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Using the energy carriers formed in the first steps of photosynthesis, the light-independent reactions, or the Calvin cycle, take in CO 2 from the environment. An enzyme, RuBisCO, catalyzes a reaction with CO 2 and another molecule, RuBP.
The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form of NADPH and ATP. This chemical energy supports the light-independent reactions and fuels the assembly of sugar molecules. The light-dependent reactions are depicted in Figure \ (\PageIndex {7}\).
The net-reaction of all light-dependent reactions in oxygenic photosynthesis is: 2H 2 O + 2NADP+ + 3ADP + 3Pi → O 2 + 2NADPH + 3ATP Key Points Light energy splits water and extracts electrons in photosystem II (PSII); then electrons are moved from PSII to cytochrome b6f to photosystem I (PSI) and reduce in energy.
Light energy initiates the process of photosynthesis when pigments absorb specific wavelengths of visible light. Organic pigments, whether in the human retina or the chloroplast thylakoid, have a narrow range of energy levels that they can absorb.
Energy from ATP and hydrogen from reduced NADP are passed from the light-dependent stage to the light-independent stage of photosynthesis. The energy and hydrogen are used during the light-independent reactions (known collectively as the Calvin cycle) to produce complex organic molecules, including (but not limited to) carbohydrates, such as:
Photosynthesis takes place in two stages: light-dependent reactions and light-independent reactions (the Calvin cycle). Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH.
The Calvin cycle, or the light-independent reactions, is the term used for the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules (Figure 6.13).
