Describe the three phases of the Calvin cycle and how the products of the light-capturing reactions participate in this process.

Verified step by step guidance

1. Step 1: The first phase of the Calvin cycle is carbon fixation. In this phase, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule, 3-PGA. This is a crucial step in the process of photosynthesis, as it is the first step in converting inorganic carbon into organic molecules that can be used by the plant.
2. Step 2: The second phase of the Calvin cycle is reduction. In this phase, ATP and NADPH from the light-dependent reactions are used to convert 3-PGA into G3P. This is a two-step process: first, ATP is used to phosphorylate 3-PGA, and then NADPH donates electrons to the phosphorylated compound, reducing it to G3P. One molecule of G3P is output from the cycle to be used by the plant, while the rest continue to the next phase.
3. Step 3: The third phase of the Calvin cycle is regeneration. In this phase, the remaining G3P molecules are rearranged into RuBP, the molecule that accepts CO2 in the first phase of the cycle. This process requires ATP from the light-dependent reactions.
4. Step 4: The products of the light-capturing reactions, ATP and NADPH, are crucial for the Calvin cycle. ATP provides the energy needed for the reactions, while NADPH provides the electrons needed for the reduction of 3-PGA to G3P.
5. Step 5: The Calvin cycle is a continuous process, with the product of the last phase (RuBP) being used in the first phase to accept more CO2. This allows the plant to continuously convert inorganic carbon into organic molecules.

Key Concepts

Calvin Cycle Phases

The Calvin cycle consists of three main phases: carbon fixation, reduction, and regeneration. In carbon fixation, carbon dioxide is incorporated into a five-carbon sugar, ribulose bisphosphate (RuBP), by the enzyme RuBisCO. During the reduction phase, ATP and NADPH produced in the light reactions convert the fixed carbon into glyceraldehyde-3-phosphate (G3P). Finally, in the regeneration phase, some G3P molecules are used to regenerate RuBP, allowing the cycle to continue.

Light-Capturing Reactions

Light-capturing reactions occur in the thylakoid membranes of chloroplasts, where sunlight is absorbed by chlorophyll and other pigments. This energy is used to split water molecules, releasing oxygen and generating ATP and NADPH. These products are essential for the Calvin cycle, providing the energy and reducing power needed for the conversion of carbon dioxide into organic molecules.

Role of ATP and NADPH

ATP and NADPH are crucial for the Calvin cycle as they supply the energy and electrons required for the reduction of 3-phosphoglycerate (3-PGA) to G3P. ATP provides the necessary energy for the phosphorylation of intermediates, while NADPH donates electrons to reduce 3-PGA. The efficient use of these molecules ensures that the cycle can effectively convert inorganic carbon into organic compounds, which are vital for plant growth and energy storage.