Photosynthesis
Photosynthesis Overview
Chapter 8
—  Energy
for all life on Earth ultimately comes
from photosynthesis
6CO2 + 12H2O
—  Oxygenic
C6H12O6 + 6H2O + 6O2
photosynthesis is carried out by
◦  Cyanobacteria
◦  7 groups of algae
◦  All land plants – chloroplasts
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Stages
Chloroplast
— 
— 
Light-dependent reactions
— 
Carbon fixation reactions or light-independent reactions
Thylakoid membrane – internal
membrane
◦  Contains chlorophyll and other
photosynthetic pigments
◦  Pigments clustered into
photosystems
◦  Require light
1. Capture energy from sunlight
2. Make ATP and reduce NADP+ to NADPH
◦  Does not require light
3. Use ATP and NADPH to synthesize organic molecules from CO2
Animation
Grana – stacks of flattened sacs
of thylakoid membrane
—  Stroma lamella – connect grana
—  Stroma – semiliquid
surrounding thylakoid
membranes
— 
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—  F.F. Blackman
Discovery of Photosynthesis
—  Jan
(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
Baptista van Helmont (1580–1644)
◦  Demonstrated that the substance of the plant was
not produced only from the soil
—  Joseph
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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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—  C. B. van
Pigments
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)
—  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
◦  Demonstrated Niel was right that light energy
could be harvested and used in a reduction
reaction
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Absorption spectrum
—  When
either
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Many pigments
a photon strikes a molecule, its energy is
—  Organisms
have evolved a variety of different
pigments
—  Only two general types are used in green plant
photosynthesis
◦  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
◦  Chlorophylls
◦  Carotenoids
—  In
some organisms, other molecules also
absorb light energy
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Structure of chlorophyll
Chlorophylls
—  Chlorophyll
a
—  Porphyrin
◦  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
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ring
◦  Complex ring structure with
alternating double and single
bonds
◦  Magnesium ion at the center
of the ring
b
—  Photons
excite electrons
in the ring
—  Electrons are shuttled
away from the ring
◦  Accessory pigment or secondary pigment
absorbing light wavelengths that chlorophyll a
does not absorb
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12
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Action spectrum
Other Pigments
—  Relative
effectiveness of different wavelengths
of light in promoting photosynthesis
—  Corresponds to the absorption spectrum for
chlorophylls
—  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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Antenna complex
Photosystem Organization
—  Also
complex
◦  Hundreds of accessory pigment molecules
◦  Gather photons and feed the captured light energy
to the reaction center
—  Reaction
called light-harvesting complex
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
—  Captures
center
◦  1 or more chlorophyll a molecules
◦  Passes excited electrons out of the photosystem
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Reaction center
Light-Dependent Reactions
—  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
1. 
Primary photoevent
◦  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. 
Capture of light energy
—  Antenna
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Chemiosmosis
◦  Produces ATP
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Cyclic photophosphorylation
Chloroplasts have two connected
photosystems
—  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
—  Oxygenic
photosynthesis
I (P700)
—  Photosystem
◦  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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Noncyclic photophosphorylation
The connection
—  Plants
—  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/ b6f complex
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 I
Photosystem II
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
— 
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
— 
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b6-f complex
◦  Proton pump embedded in thylakoid
membrane
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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
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Question 7
photophosphorylation generates
◦  NADPH
◦  ATP
—  Building
organic molecules takes more energy
than that alone
—  Cyclic photophosphorylation used to produce
additional ATP
Where does the Calvin Cycle take place?
—  A. stroma of the chloroplast
—  B. thylakoid membrane
—  C. cytoplasm surrounding the chloroplast
—  D. chlorophyll molecule
—  E. outer membranes of the chloroplast
◦  Short-circuit photosystem I to make a larger
proton gradient to make more ATP
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Carbon Fixation – Calvin Cycle
Calvin cycle
—  To
build carbohydrates cells use
—  Energy
—  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
◦  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. 
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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
Energy cycle
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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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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6
Photorespiration
—  Rubisco
Types of photosynthesis
has 2 enzymatic activities
◦  Carboxylation
—  C3
–  Addition of CO2 to RuBP
–  Favored under normal conditions
◦  Plants that fix carbon using only C3 photosynthesis (the
Calvin cycle)
◦  Photorespiration
—  C4
–  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
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
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The cost of doing business this way
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
— 
—  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 in action
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
—  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
◦  Reverse of that in most plants
—  Fix
CO2 using PEP carboxylase during the
night and store in vacuole
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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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