Carbon Fixation Reactions
To build carbohydrates, cells need:
To build carbohydrates, cells need:
1. energy 1. energy
-ATP from light-ATP from light--dependent reactionsdependent reactions 2. reduction potential
2. reduction potential
-NADPH from photosystem I-NADPH from photosystem I
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Carbon Fixation Reactions Carbon Fixation Reactions
Calvin cycle Calvin cycle
-biochemical pathway that allows for carbon -biochemical pathway that allows for carbon fixation
fixation
-occurs in the stroma -occurs in the stroma
-uses ATP and NADPH as energy sources-uses ATP and NADPH as energy sources -incorporates CO-incorporates CO22into organic moleculesinto organic molecules
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Carbon Fixation Reactions Carbon Fixation Reactions
carbon fixation
carbon fixation –– the incorporation of COthe incorporation of CO22 into organic molecules
into organic molecules
-occurs in the first step of the Calvin cycle-occurs in the first step of the Calvin cycle ribulose
ribulose--bisbis--phosphate + COphosphate + CO22 2(PGA)2(PGA) 5 carbons
5 carbons 1 carbon 3 carbons1 carbon 3 carbons The reaction is catalyzed by
The reaction is catalyzed by rubisco.rubisco.
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Carbon Fixation Reactions Carbon Fixation Reactions
The Calvin cycle has 3 phases:
The Calvin cycle has 3 phases:
1. carbon fixation 1. carbon fixation
RuBP + CO
RuBP + CO22 2 molecules PGA2 molecules PGA 2. reduction
2. reduction
PGA is reduced to G3P PGA is reduced to G3P 3. regeneration of RuBP 3. regeneration of RuBP
G3P is used to regenerate RuBP G3P is used to regenerate RuBP
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Carbon Fixation Reactions Carbon Fixation Reactions
Glucose is not a direct product of the Calvin Glucose is not a direct product of the Calvin
cycle.
cycle.
-2 molecules of G3P leave the cycle-2 molecules of G3P leave the cycle -each G3P contains 3 carbons-each G3P contains 3 carbons
-2 G3P are used to produce 1 glucose in -2 G3P are used to produce 1 glucose in reactions in the cytoplasm
reactions in the cytoplasm
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Carbon Fixation Reactions Carbon Fixation Reactions
During the Calvin cycle, energy is needed.
During the Calvin cycle, energy is needed.
The energy is supplied from:
The energy is supplied from:
--18 ATP molecules18 ATP molecules --12 NADPH molecules12 NADPH molecules
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Carbon Fixation Reactions Carbon Fixation Reactions
The energy cycle:
The energy cycle:
-photosynthesis uses the products of -photosynthesis uses the products of respiration as starting substrates respiration as starting substrates -respiration uses the products of -respiration uses the products of
photosynthesis as starting substrates photosynthesis as starting substrates
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Photorespiration Photorespiration
Rubisco has 2 enzymatic activities:
Rubisco has 2 enzymatic activities:
1. carboxylation
1. carboxylation –– the addition of COthe addition of CO22 to to RuBPRuBP
-favored under normal conditions-favored under normal conditions 2. photorespiration2. photorespiration –– the oxidation of the oxidation of
RuBP by the addition of O RuBP by the addition of O22 -favored in hot conditions-favored in hot conditions
COCO22 and Oand O22 compete for the active site on compete for the active site on RuBP.
RuBP.
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Photorespiration Photorespiration
Some plants can avoid photorespiration by Some plants can avoid photorespiration by
using an enzyme other than rubisco.
using an enzyme other than rubisco.
