Modern Concept Of Photosynthesis - 3 | Agriculture Optional Notes for UPSC PDF Download

Dark phase

The dark phase of photosynthesis, also known as the carbon fixation or photosynthetic carbon reduction (PCR) cycle, is the pathway by which photosynthetic eukaryotic organisms ultimately convert carbon dioxide (CO₂) into carbohydrates. Unlike the light reactions, the dark phase is sensitive to temperature changes but independent of light, which is why it's referred to as the "dark" reaction. However, it relies on the products of the light reaction, namely NADP.H and ATP.

The carbon dioxide fixation occurs in the stroma of chloroplasts, as it contains the necessary enzymes for CO₂ fixation and sugar synthesis.

To study the different steps of this process, techniques like radioactive tracer using 14C, chromatography, and autoradiography are employed, often using microorganisms like Chlorella and Scenedesmus in laboratory settings.

In the dark reactions, CO₂ assimilation and reduction take place through three pathways:

• Calvin Cycle: Discovered by Calvin and Benson, the Calvin cycle is known as the C₃ cycle because it involves cyclic CO₂ reduction, and the first stable product in this cycle is a 3-carbon compound, 3-phosphoglyceric acid (3-PGA). This cycle comprises three phases: carboxylation, glycolytic reversal, and the regeneration of ribulose-1,5-biphosphate (RuBP). RuBP is the CO2 acceptor molecule in this cycle, and its covalent bonding with CO2 is catalyzed by RuBP-carboxylase/oxygenase (Rubisco). The Calvin cycle operates by taking in one carbon (as CO₂) at a time and requires six turns of the cycle to produce a net gain of six carbons, such as hexose or glucose. The formation of one mole of hexose sugar (glucose) in this cycle requires 18 ATP and 12 NADPH₂. Plants that follow this pathway are referred to as C-3 plants, and about 85% of plant species fall into this category, including various cereals, groundnuts, sugar beets, and more.
• Hatch and Slack Cycle (C₄ Cycle): Introduced by Kortschak and Hart, the C₄ cycle was further elaborated by M.D. Hatch and C.R. Slack in 1966. Unlike the Calvin cycle, C₄ plants such as sugarcane, maize, and sorghum, produce a four-carbon (C₄) compound, oxaloacetic acid, as the first stable product. These plants have unique leaf anatomy called "Kranz" where bundle sheath cells surrounding the vascular bundles contain numerous chloroplasts. C₄ plants utilize a two-step process involving mesophyll cells and bundle sheath cells. Initially, phosphoenolpyruvic acid (PEP) in the mesophyll cells reacts with atmospheric CO₂ to form oxaloacetic acid (OAA), a 4-C compound. OAA is then converted to malic acid and enters the bundle sheath cells, where oxidative decarboxylation generates pyruvic acid (a 3-C compound) and releases CO₂. The released CO₂ reacts with ribulose-1,5-biphosphate (RuBP) to initiate the Calvin cycle. Pyruvic acid returns to the mesophyll cells and regenerates PEP. C₄ plants require 2 carboxylation reactions, one in mesophyll chloroplasts and another in bundle sheath chloroplasts. They are excellent photosynthesizers and do not undergo photorespiration. For the formation of one molecule of hexose (glucose), C₄ plants need 30 ATP and 12 NADPH₂.
• Pseudocyclic Photophosphorylation: Arnon and his colleagues demonstrated another form of photophosphorylation in which ATP is produced from ADP and inorganic phosphate in the absence of CO₂ and NADP when chlorophyll molecules are illuminated. This form is known as pseudocyclic photophosphorylation and relies on oxygen-dependent FMN catalysis. It does not require significant net chemical changes and can repeatedly self-repeat, appearing to need only one water molecule to meet ATP requirements.

Characteristics of C₄cycle

The C₄ pathway and Crassulacean Acid Metabolism (CAM) are two distinct mechanisms used by certain plants to optimize photosynthesis, particularly in arid or stressful environments.
Here are the characteristics of each pathway:

• C₄ Pathway:
  • Enhanced CO₂ Assimilation: C₄ plants exhibit a higher rate of CO₂ assimilation compared to C₃ plants due to several factors, including the strong affinity of PEP carboxylase for CO₂.
  • Reduced Photorespiration: C₄ plants experience less photorespiration than C₃ plants, which leads to increased production of dry matter.
  • Adaptation to Environmental Stresses: C₄ plants are better adapted to environmental stresses, making them more resilient in challenging conditions.
  • Increased ATP Requirement: The CO₂ fixation process in C₄ plants demands more ATP, especially for the conversion of pyruvic acid to phosphoenol pyruvic acid and its subsequent transport.
  • PEP as CO₂ Acceptor: Phosphoenol pyruvate (PEP) serves as the CO₂ acceptor molecule in C₄ plants, and the key enzyme is PEP-carboxylase (PEPCO). While RuBP-carboxylase is negligible or absent in mesophyll chloroplasts, it is present in bundle sheath chloroplasts.
• Crassulacean Acid Metabolism (CAM):
  • Diurnal Stomatal Behavior: CAM plants, such as cacti and succulents, have specialized stomatal behavior. They keep their stomata closed during the day to minimize water loss (termed "Scotactive stomata") and open them at night.
  • Malic Acid Storage: During the night, CAM plants store CO₂ in the form of malic acid with the help of the enzyme PEP carboxylase. This stored CO₂ is used in the Calvin cycle during the daytime, ensuring efficient photosynthesis.
  • Crassulacean Acid Metabolism: The diurnal change in acidity, initially observed in crassulacean plants like Bryophyllum, led to the name "crassulacean acid metabolism."
  • pH Fluctuation: CAM plants undergo a decrease in pH during the night when malic acid is stored and an increase in pH during the day when CO₂ is released for photosynthesis.
  • Enzymes of C₃ and C₄: CAM plants possess enzymes associated with both C₃ and C₄ pathways in their mesophyll cells, providing them with a flexible metabolism to survive in xeric (dry) habitats. These plants can also utilize CO₂ released during respiration.

Characteristics of CAM pathway

• There is decrease in pH during the night and increase in pH during the day.
• CAM plants have enzymes of both C₃ and C₄ cycle in mesophyll cells. This metabolism enable CAM plants to survive under xeric habitats. These plants have also the capability of fixing the CO₂ lost in respiration.
• Malic acid is stored in the vacuoles during the night which is decarboxylated to release CO₂ during the day.