CBSE Questions for Class 11 Medical Biology Photosynthesis In Higher Plants Quiz 3 - MCQExams.com

The most distinct feature of C$${_4}$$ plants is the presence of bundle sheath with chloroplasts in the leaves. It is like a wreath around the vascular bundle. Hence, such anatomy is called Kranz anatomy. Bundle sheath may be single layered as in maize or double layered as in sugarcane. If two-layered, the outer one is green and the inner one is thick walled and called as the mestomal sheath. Mesophyll cell chloroplasts are granal and starch lacking but the bundle sheath cell chloroplasts are grana-lacking and starch-rich. Such feature of having two types of chloroplasts is known as chloroplast dimorphism.

A metabolic pathway first defined by M.D. Hatch and C.R. Slack in 1966 to reduce photorespiration. In this pathway, the $$CO_2$$ is first added to phosphoenolpyruvate by the enzyme PEP carboxylase, producing the four carbon compound in mesophyll cells, that is later transported to bundle sheath cells to liberate the $$CO_2$$ for use in the Calvin cycle.

 Correct option: (C)

 Explanation for correct option:

•  Agranal chloroplasts are the  feature of C₄ plant 
  C₄ plants.
•  chloroplasts were found to be located in bundle sheath cells.
• The C₄ plants possess a characteristic leaf anatomy called Kranz anatomy.
• Their vascular bundles are surrounded by two rings of cells- the inner ring is called bundle sheath cells that contain starch-rich chloroplasts lacking grana, which differ from those in mesophyll cells that are present as the outer ring.

• 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.

Phosphoenol pyruvate is formed from the decarboxylation of oxloacetate and hydrolysis of one guanosine triphosphate molecule. This reaction is catalyzed by the enzyme phosphoenolpyruvate carboxylase. This reaction is a rate limiting step in gluconeogenesis.

GTP + Oxaloacetate -----> GDP + Phosphoenolpyruvate + $$CO_2$$.

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.

• 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. 

• Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. 
• More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration or photosynthesis. 
• Peter D. Mitchell proposed the chemiosmotic hypothesis in 1961. 
• The theory suggests essentially that most ATP synthesis in respiring cells comes from the electrochemical gradient across the inner membranes of mitochondria by using the energy of $$NADH_2$$ and $$FADH_2$$ formed from the breaking down of energy-rich molecules such as glucose.
• So, the correct answer is option D.

• According to the chemiosmosis theory of ATP generation. The ATP is generated utilizing the electrochemical gradient generated due to the accumulation of protons in the thylakoid lumen of the chloroplast. 
• The accumulation of protons gives rise to an electric gradient, due to the accumulation of positive charge and a chemical gradient due to the accumulation of hydrogen atoms. 
• When protons move along this electrochemical gradient through the  F$$_0$$ - F$$_1$$ ATP synthase enzyme complex, ATP generation occurs.