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Photosynthesis
Chapter 8
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
3
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
4
5
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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Discovery of Photosynthesis
• Jan Baptista van Helmont (1580–1644)
– Demonstrated that the substance of the plant
was not produced only from the soil
• Joseph Priestly (1733–1804)
– Living vegetation adds something to the air
• Jan Ingen-Housz (1730–1799)
– Proposed plants carry out a process that uses
sunlight to split carbon dioxide into carbon
and oxygen (O2 gas)
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• F.F. Blackman (1866–
1947)
– Came to the startling
conclusion that
photosynthesis is in
fact a multistage
process, only one
portion of which uses
light directly
– Light versus dark
reactions
– Enzymes involved
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• C. B. van Niel (1897–1985)
– Found purple sulfur bacteria do not release O2
but accumulate sulfur
– Proposed general formula for photosynthesis
• CO2 + 2 H2A + light energy → (CH2O) + H2O + 2 A
– Later researchers found O2 produced comes
from water
• Robin Hill (1899–1991)
– Demonstrated Niel was right that light energy
could be harvested and used in a reduction
reaction
10
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
• Photoelectric effect – removal of an
electron from a molecule by light
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12
Absorption spectrum
• When a photon strikes a molecule, its
energy is either
– Lost as heat
– Absorbed by the electrons of the molecule
• Boosts electrons into higher energy level
• Absorption spectrum – range and
efficiency of photons molecule is capable
of absorbing
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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
15
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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Pigments
Pigments:
17
• Structure of
chlorophyll
• porphyrin ring
– Complex ring structure
with alternating double
and single bonds
– Magnesium ion at the
center of the ring
• Photons excite
electrons in the ring
• Electrons are shuttled
away from the ring
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• Action spectrum
– Relative effectiveness of different
wavelengths of light in promoting
photosynthesis
– Corresponds to the absorption spectrum for
chlorophylls
19
• 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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Antenna complex
• Also called light-harvesting complex
• Captures photons from sunlight and
channels them to the reaction center
chlorophylls
• In chloroplasts, light-harvesting complexes
consist of a web of chlorophyll molecules
linked together and held tightly in the
thylakoid membrane by a matrix of
proteins
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Reaction center
• Transmembrane protein–pigment complex
• When a chlorophyll in the reaction center
absorbs a photon of light, an electron is
excited to a higher energy level
• Light-energized electron can be
transferred to the primary electron
acceptor, reducing it
• Oxidized chlorophyll then fills its electron
“hole” by oxidizing a donor molecule
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Light-Dependent Reactions
1. Primary photoevent
Capture of light energy
– Photon of light is captured by a pigment molecule
2. Charge separation
– Energy is transferred to the reaction center; an
excited electron is transferred to an acceptor
molecule
3. Electron transport
– Electrons move through carriers to reduce NADP+
4. Chemiosmosis
– Produces ATP
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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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Photosystem II
• Resembles the reaction center of purple bacteria
• Core of 10 transmembrane protein subunits with
electron transfer components and two P680
chlorophyll molecules
• Reaction center differs from purple bacteria in
that it also contains four manganese atoms
– Essential for the oxidation of water
• b6-f complex
– Proton pump embedded in thylakoid membrane
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Photosystem I
• Reaction center consists of a core
transmembrane complex consisting of 12
to 14 protein subunits with two bound P700
chlorophyll molecules
• Photosystem I accepts an electron from
plastocyanin into the “hole” created by the
exit of a light-energized electron
• Passes electrons to NADP+ to form
NADPH
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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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Production of additional ATP
• Noncyclic photophosphorylation generates
– NADPH
– ATP
• Building organic molecules takes more
energy than that alone
• Cyclic photophosphorylation used to
produce additional ATP
– Short-circuit photosystem I to make a larger
proton gradient to make more ATP
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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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Calvin cycle
• Named after Melvin Calvin (1911–1997)
• Also called C3 photosynthesis
• Key step is attachment of CO2 to RuBP to
form PGA
• Uses enzyme ribulose bisphosphate
carboxylase/oxygenase or rubisco
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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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Output of Calvin cycle
• Glucose is not a direct product of the
Calvin cycle
• G3P is a 3 carbon sugar
– Used to form sucrose
• Major transport sugar in plants
• Disaccharide made of fructose and glucose
– Used to make starch
• Insoluble glucose polymer
• Stored for later use
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Energy cycle
• Photosynthesis uses the products of respiration
as starting substrates
• Respiration uses the products of photosynthesis
as starting substrates
• Production of glucose from G3P even uses part
of the ancient glycolytic pathway, run in reverse
• Principal proteins involved in electron transport
and ATP production in plants are evolutionarily
related to those in mitochondria
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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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Types of photosynthesis
• C3
– Plants that fix carbon using only C3 photosynthesis
(the Calvin cycle)
• C4 and CAM
–
–
–
–
–
Add CO2 to PEP to form 4 carbon molecule
Use PEP carboxylase
Greater affinity for CO2, no oxidase activity
C4 – spatial solution
CAM – temporal solution
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C4 plants
• Corn, sugarcane, sorghum, and a number of
other grasses
• Initially fix carbon using PEP carboxylase in
mesophyll cells
• Produces oxaloacetate, converted to malate,
transported to bundle-sheath cells
• Within the bundle-sheath cells, malate is
decarboxylated to produce pyruvate and CO2
• Carbon fixation then by rubisco and the Calvin
cycle
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• C4 pathway, although it overcomes the problems
of photorespiration, does have a cost
• To produce a single glucose requires 12
additional ATP compared with the Calvin cycle
alone
• C4 photosynthesis is advantageous in hot dry
climates where photorespiration would remove
more than half of the carbon fixed by the usual
C3 pathway alone
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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 using PEP carboxylase during the
night and store in vacuole
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• When stomata closed during the day,
organic acids are decarboxylated to yield
high levels of CO2
• High levels of CO2 drive the Calvin cycle
and minimize photorespiration
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Compare C4 and CAM
• Both use both C3 and C4 pathways
• C4 – two pathways occur in different cells
• CAM – C4 pathway at night and the C3
pathway during the day
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