Chapter 10
Photosynthesis
Dr. Wendy Sera
Houston Community
College
Biology 1406
© 2014 Pearson Education, Inc.
You should be able to:
1. Describe the structure of a chloroplast
2. Describe the relationship between an action
spectrum and an absorption spectrum
3. Trace the movement of electrons in linear
electron flow
4. Trace the movement of electrons in cyclic
electron flow
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5. Describe the similarities and differences between
oxidative phosphorylation in mitochondria and
photophosphorylation in chloroplasts
6. Describe the role of ATP and NADPH in the
Calvin cycle
7. Describe the major consequences of
photorespiration
8. Describe two important photosynthetic
adaptations that minimize photorespiration
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1
The Process That Feeds the Biosphere
 Photosynthesis is the process that converts solar
energy into chemical energy
 Directly or indirectly, photosynthesis nourishes
almost the entire living world
Figure 10.1—How does
sunlight help build the
trunk, branches, and
leaves of this broadleaf
tree?
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Energy flow
and chemical
recycling in
ecosystems
Light
energy
ECOSYSTEM
Photosynthesis
in chloroplasts
CO2 + H2O
Organic
+O
molecules 2
Cellular respiration
in mitochondria
ATP
ATP powers most cellular work
Heat
energy
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Modes of Nutrition
 Autotrophs sustain themselves without eating
anything derived from other organisms
 Autotrophs are the producers of the biosphere,
producing organic molecules from CO2 and other
inorganic molecules
 Almost all plants are photoautotrophs, using the
energy of sunlight to make organic molecules
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2
Modes of Nutrition, cont.
 Photosynthesis occurs in plants, algae, certain
other unicellular eukaryotes, and some
prokaryotes
 These organisms feed not only themselves, but
also most of the living world
Other organisms also benefit
from photosynthesis.
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(d) Cyanobacteria
40
μm
(b) Multicellular alga
1 μm
(a) Plants
10 μm
(e) Purple sulfur
bacteria
Figure 10. 2—Photoautotrophs
(c) Unicellular eukaryotes
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Modes of Nutrition, cont.
 Heterotrophs obtain their organic material from
other organisms
 Heterotrophs are the consumers of the biosphere
 Primary Consumers
 Secondary Consumers
 Almost all heterotrophs, including humans, depend
on photoautotrophs for food and O2
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3
BioDiesel—Alternative to Fossil Fuels
 Earth’s supply of fossil fuels was formed from the
remains of organisms that died hundreds of
millions of years ago
 In a sense, fossil fuels represent stores of solar
energy from the distant past
Figure 10.3—
Alternative
fuels from
algae
© 2014 Pearson Education, Inc.
Concept 10.1: Photosynthesis converts light
energy to the chemical energy of food
 Chloroplasts are structurally similar to and likely
evolved from photosynthetic bacteria
 The structural organization of these organelles
allows for the chemical reactions of
photosynthesis
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Chloroplasts: The Sites of Photosynthesis
in Plants
 Leaves (a plant organ) are the major locations of
photosynthesis
 Chloroplasts are found mainly in cells of the
mesophyll, the interior tissue of the leaf
 Each mesophyll cell contains 30–40 chloroplasts
 CO2 enters and O2 exits the leaf through
microscopic pores called stomata
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4
Leaf cross section
Chloroplasts Vein
Mesophyll
Figure 10.4—
Zooming in on
the location of
photosynthesis
in a plant
Stomata
CO2
O2
Chloroplast Mesophyll
cell
Thylakoid
Thylakoid
Stroma Granum space
Outer
membrane
Intermembrane
space
Inner
membrane
20 μm
1 μm
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Chloroplasts, cont.
