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Fundamentals of photosynthesis
Photosynthesis is a fundamental biological process by which plants, algae, and certain bacteria convert light energy into chemical energy. Using sunlight, these organisms transform carbon dioxide (CO₂) and water (H₂O) into sugars and oxygen (O₂). The sugars serve as an energy source and building material, while oxygen is released into the atmosphere. This transformation takes place in specialized organelles called chloroplasts.
Photosynthesis is divided into two main stages: the light-dependent reactions and the Calvin-Benson-Bassham (CBB) cycle, also known as the dark reactions.
In the light-dependent reactions, light energy is harnessed to produce ATP and NADPH—energy-rich molecules that power the subsequent carbon-fixation steps. These reactions occur in the thylakoid membranes and generate oxygen as a byproduct.
The CBB cycle, taking place in the stroma of the chloroplast, uses ATP and NADPH to fix CO₂ into organic molecules. The key enzyme Rubisco catalyzes the first step, binding CO₂ to ribulose-1,5-bisphosphate. Through a series of reactions—fixation, reduction, and regeneration—the cycle ultimately produces sugar precursors, while regenerating the CO₂ acceptor to sustain the process.
A major limitation of this pathway is photorespiration. Under certain conditions, Rubisco binds O₂ instead of CO₂, leading to the formation of 2-phosphoglycolate—a compound that is both energetically costly to recycle and potentially toxic to the cell. Photorespiration can significantly reduce photosynthetic efficiency, especially under stress conditions like high temperature or drought.
Despite decades of study, many aspects of photosynthesis remain only partially understood. These include the regulation and integration of carbon fixation with other cellular processes, the dynamic organization within chloroplasts, and the plant’s ability to adapt photosynthesis to fluctuating environments.
Recent research aims to overcome these limitations, including efforts to engineer synthetic carbon fixation pathways. These could, in theory, outperform natural photosynthesis in terms of efficiency and resilience—but implementing them in living chloroplasts remains a challenge.
