Photorespiration Fall 2024 PDF

Photosynthesis C3, C4, and CAM plants STOMATA and GUARD CELLS In order for plants to uptake CO2 into the leaves, the stomata must remain open (these are on the underside of the leaf) They are protected by guard cells that regulate their opening and closing These stomata that take in CO2, and rel...

Photosynthesis C3, C4, and CAM plants STOMATA and GUARD CELLS In order for plants to uptake CO2 into the leaves, the stomata must remain open (these are on the underside of the leaf) They are protected by guard cells that regulate their opening and closing These stomata that take in CO2, and release O2, also allow for transpiration to occur When the weather is hot and dry, guard cells decrease the size of the stomata (or close them altogether) in an effort to conserve water. As a result, CO2 concentration in the leaves’ air spaces declines. Since the cell continues with photosynthesis, oxygen levels go up. Oxygen competes with CO2 in the Calvin Cycle for rubisco’s active site, and binds to it more frequently, affecting the first stage of the dark reaction Photorespiration This is called photorespiration, and affects C3 plants when the weather is hotter out 🡪 lose photosynthetic efficiency by 30%. Competitive Inhibition of Rubisco Since O2 molecules are fixed instead of CO2, an intermediate called phosphoglycolate is made. This then undergoes a series of reactions to “clean up the mess” and in the end will make 3-PGA, but unfortunately CO2 molecules are lost. This results in no glucose made, and a huge loss in productivity. Adaptations to High Temperatures: C4 Plants C4 plants have evolved a mechanism that allows them to concentrate CO2 at the site where rubisco is found in special cells called bundle sheath cells Allows them to avoid photorespiration Adaptation to hot, dry climates Many plants including sugar cane and corn  C4 pathway C4 plants have a unique leaf anatomy where a mesophyll cell is right next to a bundle sheath cell🡪 CO2 is first converted into a two different 4C molecules in mesophyll cell Then the 4C molecule is broken down into CO 2 and a 3C molecule in bundle sheath cell (so that high concentrations of CO2 here can now outcompete oxygen for rubisco) CAM Plants Water storing plants (cacti, pineapples) are members of the Crassulacea family Adaptation: open stomata at night and close them during the day (reverse!)🡪 the best evolutionary adaptation to extreme heat and dry conditions At night they take in CO2 and convert it into 4C molecules(stored in vacuoles until morning) then releases CO2 to be used in the Calvin Cycle Crassulacean Acid Metabolism (CAM)occurs only in the mesophyll cells, but the stages happen at different times of the day C3, C4, and CAM plants https://www.youtube.com/watch?v=HbL g4lMpUa8