Study Notes on Carbon Fixation in Plants

Importance of CO2: Atmospheric carbon dioxide is essential for photosynthesis, entering plants mainly through specialized pores called stomata, bordered by guard cells.
Carbon Fixation: Process of adding CO2 to organic compounds, converting CO2 gas into biologically useful forms, primarily occurs in chloroplasts. This is a redox reaction involving the reduction of carbon.

Calvin Cycle: Main pathway where CO2 is fixed into carbohydrates, initially producing 3-phosphoglycerate (3PGA).
Research Contributions: Melvin Calvin tracked carbon fixation using radioactive carbon isotopes, confirming 3PGA as the first product of carbon fixation.
Rubisco: The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) catalyzes the reaction between CO2 and ribulose bisphosphate (RuBP). It has slow kinetics, catalyzing only three reactions per second.

Photorespiration: Occurs when rubisco reacts with O2 instead of CO2, leading to inefficiency as it consumes energy and yields CO2, reducing overall fixation rates.

C4 Pathway: Separates CO2 fixation and the Calvin Cycle spatially; fixes CO2 into four-carbon compounds (e.g., oxaloacetate) using PEP carboxylase in mesophyll cells, which is then delivered to bundle-sheath cells for the Calvin Cycle.
Efficiency: Raises CO2 concentration near rubisco, improving fixation efficiency, especially in hot, dry conditions.
CAM Pathway: Similar to C4 but separates the processes temporally. CAM plants uptake CO2 at night, storing it as organic acids, which are processed during the day to feed the Calvin Cycle when stomata are closed to minimize water loss.

Stomata function to control gas exchange, aiding in photosynthesis and mitigating water loss.
Rubisco is the main enzyme for carbon fixation but is inefficient and slow.
C4 and CAM plants enhance CO2 availability, allowing for effective photosynthesis in challenging environments, such as high temperatures and low humidity sections of the day.