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Photosynthesis, Carbon Fixation And

Virtually all life on Earth ultimately depends on the light-driven fixation of carbon dioxide (CO₂) according to the following equation: 6CO₂ + 6H₂ O → C₆ H₁₂ O₆ (glucose) + 6O₂

Photosynthesis takes place in subcellular membrane-bound compartments called chloroplasts. As radiotracers such as carbon-14 became available to researchers following World War II (1939-45), one application was to define the biochemistry of photosynthetic CO₂ fixation. Major class divisions in the plant kingdom are based on how CO₂ is fixed.

C3 Photosynthesis.

Many important biological processes are sustained by cycles that continuously consume and renew one or more key intermediates while producing some other major product. Photosynthesis is sustained by the Calvin-Benson cycle.

The C₃ photosynthetic mechanism is so named because the carbon atom of a molecule of CO₂ taken up by an illuminated leaf is first detected in the three-carbon compound 3-phosphoglyceric acid (PGA). The vast majority of higher plants and algae are C₃ species. PGA is formed when CO₂ combines with a 5-carbon sugar, ribulose biphosphate (RuBP). The reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, an abundant protein in all green tissues. This multifunctional enzyme has come to be called rubisco.

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