Study Notes: CAM Plants (Crassulacean Acid Metabolism) CAM Plants Crassulacean Acid Metabolism (CAM) is a specialized photosynthetic pathway found in plants adapted to **arid and semi-arid environments**. This mechanism minimizes **water loss** by temporally separating CO2 fixation and the Calvin cycle, with CO2 being fixed at night and processed during the day. Key Features: • Temporal separation of processes: CO2 fixation occurs at night, and the Calvin cycle occurs during the day. • Stomata open at night (when temperatures are cooler), reducing water loss due to transpiration. • Adaptations to conserve water and survive in conditions of extreme heat and drought. Examples of CAM Plants: • Succulents: Crassula, Aloe, and Agave. • Cacti: Opuntia and Carnegiea gigantea. • Epiphytes: Tillandsia and Bromeliads. — Mechanism of CAM Photosynthesis The CAM pathway consists of two main phases: the Night Phase and the Day Phase. 1. Night Phase (CO2 Fixation) • Stomata open at night, allowing atmospheric CO2 to diffuse into the leaf. • CO2 is fixed into a 3-carbon molecule, phosphoenolpyruvate (PEP), by the enzyme PEP carboxylase. • This reaction forms a 4-carbon compound, oxaloacetate (OAA), which is converted into malic acid. • Malic acid is stored in the vacuoles of mesophyll cells as a temporary carbon reservoir. 2. Day Phase (Calvin Cycle) • During the day, stomata remain closed to prevent water loss. • Malic acid is transported from the vacuoles to the cytoplasm, where it is decarboxylated to release CO2 . • The released CO2 enters the Calvin cycle and is fixed by RuBisCO to produce sugars. • The light reactions of photosynthesis provide ATP and NADPH required for the Calvin cycle. — 1 Advantages of the CAM Pathway • Reduced water loss: Stomata open at night when temperatures and evaporation rates are lower. • Enhanced survival: CAM plants thrive in arid regions where C3 and even C4 plants struggle. • Efficient carbon storage: Temporary storage of CO2 as malic acid allows plants to photosynthesize even when stomata are closed during the day. Evolutionary Perspective of CAM Plants Background: • CAM photosynthesis is believed to have evolved as a survival mechanism for plants in arid environments where water availability is a limiting factor. • It represents a fine-tuned adaptation to reduce water loss while maintaining photosynthetic activity. Relationship to RuBisCO: • Like in C4 plants, the CAM pathway addresses RuBisCO’s inefficiency in distinguishing CO2 from O2 by concentrating CO2 near RuBisCO. • This minimizes photorespiration during the day, improving carbon fixation efficiency. Adaptive Significance: • CAM plants dominate desert ecosystems, where their ability to conserve water gives them a significant advantage over C3 and C4 plants. • Their metabolic flexibility allows some CAM plants to switch between C3 and CAM pathways depending on environmental conditions (e.g., Mesembryanthemum crystallinum). 2