 A chloroplast has an envelope of two membranes
surrounding a dense fluid called the stroma
 Thylakoids are connected sacs in the chloroplast
which compose a third membrane system
 Thylakoids may be stacked in columns called grana
 Chlorophyll, the pigment which gives leaves their
green color, resides in the thylakoid membranes
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Tracking Atoms Through Photosynthesis:
Scientific Inquiry
 Photosynthesis is a complex series of reactions
that can be summarized as the following equation:
6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O
 Can be simplified to:
6 CO2 + 6 H2O + Light energy → C6H12O6 + 6 O2
 Thus, the overall chemical change during
photosynthesis is the reverse of the one that
occurs during cellular respiration
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5
The Splitting of Water
Figure 10.5—Tracking atoms through photosynthesis
Reactants:
Products:
6 CO2
C6H12O6
12 H2O
6 H2 O
6 O2
 Chloroplasts split H2O into hydrogen and oxygen,
incorporating the electrons of hydrogen into sugar
molecules and releasing oxygen as a by-product
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Photosynthesis as a Redox Process
 Photosynthesis is a redox process in which H2O is
oxidized and CO2 is reduced (the opposite of
cellular respiration!)
 Photosynthesis is an endergonic process (thus,
an anabolic pathway); the energy boost is
provided by light
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Photosynthesis as a Redox Process, cont.
becomes reduced
becomes oxidized
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6
The Two Stages of Photosynthesis: A Preview
 Photosynthesis consists of the light reactions
(the photo part) and Calvin cycle (the synthesis
part)
 The light reactions (in the thylakoids)
 Split H2O
 Release O2
 Reduce the electron acceptor NADP+ to NADPH
 Generate ATP from ADP by photophosphorylation
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 The Calvin cycle (in the stroma) forms sugar from
CO2, using ATP and NADPH
 The Calvin cycle begins with carbon fixation,
incorporating CO2 into organic molecules
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Light
H2O
CO2
NADP+
LIGHT
REACTIONS
ADP
+
Pi
CALVIN
CYCLE
ATP
Thylakoid
Figure 10.6—An
overview of
photosynthesis:
cooperation of the
light reactions and
the Calvin cycle
Stroma
NADPH
Chloroplast
O2
[CH2O]
(sugar)
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7
BioFlix: Photosynthesis
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Concept 10.2: The light reactions convert solar
energy to the chemical energy of ATP and
NADPH
 Chloroplasts are solar-powered chemical factories
 Their thylakoids transform light energy into the
chemical energy of ATP and NADPH
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The Nature of Sunlight
 Light is a form of electromagnetic energy, also
called electromagnetic radiation
 Like other electromagnetic energy, light travels in
rhythmic waves
 Wavelength is the distance between crests
of waves
 Wavelength determines the type of electromagnetic
energy
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8
The Electromagnetic Spectrum
 The electromagnetic spectrum is the entire
range of electromagnetic energy, or radiation
 Visible light consists of wavelengths (including
those that drive photosynthesis) that produce
the colors we can see (380-750 nm)
 Light also behaves as though it consists of
discrete particles, called photons
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Figure 10.7—The electromagnetic spectrum
10−5 nm 10−3 nm 1 nm
Gamma
X-rays
rays
103 nm
UV
106 nm
Infrared
1m
(109 nm)
Microwaves
103 m
Radio
waves
Visible light
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
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Photosynthetic Pigments: The Light Receptors
 Pigments are substances that absorb visible light
 Different pigments absorb different wavelengths
 Wavelengths that are not absorbed are reflected or
transmitted
 Leaves appear green because chlorophyll
reflects and transmits green light
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9
Figure 10.8—Why leaves are green: interaction of light with
chloroplasts
Light
Reflected
light
Chloroplast
Absorbed
light
Granum
Transmitted
light
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Animation: Light and Pigments
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Spectrophotometry
 A spectrophotometer measures a pigment’s
ability to absorb various wavelengths
 This machine sends light through pigments and
measures the fraction of light transmitted at each
wavelength
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10
Refracting
prism
White
light
Chlorophyll
solution
2
Photoelectric
tube
Galvanometer
3
1
4
Slit moves to pass light
of selected wavelength.
Figure 10.9—Research
method: determining an
absorption spectrum
Green
light
The high transmittance (low
absorption) reading indicates that
chlorophyll absorbs very
little green light.
Blue
light
The low transmittance (high
absorption) reading indicates
that chlorophyll absorbs
most blue light.
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Absorption vs Action Spectra
 An absorption spectrum is a graph plotting a
pigment’s light absorption versus wavelength
 The absorption spectrum of chlorophyll a
suggests that violet-blue and red light work best
for photosynthesis
 An action spectrum profiles the relative
effectiveness of different wavelengths of radiation
in driving a process
Absorption
of light by
chloroplast
pigments
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Chlorophyll a
Chlorophyll b
Carotenoids
Rate of
photosynthesis
(measured by O2
release)
Figure 10.10-400
500
600
700
Which
Wavelength of light (nm)
wavelengths of (a) Absorption spectra
light are most
effective in driving
photosynthesis?
400
500
(b) Action spectrum
600
700
Aerobic bacteria
Filament of alga
400
500
(c) Engelmann’s experiment
600
700
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11
Englemann’s Experiment
 The action spectrum of photosynthesis was first
demonstrated in 1883 by Theodor W. Engelmann
 In his experiment, he exposed different segments
of a filamentous alga to different wavelengths
 Areas receiving wavelengths favorable to
photosynthesis produced excess O2
 He used the growth of aerobic bacteria clustered
along the alga as a measure of O2 production
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The Pigments of Photosynthesis
 Chlorophyll a is the main photosynthetic
pigment
 Accessory pigments, such as chlorophyll b,
broaden the spectrum used for photosynthesis
 The difference in the absorption spectrum
between chlorophyll a and b is due to a slight
structural difference between the pigment
molecules
 Accessory pigments called carotenoids absorb
excessive light that would damage chlorophyll
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CH3
CH3 in chlorophyll a
CHO in chlorophyll b
Porphyrin ring:
light-absorbing
“head” of molecule;
note magnesium
atom at center
Figure 10.11—Structure
of chlorophyll molecules
in chloroplasts of plants
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts; H atoms not
shown
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12
Video: Space-Filling Model of Chlorophyll a
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Excitation of Chlorophyll by Light
 When a pigment absorbs light, it goes from a
ground state to an excited state, which is unstable
 When excited electrons fall back to the ground
state, photons are given off, an afterglow called
fluorescence
 If illuminated, an isolated solution of chlorophyll
will fluoresce, giving off light and heat
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Figure 10.12—Excitation of isolated chlorophyll by light
Energy of electron
e−
Excited
state
Heat
Photon
Chlorophyll
molecule
Photon
(fluorescence)
Ground
state
(a) Excitation of isolated chlorophyll molecule
(b) Fluorescence
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13
A Photosystem: A Reaction-Center Complex
Associated with Light-Harvesting Complexes
 A photosystem consists of a reaction-center
complex (a type of protein complex) surrounded by
light-harvesting complexes
 The light-harvesting complexes (pigment molecules
bound to proteins) transfer the energy of photons to
the reaction center
 A primary electron acceptor in the reaction center
accepts excited electrons and is reduced as a result
 Solar-powered transfer of an electron from a
chlorophyll a molecule to the primary electron
acceptor is the first step of the light reactions
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Figure 10.13—The structure and function of a photosystem
Photosystem
Reactioncenter
complex
STROMA
Primary
electron
acceptor
e−
Transfer
of energy
Special pair of chlorophyll a molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
(a) How a photosystem harvests light
Thylakoid membrane
Lightharvesting
complexes
Thylakoid membrane
Photon
Chlorophyll
Protein
subunits
(b) Structure of a photosystem
STROMA
THYLAKOID
SPACE
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Photosystem
Photon
Thylakoid membrane
Figure
10.13—The
structure and
function of a
photosystem
Lightharvesting
complexes
STROMA
ReactionPrimary
center
complex electron
acceptor
e−
Transfer
of energy
Special pair of chlorophyll a molecules
Pigment
THYLAKOID SPACE
molecules
(INTERIOR OF THYLAKOID)
(a) How a photosystem harvests light
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14
Photosystem II
 There are two types of photosystems in the
thylakoid membrane
 Photosystem II (PS II) functions first (the
numbers reflect order of discovery) and is best at
absorbing a wavelength of 680 nm
 The reaction-center chlorophyll a of PS II is called
P680
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Photosystem I
 Photosystem I (PS I) is best at absorbing a
wavelength of 700 nm
 The reaction-center chlorophyll a of PS I is called
P700
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Linear Electron Flow
 During the light reactions, there are two possible
routes for electron flow: cyclic and linear
 Linear electron flow, the primary pathway,
involves both photosystems and produces ATP
and NADPH using light energy
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15
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
[CH2O] (sugar)
O2
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Linear Electron Flow, cont.
