In the light reactions and Calvin cycle of photosynthesis, which component is consumed (not recycled) and therefore must be continually supplied from outside the chloroplast?

• A. Carbon dioxide ($C_1{O}_2$)
• B. Adenosine diphosphate (ADP)
• C. Nicotinamide adenine dinucleotide phosphate (NADP$+$)
• D. Ribulose-1,5-bisphosphate (RuBP)

Answer: (A)

Verified step by step guidance

1. Understand the two main stages of photosynthesis: the light reactions and the Calvin cycle. The light reactions convert light energy into chemical energy (ATP and NADPH), while the Calvin cycle uses that energy to fix carbon dioxide into organic molecules.
2. