Photosynthesis Problem Set 2
Problem 9 Tutorial: C4 photosynthesis
Which statement about photosynthesis in C4 plants is NOT true?
Some plants that live in hot, dry climates maintain low oxygen levels in their leaves by keeping the stomata closed to prevent water loss. To get adequate CO2 for photosynthesis, C4 plants have adapted to photorespiration by modifying the Calvin-Benson cycle. C4 plants have a special leaf anatomy, with prominent bundle sheath cells surrounding the leaf veins. Photorespiration is minimal in C4 plants compared to C3 plants, and CO2 is actively concentrated in these bundle sheath cells.
C4 plants have evolved a special, energy-requiring pathway to create high, local concentrations of CO2 for the Calvin-Benson Cycle. C4 plants initially fix CO2 at low intracellular CO2 concentrations in mesophyll cells as 4-carbon compounds, using the energy from the hydrolysis of 1 ATP for each CO2 fixed. CO2 is then released in the bundle sheath cells where the Calvin-Benson Cycle reactions occur. The path of CO2 from air, to initial fixation in mesophyll cells, to release in bundle sheath cells and entry into the Calvin-Benson Cycle is shown in the following diagram:
C4 photosynthesis is an adaptation for plants living in hot, arid climates like Tucson, Arizona. C4 plants initially fix CO2 in mesophyll cells as 4-carbon compounds, and later release CO2 in bundle sheath cells. There is an additional ATP requirement for each carbon dioxide utilized in this pathway.
High levels of oxygen at the site of the RUBISCO reaction are the cause of photorespiration. C4 plants largely bypass photorespiration by using an extension of the Calvin-Benson cycle to pump only CO2, and not oxygen, into the bundle sheath cells where the RUBISCO reaction occurs. C4 plants can maintain a high, local concentration of CO2 for RUBISCO activity without raising cellular oxygen levels.
University of Arizona
Friday, October 4, 1996
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