Tag: photosynthesis

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 + $CO _2$.

Certain plants especially succulents which grow under extremely xerophytic conditions, fix atmospheric carbon dioxide in the dark. Since the process was first observed in the plants belonging to family Crassulaceae such as Bryophyllum, Kalanchoe, Sedum etc., it was termed crassulacean acid metabolism (CAM). Such plants are known as CAM plants. The most characteristic feature of these plants is that their stomata remain open at night but closed during the day. Thus, CAM is a kind of adaptation in succulents to carry out photosynthesis without much loss of water. This helps in conserving water in these plants. 

CAM plants are mainly succulent xerophytes. Besides C3 and C4 cycles, Crassulacean acid metabolism is a third mechanism for fixing carbon dioxide. It is found not only in members of the family Crassulaceae eg., Kalanchoe, Sedum, Crassula, but also in the members of family Cactaceae example Opuntia. Besides it also occurs in some members of Liliaceae, Orchidaceae, Bromeliaceae, Euphorbia sp and pteridophytes like Isoetes. In CAM plants the stomata are open at night and during day time the stomata remain close.

CAM photosynthesis is a carbon fixation pathway present in some plants, such as desert plants. These plants fix carbon dioxide during night, storing it as the four carbon acid malate. The malate is then reduced to a three carbon compound oxaloacetate and the carbon dioxide. This carbon dioxide is released during the day, where it is concentrated around the enzyme RuBP carboxylase and increases the efficiency of photosynthesis. The carboxylating enzyme used during day is RuBP carboxylase and during the night its PEP carboxylase.

Answer is option C.

In CAM plants there is temporal separation between initial carbon dioxide fixation and Calvin cycle. In these plants, during night time the stomata are open and atmospheric carbon dioxide is fixed into organic acids like malic acid. The organic acids are stored in cell vacuoles during night time. During day time the stomata are closed and organic acids are decarboxylated to release carbon dioxide. The released carbon dioxide is fixed by the RuBisCO and usual reactions of the Calvin cycle.

• Kranz anatomy is a special structure of leaves in C4 PLANTS e.g. maize, sugarcane, sorghum, and millets.
  
• Mustard and Spinach belongs to C3 plants.
  
• Tulip is an example of a CAM plant. 

Crassulacean acid metabolism is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide. The $CO _2$ is stored as the four-carbon acid malate and then used during photosynthesis during the day. As photosynthesis occurs during night, the concentration of organic acid, which is the product of photosynthesis increases during night. Thus, option C is correct answer.

Ranson and Thomas coined the term CAM (Crassulacean Acid Metabolism)  in 1940. CAM pathway is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide.

C- Plants using PEP carboxylase have a ring of specialised cells, called bundle sheath cells, around the vascular bundles (veins) in their leaves. PEP carboxylase forms part of a carbon dioxide pumping mechanism. Oxaloacetate can be converted to malate in the leaf mesophyll cells and transferred to the bundle sheath cells. Here 4 carbon malate is converted to 3 carbon pyruvate, yielding carbon dioxide and reducing NADP to NADPH for use in the C3 carbon fixing cycle.