Summary
Highlights
Photosynthesis is the cellular process by which plants use sunlight, water, and carbon dioxide to produce their own nourishment. The word 'photosynthesis' means 'to build with light'. This process forms the base of the entire food chain, with plants creating energy-rich molecules that other organisms consume.
Photosynthesis consists of two main stages: the light reactions, which convert solar energy into chemical energy, and the Calvin Cycle, which uses this chemical energy to build G3P (a precursor to glucose). Both stages occur in chloroplasts, organelles found in plant cells. Chloroplasts contain thylakoids, stacked in grana, and embedded with chlorophyll.
Chlorophyll is a special pigment molecule in the thylakoid membrane that absorbs sunlight. When chlorophyll absorbs a photon, one of its electrons jumps to an excited state. This excitation energy is passed between chlorophyll molecules within a photosystem until it reaches a reaction center, where the high-energy electron is transferred to a primary electron acceptor.
The light reactions involve a series of redox reactions in Photosystem II and Photosystem I. In Photosystem II, electrons flow through an electron transport chain, producing ATP. In Photosystem I, another electron transport chain converts NADP+ into NADPH. Overall, light reactions use sunlight and water to generate oxygen, ATP, and NADPH.
The Calvin cycle occurs in the stroma and is an anabolic process that builds organic molecules from smaller components, requiring the ATP and NADPH produced in the light reactions. It has three phases: carbon fixation (CO2 is captured by RuBisCo), reduction (molecules are reduced by ATP and NADPH), and regeneration of RuBP. The cycle produces G3P, which can be used to generate glucose and other organic compounds.
In summary, photosynthesis involves light and water entering during the light reactions to produce oxygen, ATP, and NADPH. These products then fuel the Calvin cycle, where carbon dioxide is taken in to produce organic products like sugars. This process is essentially the reverse of cellular respiration, involving a series of redox reactions where electrons flow in opposite directions to build sugars.