This is a distinctly different pathway to that followed by C4 and CAM plants.The basic process for photosynthesis was understood early in the 18th century.
However some stages of photosynthesis are still not fully understood.If the carbon dioxide level is too low, oxygen will replace it and carbon will be lost from the cycle, sugars drained, toxins produced, and photosynthesis inhibited.
The CO concentrates around the enzyme RuBisCo and photosynthesis via the Calvin cycle results.Day Malic acid --> Malate decarboxylated --> PEP + CO (for Calvin cycle)In the late afternoon the stomata open and this day/night cycle repeats.The water efficiency of this process is demonstrated by the fact that C3 plants lose 97% of their water by transpiration whereas CAM plants loose little to none.
Briefly the CAM pathway involves the plant shutting stomata during the day to reduce water loss, opening them at night to collect CO, and storing the CO as the 4C molecule malic acid.
Imagine what that might do for rice production and the worlds food problems.References: As with Photosynthesis 1, this presentation was gleaned from the following scientific articles and internet pages:
Sage et al, 2011, The C4 plant lineages of planet Earth.
These plants usually have a rapid response to rain showers.Plants using the last three CAM types are usually found in areas where water shortage alternates with water excess.
CAM plants not only are good at retaining water but also use very efficiently. However, because their stomata are closed by day, they are less efficient at CO2 absorption. This limits the amount of carbon they have available for growth.
Note that photorespiration which is in general caused by the uptake of O (oxygen) instead of CO by the RuBisCo enzyme, undoes the good work of photosynthesis in the C3 plant.
Under these conditions C4 photosynthesis has developed a number of times in a number of plant lines in the 25-30 million years since the late Oligocene, getting to todays numbers by the end of the Miocene.Assuming that low CO is a pre-condition for the development of C4 plants, paramaters such as increasing aridity, high light habitats, increasing temperature and seasonality, fire, and the distribution of grazing animals, are all thought to play an important part in this evolutionary trend.At temperatures 22C - 30C, Quantum yields for C3 and C4 plants are the same
Temperatures above 30C, quantum yields are greater in C4 plants
Temperatures below 22C, quantum yields are greater in C3 plants.
The CO is then fixed by the RuBisCo in the bundle sheath cells, and photosynthesis occurs with its resultant sugar via the C3 pathway and the Calvin cycle.The direct C4 pathway...
CO + PEP --> oxaloacetic acid --> malic acid --> to bundle sheath cellsThen in bundle sheath cells, in the presence of RuBisCo, using the Calvin cycle...
CO + PEP --> photosynthesis --> SugarThe combined efficiency of PEP in fixing CO, together with the tightly packed double ring of bundle sheath cells and mesophyll cells (called Kranz anatomy.....
Cross section C4 leaf.
Whereas the RuBisCo in the C3 plant fixes the CO (rather poorly) and prepares for the photosynthesis process in all the chloroplasts in all the mesophyll cells, the C4 plant has a more efficient way of fixing the CO.It has a much more efficient enzyme called PEP which compared to RuBisCo has a much greater affinity with CO.
Often the benefit of continued metabolism (survival) is at the expense of quantum yield (growth).
Plants which can switch photosynthetic pathways between CAM and C3 depend on environmental factors for the switch e.g.
CAM was first discovered in the late 1940s. It was observed by the Ranson and Thomas, in the family of (which includes and ). Its name refers to acid metabolism in Crassulaceae, not the of Crassulacean acid.
Crassulacean acid metabolism, also known as CAM photosynthesis, is a pathway present in some . These plants fix (CO2) during the night, storing it as the four-carbon acid . The CO2 is released during the day, where it is concentrated around the , increasing the . The CAM pathway allows to remain shut during the day, reducing ; therefore, it is especially common in plants adapted to conditions.