The evolutionary importance of the C-4 pathway becomes obvious when one understa

Q: The evolutionary importance of the C-4 pathway becomes obvious when one understa

ANSWER C-4 plants avoid photorespiration by first fixing CO2 using the enzyme PEP carboxylase (this is actually better as it has higher affinity for carbon dioxide than Rubisco), creating high concentrations of CO2 inside the leaf (consequence of greater affinity), running the Calvin Cycle within specialized bundle-sheath cells (fact). Hence: d. All of the above

ID: 203542 • Letter: T

Question

The evolutionary importance of the C-4 pathway becomes obvious when one understands that photosynthesis in C-3 plants is accompanied by photorespiration: a light-dependent process where rubisco binds competitively to 02 instead of CO2, wasting a turn of the Calvin cycle and eventually resulting in the release of up to 25 percent or more of the previously reduced CO2 from the plant (Foyer 1984; Salisbury & Ross 1985). Thus, photorespiration, which is enhanced by bright light, high 02/CO2 ratios and warm temperatures, is considered to be a wasteful process that can significantly reduce the growth rate and therefore productivity of plants, particularly those in the C-3 group. C-4 plants appear to exhibit little, if any, photorespiration under normal environmental conditions and even in bright light, low CO2 levels and warm temperatures. The physiology is complex, but C-4 plants do not normally show the overall negative effects of photorespiration for at least two reasons: 1. The escape of photorespired CO2 released in the bundle-sheath cells is prevented by subsequent CO2 assimilation in the surrounding, nonphotorespiratory mesophyll cells; unlike rubisco, the carboxylation activity of PEP carboxylase is not inhibited by oxygen. 2. Carbon assimilation in mesophyll cells produces relatively high concentrations of CO2 around rubisco in the bundle-sheath cells, a condition favoring carboxylation over oxygenation activity, or photosynthesis over photorespiration.

C-4 plants avoid photorespiration by:

a. First fixing CO2 using the enzyme PEP carboxylase

Explanation / Answer

ANSWER
C-4 plants avoid photorespiration by first fixing CO2 using the enzyme PEP carboxylase (this is actually better as it has higher affinity for carbon dioxide than Rubisco), creating high concentrations of CO2 inside the leaf (consequence of greater affinity), running the Calvin Cycle within specialized bundle-sheath cells (fact).
Hence: d. All of the above

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