Primary
acceptor
e−
1
2
P680
Light
 There are 8 steps in linear
electron flow:
1. A photon hits a pigment and
its energy is passed among
pigment molecules until it
excites P680
2. An excited electron from
P680 is transferred to the
primary electron acceptor
(we now call it P680+)
Pigment
molecules
Photosystem II
(PS II)
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Linear Electron Flow, cont.
3. H2O is split by enzymes,
and the electrons are
transferred from the
hydrogen atoms to
P680+, thus reducing it to
P680
Primary
acceptor
2 H+
+
½
O2
1
H2 O
e−
2
3
e−
e−
P680
Light
Pigment
molecules
Photosystem II
(PS II)
• P680+ is the strongest
known biological
oxidizing agent
• O2 is released as a byproduct of this reaction
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16
Linear Electron Flow, cont. 4. Each electron “falls”
4
Primary
acceptor
2 H+
+
½ O2
e−
H2 O
Pq
2
Cytochrome
complex
3
Pc
e−
e−
1
down an electron
transport chain from
the primary electron
acceptor of PS II to
PS I
Electron
transport
chain
P680
5. Energy released by
the fall drives the
creation of a proton
gradient across the
thylakoid membrane
5
Light
ATP
Pigment
molecules
 Diffusion of H+
(protons) across the
membrane drives
ATP synthesis
Photosystem II
(PS II)
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Linear Electron Flow, cont.
4
Primary
acceptor
2 H+
+
½ O2
1
H2 O
e−
2
Primary
acceptor
e−
Pq
Cytochrome
complex
3
e−
e−
Electron
transport
chain
Pc
P680
P700
5
Light
Light
6
ATP
Pigment
molecules
Photosystem II
(PS II)
Photosystem I
(PS I)
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6. In PS I (like PS II), transferred light energy excites
P700, which loses an electron to an electron
acceptor
 P700+ (P700 that is missing an electron) accepts
an electron passed down from PS II via the
electron transport chain
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17
Linear Electron Flow, cont.
4
Primary
acceptor
2 H+
+
½ O2
1
H2 O
2
e−
Electron
transport
chain
e−
e−
Fd
e−
Pq
e− −
e
Cytochrome
complex
3
8
NADP+
reductase
Pc
P680
Electron
transport
chain
7
Primary
acceptor
NADP+
+ H+
NADPH
P700
5
Light
Light
6
ATP
Pigment
molecules
Photosystem II
(PS II)
Photosystem I
(PS I)
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7. Each electron “falls” down an electron transport
chain from the primary electron acceptor of PS I
to the protein ferredoxin (Fd)
8. The electrons are then transferred to NADP+ and
reduce it to NADPH
 The electrons of NADPH are now available for
the reactions of the Calvin cycle
 This process also removes an H+ from the stroma
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The energy changes of electrons during linear
flow through the light reactions can be shown
in a mechanical analogy
e−
e−
e−
e−
Mill
makes
ATP
e−
NADPH
e−
e−
ATP
Photosystem II
Photosystem I
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18
Cyclic Electron Flow
 In cyclic electron flow, electrons cycle back from
Fd to the PS I reaction center
 Cyclic electron flow uses only photosystem I and
produces ATP, but not NADPH
 No oxygen is released
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Figure 10.16—Cyclic electron flow
Primary
acceptor
Primary
acceptor
Fd
Fd
Pq
NADP+
+ H+
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Photosystem I
Photosystem II
ATP
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 Some organisms such as purple sulfur bacteria
have PS I, but not PS II
 Cyclic electron flow is thought to have evolved
before linear electron flow
 Cyclic electron flow may protect cells from
light-induced damage
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19
