Photosynthesis
3CO2 + 6H2O
C3H6O3 + 3O2 + 3H2O
Photosynthesis
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Two major steps:
– Energy transduction reactions or light reactions
– Carbon-fixation reactions or dark reactions or Calvin Cycle
Light reactions
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Light energy forms ATP and NADPH
Water molecule is split and oxygen is released
Dark reactions
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Energy from ATP and NADPH used to synthesize organic molecules
What is light?
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Visible light-portion of electromagnetic spectrum that causes sensation of
vision in humans
~400-700 nm
What is light?
• Light has wave properties nanometers (nm), or in frequency of peaks (Hz)
Light
• Light also has particle properties-hits earth as "packets" of energy called
photons; the energy of 1 photon=1 quantum; quantum energy is inversely
What is light?
• Light sources vary:
• Sunlight varies in distribution of wavelengths throughout the day; enriched in
red at noon, far-red at sunset or in shade
• Fluorescent lights are enriched in "cool" blue
• Incandescent lights are enriched in "warm" red
How is light characterized and measured?
• Light quantity-how many total photons impact a surface over time
• Photon Fluence Rate=mol photons/m2*sec, or in energy equivalents=
J/m2*sec,
• Quantum sensors commonly measure quantity of PAR (400-700nm) striking a
2*sec PAR on a clear summer day)
surface (commonly 2000
Light characterization
• Light quality photons/m2
• Quantum radiometers measure light quality
• Both quantity and quality vary daily with changes in atmosphere, clouds,
shading, sun angle
Light quality effects
• Red/far-red light ratio (R/FR)-important signal for plant development, ratio
lower under shade or at end of day
• Infrared-long wavelengths largely reflected by earth, but absorbed by CO2 and
water vapor in atmosphere, leading to heat buildup and the Greenhouse Effect
Light quality effects
• Ultraviolet (UV)-short wavelengths, higher energy, mutagenic
• Most uv is absorbed by ozone in stratosphere but required for some plant
processes
• Hole in stratospheric ozone layer means more UV radiation for the earth = skin
cancer, reduced plant productivity
How is light energy captured and transferred?
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Photoreceptors (pigments)-required to capture light energy
Chlorophyll-main photosynthetic pigment and best studied pigment
Chlorophyll
• Synthesized as protochlorophyll in the dark- requires light for conversion to
chlorophyll
• Porphyrin head (N and Mg-containing), plus long hydrocarbon tail
• Loss of Mg upon extraction forms pheophytin
• Insoluble in water, found in chloroplast membrane lipids, associated w/proteins
Chlorophyll
• Several forms:
• a (all plants, algae, cyanobacteria) abs. peaks at 420 (blue) and 670 (red)
– ¾ of chlorophyll in leaves of green plants
• b (plants, green algae) abs. peaks at 490 and 650
• c (brown algae)
Accessory pigments
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A pigment not directly involved in photosynthetic energy transduction
Serves to broaden the range of light that can be used in photosynthesis
Other pigments
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Carotenoids
– Red, orange or yellow lipid-soluble pigments found in all chloroplasts
• Carotene
• Xanthaophylls
Energy capture and transfer
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an electron in pigment
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Excites molecule from low energy stable ground state to a higher energy,
unstable excited state
Energy transfer
• Energy dissipates from unstable excited state:
• 1. Some energy lost as combination of heat and as a lower energy photon =
fluorescence
• 3. Some energy transferred between pigments = resonance energy transfer
• 4. Electron given up to an acceptor molecule as pigment is photooxidized =
main act of photosynthesis
Light-dependent reactions of photosynthesis
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Light energy (captured by chlorophyll) is used to extract low energy electrons
