Crassulacae Acid Metabolism in Cactus and Succulents ... 2/3

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Biochemistry of CAM

As mentioned above in normal photosynthesis plants breathe through stomata (little holes) in the leaves taking in Carbon dioxide (CO2) from the atmosphere, combining it with water (H2O) in the presence of chlorophyll and enzymes and energy from the sun to make sugars (CHO).

• 6 H2O + 6 CO2 + Sunlight ---> Sugars (C6H12O6) + 6 O2

Because sunlight is needed to power this chemical reaction, it occurs during the day, at night the stomata are closed. It is also called a C3 cycle as the first chemical made by the plant is a 3 chain Carbon molecule. The only cacti to use this process is the primitive Pereskia.

Having stomata open during the day also means water can escape from the plant, especially where the air outside is hot and dry and of a low humidity ... like in the desert. This is not good if water is a scarcity in the first place.

CAM plants take in Carbon dioxide from the atmosphere during the evening/night and early morning fixing it within the plant as an organic acid (malic acid) with the help of the enzyme phosphoenolpyruvate carboxylase (PEPC), while the outside humidity is high (quite often in the "desert" there is an overnight dew). During the early evening and early morning (daylight hours) the CAM plant can also have a normal C3 metabolism, converting the CO2 directly to sugars (as well as storing it for tommorrows metabolism in the evening).
During the day the organic acid is broken down internally with the help of C3 carboxylase and Rubiscoto enzymes to release Carbon dioxide within the plant, when sunlight is present to provide energy for the chemical reaction to make sugars.
This means the stomata can be open during the evening/night/early morning when outside temperatures are low and humidity is (relativity) high and the moisture is not "sucked" out of the plant. Meanwhile during the day stomata remain closed using the internally stored and released Carbon dioxide, thereby sealing the plant off from the outside world, and preventing water loss, in fact up to six to ten times less loss of water compared to normal respiration.

• Night: CO2 + H2O + phosphoenolpyruvate carboxylase (PEPC) ----> Organic Acid ... Stomata open.
  
  
• Day: Organic Acid Breaks down (C3 carboxylase, Rubisco) releasing ... CO2 + H2O + Sunlight ----> Sugars (CHO) + O2 ... Stomata Closed

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