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Photosynthesis:
Energy from the Sun
8
Identifying Photosynthetic
Reactants and Products
• Photosynthesis, the biochemical process by
which plants capture energy from sunlight and
store it in carbohydrates, is the very basis of life
on Earth.
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Identifying Photosynthetic
Reactants and Products
• By the 1800s, scientists had learned:
 Three ingredients are needed for
photosynthesis: water, CO2, and light.
 There are two products: carbohydrates and O2.
 The water, which comes primarily from the soil,
is transported through the roots to the leaves.
 The CO2 is taken in from the air through
stomata, or pores, in the leaves.
Figure 8.1 The Ingredients for Photosynthesis
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Identifying Photosynthetic
Reactants and Products
• By 1804, scientists had summarized the overall
chemical reaction of photosynthesis:
• CO2 + H2O + light energy  sugar + O2
• More recently, using H2O and CO2 labeled with
radioactive isotopes, it has been determined that
the actual reaction is:
• 6 CO2 + 12 H2O  C6H12O6 + 6 O2 + 6 H2O
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The Two Pathways of Photosynthesis:
An Overview
• Photosynthesis occurs in the chloroplasts of green
plant cells and consists of many reactions.
• Photosynthesis can be divided into two pathways:
 The light reaction is driven by light energy
captured by chlorophyll. It produces ATP and
NADPH + H+.
 The Calvin–Benson cycle does not use light
directly. It uses ATP, NADPH + H+, and CO2 to
produce sugars.
Figure 8.3 An Overview of Photosynthesis (Part 1)
Figure 8.3 An Overview of Photosynthesis (Part 2)
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The Interactions of Light and Pigments
• Visible light is part of the electromagnetic radiation
spectrum.
• It comes in discrete packets called photons.
Figure 8.5 The Electromagnetic Spectrum
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The Interactions of Light and Pigments
• When a photon and a pigment molecule meet,
one of three things happens: The photon may
bounce off, pass through,or be absorbed by the
molecule.
• If absorbed, the energy of the photon is acquired
by the molecule.
• The molecule is raised from its ground state to an
excited state of higher energy.
Figure 8.4 Exciting a Molecule
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The Interactions of Light and Pigments
• Molecules that absorb wavelengths in the visible
range are called pigments.
• When a beam of white light shines on an object,
and the object appears to be red in color, it is
because it has absorbed all other colors from the
white light except for the color red.
• In the case of chlorophyll, plants look green
because they absorb green light less effectively.
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The Interactions of Light and Pigments
• A molecule can absorb radiant energy of only
certain wavelengths.
• If we plot the absorption by the compound as a
function of wavelength, the result is an
absorption spectrum.
• If absorption results in an activity of some sort,
then a plot of the effectiveness of the light as a
function of wavelength is called an action
spectrum.
Figure 8.6 Absorption and Action Spectra
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The Interactions of Light and Pigments
• Plants have two predominant chlorophylls:
chlorophyll a and chlorophyll b.
• These chlorophylls absorb blue and red
wavelengths, which are near the ends of the
visible spectrum.
Figure 8.7 The Molecular Structure of Chlorophyll
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The Interactions of Light and Pigments
• A pigment molecule enters an excited state when
it absorbs a photon.
• If the molecule does not return to ground state,
the pigment molecule may pass some of the
absorbed energy to other pigment molecules.
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The Interactions of Light and Pigments
• Pigments in photosynthetic organisms are
arranged into systems.
• In these systems, pigments are packed together
and attached to thylakoid membrane proteins to
enable the transfer of energy.
• The excitation energy is passed to the reaction
center of the complex.
Figure 8.8 Energy Transfer and Electron Transport
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The Interactions of Light and Pigments
• Excited chlorophyll (Chl*) in the reaction center
acts as a reducing agent.
• Chl* can react with an oxidizing agent in a
reaction such as:
 Chl* + A  Chl+ + A–.
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The Light Reactions: Electron Transport,
Reductions, and Photophosphorylation
• The energized electron that leaves the Chl* in the
reaction center immediately participates in a
series of redox reactions.
• The electron flows through a series of carriers in
the thylakoid membrane (electron transport).
• Two energy rich products of the light reactions,
NADPH + H+ and ATP, are the result.
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The Light Reactions: Electron Transport,
Reductions, and Photophosphorylation
• There are two different reactions in
photosynthesis:
 Light Reactions: produces NADPH + H+ and
ATP and O2.
 Dark Reactions: uses CO2 , produces only
ATP.