from water, transferring them through photosynthetic electron transport to
produce a strong reductant (NADPH); ATP synthesis is coupled to this process
Photosystems
• Higher plant photosynthesis consists of two Photosystems (PSII and PSI),
which are big chlorophyll-protein complexes embedded in the thylakoid
membranes
• Each reaction center of PSII and PSI consists of 2 molecules of chlorophyll a
complexed with protein and surrounded by "antennae“ molecules
Photosystems
• Antennae consist of 250-400 pigment molecules, which capture energy and
funnel energy to each reaction center through resonance energy transfer
• Reaction center of PSII absorbs maximally at 680nm, PSI absorbs maximally
at 700nm
Lateral heterogeneity
• PSII is located mostly in membranes in the appressed regions of the
thylakoids (within the grana)
• PSI is located mostly on the regions of the thylakoid membrane exposed to the
stroma
• PQ, PC, and fd carry electrons and diffuse to where needed
Photosynthetic electron transport
1. Antennae funnel energy to Reaction Center P680 in PSII exciting P680 to a
higher energy state
2. P680+ donates an electron to pheophytin (the first electron acceptor)
3. At the same time, the Water splitting complex oxidizes water into O2 and H+
ions and then donates electrons originating from the water to P680 to make up
for the electron it just gave up
Photosynthetic electron transport
4. Pheophytin to Plastoquinone pool (PQ) to iron-sulfur molecule (Fe-S) on the
cytochrome b6/f complex, where Fe-S donates electron to cyt f
5. cyt f donates electron to the copper binding protein Plastocyanin (PC)
6. PC diffuses along the lumen side of the membrane and donates an electron to
Reaction Center P700 in PSI
Photosynthetic electron transport
7. To continue electron transport, P700 must be excited by energy captured by
the antennae of PSI
8. P700+ donates an electron to ferredoxin-NADP+ reductase (iron-containing
protein/enzyme)
9. Fd-NADP+ reductase donates an electron to the final electron acceptor
NADP+, reducing it to NADPH
Photosynthetic electron transport
• NADPH is a strong reductant used to reduce CO2 in the stroma (next lecture)
• 692 kJ of light energy used to excite each pair of electrons at the start of the
process (only a portion of that gets conserved in chemical bonds)
• Total energy from light reactions is 6 ATP and 6 NADPH
Z-scheme
Photophosphorylation
• As photosynthetic electron transport progresses, the proton gradient formed in
the lumen is used to drive ATP synthesis
• Oxidation of water increases H+ in the lumen; H+ are also pumped into the
lumen from the stroma through the PQ-cytochrome pump
• H+ can be 3-4 thousand times higher on the lumen side of the membrane
creating an electrochemical gradient
Cyclic photophosphorylation
• More primitive
• Produces ATP for dark cycle (requires 3:2 ratio of ATP:NADPH)
Carotenoids
• Carotenoids are closely associated with chlorophyll; may help to capture light
• Also protect photosystems from photooxidative damage
Photooxidation
• Anthocyanins (that accumulate during Fall) are thought to protect against
photooxidation during sunny, but cool periods during the fall
The dark reactions
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The Calvin cycle - the C3 pathway for photosynthetic carbon fixation
Elucidation by Melvin Calvin led to Nobel Prize
3CO2 + 6NADPH + 9ATP
Glyceraldehyde 3-phosphate +
6NADP+ + 9ADP + 9Pi +3H2O
CO2 uptake
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Diffuses in through stomata into air spaces
Diffuses freely through membranes into mesophyll cells
Diffuses through cytoplasm to chloroplast
Reaction 1 of the Calvin Cycle
• Carboxylation (adding a carboxyl group from CO2) of the 5 carbon ribulose1,5-bisphosphate (RuBP) to form 2 molecules of 3-Phosphoglycerate (3-PGA)
• First detectable product of Calvin Cycle is 3-PGA - C3 pathway
Rubisco
• Catalyzed by ribulose bisphosphate carboxylase-oxygenase (Rubisco)-the
most abundant protein on earth)
• Has a binding site for both CO2 and O2
Reaction 2 of the Calvin Cycle
• Formation of a triose-phosphate called glyceraldehyde-3-phosphate (G3P)
