Transcript CHAPTER 8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

CHAPTER 8
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Photosynthesis Overview
• Energy for all life on Earth ultimately
comes from photosynthesis
6CO2 + 12H2O
C6H12O6 + 6H2O + 6O2
• Oxygenic photosynthesis is carried out by
– Cyanobacteria
– 7 groups of algae
– All land plants – chloroplasts
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Chloroplast
• Thylakoid membrane – internal membrane
– Contains chlorophyll and other photosynthetic
pigments
– Pigments clustered into photosystems
• Grana – stacks of flattened sacs of
thylakoid membrane
• Stroma lamella – connect grana
• Stroma – semiliquid surrounding thylakoid
membranes
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Stages
• Light-dependent reactions
– Require light
1.Capture energy from sunlight
2.Make ATP and reduce NADP+ to NADPH
• Carbon fixation reactions or lightindependent reactions
– Does not require light
3.Use ATP and NADPH to synthesize organic
molecules from CO2
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Pigments
• Molecules that absorb light energy in the
visible range
• Light is a form of energy
• Photon – particle of light
– Acts as a discrete bundle of energy
– Energy content of a photon is inversely
proportional to the wavelength of the light
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• Organisms have evolved a variety of
different pigments
• Only two general types are used in green
plant photosynthesis
– Chlorophylls
– Carotenoids
• In some organisms, other molecules also
absorb light energy
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Chlorophylls
• Chlorophyll a
– Main pigment in plants and cyanobacteria
– Only pigment that can act directly to convert
light energy to chemical energy
– Absorbs violet-blue and red light
• Chlorophyll b
– Accessory pigment or secondary pigment
absorbing light wavelengths that chlorophyll a
does not absorb
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• Carotenoids
– Carbon rings linked to
chains with alternating
single and double
bonds
– Can absorb photons
with a wide range of
energies
– Also scavenge free
radicals – antioxidant
• Protective role
• Phycobiloproteins
– Important in low-light
ocean areas
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Photosystem Organization
• Antenna complex
– Hundreds of accessory pigment molecules
– Gather photons and feed the captured light
energy to the reaction center
• Reaction center
– 1 or more chlorophyll a molecules
– Passes excited electrons out of the
photosystem
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Cyclic photophosphorylation
• In sulfur bacteria, only one photosystem is
used
• Generates ATP via electron transport
• Anoxygenic photosynthesis
• Excited electron passed to electron
transport chain
• Generates a proton gradient for ATP
synthesis
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Chloroplasts have two connected
photosystems
• Oxygenic photosynthesis
• Photosystem I (P700)
– Functions like sulfur bacteria
• Photosystem II (P680)
– Can generate an oxidation potential high enough to
oxidize water
• Working together, the two photosystems carry out
a noncyclic transfer of electrons that is used to
generate both ATP and NADPH
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• Photosystem I transfers electrons
ultimately to NADP+, producing NADPH
• Electrons lost from photosystem I are
replaced by electrons from photosystem II
• Photosystem II oxidizes water to replace
the electrons transferred to photosystem I
• 2 photosystems connected by cytochrome/
b6-f complex
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Noncyclic photophosphorylation
• Plants use photosystems II and I in series
to produce both ATP and NADPH
• Path of electrons not a circle
• Photosystems replenished with electrons
obtained by splitting water
• Z diagram
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Chemiosmosis
• Electrochemical gradient can be used to
synthesize ATP
• Chloroplast has ATP synthase enzymes in
the thylakoid membrane
– Allows protons back into stroma
• Stroma also contains enzymes that
catalyze the reactions of carbon fixation –
the Calvin cycle reactions
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Carbon Fixation – Calvin Cycle
• To build carbohydrates cells use
• Energy
– ATP from light-dependent reactions
– Cyclic and noncyclic photophosphorylation
– Drives endergonic reaction
• Reduction potential
– NADPH from photosystem I
– Source of protons and energetic electrons
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3 phases
1. Carbon fixation
– RuBP + CO2 → PGA
2. Reduction
– PGA is reduced to G3P
3. Regeneration of RuBP
– PGA is used to regenerate RuBP
•
•
3 turns incorporate enough carbon to produce a
new G3P
6 turns incorporate enough carbon for 1
glucose
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Photorespiration
• Rubisco has 2 enzymatic activities
– Carboxylation
• Addition of CO2 to RuBP
• Favored under normal conditions
– Photorespiration
• Oxidation of RuBP by the addition of O2
• Favored when stoma are closed in hot conditions
• Creates low-CO2 and high-O2
• CO2 and O2 compete for the active site on
RuBP
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C4 plants
• Corn, sugarcane, sorghum, and a number of
other grasses
• Initially fix carbon in mesophyll cells
• transported to bundle-sheath cells
• Carbon fixation then by rubisco and the Calvin
cycle
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CAM plants
• Many succulent (water-storing) plants,
such as cacti, pineapples, and some
members of about two dozen other plant
groups
• Stomata open during the night and close
during the day
– Reverse of that in most plants
• Fix CO2 during the night and store in
vacuole
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