photosynthesis calvin cycle
TRANSCRIPT
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Calvin CycleAdaptationsFactors Affecting Rate
Photosynthesis Part 2
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Calvin Cycle
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Calvin cycle occurs in the stroma
Intermembrane Space
/Thylakoid space
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Calvin Cycle Overview
Cyclical process with 3 phases:Carbon fixation: incorporation of CO2Reduction: utilization of energy
molecules to form organic compoundRenegeration: regenerates
molecules for another cycle
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Calvin Cycle Overview
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Phase 1: Carbon FixationCO2 (1c) + 1,5-
RuBP (5c) = short lived 6C intermediate
6C molecule split into two 3C molecule known as 3-PGA/PGA/3PG
Above reaction occurs 3x
reaction type: synthesis
enzyme: synthase (Rubisco)
energy: absorbed
(3-PGA or PGA or 3PG)
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Rubisco
ribulose bisphosphate carboxylase / oxygenase
large, slow reacting enzymemost enzymes process 1000 reactions /
secondrubisco processes 3 reactions / second
plants need large amounts of rubisco for Calvin cyclehalf the protein in a leafmost abundant protein on Earth
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Phase 2: Reduction
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Calvin Cycle: Energy Utilization
ATP phosphorylates each 3-carbon molecule
reaction type: phosphorylation
enzyme: kinaseenergy: absorbed
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Calvin Cycle: Energy Utilization
NADPH used to synthesize G3P
reaction type: redox
enzyme: dehydrogenase
energy: absorbed
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Phase 3: RegenerationOf the 6 G3P produced 1 of them exits
the cycle to eventually become glucose and other types of organic compounds
The other 5 G3P continue in the cycle to help regenerate the starting substances
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Phase 3: Regeneration
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Calvin Cycle: Regenerate Molecules
G3P resynthesized to 1,5-RuBP
5 x 3C (G3P) 3 x 5C (RuBP)
15 C in totalUses ATP
reaction type: synthesis
enzyme: synthaseenergy: absorbed
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Adaptations to LimitationsPhotorespiration limitations:
C3 plant Adaptations to hot, arid conditions:
C4 plantsCAM plants
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C3 PlantsPlants whose
first organic product of carbon fixation is a 3-carbon compound
C3 plants undergo photosynthesis as described
(3-PGA or PGA or 3PG)
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C3 Plants
Stomata are open during the day / closed at night
What happens to stomata in hot, arid conditions?
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Stomata
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C3 Plant Limitations
In hot, arid conditions, plants close the stomata to prevent water loss
What affect does that have on CO2 and O2 concentration?
http://evolution.berkeley.edu/evolibrary/images/interviews/stoma_diagram.gif
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C3 Plant Limitations
Closed stomata CO2 decreases:Slows/stops Calvin cycle
Closed stomata O2 increases:rubisco binds to O2 rather than CO2 skip the Calvin cycle glucose is not produced
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Rubisco2 possible substrates: CO2 or O2Carboxylase: binds CO2 yields 2x PGAOxygenase: binds O2 yields 1x PGA and PG
http://ucce.ucdavis.edu/files/repository/calag/fig6302p68a.jpg
X
Photosynthesis
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Rubisco Reaction
http://www.chemgapedia.de/vsengine/media/width/750/height/409/vsc/de/ch/8/bc/stoffwechsel/photosynthese/flash/rubisco_mech8.svg.jpg
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Phosphoglycollate (PG)
cannot be converted directly into sugars
is a wasteful loss of carbonto retrieve the carbon from it, plants
must use an energy-expensive process called photorespiration
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Photorespiration (aka photo-oxidation)
“photo”: occurs in the light unlike photosynthesis, produces no organic
fuel (no Calvin cycle)“respiration”: consumes oxygen (by
rubisco)unlike cellular respiration, generates no ATP
wastes energy and reducing powerresults in the production of dangerous
reactive oxygen species (H2O2) in the peroxisome
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Why does photorespiration exist?Evolutionary baggage:
Metabolic relic from earlier time when atmosphere had less O2 and more CO2
With so much CO2, rubisco’s inability to exclude O2 had little impact on photosynthesis
Modern times:Rubisco’s affinity for O2 has a negative
impact on crop yields
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C3 Plants: Agricultural Crops
Hot arid dry conditions are detrimental to C3 plantsThese conditions have a negative impact for farmers and
consumers of C3 agricultural crops: rice, wheat, soy
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Evolutionary Efficiency
Atmospheric O2 is 500x higher than CO2
Yet rubisco fixes on average 4 CO2 for every O2
