Why is phorespiration avoided in C 4 plants?

Photorespiration is the process of oxidation that is initiated in the chloroplast and takes place only during the day. RuBisCO, the enzyme required for CO 2 fixation in the Calvin cycle, is responsible for the photorespiration. The active site of the enzyme RuBisCO can bind to both CO 2 and O 2 . In C 4 plants, a special anatomical feature is seen, called Kranz anatomy. The large bundle sheath cells are arranged in two or three layers around the vascular bundles. Two types of chloroplast are found in C 4 plants, the chloroplast present in mesophyll cell is small and has well-developed grana while the chloroplast found in bundle sheath cell is large and without grana. Thylakoids are found in the form of stroma lamellae. Hence, in C 4 plants, the light reaction takes place in mesophyll cells. In C 4 plants, PEP carboxylase (PEPCase) enzyme catalyses the carboxylation and fixes CO 2 at low concentration in the mesophyll cells. In the bundle sheath cells, CO 2 concentration is more hence, RuBisCO functions as carboxylase, and photorespiration is avoided.

Name the CO 2 acceptor in the mesophyll cells during HSK pathway.

The Hatch-Slack pathway (HSK) is also called the C 4 cycle in which the first stable product is a 4 carbon compound, oxaloacetic acid. This cycle is present in most of the monocot plants in which two types of photosynthetic cells found in their leaves, i.e, mesophyll cells and bundle sheath cells. In the C 4 cycle, carbon dioxide from the environment reaches to mesophyll cells of the leaf through stomata. This carbon dioxide absorbed by phosphoenolpyruvic acid (PEP) and in the presence of the PEP Carboxylase enzyme, converts it into oxaloacetic acid. Hence, the CO 2 acceptor in the mesophyll cells during HSK pathway is phosphoenolpyruvic acid (PEP).

Give any two examples of C 4 plants.

The C 4 cycle is also called Hatch-Slack pathway in which the first stable product is a 4 carbon compound, oxaloacetic acid. This cycle is present in most of the monocot plants in tropical region in which two types of photosynthetic cells found in their leaves, i.e, mesophyll cells and bundle sheath cells. Examples: Maize, sugarcane, millet, sorghum, etc.

Explain the mechanism of the C 4 cycle.

The C 4 pathway acts as a mechanism to build up high concentrations of carbon dioxide in the chloroplasts of the bundle sheath cells. The mechanism takes place as follows: The C 4 pathway initiates with a molecule called phosphoenolpyruvate (PEP) which is a 3-carbon molecule. This is the primary CO 2 acceptor and the carboxylation takes place with the help of an enzyme called PEP carboxylase. They yield a 4-C molecule called oxaloacetic acid (OAA). This is converted into another 4-carbon compound known as malic acid. Later, they are transferred from mesophyll cells to bundle sheath cells. Here, OAA is broken down to yield carbon dioxide and a 3-C molecule. The CO 2 thus formed is utilised in the Calvin cycle whereas the 3-C molecule is transferred back to mesophyll cells for regeneration of PEP.

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Dimorphic chloroplasts are present in

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Important Questions on Photosynthesis in Higher Plants

HARD

Biology>Plant Physiology>Photosynthesis in Higher Plants>Biosynthetic Phases of Photosynthesis

Why is phorespiration avoided in C₄ plants?

HARD

Biology>Plant Physiology>Photosynthesis in Higher Plants>Biosynthetic Phases of Photosynthesis

A plant in your garden avoids photorespiratory losses, has improved water use efficiency, shows high rates of photosynthesis at high temperatures and has improved efficiency of nitrogen utilization. In which of the following physiological groups would you assign this plant?

EASY

Biology>Plant Physiology>Photosynthesis in Higher Plants>Biosynthetic Phases of Photosynthesis

Name the CO₂ acceptor in the mesophyll cells during HSK pathway.

MEDIUM

Biology>Plant Physiology>Photosynthesis in Higher Plants>Biosynthetic Phases of Photosynthesis

Choose the correct option for the given figure and the “a” part highlighted in it.

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