from 3-PGA via two reactions mediated by two enzymes, NADPH and ATP
RuBP regeneration
• Regeneration of the 5-carbon acceptor RuBP is accomplished by various
reactions to form Ru5P, which is then phosporylated to RuBP requiring ATP
• Three turns of entire cycle regenerates the 3 molecules of RuBP, plus one
additional G3P
• 3 ATP + 2 NADPH are needed per CO2 fixed
Fates of G3P
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Stored as starch in the chloroplast
Exported to cytosol, converted to sucrose, then exported and partitioned to
various tissues
Converted to pyruvate and respired, yielding ATP and carbon skeletons
Regulation of the
Calvin Cycle
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Regulation of cycle keeps carbon fixation compatible with carbon metabolism
Autocatalysis of RuBP
• At night, Calvin cycle shuts down, RuBP regeneration stops, and the number
of acceptor molecules for CO2 becomes limiting
• In morning, RuBP can autocatalyze from G3P
• Other regulatory control points occur at key enzymatic steps
Regulation of Rubisco
• Light regulated; turns off in darkness, on in light
• Stroma pH: increases from 7.1 in dark to 7.9 in light (as protons move across
the thylakoid membrane), and Rubisco is more active at pH 7.9
• Stroma Mg 2+ concentration: increases from 1-2 mM in dark to 3-5 mM in light;
Rubisco activity requires Mg+
• CO2 presence activates Rubisco in morning through Rubisco activase
Photorespiration
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The problem of photorespiration and potential solutions
We know that photosynthesis evolves O2 and uses CO2, but it can also use O2
and evolve CO2
Dual affinity of Rubisco
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Rubisco has O2 and CO2 binding sites
O2 binding favored under high O2, low CO2, and high temperature
Can result in losses of 50% or more of recently fixed carbon
Evolutionary solutions to photorespiration
• Affinity for oxygen may be an evolutionary "hangover” from plant evolution in
high CO2 environments
The C4 pathway
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A CO2 concentrating mechanism offers a solution
Initial evidence for alternative pathway found by noticing incorporation of
radiolabelled CO2 into C4 acids (e.g. oxaloacetate) rather than the C3 acid 3PGA
C4 Pathway
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Insensitive to O2 levels, and can operate at low CO2 levels-favored under high
light and high temperatures
Found in 18 families of tropical and subtropical plants (1500 spp.)
C4 Anatomy
• Anatomical component - two distinct types of photosynthetic cells "Kranz
anatomy"
• A "wreath" of bundle sheath cells containing many chloroplasts surround
vascular tissue in leaf veins
• Mesophyll cells surround the bundle sheath cells
Kranz anatomy
C4 pathway
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In mesophyll cells:
CO2 is fixed to phosphoenolpyruvate (PEP) by the enzyme PEP carboxylasese
to create oxaloacetate (OAA)
OAA reduced to malate
Malate moves from mesophyll cells to bundle sheath cells
C4 pathway
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In bundle sheath cells:
Malate is decarboxylated to release CO2 and pyruvate, which then enters the
Calvin cycle
Pyruvate moves back to mesophyll cells and regenerates PEP
C4 pathway
• C4 pathway is more costly compared to C3 pathway
• Why do it?
– High ratio of CO2/O2 in bundle sheath cells favors carboxylation by rubisco
rather than photo respiration
– Any CO2 lost in photorespiration in interior bundle sheath cells can be
captured by other cells
C4 pathway
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Due to more efficient use of CO2, stomata can be smaller and closed more
often
An advantage in dry, hot areas
Some C4-C3 intermediates in higher plants
Crassulacean Acid Metabolism (CAM)
• Specialized form of C4 photosynthesis
• Found in all succulents-e.g. cacti, Aloe vera
CAM
• Stomata open only at night; CO2 uptake at night – conserves water loss
• CO2 is "stored" in organic acids (malate) in vacuole until they can be
decarboxylated during the day releasing CO2 to be fixed using light energy
• No special anatomy; timing separates functions