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Lecture 23

BI110 Lecture 23: Biology Note 23 (module 31)

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Matthew Smith

Module 31: C , 3 ,4and CAM Photosynthesis - different types of photosynthesis that occur in plants (already have talked about C photosythesis) 3 Rubisco has a Carboxylase & Oxygenase Activity - Carboxylation Reaction-represents the GAIN of carbon  Happens in first step of calvin cycle (Carboxylation Reaction – represents the GAIN of carbon)  Takes RuBP a+CO2 and splits them into two 3PGA molecules - Oxygenation Reaction- represents the LOSS of carbon  RuBP-O2 and splits them into one 3PGA and one phosphoglycolate  Phosphoglycolate gets converted into glycolate  Glycolate gets released as CO2  Not a productive activity (called Photorespiration) • Rubisco can use RuBP with CO2 or O2 Oxygenase activity of Rubisco - O2 acts as a competitive inhibitor of Rubisco (competing with Co2 to bind to the binding site of Rubisco)  ancient enzyme, developed when very little O2 in the atmosphere - Rubisco active site has greater affinity for CO2 than O2 (will use CO2 more than O2) - Atmosphere is 21% O2 and 0.04% CO2 (more availability to bind to O2 but wants to use CO2 more) - At moderate temps  Carboxylation 75%, oxygenation 25% - Plant species have evolved mechanisms to reduce the oxygenation reaction  increase ratio of CO2/O2 (aka more CO2) where Calvin cycle (rubisco) takes place (so that the carboxylation reacton takes place more than the oxygenation reaction) Dilemma of Plants in Hot Dry Climates - Plants have to open their stomata (pores) to allow CO2 to get in - But plants that live in hot environments will lose water when they open their stomata due to evaporation - So the problem is that they need to open stomata to let the gas in but they don’
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