oxaloacetate, making the process a cycle
• The NADH and FADH2 produced by the cycle relay
electrons extracted from food to the electron transport
chain
37. During oxidative phosphorylation,
chemiosmosis couples electron transport to
ATP synthesis
• Following glycolysis and the citric acid cycle, NADH and
FADH2 account for most of the energy extracted from
food
• These two electron carriers donate electrons to the
electron transport chain, which powers ATP synthesis
via oxidative phosphorylation
38. The Pathway of Electron Transport
• The electron transport chain is in the cristae of the
mitochondrion
• Most of the chain’s components are proteins, which
exist in multiprotein complexes
• The carriers alternate reduced and oxidized states as
they accept and donate electrons
• Electrons drop in free energy as they go down the
chain and are finally passed to O2, forming water
39. ATP ATP ATP
Glycolysis
Oxidative
phosphorylation:
electron transport
and chemiosmosis
Citric
acid
cycle
NADH
50
FADH2
40 FMN
Fe•S
I FAD
Fe•S II
III
Q
Fe•S
Cyt b
30
20
Cyt c
Cyt c1
Cyt a
Cyt a3
IV
10
0
Multiprotein
complexes
Freeenergy(G)relativetoO2(kcal/mol)
H2O
O22 H+
+ 1
/2
•The electron
transport chain
generates no ATP
•The chain’s
function is to
break the large
free-energy drop
from food to O2
into smaller steps
that release
energy in
manageable
amounts
40. Chemiosmosis: The Energy-Coupling
Mechanism
• Electron transfer in the electron transport chain causes
proteins to pump H+
from the mitochondrial matrix to
the intermembrane space
• H+
then moves back across the membrane, passing
through channels in ATP synthase
• ATP synthase uses the exergonic flow of H+
to drive
phosphorylation of ATP
• This is an example of chemiosmosis, the use of energy
in a H+
gradient to drive cellular work
41. Figure 9.14 INTERMEMBRANE
SPACE
Rotor
H+ Stator
Internal rod
Catalytic
knob
ADP
+
P i
MITOCHONDRIAL
MATRIX
ATP
A rotor within the
membrane spins
as shown when H+
flows past
it down the H+
gradient.
A stator anchored
in the membrane
holds the knob
stationary.
A rod (or “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.
43. • The energy stored in a H+
gradient across a membrane
couples the redox reactions of the electron transport
chain to ATP synthesis
• The H+
gradient is referred to as a proton-motive force,
emphasizing its capacity to do work
45. 2. The hydrogens taken from glucose or a
breakdown product of glucose are added to
oxygen, releasing energy to
a. actively transport H+
into the intermembrane
space.
b. actively transport NAD+
into the intermembrane
space.
c. actively transport Na+
into the matrix.
d. power facilitated
diffusion of H+
into
the matrix.
e. actively transport
H+
into the matrix.
46. 2. The hydrogens taken from glucose or a
breakdown product of glucose are added to
oxygen, releasing energy to
a. actively transport H+
into the intermembrane
space.
b. actively transport NAD+
into the intermembrane
space.
c. actively transport Na+
into the matrix.
d. power facilitated
diffusion of H+
into
the matrix.
e. actively transport
H+
into the matrix.
49. Fermentation enables some cells to
produce ATP without the use of oxygen
• Cellular respiration requires O2 to produce ATP
• Glycolysis can produce ATP with or without O2 (in
aerobic or anaerobic conditions)
• In the absence of O2, glycolysis couples with
fermentation to produce ATP
• Fermentation consists of glycolysis plus reactions that
regenerate NAD+
, which can be reused by glycolysis
• Two common types are alcohol fermentation and lactic
acid fermentation
50. Figure 9.17
2 ADP + 2 P i 2 ATP
Glucose
2 NAD+
NADH2
+ 2 H+
2 Pyruvate
CO22
2 Ethanol
(a) Alcohol fermentation
2 Acetaldehyde 2 Lactate
Lactic acid fermentation
2 ADP + 2 P i 2 ATP
GLYCOLYSIS
NAD+
+ 2 H+
NADH22
2 Pyruvate
(b)
GLYCOLYSIS Glucose
51. Fermentation and Cellular Respiration
Compared
• Both processes use glycolysis to oxidize glucose and
other organic fuels to pyruvate
• The processes have different final electron acceptors:
an organic molecule (such as pyruvate) in fermentation
and O2 in cellular respiration
• Cellular respiration produces much more ATP
52. • Yeast and many bacteria are facultative anaerobes,
meaning that they can survive using either
fermentation or cellular respiration
• In a facultative anaerobe, pyruvate is a fork in the
metabolic road that leads to two alternative catabolic
routes
53. LE 9-18
Pyruvate
Glucose
CYTOSOL
No O2 present
Fermentation
Ethanol
or
lactate
Acetyl CoA
MITOCHONDRION
O2 present
Cellular respiration
Citric
acid
cycle
54. The Evolutionary Significance of
Glycolysis
• Glycolysis occurs in nearly all organisms
• Glycolysis probably evolved in ancient prokaryotes
before there was oxygen in the atmosphere
• Very little O2 was available in the atmosphere until
about 2.7 billion years ago, so early prokaryotes likely
used only glycolysis to generate ATP
• Gycolysis and the citric acid cycle are major
intersections to various catabolic and anabolic
pathways
55. The Versatility of Catabolism
• Catabolic pathways funnel electrons from many kinds
of organic molecules into cellular respiration
• Glycolysis accepts a wide range of carbohydrates
• Proteins must be digested to amino acids; amino
groups can feed glycolysis or the citric acid cycle
• Fats are digested to glycerol (used in glycolysis) and
fatty acids (used in generating acetyl CoA)
• An oxidized gram of fat produces more than twice as
much ATP as an oxidized gram of carbohydrate (has
more reducing power because less oxidized)
