photosynthesis: the light- independent reactions biol 3470 plant physiol biotech 5.5 to 5.12 lecture...
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![Page 1: Photosynthesis: the light- independent reactions Biol 3470 Plant Physiol Biotech 5.5 to 5.12 Lecture 9 Thurs. Feb. 2, 2006 From Rost et al., “Plant Biology”,](https://reader036.vdocument.in/reader036/viewer/2022062421/56649c7e5503460f94933b2b/html5/thumbnails/1.jpg)
Photosynthesis: the light-
independent reactions
Biol 3470
Plant Physiol Biotech
5.5 to 5.12
Lecture 9
Thurs. Feb. 2, 2006
From Rost et al., “Plant Biology”, 2nd edn
![Page 2: Photosynthesis: the light- independent reactions Biol 3470 Plant Physiol Biotech 5.5 to 5.12 Lecture 9 Thurs. Feb. 2, 2006 From Rost et al., “Plant Biology”,](https://reader036.vdocument.in/reader036/viewer/2022062421/56649c7e5503460f94933b2b/html5/thumbnails/2.jpg)
The process of carbon fixation in plants goes by many names
• Including:– The dark reactions– The enzymatic reactions of photosynthesis– Reductive pentose phosphate cycle– C3 cycle– The photosynthetic carbon reduction (PCR)
cycle (in the textbook)
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The PCR cycle converts atmospheric carbon to organic molecules
• Convert CO2 to stable phosphorylated carbon intermediates (specifically- a three-carbon carbohydrate, 3-PGA)
• Uses the energy produced in the light-dependent reactions to reduce CO2
– Convert less complex → more complex molecules– Fight entropy
• Pathway elicited in late 1940s and early 50s by U.S. plant physiologist Melvin Calvin using labeled 14CO2 – feed plants 14CO2 – Allow metabolism– Kill, extract, examine small carbohydrates that contain 14C using
paper chromatography and autoradiography
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The PCR cycle contains 3 distinct segmentsStep 1: Carboxylation fixes CO2
using the enzyme rubisco• 14CO2 fixed first into 3-
phosphoglycerate (3 Cs ≡ C3 cycle)– 3-PGA is the first organic product of
the PCR cycle
• Given this product and reactant, one would assume the plant substrate of the PCR cycle would have ___Cs
1
3 2
• Actually, the plant substrate for the PCR cycle is a five-carbon substrate, RuBP
Fig. 5.8
Fig. 5.9
Unstable intermediate is hydrolyzed
(2x )
Rubisco reaction
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The key enzyme regulating carbon uptake by the PCR cycle is rubisco
Rubisco• Enzyme with a high affinity for CO2• Present in high amounts in the chloroplast stroma• Its activity maintains a CO2 gradient from the atmosphere • ΔGº′ = -35 kJ/mol (energetically favourable to occur
spontaneously)• But its activity requires ATP + NADPH made in the light
reactions elsewhere in PCR cycle
• e.g. in 2nd step: reduction of 3-PGA to G3P
• This is Step 2: Reduction
Fig. 5.10
Phosphorylate!
Reduce!
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The final step in the PCR cycle regenerates the rubisco substrate
• This is accomplished via Step 3: Regeneration– Requires 1 ATP per CO2
• Note that the PCR cycle is autocatalytic– This means that it operates more quickly if
CO2 and/or RuBP pools are low (e.g. in the morning, when the RuBP supply is depleted)
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PCR cycle activity must be integrated with plant carbon metabolism as a whole
• These include respiration (glycolysis) and macromolecule synthesis (for nucleic acids, lipids, carbohydrates, proteins)
• Thus, the PCR cycle activity must be regulated by a number of mechanisms• The plant wants to keep CO2 fixation rate high to make more organic carbon
Fig. 5.11
Rubisco
Auto
cata
lytic
RuB
P
rege
nera
tion
2. Reduction
3. Regener-ation
1. Carbox-ylation
• The carbon from 5 of every 6 molecules of G3P needs to be recycled to make RuBP and keep the cycle spinning
• Only around one-sixth of the carbon fixed is exported from the leaf and supports growth and metabolism
• To keep high CO2 fixation, the plant can prevent G3P export
The PCR cycle consumes the ATP and NADPH produced in the light-dependent reactions
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Mechanisms of regulation of PCR cycle activity
• The activity of rubisco is regulated by light• Complex mechanism driven by uptake of protons by
thylakoid lumen between1. Mg2+→ moves lumen → stroma to compensate for H+ uptake by
thylakoids in light inactive active
(light)
Dark stroma pH = 5.0Light
Fig. 5.14
• Stromal pH ↑ activates rubisco
2. CO2 → binds to activating site on rubisco (not active site!) ≡ CARBAMYLATION
3. pH increase favours carbamylation (H+ sink in lumen)
Rubisco is now catalytically ready to fix atmospheric CO2!
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Plant cells also respire: convert O2→CO21. Via mitochondiral respiration at night
– This is oxidative phosphorylation to generate ATP in the dark (R on diagram)
– This also happens in the light!
2. Via rubisco → can use O2 as a substrate in photorespiration (PR)
• Therefore, measuring NET gas exchange in photosynthetic organs is difficult!
