Chapter 7: Photosynthesis (Outline)
 Photosynthetic Organisms
 Flowering Plants as Photosynthesizers
 Photosynthesis
 Light Reactions
 Noncyclic Pathway
 Calvin Cycle Reactions
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Carbon Dioxide Fixation
Reduction of Carbon Dioxide
Regeneration of RuBP
C4
CAM
Photosynthetic Organisms
 Organisms cannot live without energy
 Almost all the energy that organisms used
comes from the solar energy
 Photosynthesis:
 A process that captures solar energy and
transforms it into chemical energy (carbohydrate)
 Occurrs in higher plants, algae, mosses,
photosynthetic protists and some bacteria
(cyanobacteria)
Photosynthetic Organisms
 Autotrophs –
organisms having
the ability to
synthesize
carbohydrates
 Heterotrophs – are
consumers that use
carbohydrates
produced by
photosynthesis
Flowering Plants & Photosynthesis
 Photosynthesis takes place in the green
portions of plants
 Leaf of flowering plant contains mesophyll tissue
with chloroplasts
 Specialized to carry on photosynthesis
 CO2 enters leaf through stomata
 Diffuses into chloroplasts in mesophyll cells
 In the stroma, CO2 is combined with H2O to form
C6H12O6 (sugar)
 Chlorophyll is capable of absorbing solar energy,
which drives photosynthesis
Photosynthesis
 The process of photosynthesis is an example
of a redox reaction (i.e. movement of
electrons from one molecule to another)
 In living things, electrons are accompanied by
hydrogen ions (H+ + e)
 Oxidation is the loss of hydrogen atoms;
Reduction is the gain of hydrogen atoms
Photosynthesis
 Carbon dioxide reduction requires energy and
hydrogen atoms
 Solar energy will be used to generate ATP needed
to reduce CO2 to sugar
 Electrons needed to reduce CO2 will be carried by
a coenzyme (NADP+)
 NADP+ is the active redox coenzyme of
photosynthesis
 When NADP+ is reduced, it accepts 2 e- and H+
 When NADP+ is oxidized, it gives up its electrons
Photosynthetic Reactions
 In the 1930’s, C. B. van Niel studying
photosynthesis in bacteria discovered that CO2
was not split in the process of carbohydrate
synthesis
 Researchers later confirmed using the isotope
oxygen (18O) that O2 given off from
photosynthesis comes from water not CO2
 When water splits, O2 is released and the
hydrogen atoms (2 e- + H+) are taken up by
NADPH
Photosynthetic Reactions
 Light Reactions:
 Chlorophyll present in thylakoid membranes
absorbs solar energy
 This energizes electrons
 Electrons move down electron transport chain
 Pumps H+ into thylakoids
 Used to make ATP out of ADP and NADPH out of
NADP+
 Calvin Cycle Reactions:
 In the stroma, CO2 is reduced to a carbohydrate
 Reduction requires the ATP and NADPH
produced in the light reactions
Photosynthesis Overview
Photosynthetic Pigments
 Pigments:
 Molecules that absorb some colors in rainbow
(visible light) more than others, why?
 Colors least absorbed reflected/transmitted most
 Absorption Spectra
 Graph showing relative absorption of the various
colors of the rainbow
 Chlorophylls a and b (main pigments) are green
because they absorb much of the reds and blues
of white light
 Carotenoids are accessory pigments
Phosynthetic Pigments and
Photosynthesis
Spectrophotometer
 An instrument which measures the amount of
light of a specified wavelength which passes
through a medium
 Amount of light
absorbed at each
wavelength is
plotted on a graph,
resulting in an
absorption spectra
Light Reactions
 The light reactions utilize two photosystems:
 Photosystem I (PSI)
 Photosystem II (PSII)
 A photosystem consists of
 Pigment complex (molecules of chlorophylls a &
b the carotenoids) and electron acceptor
molecules that help collect solar energy like an
antenna
 Occur in the thylakoid membranes
The Noncyclic Electron Pathway
 Uses two photosystems, PS I and PS II
 Energy enters the system when PS II becomes
excited by light
 Causes an electron to be ejected from the
reaction center (chlorophyll a)
 Electron travels down electron transport chain to
PS I
 PS II takes replacement electron from H2O,
which splits, releasing O2 and H+ ions
 The H+ ions concentrate in thylakoid chambers
 Which causes ATP production, how?
