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How this giant cactus living in the desert, one of the harshest environment in the planet, performs photosynthesis
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Survival in desert is equal to their ability to conserve water. The major problem faced by these plants is, during daytime, because of high temperature, these plants must close stomata to prevent water loss by transpiration
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Then how do they get carbon dioxide for Calvin cycle or synthesis of glucose
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These amazing plants evolved an extraordinary pathway called CAM pathway. For easy understanding, we have divided this entire pathway into simple steps
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Let's begin. So this is a mesophyll cell. Step 1 is carbon dioxide enters through stomata at night
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In CAM plants, the stomata is scotoactive, that means it can open during night time
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When temperature drops during night time, the stomata opens and carbon dioxide enters through the stomata
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Next step is primary carboxylation or initial carbon fixation. Step 2, carbon dioxide combines with PEP, phosphoenolpyruvate, which is a 3 carbon compound, forming 4 carbon oxaloacetate
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The enzyme involved is PEP carboxylase, as it is fixing carbon dioxide, that's why it is called as carboxylase
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Here carbon dioxide combines with water forming bicarbonate, combines with PEP forming oxaloacetate, which is a 4 carbon compound
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Step 3, oxaloacetate is converted to malic acid. It is a dehydrogenation reaction
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The enzyme involved is malate dehydrogenase. It is also a 4 carbon compound
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Then this malate is transported to the vacuole where it is stored during night time
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So malate is stored in the vacuole. During day time, as the temperature rises, stomata closes
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So that carbon dioxide cannot enter through stomata. Stomata is closed during day time
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Then the question is, carbon dioxide is essential for Calvin cycle. So without opening stomata, how this chloroplast or stroma of chloroplast gets carbon dioxide for Calvin cycle
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Here comes the role of stored malate. Malate that is stored in the vacuole is taken out and that is decarboxylated
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Step 4 is decarboxylation of malate forming pyruvate. So malate is converted to pyruvate or decarboxylated
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A carbon dioxide molecule is released and forming pyruvate which is a 3 carbon compound
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And the carbon dioxide that is released enters the stroma of the chloroplast and performs Calvin cycle
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So this is secondary carboxylation or Calvin cycle where carbon dioxide combines with RuBP which is a 5 carbon compound
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And the first stable compound is phosphoglyceric acid and this reaction ultimately synthesizes glucose
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So this takes place in the stroma of chloroplast. So by this pathway, these plants can carry out Calvin cycle even without opening the stomata
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And the final step is regeneration of this phosphoenolpyruvate. To this pyruvate molecule, a phosphate is added
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ATP is utilized for the process and the enzyme is pyruvate phosphate di kinase and forms PEP or phosphoenolpyruvate
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Thus completing the pathway. The major difference between C4 cycle and CAM pathway as you see
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initial carbon dioxide fixation takes place during night time and Calvin cycle takes place during day time
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Both are separated in time. So hope you are clear with this pathway
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Finally, before concluding, let me share some examples of CAM plants. Common CAM plants include pineapple, cactus, then acave, orchid, aloe etc
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These are all CAM plants. They are called as CAM plants because scientists first observed this pathway in the family members of chrysoleceae
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Approximately there are 1700 CAM species. Majority are succulents and finally the speciality of the stomata and finally scotoactive stomata
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Stomata that opens during night and closes during daytime is called as scotoactive
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So CAM plants possess scotoactive stomata that helps in opening during night, receiving carbon dioxide during night time and storing it as malate
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And during day time, this stomata closes to prevent water loss by transpiration
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From malate decarboxylation, carbon dioxide is released and that runs the Calvin cycle
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Hope you are clear. Take care. Stay blessed. Thank you so much
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