• We can define an apparent photosynthesis rate
= CO2 fixation rate – CO2 evolution rate =gross p’syn – (mt R + PR)
• At a low atmospheric [CO2] these values (GP) and (R+PR) are equal
• this is the CO2 compensation point
Fig. 5.16
(mt)
(mt)
(Rubisco O2-ase)
(Rubisco CO2-ase)
(mt)
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Photorespiration is due to rubisco’s oxygenase activity
• Makes 2-phospho-glycolate (2C) + 3-PGA from RuBP + O2
• This C in 2-P-glycolate not wasted but reassimilated by exchange of intermediates with 2 other organelles– Peroxisome– Mitochondria
Fig. 5.18
Exported or recycled to regenerate RuBP
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The function of photorespiration is not immediately obvious
Energetically wasteful, so why do it? Thoughts and theories…
1. [O2] in atmosphere has been low during most of evolutionary history
• Therefore PR is an evolutionary relic?• No! PR mutants are lethal! → Therefore, PR is essential• No evolutionary pressure to get rid of O2-ase function
2. The salvage cycle does a good job of recovering photorespired C
• Each 2 turns of the 2-P-glycolate salvage cycle forms 4 3-PGA• 1 lost, 3 returned to the PCR cycle• Complex salvage pathway works well!
3. Metabolic safety valve?• PR protects against photoxidative damage by allowing P.E.T. to
continue at low [CO2]• e.g., under high light + low CO2 (photoinhibitory conditions, stomata
closed, water-stressed)
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The chloroplast oxidative pentose phosphate cycle allows plants to make NADPH in the dark
• Shares intermediates with PCR• Both at once: FUTILE CYCLE!
– Use 3 ATP
– No CO2 fixation!
• Both pathways are light-regulated
PCR
a/k/a RPPC
OPPC
• Light induces changes the structure of the disulfide bonds of the pathways’ enzymes– PCR cycle enzymes active when reduced– OPPC cycle enzymes active when oxidized
Light Dark
PCR enzymes
√ X
OPPC enzymes
X √
Fig. 5.20
•Why have an OPPC?–Make NADPH in dark–Make ribose and deoxyribose for nucleic acid synthesis
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1. Mesophyll• fewer chloroplasts
2. Bundle sheath cells• lots of chloroplasts• surround vascular tissue• thick cell walls prevent
diffusion of CO2 out of BS cells and traps photorespired CO2
• No mesophyll cells are more than 2-3 cells away from BS
• This ensures quick export of fixed CO2 as sucrose
• Many chloroplasts needed to fix high [CO2]
How can plants minimize PR and maximize GP?Plants are separated into 2 main groups based on their ability to do this:• Plants where 3-C 3-PGA is product of CO2 fix’n = C3• Plants where 4-C oxaloacetate is product of CO2 fix’n = C4• C4 plants have 2 distinct photosynthetic tissues
– Leaf anatomy differs from C3 leaf
Fig. 5.21: C4 leaf X-section (e.g., maize)
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C4 plants are present in all 18 plant families• This includes flowering plants as wellC4 plants are:• Better at CO2 fixation (up to 3X more efficient)• Better at drought stress• Concentrate CO2 at rubisco active site and thus minimize
CO2 loss!• How do they do this?
– Fix CO2 into C4 organic acid in the mesophyll cell using PEP carboxylase (not rubisco!)
– Use a transporter to move the acid into the bundle sheath cell
– Release CO2 there
– Fix CO2 via PCR
– Recycle C3 acid released (pyruvate) back to mesophyll
PEP carboxylase malate malate
pyruvatepyruvate
Malic Enzyme
Fig. 5.22: the C4 carbon fixation pathway
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• C4 metabolism pluses:– CO2 outcompetes O2 at
rubisco active site: Less PR! – Much lower compensation
point• maintain high CO2 fixation rates
when stomata are partially closed → conserves H2O
– Lower transpiration ratio = less H2O transported per CO2 assimilated
Using the C4 pathway to fix carbon is not always an advantage for the plant
• C4 metabolism minuses:– Need to “spend” 2 ATP per CO2 to recycle C3 acid back to the
mesophyll cells• C3 plants often have an ecological advantage
– Grow better in cooler climates and low irradiance– Higher CO2 assimilation rate in environments with lots of water
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How do plants grow in the desert?• Use CAM metabolism: conserves
H2O• CAM plants have an inverted
stomatal cycle – Night: open– Day: closed
• Therefore CO2 uptake at night → accumulate malate in vacuole
• During day → convert malate to starch via PCR cycle
• Need PEPC as in C4 photosynthesis– Requires lots of PEP (PEPC
substrate), provided from glycolytic breakdown of starch
• CAM is similar to C4, but:– No specialized anatomy (specialized
cell types)– No closed cycle of carbon
intermediates
Night Day
PEPC
Large, watery
Decarbox-
ylation
Malic
enzyme
export
Fig. 5.26
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CAM plants are evolutionarily adapted to live in low water environments
• CAM plants have even lower transpiration ratios than C4 plants BUT– Only fix <1/2 C of C3 and <1/3
of C4 plants → slow growers– But can
• continue CO2 uptake under H2O stress
• Reassimilate respired CO2
• Some plants can “switch on” CAM metabolism (facultative vs. obligatory)
From http://www.arizonensis.org/images/plantae/cereus_gigant.jpg