-PEP carboxylase-PEP carboxylase
-CO-CO22 is added to phosphoenolpyruvate (PEP)is added to phosphoenolpyruvate (PEP) -a 4 carbon compound is produced-a 4 carbon compound is produced
-CO-CO22 is later released from this 4-is later released from this 4-carbon carbon compound and used by rubisco in the compound and used by rubisco in the Calvin cycle
Calvin cycle
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Photorespiration Photorespiration
CC44plantsplants
-use PEP carboxylase to capture CO-use PEP carboxylase to capture CO22 -CO-CO22 is added to PEP in one cell type is added to PEP in one cell type
(mesophyll cell) (mesophyll cell)
-the resulting 4-the resulting 4--carbon compound is moved carbon compound is moved into a bundle sheath cell where the CO into a bundle sheath cell where the CO22is is released and used in the Calvin cycle
released and used in the Calvin cycle
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Photorespiration Photorespiration
CAM plants CAM plants
-CO-CO22 is captured at night when stomata are is captured at night when stomata are openopen
-PEP carboxylase adds CO-PEP carboxylase adds CO22 to PEP to to PEP to produce a 4 carbon compound
produce a 4 carbon compound
-this compound releases CO-this compound releases CO22 during the day during the day -CO-CO22 is then used by rubisco in the Calvin is then used by rubisco in the Calvin
cycle cycle
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Photosynthesis
The Source of most Biological Energy Trapped in Photosynthesis
Energy Converted to Chemical Bonds
Light: An Energy Waveform With Particle Properties Too
wavelength (nm)
10-9meter 0.000000001 meter!
400 500 600 700 nm wavelength
violet blue green yellow orange red
Light: An Energy Waveform With Particle Properties Too
wavelength (nm)
10-9meter 0.000000001 meter!
400 500 600 700 nm wavelength
visible spectrum
White Light
Leaf Pigments Absorb Most
Colors
Green is reflected!
Light: An Energy Waveform With Particle Properties Too
amplitude brightness intensity
Many metric units for different purposes
We will use an easy-to-remember English unit: foot-candle 0 fc = darkness
100 fc = living room 1,000 fc = CT winter day
10,000 fc = June 21, noon, equator, 0 humidity
What wavelengths of light drive photosynthesis?
wavelength (nm)
400 500 600 700 nm visible spectrum
green light reflected
Action Spectrum
some still drives photosynthesis
Photosynthetic Rate
0 100%
Light beyond 700 nm has insufficient energy to drive photosynthesis
Anten
In each energy transfer some energy is lost as heat:
2ndlaw of thermodynamics.
But enough energy is passed to P680 to eject an electron to the electron transport system.
CH2 pigments are
amphipathic
What intensities of light drive photosynthesis?
Light Intensity (fc)
0 10 100 1,000 10,000 fc add to reserve
grow reproduce
Using reserves and
may die
The example plant shown here “breaks even” at an intensity we have in our homes…a house plant!
What intensities of light drive photosynthesis?
Light Intensity (fc)
0 10 100 1,000 10,000 fc
The second example plant shown here cannot survive in our homes…it is a sun-loving crop plant!
Photosynthesis B
Shade tolerant plant dies in intense light!
reducing The Light Reactions: An Energy Diagram
ADP+Pi ATP
The PCR Cycle has Three Phases
P-C-C-C-C-C-P
sucrose for transport starch for storage
ADP
Let’s Do Some Stoichiometry:
sucrose for transport starch for storage
ADP
To take off 3 carbons:
3
sucrose for transport starch for storage
ADP
To take off 3 carbons:
3
sucrose and starch are not
3-carbon compounds!
The PCR Cycle and Light Reactions are interdependent
The PCR Cycle cannotoperate in darkness!
“Dark Reactions?”
RuBisCO: an ancient enzyme with a modern problem RuBP + CO2
RuBP + O2
2 x P-C-C-C (Phosphoglycerate)
P-C-C-C (a Phosphoglycerate)
+ P-C-C 2 x CO2
• Early in evolution of photosynthesis the atmosphere was anaerobic, so RuBisCo evolved without a problem.
• As photosynthesis was successful, competitive inhibition from oxygen was essentially a negative feedback.
• Evolution has not yet replaced RuBisCO.
• But several workarounds have evolved…
RuBisCO often constitutes up to 50% of the protein in a plant…to ensure enough photosynthesis is achieved