Crassulacean acid metabolism (CAM pathway)
Crassulacean acid metabolism is a mechanism of photosynthesis which occurs in succulents. Succulents are plants that generally grow in acid habitat or in extreme xeric habitat. Before the discovery of C4 pathway some succulents belonging to family Crassulaceae were known to fix carbon dioxide in dark leading to the synthesis of organic acids at night. Hence this process was termed as Crassulacean acid metabolism (CAM) and these plants were termed as CAM plants.
Many plants of xerophytic habitats have unique mechanism of storing large quantities of carbon dioxide at night. During the day light hours the photosynthetic carbon reduction (PCR) cycle uses the stored CO2. Since then this specialized method of photosynthesis has been studied in some 21 different families of flowering plants. Of these, Crassulaceae (e.g., Crassula, Kalanchoe, Sedum, Sempervivum, etc) and Cactaceae (e.g., Cereus, Mamillaria, Opuntia, etc) exhibit only CAM cycle, whereas rest of the families exhibit both C3 and C4 cycles.
Plants exhibiting CAM pathway show special adaptation for extremely dry or xerophytic habitats. They possess unique structural and physiological features which help in water conservation. All these plants have thick, fleshy succulent leaves. Although succulence is a prerequisite of CAM, but not all succulent plants exhibit CAM.
In CAM plants carbon dioxide fixation takes place through C4 cycle. But unlike other C4 plants the stomata of succulents open at night and close during the day. This helps in reduction of stomatal transpiration during day and efficient carbon dioxide fixation at night.
Following are the two metabolic phases of Crassulacean acid metabolism (CAM pathway)
Acidification (Phase I) during night time :
Stomata are open at night. Acidification takes place at night. During this phase carbon dioxide fixation takes place. Phosphoenol pyruvic acid (PEP) produced by glycolysis helps in the fixation of CO2. Carboxylation of phosphoenol pyruvic acid (PEP) takes place in the presence of an enzyme PEP carboxylase. This results in the formation of oxaloacetic acid (OAA). Oxaloacetic acid (OAA) in the presence of NADPH and the enzyme malic dehydrogenase converts to malic acid. This malic acid accumulates in the leaves. The end product of dark fixation of carbon dioxide is malic acid. The cell vacuoles store it.
PEP+HCO3–(CO2+H2O) → Oxaloacetic acid (OAA)+H3PO4
Oxaloacetic acid (OAA)+NADPH → Malic acid+NADP+
Deacidification (Phase II) during day time:
During daytime, stomata closes. However light is available for photosynthesis. The cell vacuoles release malic acid. With the help of malic enzyme decarboxylation of malic acid takes place. The decarboxylation of malic acid results in the production of pyruvic acid. As a result NADP reduces to NADPH and it evolves carbon dioxide. This process is termed as deacidification.
Malic acid (malate)+NADP+ → Pyruvic acid (Pyruvate)+CO2+NADPH
The carbon dioxide diffuses into the chloroplast. Fixation of carbon dioxide through Calvin cycle takes place with the help of RuBP and Rubisco. Crassulacean plants develop a unique mechanism to perform photosynthesis without much loss of water. They perform acidification or dark fixation of carbon dioxide during night (Phase I) and deacidification (Phase II) during day to release carbon dioxide for actual photosynthesis.
Ecological significance of Crassulacean acid metabolism (CAM pathway)
CAM plants have adaptations to exceptionally dry habitats. Many of them are true desert plants, growing in sandy soils. Stomata of these plants open at night when conditions favouring water loss are at a minimum. During day light hours the stomata close to reduce water loss. Photosynthesis can proceed utilizing carbon dioxide (by decarboxylating organic acids).
This photosynthetic strategy allows photosynthesis to occur with absolute minimum loss of water. This can be well demonstrated by comparing the transpiration ratios of C3, C4 and CAM plants. Transpiration ratio is the comparision of gram of water lost for per gram dry weight increase. It can also be represented as:
TR=mols of H2O transpired/mols of CO2 assimilated
The transpiration ratio for plants possessing CAM cycle is 18-125:1 (i.e., for each gram of dry matter synthesized 18 to 125 gms of water is lost). For C3 plants the ratio is 450-950:1 and for C4 plants it is 250-300:1.
Comparision between CAM and C4 plants
The CAM and C4 plants show some similarities and dissimilarities.
- Both CAM and C4 cycles use cytoplasmic phosphoenol pyruvate carboxylase (PEPcase) to form 4-C acids from phosphoenol pyruvic acid (PEP) and bicarbonate ions (HCO3–) in mesophyll cells.
- In both the cases, decarboxylation of 4-C acids occurs to yield CO2 for use in Calvin cycle.
- CAM plants occur in angiosperm, gymnosperm and some pteridophytes, whereas C4 plants occur in angiosperms only.
- In CAM plants the stomata closes during day and opens at night, whereas in C4 plants the stomata opens during day and closes during night.
- Succulence is a prerequisite for CAM, but not for C4 cycle.
- Kranz anatomy occurs in C4 plants, but not in CAM plants.
- The CAM plants have a low transpiration ratio in comparison to C4 plants.
- In CAM plants, the carboxylation of PEP and decarboxylation of 4-C acids, occur in the same mesophyll cells. Here, the carboxylation of PEP takes place at night and decarboxylation of 4-C acids occurs during the day time. But in case of C4 plants, carboxylation of PEP occurs in the mesophyll cells while decarboxylation of 4-C acids takes place in bundle sheath cells in quick succession.
Aquatic CAM photosynthesis:
The discovery of aquatic CAM photosynthesis was during the study of biochemical anaerobic metabolism. CAM occurs in all aquatic species of Isoetes. It also occurs in some species within the genera Crassula, Eleocharis,Littorella, Sagittaria and Scirpus.
The low availability of CO2 limits photosynthesis by aquatic plants . CAM enhances underwater photosynthesis and diminishes photorespiration in the aquatic plant Isoetes australis.
Aquatic plants have adaptations which facilitate inorganic carbon uptake, as addition of carbon for underwater photosynthesis is greatly hampered by slow diffusion of gases in water. Morphological adaptations include filamentous leaves. This reduces the thickness of diffusive boundary layers and thin cuticles to lower resistance to CO2 uptake from water.
Physiological adaptations to deal with low CO2 availabilities include those that increase CO2 at Rubisco, commonly referred to as carbon-concentrating mechanisms (CCMs) such as CAM.
In addition to enhancing photosynthesis underwater, CCMs also reduces photorespiration in aquatic plants. Photorespiration results from the oxygenase activity of Rubisco and low CO2:O2 ratio. Thus, with CCMs such as CAM, a more favourable CO2:O2 ratio can be maintained at Rubisco and thus diminish photorespiration.
CAM in aquatic plants, as in terrestrial plants, involves night-time fixation of CO2 into malate or malic acid, followed by daytime decarboxylation of malate so that CO2 is supplied to the C3 pathway without the need for simultaneous inward diffusion of external CO2.
Experiments suggests that the tissues high in malate have enhanced underwater net photosynthesis at low external CO2 concentrations and photorespiration diminishes by CAM.