Test: Dark Reactions (Photosynthesis) - NEET MCQ

During the dark phase of photosynthesis, carbon dioxide combines with RUBP or RUDP (ribulose bi/di phosphate) in the carboxylation reaction catalyzed by enzyme RUBISCO (ribulose-1,5-bisphosphate carboxylase/oxygenase). The carboxylation of RUBP generates 3 PGA or 3 phosphoglyceric acid, which is the first stable product of dark phase.

Photosynthesis occurs in two phases-light reaction and dark reaction. During light reaction water undergoes photolysis releasing oxygen and reduced NADPH2 and ATP molecules are synthesized. During the dark reaction, NADPH2 and ATP are used to reduce carbon dioxide into sugars.

During the dark phase of photosynthesis, carbon dioxide combines with RUBP in the carboxylation reaction catalyzed by enzyme RuBisCo. The carboxylation of RUBP generates 3 PGA or 3 phosphoglyceric acid, which is the first stable product of dark phase.

The carbon dioxide is fixed during dark phase of photosynthesis using reducing and assimilatory power synthesized during light phase. The initial carboxylating enzyme is PEP carboxylase in C4 plants. It carboxylates PEP or phosphoenolpyruvate and produces organic acids having four carbon atoms.

3-phosphoglycerate (PGA, a carboxylic acid) is first phosphorylated into 1,3-bisphosphoglycerate by enzyme 3-phosphoglycerate kinase. This is followed by reduction of 1,3-bisphosphoglycerate catalyzed by glyceraldehyde 3-phosphate dehydrogenase enzyme into glyceraldehyde 3-phosphate (aldehyde). NADPH serves as electron donor here. Thus, the correct answer is B.

C3 plants include most of the temperate crops and all the woody trees. Example: beans, rice, wheat, potatoes

C4 plants constitute crops growing in a hot and dry climate. These are mostly grasses and some shrubs.  Example: maize, sugarcane, amaranth

Solution:

In C4 plants, carbon fixation is a two-step process. The initial fixation of carbon dioxide occurs in the mesophyll cells, where it is converted into a 4-carbon compound. This compound is then transported to the bundle sheath cells, where carbon dioxide is released and refixed by the Calvin cycle in the chloroplasts of these cells. The bundle sheath cells play a crucial role in the C4 pathway, allowing the plant to efficiently fix carbon even under conditions of low atmospheric CO2 concentration.

Therefore, the correct answer is Option C: Bundle sheath cells.

Hatch-Slack pathway also called C4 carbon fixation is a photosynthetic process in some plants. It is the first step in extracting carbon from carbon dioxide to be able to use it in sugar and other biomolecules. It is one of the three known processes for carbon fixation. C4 refers to the four-carbon molecule that is the first product of this type of carbon fixation.

Word "Kranz" means a ring or wreath and refers to the thick walled bundle sheath cells present around vascular bundles in the shape of a ring in C4 plants. The bundle sheath cells have agranal chloroplast and fix carbon dioxide with the help of RuBisCO enzyme. The thin walled mesophyll cells carry out initial carbon dioxide fixation with the help of PEP carboxylase enzyme. This special arrangement of cells and presence of dimorphic chloroplasts helps in minimizing photorespiration in C4 plants.

The C₄ plants are called so because the first stable product of this process are the four carbon atom containing organic acids like oxalo-acetic acid and malic acid. The initial acceptor of carbon dioxide is phosphoenol pyruvic acid (PEP) and the initial carboxylating enzyme is PEP carboxylase. The initial carboxylation reaction occurs in the mesophyll cells. In the C₄ organic acids are decarboxylated in the bundle sheath cells to release carbon dioxide. In the bundle sheath cells the carbon dioxide is fixed with the help of RuBisCO enzyme.

In the majority of the plants, the first stable intermediate of carbon dioxide reduction is a three-carbon compound. Hence, this type of carbon reduction is called as C3 cycle. In C3 cycle the initial carboxylation is brought about by RuBisCO, which carboxylates Ribulose- 1,5 bisphosphate (RUBP) into phosphoglyceric acid. The phosphoglyceric acid is later on reduced to phosphoglyceraldehyde, which is used to synthesize sugars and recycle RUBP.

