Carbon dioxide is taken up directly by Rubisco in C3 metabolic pathway whose biochemical reactions occur within a cell simultaneously. The fixation of carbon dioxide was distributed into different cells during C4 cycle. In water-scarce areas, plants also have evolved a method called CAM pathway (Crassulacean acid metabolism).
CAM Pathway: Day and Night
The carbon fixation occurs at different times, though its location always remains within the mesophyll cells. It was discovered long before that there is a diurnal fluctuation of organic acids in some desert plant tissues. Organic acids accumulate at night and gradually disappear during daytime. Some desert plants have such a high concentration that their sap tastes sour. This phenomenon was first observed in Crassulaceae family, hence itās called Crassulacean Acid Metabolism.
To prevent water loss via transpiration in extremely arid and hot environments, CAM plants keep their stomata tightly closed not only at noon but throughout the entire daytime. Thus, carbon fixation can only occur in cool and humid night. While light and dark reactions take place under daylight.
At night, they open stomata to greedily absorb carbon dioxide from the air which becomes soluble bicarbonate in cytoplasm. The subsequent steps are similar to C4 cycle. PEP carboxylase fixes bicarbonate into oxaloacetate that is then reduced to malate stored in the large central vacuoles. Thereby, CAM plant tissues taste very acidic at night. Once daylight arrives, the stomata close to prevent water evaporation, and malate is transported to cytoplasm where it is broken down into pyruvate and carbon dioxide. PEP carboxylase activity is inhibited by malate to prevent them from competing with Rubisco for substrate. The carbon dioxide concentrated around Rubisco is highly beneficial for C3 cycle and photorespiration is suppressed. The light reactions produce ample ATP and NADPH to convert pyruvate back into PEP that is used in the next round of CAM pathway. By evening, the acidity of these plants decreases, and they may even taste sweet due to sugar accumulation.
Types of CAM Plants: Obligate and Facultative
The stomata is closed completely to eliminate transpiration, and the low temperatures and high humidity at night are conducive to maintaining low transpiration level. Therefore, another notable feature of CAM plants is their extreme water conservation. For every gram of COā fixed, C3 plants consume 400-500 grams of water. This figure is halved for C4 plants, while this figure is only 50-100 in CAM plants.
Obligate CAM: Desert Plantšµ
Hot and dry deserts or semi-deserts are CAM plants paradise where drought, high temperatures, and saline soils are forbidden zones for C3 and C4 plants. Besides the unique nocturnal mechanism, they also have other traits to adapt to these harsh natural conditions. Large central vacuoles store water and malate, while thick cuticles and biological waxes reduce water evaporation, making their stems and leaves succulent. Many desert plants, like the creosote bush, position their leaf edges toward the sun. This reduces surface area exposed to intense sunlight directly, thereby leaf temperature is lowered and water loss is minimized. Cacti leaves have even degenerated into spines outright. CAM plants not only have a high concentration of cell sap but the special proteins also limit salt ions to penetrate their cell membrane.
The stomata are closed all day long to result in no gas exchange under extreme conditions. Cellular respiration and CAM pathway complete the carbon cycle within their bodies. This ensures their survival during months-long drought. Desert plants are almost all obligate CAM pathway users. It means they adopt CAM photosynthesis throughout their lifetime, even if they become potted plants in excellent living conditions.
Facultative CAM: Epiphyte, Aquatic Plant,š
Some plants are facultative CAM plants. They switch to C3 photosynthesis to produce organic matter rapidly when conditions are favorable. CAM metabolism is expressed under significant survival stress. They mostly inhabit tropical rainforests as epiphytes. Although tropical rainforests are the wettest places on earth, their roots only anchor them to trees or crevices in rocks. Roots not touching the soil mean that ingesting water isn't easy, so they still retain CAM cycles to compensate for this deficiency. Some air plants have thick hairs on their leaves to capture water and nutrients from atmosphere, such as Tillandsia. Bromeliads use succulent leaves to create a cup above the stem to store rainwater.
It's interesting that some aquatic plants also use CAM cycle. Why is this, they aren't water deficient? These plants typically inhabit shallow ponds or small lakes. Their entire bodies are submerged during the rainy season. Phytoplankton floating on water surface will preferentially acquire carbon dioxide dissolved in water. Additionally, the water area is so small that carbon dioxide is depleted before noon. Conversely, fixing carbon at night is much easier, since thereās no photosynthesis and all organisms conduct aerobic respiration at this time. They are also facultative CAM plants whose stems and leaves emerge from water during dry season to intake atmospheric carbon dioxide directly. At this time, the C3 cycle or Calvin cycle is the primary choice. These plants are mostly submerged species, such as Isoetes.
There are no strong genetic relationships among these three groups of plants. It is believed that parallel evolution has led to diverse plants that exhibit CAM photosynthesis.