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The Light Reactions: Electron Transport,
Reductions, and Photophosphorylation
• In the light reactions, two photosystems are used.
• Photosystems are light-driven molecular units
that consist of many chlorophyll molecules and
accessory pigments bound to proteins in separate
energy-absorbing antenna systems.
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The Light Reactions: Electron Transport,
Reductions, and Photophosphorylation
• Photosystem I uses light energy to reduce
NADP+ to NADPH + H+.
• The reaction center contains a chlorophyll a
molecule called P700 because it best absorbs light
at a wavelength of 700 nm.
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The Light Reactions: Electron Transport,
Reductions, and Photophosphorylation
• Photosystem II uses light energy to oxidize water
molecules, producing electrons, protons, and O2.
• The reaction center contains a chlorophyll a
molecule called P680 because it best absorbs light
at a wavelength of 680 nm.
Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 1)
Figure 8.10 Cyclic Electron Transport Traps Light Energy as ATP
NADP+ →
NADPH
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The Light Reactions: Electron Transport,
Reductions, and Photophosphorylation
• To produce ATP, high-energy electrons move
through a series of redox reactions, releasing
energy that is used to transport protons across
the membrane.
• Active proton transport results in the protonmotive force: a difference in pH and electric
charge across the membrane.
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The Light Reactions: Electron Transport,
Reductions, and Photophosphorylation
• The electron carriers in the thylakoid membrane
are oriented so as to move protons into the
interior of the thylakoid, and the inside becomes
acidic with respect to the outside.
• This difference in pH leads to the diffusion of H+
out of the thylakoid through ATP synthase.
Figure 8.11 Chloroplasts Form ATP Chemiosmotically
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Making Carbohydrate from CO2:
The Calvin–Benson Cycle
• The dark reactions of the Calvin-Benson cycle take
place in the stroma of the chloroplasts.
• This cycle does not use sunlight directly; but it
requires the ATP and NADPH + H+ produced in the
light reactions.
• Thus the Calvin-Benson reactions require light
indirectly and takes place only in the presence of
light.
• The Calvin-Benson cycle uses CO2.
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Making Carbohydrate from CO2:
The Calvin–Benson Cycle
• The initial reaction of the Calvin–Benson cycle
fixes one CO2 into a 5-carbon compound.
• The enzyme that catalyzes the fixation of CO2 is
called rubisco.
• Rubisco is the most abundant protein in the world.
Figure 8.14 RuBP Is the Carbon Dioxide Acceptor
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Making Carbohydrate from CO2:
The Calvin–Benson Cycle
• The Calvin–Benson cycle consists of three
processes:
 Fixation of CO2, by combination with RuBP
(catalyzed by rubisco)
 Conversion of fixed CO2 into carbohydrate
(G3P) (this step uses ATP and NADPH)
 Regeneration of the CO2 acceptor RuBP by ATP
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Making Carbohydrate from CO2:
The Calvin–Benson Cycle
• The end product of the cycle is glyceraldehyde 3phosphate, G3P.
• There are two fates for the G3P:
 One-third ends up as starch, which is stored in
the chloroplast and serves as a source of
glucose.
 Two-thirds is converted to the disaccharide
sucrose, which is transported to other organs.
Figure 8.13 The Calvin-Benson Cycle
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Making Carbohydrate from CO2:
The Calvin–Benson Cycle
• The products of the Calvin–Benson cycle are
vitally important to the biosphere as they are the
total energy yield from sunlight conversion by
green plants.
• Most of the stored energy is released by
glycolysis and cellular respiration by the plant
itself.
• Some of the carbon of glucose becomes part of
amino acids, lipids, and nucleic acids.
• Some of the stored energy is consumed by
heterotrophs, where glycolysis and respiration
release the stored energy.
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Metabolic Pathways in Plants
• Green plants are autotrophs and can synthesize
all their molecules from three simple starting
materials: CO2, H2O, and NH4.
• To satisfy their need for ATP, plants, like all other
organisms, carry out cellular respiration.
• Both aerobic respiration and fermentation can
occur in plants, although respiration is more
common.
• Cellular respiration takes place both in the dark
and in the light.
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Metabolic Pathways in Plants
• Energy flows from sunlight to reduced carbon in
photosynthesis to ATP in respiration.
• Energy can be stored in macromolecules such as
polysaccharides, lipids, and proteins.
• For plants to grow, energy storage must exceed
energy released or overall carbon fixation by
photosynthesis must exceed respiration.
• The capture and movement of sun energy
becomes the basis for ecological food chains.