The dark reaction of photosynthesis is not dependent on sunlight but it does not mean it does not occur in its presence. The dark reaction occurs both in the presence and absence of sunlight. This reaction occurs in the stroma of the chloroplasts which are cell organelles known as the kitchen of the cells. The CO2 taken in by the plant combines a 5 carbon compound known as ribulose 1,5- biphosphate to form two molecules of 3-phosphoglycerate(3PGA).  This step is catalyzed by the enzyme ribulose biphosphate carboxylase or RUBISCO(RuBp).

Photosynthesis comprises of light phase and dark phase. During light phase reducing power and assimilatory powers are synthesized, which are used in the dark phase to reduce carbon dioxide to carbohydrates. The dark phase comprises of initial carboxylation reaction, reduction of phosphoglyceric acid to phosphoglyceraldehyde and synthesis of carbohydrates and regeneration of ribulose 1,5 bisphosphate from phosphoglyceraldehyde. The initial carboxylation comprises of reaction between carbon dioxide and ribulose 1,5 bisphosphate to generate two molecules of phosphoglyceric acid. This reaction is catalyzed by ribulose 1,5 bisphosphate carboxylase oxygenase (RuBisCO) enzyme.

C₄ plants utilize the C₄ carbon obsession pathway to expand their photosynthetic proficiency by diminishing or stifling photorespiration, which predominantly happens under low climatic.
These are significant harvests, for example, maize, sorghum and sugarcane,and Atriplex yet in addition weeds and obtrusive plants.
C₄ photosynthesis has various unmistakable properties that empower the catch .
The result of ongoing endeavors to build C₄ chemicals into C₃ plants is Be that as it may, in malate‐forming C₄ plants, for example, maize, oxaloacetate take-up would not . it must be borne as a main priority that three of these proteins.  
C₄ species are the monetarily significant harvests corn or maize (Zea mays), sugarcane (Saccharum officinarum), (Sorghum bicolor), and millets, just as the switchgrass (Panicum virganum) which has been used as a wellspring of biofuel.
Atriplex are eatable, and likely more. Atriplex is the old named utilized by Pliny for the orach, additionally know as A.
Hence, the correct answer is Option D. 

In the leaves of C4 plants, bundle sheath cells are present in the form of rings around the vascular bundle. The arrangement is called kranz and the anatomy is called kranz type of anatomy.
So, the correct option is ‘Leaves’.

C4 plants are adapted to hot and dry climate regions. Sugarcane, Maize, Sorghum, etc., are the examples of C4 plants. In these plants, the Calvin cycle occurs in the bundle-sheath cells. C4 plants have a special type of anatomy called Kranz anatomy. They can tolerate higher temperatures because of Kranz anatomy. 'Kranz' means wreath and is a reflection of the arrangement of cells. Kranz anatomy is characterized by having a large number of chloroplasts, grana are absent, thick walls impervious to gaseous exchange and no extracellular spaces.

In C4 plants, PEP carboxylase fixes PEP with carbon dioxide in the mesophyll cells of the leaf and results in four carbon compound, malate which is shuttled into the bundle sheath cells. Thus, option B is correct. PGA, RDP, RMP are the intermediate products of C3​ pathway. 
Therefore, the correct answer is option C.

Phosphoenolpyruvate carboxylase is also known as PEP carboxylase is an enzyme in the family of carboxy-lyases that catalyzes the addition of CO2 to phosphoenolpyruvate (PEP) to form the four-carbon compound oxaloacetate: 

PEP + CO2 → oxaloacetate + Pi 

This reaction is used for carbon fixation in so-called "CAM" and "C4" plants where it plays a key role in photosynthesis. The enzyme is also found in some bacteria, but not in animals or fungi .

• Due to their reduced rate of photorespiration and high rate of photosynthesis, C₄ plants are more efficient than C₃ plants. 
• So the yield of C₃ plants is lower as compared to C₄ plants.

