Textbook Question
Which of the following are produced by reactions that take place in the thylakoids and consumed by reactions in the stroma?
a. CO₂ and H₂O
b. ATP and NADPH
c. ATP, NADPH, and CO₂
d. ATP, NADPH, and O₂
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9. Photosynthesis
Calvin Cycle
4:06 minutesProblem 6
Textbook Question
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 guidance1
Identify the three phases of the Calvin cycle: The Calvin cycle consists of three main phases: carbon fixation, reduction, and regeneration of the ribulose-1,5-bisphosphate (RuBP).
Explain the carbon fixation phase: In this initial phase, carbon dioxide (CO2) from the atmosphere is attached to a five-carbon sugar named ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), resulting in a six-carbon compound that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
Describe the reduction phase: The 3-PGA molecules are then phosphorylated by ATP (produced during the light reactions of photosynthesis) and reduced by NADPH (also produced during the light reactions) to form glyceraldehyde-3-phosphate (G3P). This is a key step where the energy and reducing power from the light reactions are used to form a carbohydrate.
Outline the regeneration of RuBP: Some of the G3P molecules go on to form glucose and other carbohydrates, while the majority are used to regenerate RuBP, enabling the cycle to continue. This regeneration process requires further ATP from the light reactions.
Connect the light reactions to the Calvin cycle: The ATP and NADPH, which are essential for the reduction and regeneration phases of the Calvin cycle, are products of the light-capturing reactions of photosynthesis. These molecules provide the necessary energy and reducing power that drive the synthesis of carbohydrates in the Calvin cycle.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
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.
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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 crucial for the Calvin cycle, as they provide the energy and reducing power needed for the conversion of carbon dioxide into organic molecules during the reduction phase.
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Role of ATP and NADPH
ATP and NADPH are the primary energy carriers produced during the light-capturing reactions of photosynthesis. ATP provides the necessary energy for the conversion of 3-phosphoglycerate into G3P in the reduction phase of the Calvin cycle, while NADPH supplies the electrons needed for this reduction process. Together, they facilitate the transformation of inorganic carbon into organic compounds, which are essential for plant growth and energy storage.
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