光合作用英文版 Photosynthesis幻灯片.pptVIP

* * * * Affinity of PEP Carboxylase for CO2 is much higher than its affinity for O2. Competing Reactions Rubisco grabs CO2, “fixing” it into a carbohydrate in the light independent reactions. O2 can also react with rubisco, inhibiting its active site not good for glucose output wastes time and energy (occupies Rubisco) Photorespiration When Rubisco reacts with O2 instead of CO2 Occurs under the following conditions: Intense Light (high O2 concentrations) High heat Photorespiration is estimated to reduce photosynthetic efficiency by 25% Why high heat? When it is hot, plants close their stomata to conserve water They continue to do photosynthesis ? use up CO2 and produce O2 ? creates high O2 concentrations inside the plant ? photorespiration occurs C4 Photosynthesis Certain plants have developed ways to limit the amount of photorespiration C4 Pathway* CAM Pathway* * Both convert CO2 into a 4 carbon intermediate ? C4 Photosynthesis Leaf Anatomy In C3 plants (those that do C3 photosynthesis), all processes occur in the mesophyll cells. Image taken without permission from /thelifewire| Mesophyll cells Bundle sheath cells C4 Pathway In C4 plants photosynthesis occurs in both the mesophyll and the bundle sheath cells. Image taken without permission from /thelifewire| C4 Pathway CO2 is fixed into a 4-carbon intermediate Has an extra enzyme– PEP Carboxylase that initially traps CO2 instead of Rubisco– makes a 4 carbon intermediate C4 Pathway The 4 carbon intermediate is “smuggled” into the bundle sheath cell The bundle sheath cell is not very permeable to CO2 CO2 is released from the 4C malate ? goes through the Calvin Cycle C3 Pathway How does the C4 Pathway limit photorespiration? Bundle sheath cells are far from the surface– less O2 access PEP Carboxylase doesn’t have an affinity for O2 ? allows plant to collect a lot of CO2 and concentrate it in the bundle sheath cells (where Rubisco is) CAM Pathway Fix CO2 at night and store as a 4 carbon molecule Keep stoma