A Comparison of Chemiosmosis in Chloroplasts
and Mitochondria
 Chloroplasts and mitochondria generate ATP by
chemiosmosis, but use different sources of
energy
 Mitochondria transfer chemical energy from food
to ATP; chloroplasts transform light energy into
the chemical energy of ATP
 Spatial organization of chemiosmosis differs
between chloroplasts and mitochondria but also
shows similarities
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 In mitochondria, protons are pumped to the
intermembrane space and drive ATP synthesis as
they diffuse back into the mitochondrial matrix
 In chloroplasts, protons are pumped into the
thylakoid space and drive ATP synthesis as they
diffuse back into the stroma
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Figure 10.17—
Comparison of
chemiosmosis in
mitochondria and
chloroplasts
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrane
space
Inner
membrane
H+
Diffusion
Electron
transport
chain
Thylakoid
space
Thylakoid
membrane
ATP
synthase
Matrix
Stroma
ADP + P i
Key
Higher [H+]
Lower [H+]
H+
ATP
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20
Figure 10.18—The light reactions and chemiosmosis: Current
model of the organization of the thylakoid membrane
Photosystem II
4 H+
Light
Cytochrome
complex
Light
NADP+
reductase
3
Photosystem I
Fd
Pq
H2O
e
e
NADPH
Pc
2
1 ½ O2
THYLAKOID SPACE
+2 H+
(high H+ concentration)
NADP+ + H+
4 H+
To
Calvin
Cycle
Thylakoid
membrane
STROMA
(low H+ concentration)
ATP
synthase
ADP
+
Pi
H+
ATP
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 ATP and NADPH are produced on the side facing
the stroma, where the Calvin cycle takes place
 In summary, light reactions generate ATP and
increase the potential energy of electrons by
moving them from H2O to NADPH
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Concept 10.3: The Calvin cycle uses the
chemical energy of ATP and NADPH to reduce
CO2 to sugar
 The Calvin cycle, like the citric acid cycle,
regenerates its starting material after molecules
enter and leave the cycle
 The cycle builds sugar from smaller molecules by
using ATP and the reducing power of electrons
carried by NADPH
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21
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
[CH2O] (sugar)
O2
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 Carbon enters the cycle as CO2 and leaves as a
sugar named glyceraldehyde 3-phospate (G3P)
 For net synthesis of 1 G3P, the cycle must take
place three times, fixing 3 molecules of CO2
 The Calvin cycle has three phases
1. Carbon fixation (catalyzed by rubisco)
2. Reduction
3. Regeneration of the CO2 acceptor (RuBP)
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Input 3 CO2, entering one per cycle
Figure 10.19—
The Calvin
cycle
Phase 1: Carbon fixation
Rubisco
3 P
3 P
P
6
P
P
3-Phosphoglycerate
RuBP
6
ATP
6 ADP
Calvin
Cycle
3 ADP
3
ATP
6 P
P
1,3-Bisphosphoglycerate
6 NADPH
Phase 3:
Regeneration
of RuBP
6 NADP+
6 Pi
P
5
G3P
6
P
G3P
1
P
G3P
Output
Phase 2:
Reduction
Glucose and
other organic
compounds
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22
Concept 10.4: Alternative mechanisms of
carbon fixation have evolved in hot, arid
climates
 Dehydration is a problem for plants, sometimes
requiring trade-offs with other metabolic
processes, especially photosynthesis
 On hot, dry days, plants close stomata, which
conserves H2O, but also limits photosynthesis
 The closing of stomata reduces access to CO 2 and
causes O2 to build up
 These conditions favor an apparently wasteful
process called photorespiration
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Photorespiration: An Evolutionary Relic?
