Photorespiration and C4 CAM
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29 Terms
1
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What do plants show when lights are turned off?
\-brief apparent burst of respiration
\-very rapid respiration occuring in light alongside photosynthesis
\-does not fall off as rapidly as photosynthesis when lights go off
\-very rapid respiration occuring in light alongside photosynthesis
\-does not fall off as rapidly as photosynthesis when lights go off
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Diagram of plants in light and darkness
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true respiration vs photorespiration
\-true respiration saturates at around 5% O2
\-photorespiration keeps increasing with O2
\-true respiration is largely controlled by ATP requirement
\-photorespiration is limited only by O2 availability
\-photorespiration probs wastes half the potential productivity of most plants
\-photorespiration keeps increasing with O2
\-true respiration is largely controlled by ATP requirement
\-photorespiration is limited only by O2 availability
\-photorespiration probs wastes half the potential productivity of most plants
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What is oxygen?
\-alternative substrate for RuBisCO
\-Ribulose-1, 5-bis-phosphate carboxylase/oxygenase RuBP+O2 => PGA + phosphoglycolate (PG)
\-Ribulose-1, 5-bis-phosphate carboxylase/oxygenase RuBP+O2 => PGA + phosphoglycolate (PG)
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What did RuBisCO evolve in ?
\-anaerobes that did’t have to worry about discriminating against oxygen
\-RuBisCO evolved well before oxygenic photosynthesis
\-oxygen in the atmosphere didn’t accumulate until even later: 1500 Ma
\-RuBisCO of anaerobic photosynthetic baceria is even less discriminating than tha of oxygenic photosynhesisers
\-RuBisCO evolved well before oxygenic photosynthesis
\-oxygen in the atmosphere didn’t accumulate until even later: 1500 Ma
\-RuBisCO of anaerobic photosynthetic baceria is even less discriminating than tha of oxygenic photosynhesisers
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What is RuBisCO’s past history?
\-start off with no CO2 fixation at all
\-evolutionary trajectory goes to local optimum where RuBisCO and Calvin-Benson cycle
\-evolutionary trajectory goes to local optimum where RuBisCO and Calvin-Benson cycle
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With a typical ration of about 1 oxygenase : 2 carboxylase reactions the Calvin-Benson cycle would grind to a halt if…
we didn’t recycle PG somehow
8
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What do aerobic photosyntheic organisms contain?
\-v high conc of RuBisCO
\-increased specificity for CO2 and O2
\-reduced access for CO2 to active sie
\-RuBisCO is slow
\-plants compensate by increasing \[RuBisCO\]
\-greenhouses can be enriched artificially to 0.12% CO2 to increase competitiveness of CO2 over O2
\-higher specificity means slower reactions and more RuBisCO
\-increased specificity for CO2 and O2
\-reduced access for CO2 to active sie
\-RuBisCO is slow
\-plants compensate by increasing \[RuBisCO\]
\-greenhouses can be enriched artificially to 0.12% CO2 to increase competitiveness of CO2 over O2
\-higher specificity means slower reactions and more RuBisCO
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What can algae and bacteria concentration?
\-CO2 and RuBisCO
\-some algae have HCO3(-) ion pumps in cell membrane and carbonic anhydrase packed into pyrenods in the stroma
\-some bacteria pack RuBisCO into icosahedral carboxysomes along with carbonic anhydrase
\-some algae have HCO3(-) ion pumps in cell membrane and carbonic anhydrase packed into pyrenods in the stroma
\-some bacteria pack RuBisCO into icosahedral carboxysomes along with carbonic anhydrase
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What happens to phosphoglycolate in land plants?
\-must be salvaged into the Calvin-Benson cycle
\-complex process: three organelles, dozens of enzymes
\-consumes ATP and reducing equivalents (Fd)
2PG + O2 + 2ATP + 2Fdred => PGA + CO2 + 2ADP + 3Pi + 2Fdox
\-complex process: three organelles, dozens of enzymes
\-consumes ATP and reducing equivalents (Fd)
2PG + O2 + 2ATP + 2Fdred => PGA + CO2 + 2ADP + 3Pi + 2Fdox
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Draw calvin benson cycle
12
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Different parts of the PG salvage pathway occur in 3 diff organelles, draw it
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What happens in the plastid?
\-ammonia is recycled into an amino group in the plastid using the glutamine synthesase pathway
\-consumes reducing equivalents (cofactor/currency that donates electrons and which could be interchanged with NAD(P)H (ex: thiol groups)
\-consumes reducing equivalents (cofactor/currency that donates electrons and which could be interchanged with NAD(P)H (ex: thiol groups)
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What is the PG salvage pathway?
