Photosynthesis takes place in three stages:
Light-dependent
reactions
Light-independent
reactions
The Calvin cycle
1. Capturing energy from
sunlight
2. Using energy to make
ATP and NADPH
3. Using ATP and NADPH
to power the synthesis of
carbohydrates from CO2
6 CO2 + 12 H2O + Light energy
carbon
dioxide
water
C6H12O6 + 6 H2O + 6 O2
glucose
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water
oxygen
6.5 Building New Molecules
Light-dependent reactions provide the raw material
1. ATP serves as the source of energy
2. NADPH provides the reducing power
The Calvin Cycle (C3 photosynthesis) is the pathway
that assembles the new molecules
It takes place in the stroma of the chloroplast
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6.6 An Overview of
Cellular Respiration
Cellular respiration is the harvesting of energy from
breakdown of organic molecules produced by plants
The overall process may be summarized as
C6H12O6 + 6 O2
glucose
oxygen
6 CO2
carbon
dioxide
+
6 H2O
water
+
energy
(heat or ATP)
Cellular respiration is carried out in two stages:
Occurs in the cytoplasm
1. Glycolysis
2. Oxidation
Occurs in the mitochondria
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6.7 Using Coupled Reactions
to Make ATP
Glycolysis is the first stage in cellular respiration
Takes place in the cytoplasm
Occurs in the presence or absence of oxygen
Involves ten enzyme-catalyzed reactions
These convert the 6-carbon glucose into two
3-carbon molecules of pyruvate
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Fig. 6.16 An overview of
aerobic respiration
Oxygen is the terminal
electron acceptor in
aerobic respiration
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Fig. 6.18 Glycolysis
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Fig. 6.18 Glycolysis
This coupled reaction
is called substrate-level
phosphorylation
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Fig. 6.16 An overview of
aerobic respiration
Oxygen is the terminal
electron acceptor in
aerobic respiration
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The Krebs Cycle
Takes place in the mitochondria
It consists of nine enzyme-catalyzed reactions that
can be divided into three stages
Stage 1
Acetyl coA binds a four-carbon molecule
producing a six-carbon molecule
Stage 2
Two carbons are removed as CO2
Stage 3
The four-carbon starting material is regenerated
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Fig. 6.22 How the Krebs cycle works
1
3
2
CoA–
(Acetyl-CoA)
4-carbon molecule
(Starting material)
6-carbon
molecule
6-carbon molecule
NADH
CO2
4-carbon
molecule
ATP
5-carbon
molecule
4-carbon molecule
(Starting material)
NADH
FADH2
4-carbon molecule
NADH
CO2
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Fate of Glucose
Glucose is entirely consumed in the process of
cellular respiration
It is converted to six molecules of CO2
Its energy is preserved in
Four ATP molecules
Ten NADH electron carriers
Two FADH2 electron carriers
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Fig. 6.16 An overview of
aerobic respiration
Oxygen is the terminal
electron acceptor in
aerobic respiration
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6.9 Using the Electrons to Make ATP
The NADH and FADH2 carry their high-energy
electrons to the inner mitochondrial membrane
There they transfer them to a series of membraneassociated carriers – the electron transport chain
Three of these carriers are protein complexes
that pump protons out of the matrix
The electrons are finally donated to oxygen to form
water
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Fig. 6.24 The electron transport chain
Intermembrane space
H+
H+
H+
Inner
mitochondrial
membrane
e–
NADH activates
all three pumps
e–
FADH2
NADH +H+
NAD+
Protein
complex I
FADH2 activates
only two pumps
2H+ + ½O2
Protein
complex II
Mitochondrial matrix
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Protein
complex III
H2O
The proton pumps lead to an increase in proton
concentration in the intermembrane space
The proton
gradient induces
the protons to
reenter the matrix
through ATP
synthase channels
The proton reentry
drives the
synthesis of ATP
by chemiosmosis
Fig. 6.25
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Fig. 6.26 An overview of the electron transport chain and chemiosmosis
Intermembrane space
Pyruvate from
cytoplasm
H+
H+
NADH
e–
H+
1. Electrons are harvested
and carried to the transport
system.
Acetyl-CoA
NADH
2. Electrons provide
energy to pump
protons across the
membrane.
e–
H2O
e–
Krebs
cycle
FADH2
3. Oxygen joins with
protons to form water.
1
2
+
O2
O2
2H+
CO2
32
2
ATP
ATP
Mitochondrial matrix
4. Protons diffuse back
in, driving the synthesis
of ATP.
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H+
ATP
synthase
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Glycolysis + Mitochondrial reactions = 36 ATP
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Other Sources of Energy
Food sources, other than sugars, can be used in
cellular respiration
These complex molecules are first digested into
simpler subunits
These subunits are modified into intermediates
These intermediates enter cellular respiration
at different steps
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Fermentation
The use of organic terminal electron acceptors
The electrons carried by NADH are donated to a
derivative of pyruvate
This allows the regeneration of NAD+ that keeps
glycolysis running
Two types of fermentation are common among
eukaryotes
Lactic fermentation
Ethanolic fermentation
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Pyruvate
is oxidized
Pyruvate
is reduced
Occurs in animal
muscle cells
Occurs in
yeast cells
Fig. 6.19
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Fermentation (No O2)
Removes byproducts of glycolysis
which allows glycolysis to continue.
No energetic yield (ATP not
produced), but by allowing glycolysis
to continue, cell can still receive 2
ATP.
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Fig. 6.16 An overview of
aerobic respiration
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