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Photosynthetic Adaptations2 other plant types have adapted
photosynthesis to dry, arid conditions:C4 plants – spatial (structural)
separationCAM plants – temporal (behavioural)
separation
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PEP Carboxylase
Very high affinity for CO2Can fix carbon even when CO2 levels
decrease and O2 levels rise
PEP (3C) + CO2 OAA (4C)OAA MalateMalate pyruvate (3C) + CO2Pyruvate PEP
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C4 PlantsPreface the Calvin cycle with an alternate
mode of carbon fixation that produces a 4-carbon compound as their first organic product
Agricultural C4 crops: sugar cane, corn
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C4 Plant: Leaf Structure
Bundle-sheath cells tightly packed around vascular bundle
Mesophyll cells loosely arranged near the leaf surface
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C4 Plant Adaptation
Calvin cycle confined to bundle-sheath cells
Preceded by incorporating CO2 into organic compounds in the mesophyll
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C4 Plant AdaptationCO2 added to a 3-
carbon phosphoenolpyruvate (PEP) to form a 4-carbon oxaloacetate (OAA)
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C4 Plant Adaptation4C compound is exported
to bundle-sheath cells where it releases CO2 and a 3C pyruvate
CO2 is assimilated into the Calvin cycle by rubisco
Pyruvate returns to the mesophyll cell to be converted back to PEP (3C)
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C4 Plant AdaptationMesophyll cells act as
a CO2 pumpKeeps concentration
of CO2 in bundle-sheath cells high (and oxygen low) so that rubisco can bind CO2
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CAM PlantsCAM: Crassulacean acid metabolismSucculent (water-storing) plantsExample: cacti, pineapples
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CAM Plant AdaptationNighttime: Stomata openTake up CO2 and added
to a 3-carbon PEP to form a 4-carbon OAA
Enzyme: PEP carboxylase
Further converted to malate (malic acid) which is stored in vacuoles of mesophyll cells
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CAM Plant AdaptationDaytime: Stomata closedConserve waterno CO2 uptakeLight reactions make
ATP and NADPHCO2 in organic acid
stored in vacuoles from the night is released to run the Calvin cycle
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CAM Plant Adaptation
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CAM Plant AdaptationNighttime: CO2
incorporation into organic acids, stored in vacuole
Daytime: Calvin cycle with energy from light reaction and CO2 from night storage
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C4 and CAM PlantsCO2 is incorporated into organic intermediates
before entering Calvin cycleC4: initial carbon fixation separated structurallyCAM: initial carbon fixation separated temporally
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Factors Affecting PhotosynthesisLight intensityCarbon dioxide concentrationTemperature
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Light Intensity Low-mid intensity: linear
response Higher intensity:
saturation due to other limiting factors in photosynthesis
Highest intensity: photorespiration
light compensation point: minimum light intensity at which the leaf shows a net gain of carbon
http://www2.geog.ucl.ac.uk/~plewis/geogg124/_images/chapin1.png
Light intensity
Rat
e of
Pho
tosy
nthe
sis
Photo-respiration
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Light Intensityfactors contributing to saturation
Rate at which photosynthesis is saturated is increased with temperature and CO2 levels
http://image.tutorvista.com/content/nutrition/light-intensity.jpeg
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CO2 Concentrations
Low concentration: CO2 diffusion limits rate
High concentration: saturation due to other limiting factors in photosynthesis
http://www2.geog.ucl.ac.uk/~plewis/geogg124/_images/chapin2.png
CO2 concentration
Rat
e of
Pho
tosy
nthe
sis
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CO2 Concentrationsfactors contributing to saturation
Rate at which photosynthesis is saturated is increased with light intensity
http://images.tutorvista.com/content/photosynthesis/carbondioxide-concentration-c3-c4-species.jpeg
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Temperature
Each type of plant has a temperature range where photosynthesis is optimal
http://www2.geog.ucl.ac.uk/~plewis/geogg124/_images/chapin6.png
Antarctic Lichen
Cool coastal dune plant
Evergreen desert shrub
Summer-active desert
perennial
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Temperaturefactors contributing to maximum rate of photosynthesis
Increased CO2 levels increases the maximal rate of photosynthesis
http://www.co2science.org/education/reports/extinction/figures/fig1.jpg
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Photosynthesis Rate FactorsFactor Effect
onRate of photosynthesis
Light intensity
Light reactio
ns
Increase to a plateau (saturation) since Calvin cycle cannot keep up with light reactions
CO2 levels Calvin cycle
Increase to a plateau since light reactions can not keep up with Calvin cycle
Temperature
Maximum rate can be increased by increasing CO2