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From night to day: How do plants cleverly use carbon dioxide in two time periods?
In dry environments, some plants have evolved a unique carbon fixation pathway called Crassulacean Acid Metabolism (CAM). This method of photosynthesis allows plants to perform photosynthesis during the day and perform gas exchange at night, thereby cleverly utilizing carbon dioxide (CO2). This process not only demonstrates the wisdom of nature, but also reveals the ability of plants to adapt to extreme environments.
CAM is an adaptable photosynthesis mechanism that allows plants to survive in water-scarce environments and effectively utilize limited carbon dioxide resources.
Historical background
CAM's annotation dates back to 1804, when scientists observed plant respiration and its acidity. With the deepening of scientific research, related research gradually evolved, and around 1940, the term "succulent plant acid metabolism" was first introduced into the scientific community. This discovery is mainly based on the study of a variety of plants, especially the Nautilus family (Crassulaceae) to which succulents belong.
Double cycle overview
The CAM process can be divided into two parts: night and day. At night, the plant's stomata open, allowing carbon dioxide to enter and fix organic acids through reaction with enol phosphate (PEP). These organic acids will be stored in the vacuole for later use. In contrast, during the day, the plant's stomata close to retain moisture and then release stored organic acids, which are then reconverted into carbon dioxide and entered into the Calvin cycle of photosynthesis.
What are the benefits
Plants using CAM can keep most stomata closed during the day, significantly reducing water loss due to evapotranspiration. This is crucial for plants that live in dry environments, allowing them to continue growing even when water is extremely limited. In contrast, plants using only C3 carbon fixation will lose about 97% of the water absorbed by the roots, which is undoubtedly a high-cost process.
Comparison of CAM and C4 metabolism
While both CAM and C4 are designed to increase the efficiency of RuBisCO, they differ in how they concentrate carbon in time and space. CAM provides carbon dioxide during the day, while C4 structurally increases the concentration of carbon dioxide. In addition, some plants can even perform C4 and CAM photosynthesis simultaneously in the same leaf, which means they can flexibly adjust their carbon fixation mechanism according to environmental changes.
Biochemical processes
In plants using CAM, the storage and reduction processes of CO2 must be precisely controlled in space and time. At night, plants open their stomata and carbon dioxide enters the cells. After being catalyzed by a series of enzymes, organic acids are formed and stored in the vacuoles. As day comes, the stomata close and the stored organic acids are converted into carbon dioxide, which then participates in the Calvin cycle to create energy and carbohydrate synthesis.
Diversity utilization of plants
The extent to which plants use CAM varies. Some plants, such as "strong CAM plants", rely entirely on this mechanism for photosynthesis, while others selectively use the CAM or C3/C4 mechanism according to environmental changes. This shows that plants' adaptability and survival strategies are diverse and flexible.
The wonders of aquatic CAM
Surprisingly, CAM photosynthesis does not only exist in terrestrial plants, but this mechanism can also be found in aquatic plants. Carbon dioxide diffuses much slower in water than in air, so some aquatic plants choose to store carbon dioxide at night to resist competition in the water. This phenomenon is particularly evident in the summer, when the demand for carbon dioxide in the water increases, and nighttime CO2 capture becomes even more important.
Ecology and taxonomic distribution
The plants of CAM are mainly distributed among succulents and epiphytes, which exhibit extraordinary survival strategies in the face of drought. Many trees, such as Clusia, also exhibit dual carbon fixation capabilities, which allows them to freely switch photosynthetic mechanisms according to changes in the environment. Research shows that CAM evolved multiple times, with more than 16,000 plant species exhibiting this characteristic so far.
In the process of such environmental adaptation, we can't help but think: How do these plants maximize the use of limited resources to gain advantages in the competition for survival?
