Photosynthesis in Higher Plants Test - 21

• Photorespiration occurs when carbon dioxide level becomes low inside a leaf. This happens on hot dry days, when a plant is forced to close its stomata to prevent excess water loss. 
• If the plant continues to attempt to fix $$CO_2$$, when its stomata are closed, the $$CO_2$$ will get used up and the $$O_2$$ ratio in the leaf increase relative to $$CO_2$$ concentrations. 
• Thus, the enzyme RuBP oxygenase fixes oxygen instead of carbon dioxide and produces two molecules of phosphoglycolate (2C).  It is also called $$C_2$$ cycle.

Correct Option: D

Explanation:

• Kranz anatomy is a leaf anatomy that is unique to C4 plants. It is a reflection of the arrangement of cells and wreath (ring).
• Bundle sheath cells of a considerable size can form several layers surrounding vascular bundles.
• These cells are distinguished by possessing a significant number of agranal chloroplasts and strong, gaseous-resistant walls. There are no intercellular gaps and no exchange.

Hence, the correct option is “thick walls, no intercellular spaces and large number of chloroplasts”, as it clearly describes the Kranz anatomy in the C4 plants.

Peroxisomes are small organelles found in the photosynthetic mesophyll cells in plants. They play vital role in the process of photorespiration. These are also involved in metabolism, development and responses to stresses. These are involved in oxidation of certain biomolecules like fatty acids.

RUBISCO is a very large four-subunit enzyme present in the chloroplast stroma and catalyze carboxylation of RuBP followed by immediate splitting of unstable product into 3 phosphoglyceraldehyde. PEP carboxylate enzyme of C4 carries out carboxylation of phosphoenolpyruvate into oxaloacetate. Unlike RUBISCO, it does not have oxidation acitivity and no photorespiration. Correct answer is B.

Photorespiration is a process in plant metabolism, which attempts to reduce the consequences of a wasteful oxygenation reaction by the enzyme RuBisCO. This process reduces efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in $$C_3$$ plants. Photorespiration involves a complex network of enzyme reactions, that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.

RUBISCO has affinity for both oxygen and carbon dioxide. Photorespiration is oxidation of RuBP, rather than carboxylation, by enzyme RUBISCO and formation of 2-phosphoglycolate. Under standard 25°C, the rate of the carboxylation reaction is four times that of the oxidation reaction and 20% of fixed carbon is lost in photorespiration.  Spatial separation of carbon dioxide fixation and RUBISCO activity in mesophyll and bundle sheath cells of $$C_4$$ plants respectively avoids photorespiration by concentrating $$CO_2$$ in bundle sheath cells to scale down the oxygenase activity of RUBISCO and increase the RUBISCO efficiency which is not the case in $$C_3$$ plants. Correct answer is D.

$$C_4$$ plants use a supplementary method of $$CO_2$$ uptake, which forms a 4-carbon molecule instead of the two 3-carbon molecules of the Calvin cycle. Hence, these plants are called $$C_4$$ plants. They can effectively fix carbon in low carbon dioxide concentration as they have kranz anatomy.

• Photorespiration is a process in C$$_3$$ plants that inhibits the rate of photosynthesis in the presence of high oxygen concentration in light. In the presence of a high concentration of oxygen, C$$_3$$ plants start releasing carbon dioxide in light during an extra input of oxygen. 
• It is an enzymatic process that involves specialized reactions which take place in three organelles in plants namely, chloroplasts, mitochondria, and peroxisomes. 
• In the presence of low carbon dioxide concentration, the enzyme RuBisCO acts as an oxygenase and splits a molecule of RUBP into 2-phosphoglycolic acid and 3-phosphoglyceric acid. 

Photorespiration involves a complex network of enzyme reactions, that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria. Photorespiration reduces the efficiency of photosynthesis.