The Noncyclic Electron Pathway
 PS I captures light energy and ejects the
energized electron (e-) from the reaction
center
 Transferred permanently by electron
acceptors to a molecule of NADP+
 Causes NADPH production
NADP+ + 2 e- + H+
 NADPH and ATP produced are used by the
Calvin cycle reaction (reduction of CO2) in
the stroma
Organization of the
Thylakoid Membrane
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PS II:
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Electron transport chain:
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Consists of Pq (plastoquinone) and cytochrome complexes
Carries electrons from PS II to PS I
Pumps H+ from the stroma into thylakoid space
PS I:
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Pigment complex and electron-acceptors
Adjacent to an enzyme that oxidizes water
Oxygen is released as a gas (by-product)
Pigment complex and electron acceptors
Adjacent to enzyme that reduces NADP+ to NADPH
ATP synthase complex:
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Has a channel for H+ flow
Which drives ATP synthase to join ADP and Pi
Organization of a Thylakoid
ATP Production
 Thylakoid space acts as a reservoir for H+ ions
 Each time water is oxidized, two H+ remain in
the thylakoid space
 Electrons yield energy
 Used to pump H+ across thylakoid membrane
 Move from stroma into the thylakoid space
 Flow of H+ back across thylakoid membrane
 Energizes ATP synthase
 Enzymatically produces ATP from ADP + Pi
 This method of producing ATP is called
chemiosmosis
Calvin Cycle Reactions:
Overview of C3 Photosynthesis
 A cyclical series of reactions
 Utilizes atmospheric carbon dioxide to
produce carbohydrates
 Known as C3 photosynthesis
 Involves three phases:
 Carbon dioxide fixation
 Carbon dioxide reduction
 RuBP Regeneration
Calvin Cycle Reactions:
Phase 1: Carbon Dioxide Fixation
 CO2 enters the stroma of the chloroplast via
the stomata of the leaves
 CO2 is attached to 5-carbon RuBP molecule
 Result in an intermediate 6-carbon molecule
 This splits into two 3-carbon molecules (3PG)
 Reaction is accelerated by RuBP Carboxylase
(Rubisco)
 CO2 now “fixed” because it is part of a
carbohydrate
Calvin Cycle Reactions:
Phase 2: Carbon Dioxide Reduction
 ATP phosphorylates each 3PG molecule and
creates 1,3-bisphosphoglycerate (BPG)
 BPG is then reduced by NADPH to
glyceraldehyde-3-phosphate (G3P)
 NADPH and some ATP used for the reduction
reactions are produced in light reactions
Calvin Cycle Reactions:
Phase 3: Regeneration of RuBP
 RuBP used in CO2 fixation must be replaced
 Every three turns of Calvin Cycle,
 Five G3P molecules are used
 To remake three RuBP molecules
 5 X 3 (carbons in G3P) = 3 X 5 (carbons in RuBP)
The Calvin
Cycle
C3 Plants
 C3 plants include more than 95% percent of
the plant species on earth (e.g. trees)
 Called C3 because the CO2 is first incorporated
into a 3-carbon compound
CO2 + RUBP
Rubisco
2 3PG
 In hot dry conditions, the photosynthetic
efficiency of C3 plants suffers because of
photorespiration
 Stomata must close to avoid wilting
 CO2 decreases and O2 increases
 O2 starts combining with RuBP instead of CO2
C4 Photosynthesis: C4 Plants
 Use a supplementary method of CO2 uptake
forming a 4-carbon molecule instead of the two
3-carbon molecules
 In C4 plants
 CO2 is inserted into a 3-carbon molecule called
phosphoenolpyruvate (PEP) forming
 Oxaloacetate, a C4 molecule in the mesophyll cells
 Oxaloacetate is converted into malate, which is
transported into the bundle sheath cells
 Here the 4-carbon molecule is broken down into
CO2, which enters the Calvin cycle to form sugars &
starch
Chloroplast distribution in
C4 vs. C3 Plants
CO2 Fixation in C4 vs. C3 Plants
 In hot and dry climates
 C4 plants avoid
photorespiration as PEP in
mesophyll cells does not
bind to O2
 Net productivity of C4 is
about 2-3 times of C3
plants
 In cool, moist conditions,
CO2 fixation in C3 is three
times more efficient than
in C4
CAM Photosynthesis
 Called CAM after the plant family in which it was first
found (Crassulaceae)
 CAM plants partition carbon fixation by time
 Crassulacean-Acid Metabolism
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During the night through their open stomata
 CAM plants use PEPCase to fix CO2
 Forms C4 molecules (malate)
 Stored in large vacuoles in mesophyll cells
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During daylight
 NADPH and ATP are available
 Stomata closed for water conservation
 C4 molecules release CO2 to Calvin (C3) cycle
CO2 Fixation in a CAM Plant
 Adaptive Value:
Better water use efficiency
than C3 plants under arid
conditions due to opening
stomata at night when
transpiration rates are
lower (no sunlight, lower
temperatures, lower wind
speeds, etc.)