 In most plants (C3 plants), initial fixation of CO2,
via rubisco, forms a 3-carbon compound
(3-phosphoglycerate)
 In photorespiration, rubisco adds O2 instead of
CO2 in the Calvin cycle, producing a two-carbon
compound
 Photorespiration consumes O2 and organic fuel
and releases CO2 without producing ATP or sugar
 In many plants, photorespiration is a problem
because on a hot, dry day it can drain as much as
50% of the carbon fixed by the Calvin cycle
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C4 Plants: Sugarcane, Corn, & many Grasses
 C4 plants minimize the cost of photorespiration by
incorporating CO2 into 4-carbon compounds
 There are two distinct types of cells in the leaves
of C4 plants:
 Bundle-sheath cells are arranged in tightly packed
sheaths around the veins of the leaf
 Mesophyll cells are loosely packed between the
bundle sheath and the leaf surface
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23
 Sugar production in C4 plants occurs in a 3-step
process:
1. The production of the four carbon precursors is
catalyzed by the enzyme PEP carboxylase in
the mesophyll cells
 PEP carboxylase has a higher affinity for CO2
than rubisco does; it can fix CO2 even when CO2
concentrations are low
2. These four-carbon compounds are exported to
bundle-sheath cells
3. Within the bundle-sheath cells, they release
CO2 that is then used in the Calvin cycle
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C4 leaf anatomy
Photosynthetic
cells of
C4 plant
leaf
Mesophyll
cell
The C4 pathway
Mesophyll
cell PEP carboxylase
Bundlesheath
cell
Oxaloacetate (4C)
Vein
(vascular
tissue)
CO2
PEP (3C)
ADP
Malate (4C)
ATP
Pyruvate
CO (3C)
2
Stoma
Figure 10.20—C4 leaf anatomy
and the C4 pathway
Bundlesheath
cell
Calvin
Cycle
Sugar
Vascular
tissue
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Rising CO2 Levels
 Since the Industrial Revolution in the 1800s,
CO2 levels in the atmosphere have risen greatly
 Increasing levels of CO2 may affect C3 and C4
plants differently, perhaps changing the relative
abundance of these species
 The effects of such changes are unpredictable
and a cause for concern
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24
CAM Plants—Cacti, Pineapples, many
Succulents
 Some plants, including succulents, use
crassulacean acid metabolism (CAM) to
fix carbon
 CAM plants open their stomata at night,
incorporating CO2 into organic acids
 Stomata close during the day, and CO2 is released
from organic acids and then used in the Calvin
cycle
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Figure 10.21—C4 and CAM photosynthesis compared
Sugarcane
Pineapple
1
CO2
C4
Mesophyll Organic
cell
acid
CO2 2
Bundlesheath
cell
Calvin
Cycle
Sugar
(a) Spatial separation
of steps
CO2
Organic
acid
1
CAM
Night
CO2 2
Calvin
Cycle
Day
Sugar
(b) Temporal separation
of steps
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The Importance of Photosynthesis: A Review
 The energy entering chloroplasts as sunlight gets stored as
chemical energy in organic compounds
 Globally, photosynthesis makes 180 billion metric tons of
carbohydrates per year—the most productive chemical process
on Earth.
 Plants consume about 50% of the sugar they make as fuel
for their own cellular respiration (to regenerate ATP for
other cellular processes).
 Plants store the remaining 50% of the sugar as starch or other
sugars in structures such as roots, tubers, seeds, and fruits
 Sugars made in the chloroplasts supply all organisms on Earth
with chemical energy and carbon skeletons to synthesize their
own organic molecules, as well as the O2 in our atmosphere
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25
O2
Figure 10.22—A review of
photosynthesis (part 1)
CO2
H2O
Sucrose
(export)
Mesophyll cell
H2O
Chloroplast
CO2
Light
NADP+
LIGHT
REACTIONS:
Photosystem II
Electron transport
chain
Photosystem I
Electron transport
chain
H2O
O2
ADP
+
Pi
3-Phosphoglycerate
RuBP CALVIN
CYCLE
ATP
G3P
NADPH
Starch
(storage)
Sucrose (export)
© 2014 Pearson Education, Inc.
Figure 10.22—A review of photosynthesis (part 2)
LIGHT REACTIONS
• Are carried out by molecules
in the thylakoid membranes
• Convert light energy to the
chemical energy of ATP
and NADPH
• Split H2O and release O2
to the atmosphere
CALVIN CYCLE REACTIONS
• Take place in the stroma
• Use ATP and NADPH to convert
CO2 to the sugar G3P
• Return ADP, inorganic phosphate,
and NADP+ to the light reactions
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
26