51. During photorespiration a. sugar synthesis is made more efficient b. rubisco

Question

51. During photorespiration a. sugar synthesis is made more efficient b. rubisco binds O instead of CO c. light reactions are uncoupled from the Calvin Cycle d. the Calvin Cycle is stopped e. loss of water, which only happens in succulents 52 Why are C4 plants able to photosynthesize with no apparent photorespiration? a. They do not participate in the Calvin cycle b. They use PEP carboxylase to initially fix CO2 C. They conserve water more efficiently. d. They exclude oxygen from their tissues e. The organization of the chloroplasts is less compacted 53. CAM plants keep stomata closed in the daytime, thus reducing loss of water They can do this because they a. fix CO2 into organic acids during the night b. use CO2 from PEP instead of RuBP at the start of the Calvin Cycle c. fix CO2 in the Calvin Cycle d. open their stomata at night e. all of the above 54. Mitochondria and chloroplasts have similar structures. The current hypothesis suggests that this is because a. both require lots of membrane area so evolved the same way b. each evolved from vesicles by involuting cell membranes c. both have evolved from once free-living prokaryotes d. both evolved from primitive eukaryotic cells e. they both have elements of extraterrestrial life forms 55. The likely force driving the evolution of C4 and CAM plants is a. adapting to hot, dry conditions b. adapting to periodic floods c. the increases in atmospheric CO2 and 02 d. the type of soil e. seasonal temperature changes

Explanation / Answer

51. b. Rubisco binds to oxygen instead of carbondioxide during photorespiration. The reaction that uses CO2 is the first step of the Calvin cycle and leads to the production of sugar whereas the reaction that uses O2 is the first step
of the photorespiration pathway, which wastes energy and "undoes" the work of the Calvin cycle.

52. ans b.
In C4 plants, carbon dioxide combines with a compound called phosphoenolpyruvate (PEP) using an enzyme called phosphoenolpyruvate carboxylase to form oxaloacetate. Oxaloacetate is then converted to malate and is
transported into the bundle sheath cells (site of carbon dioxide fixation by RuBisCO) where oxygen concentration is low to avoid photorespiration.

53. ans e: all of the above

During night, CAM plants has its stomata open, CO2 enters and it is fixed as organic acids which are stored in vacuoles. During the day the stomata are closed to prevent water loss and the carbon is released to the Calvin cycle so that photosynthesis may take place. The carbon dioxide is fixed in the cytoplasm of mesophyll cells by a PEP reaction similar to that of C4 pathway. But, unlike the C4 mechanism, the resulting organic acids are stored in vacuoles for later use; that is, they are not immediately passed on to the Calvin cycle.

54. ans c: Chloroplasts and mitochondria are both thought to have evolved through the process of endosymbiosis. Scientists think that through millions of years of evolution, the aerobic bacteria became mitochondria and the photosynthetic bacteria became chloroplasts.

55. ans a

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