The net result of the process is a conversion of a 3- phosphoglycerate molecule into a molecule of the three-carbon sugar glyceraldehyde 3- phosphate (G3P). In three turns of the cycle, six molecules of G3P are produced, six ATP is converted to ADP and Pi and six NADPH are converted to NADP+ and H+.

In the majority of the plants, the first stable intermediate of carbon dioxide reduction is a three-carbon compound. Hence, this type of carbon reduction is called as C3 cycle. In C3 cycle, the initial carboxylation is brought about by RuBisCo, which carboxylates Ribulose 1,5 bisphosphate (RUBP) into phosphoglyceric acid. The phosphoglyceric acid is later on reduced to phosphoglyceraldehyde, which is used to synthesize sugars and recycle RUBP. Simple sugars like glucose and fructose are first produced. They are converted to sucrose for transportation and later on into starch for long term storage.

Photosynthesis is a redox reaction in which carbon dioxide is reduced to sugars in dark reaction and water is oxidised to release oxygen in light reaction of photosynthesis. C¹⁴ label was used to trace the path of carbon in the dark reaction in C³ plants by Calvin, Benson and Basham. The O¹⁸ label was used by Rumen and Kamen to experimentally prove that the source of oxygen released during light reaction of photosynthesis is water.

The thick walled bundle sheath cells present around vascular bundles in the shape of a ring in C4 plants have chloroplast. The bundle sheath cells have agranal chloroplast and fix carbon dioxide with the help of RuBisCO enzyme. The thin walled mesophyll cells carry out initial carbon dioxide fixation with the help of PEP carboxylase enzyme. This special arrangement of cells and presence of dimorphic chloroplasts helps in minimizing photorespiration in C4 plants.

The thick walled bundle sheath cells present around vascular bundles in the shape of a ring in C4 plants have chloroplast. The bundle sheath cells have agranal chloroplast and fix carbon dioxide with the help of RuBisCO enzyme. The thin walled mesophyll cells carry out initial carbon dioxide fixation with the help of PEP carboxylase enzyme. This special arrangement of cells and presence of dimorphic chloroplasts helps in minimizing photorespiration in C4 plants.

The thick walled bundle sheath cells present around vascular bundles in the shape of a ring in C₄ plants have chloroplast. The bundle sheath cells have agranal chloroplast and fix carbon dioxide with the help of RuBisCO enzyme. The thin walled mesophyll cells carry out initial carbon dioxide fixation with the help of PEP carboxylase enzyme. This special arrangement of cells and presence of dimorphic chloroplasts helps in minimizing photorespiration in C₄ plants. Thus two types of photosynthetic cells occur in C₄ plants- one type mesophyll cells, which carry on initial carbon dioxide fixation and another bundle sheath cells, which carry on carbon dioxide fixation using RuBisCO enzyme.

The C4 plants are characterised by Krantz anatomy. These plants are characterised by dimorphic chloroplasts. These plants have thick walled bundle sheath cells lining the vascular supply. The bundle sheath cells have large agranal chloroplasts which fix carbon dioxide into sugars with the help of RUBISCO enzyme. The thin walled mesophyll cells have grana in the chloroplast and carry on initial carbon dioxide fixation with the help of PEP carboxylase enzyme.

Therefore, the correct answer is option A.

The C₄ plants are characterised by Krantz anatomy. These plants are characterised by dimorphic chloroplasts. These plants have thick walled bundle sheath cells lining the vascular supply. The bundle sheath cells have large agranal chloroplasts, which fix carbon dioxide into sugars with the help of RuBisCO enzyme. The thin walled mesophyll cells have grana in the chloroplast and carry on initial carbon dioxide fixation with the help of PEP carboxylase enzyme. Thus the bundle sheath cells get high concentration of carbon dioxide and are not exposed to oxygen released from grana during light reaction. This adaptation reduces photorespiratory losses in C₄ plants. Thus improving overall efficiency of photosynthesis in these plants.

In the C4 pathway, initial carbon fixation takes place in mesophyll cells and the Calvin cycle takes place in bundle-sheath cells. PEP carboxylase attaches an incoming carbon dioxide molecul to the three-carbon molecule PEP, producing oxaloacetate (a four-carbon molecule).