\-quick suboptimal fix
\-starting point of no PG recovery pathway in an oxygen-rich atmosphere with just one extra enzyme we can get it to a local optimum: glycine/serine pathway
\-starting point of no PG recovery pathway in an oxygen-rich atmosphere with just one extra enzyme we can get it to a local optimum: glycine/serine pathway
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What is C4 metabolism?
\-neater adaptation against photorespiration found in many sub/tropical plants
\-C4 plants: maize (Zea mays), sugarcane (Saccharum officinarum)
\-C3 plants: wheat (Triticium aestivum), rice (Oryza sativa), potato (Solanum tuberosum)
\-C4 plants: maize (Zea mays), sugarcane (Saccharum officinarum)
\-C3 plants: wheat (Triticium aestivum), rice (Oryza sativa), potato (Solanum tuberosum)
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How was C4 discovered?
\-discovered by Hatch and Slack feeding 14CO2 to sugarcane
\-first products were C4 acids not C3 PGA
* malic acid
* oxaloacetic acid
* aspartic acid
\-first products were C4 acids not C3 PGA
* malic acid
* oxaloacetic acid
* aspartic acid
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What is the CO2 compensation point?
\-point needed for net photosynthesis
\-net photosynthesis= gros photosynthesis-respiration(s)
\-C3 photorespiration: no net photosynthesis at low \[CO2\]
\-compensation points are:
* C3: 0.005%
* C4: 0%
\-net photosynthesis= gros photosynthesis-respiration(s)
\-C3 photorespiration: no net photosynthesis at low \[CO2\]
\-compensation points are:
* C3: 0.005%
* C4: 0%
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What does suppression of photorespiration by C4 allow for plants?
\-allows plants to withstand water-stress
\-stomata narrow/close during water stress
\-CO2 conc in mesophyll falls to compensation point
\-lower in C4 plants
\-only half as much water lost
\-stomata narrow/close during water stress
\-CO2 conc in mesophyll falls to compensation point
\-lower in C4 plants
\-only half as much water lost
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What type of anatomy do C4 plants show?
\-kranz anatomy: photosynthetic tissue is concentrated around veins
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What does he initial CO2 assimilaion in C4 plants?
\-PEP carboxylase in mesophyll not RuBisCO
\-RuBisCO is still needed but is seperated into bundle sheath
\-PEP + CO2 + H2O => Oxaloacetate + Pi
\-RuBisCO is still needed but is seperated into bundle sheath
\-PEP + CO2 + H2O => Oxaloacetate + Pi
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Diagram of general anatomy of the C4 biochem pump
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How does a C4 biochemical pump works?
* CO2 fixation by PEP carboxylase occurs in the mesophyll
* resulting C4 acids translocated into bundle-sheath and decarboxylated
* CO2 enters Calvin cycle via RuBisCO as usual
* C3 fragment is returned to the mesophyll and metabolised back to PEP
* resulting C4 acids translocated into bundle-sheath and decarboxylated
* CO2 enters Calvin cycle via RuBisCO as usual
* C3 fragment is returned to the mesophyll and metabolised back to PEP
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What type of adaotation is C4?
\-recent (20 Ma) to high T and low CO2 conc
\-evolved convergently over 60 occasions
\-evolved convergently over 60 occasions
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13C vs 12C
\-RuBisCO discriminates little between 13C and 12C preferentially incorporating the latter into PGA
\-PEP carboxylase doesn’t discriminate agains 13C nearly so much so the ratio of 12C to 13C can be used to determine whether fossil plant material used C3 or C4 photosynthesis
\-PEP carboxylase doesn’t discriminate agains 13C nearly so much so the ratio of 12C to 13C can be used to determine whether fossil plant material used C3 or C4 photosynthesis
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What is a crassulacean acid metabolism (CAM)?
\-like C4++
\-mostly succulent xerophytes or epiphytes (ex: cactuses, crassula, pineapple)
\-NADP-ME but no kranz anatomy
\-has evolved on more than 30 occasions
\-slow-growing and few crops
\-mostly succulent xerophytes or epiphytes (ex: cactuses, crassula, pineapple)
\-NADP-ME but no kranz anatomy
\-has evolved on more than 30 occasions
\-slow-growing and few crops
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What does the vacuolar sap of CAM plants undergo?
Diurnal cycles in pH
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CAM vs C4: how do they seperate PEP carboxylase and RuBisCO?
\- CAM: temporally
\-C4: spatially
\-C4: spatially
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How does the CAM storage mechanism work?
\-night: stomata opens and CO2 fixed by PEP carboxylase
\-day: stomata closed, stored malatate in vacuole and consumed by RuBisCO
\-day: stomata closed, stored malatate in vacuole and consumed by RuBisCO
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Draw CAM storage mechanism work
Note 
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