Question

Photorespiration is favored by
(a)Low temperatures
(b)Low light intensity
(c)High oxygen and low carbon dioxide
(d)Low oxygen and high carbon dioxide

Answer 1

Hint: Photorespiration (also known as the photosynthetic carbon oxidation cycle) refers to a phase in the metabolism of plants where the RuBisCO enzyme oxygenates RuBP, wasting some of the photosynthesis-generated energy. A main step in the Calvin-Benson cycle, the desired reaction is the addition of carbon dioxide to RuBP (carboxylation), but approximately 25 percent of RuBisCO reactions instead add oxygen to RuBP (oxygenation), producing a product that cannot be used throughout the Calvin-Benson cycle.

Complete answer:
When the levels of carbon dioxide within a leaf are minimal, photorespiration occurs. This occurs when a plant is forced to close its stomata on hot, dry days to avoid excess water loss. The carbon dioxide will be used up if the plant wants to try to fix carbon dioxide as its stomata are closed, and the oxygen ratio in the leaf will rise compared to concentrations of carbon dioxide.

Rubisco begins to mix oxygen with RuBP instead of carbon dioxide when the amount of carbon dioxide within the leaf decreases to about 50 ppm. The net effect of this is that only one molecule of PGA is produced and a toxic 2C molecule called phosphoglycolate is produced instead of producing two 3C PGA molecules. Photorespiration takes place during hot, dry days when the temperature and strength of the light are high. In low oxygen levels and high carbon dioxide levels, photorespiration does not occur.

Additional information: This process decreases photosynthesis efficiency, potentially reducing photosynthetic production in $C_3$ plants by 25 percent. A complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes, and mitochondria are involved in photorespiration.
RuBisCO's oxygenation reaction is a wasteful process as 3-phosphoglycerate is produced at a lower rate and higher metabolic cost compared to the activity of RuBP carboxylase. While photorespiratory carbon cycling ultimately results in the formation of $G_3P$, approximately 25 percent of photorespiration-fixed carbon is re-released as Carbon dioxide and ammonia as nitrogen. At a significant cost to the cell, ammonia must then be detoxified. A direct cost of one ATP and one NAD(P)H is also incurred for photorespiration.
So, the correct answer is ‘(d) Low oxygen and high carbon dioxide’.

Note: The copper-containing enzyme involved in the first major stage of carbon fixation is ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco). RuBisCO is biologically significant since it catalyzes the primary chemical reaction by which inorganic carbon enters the biosphere. Unlike RuBisCO, phosphoenolpyruvate carboxylase fixes carbon only temporarily.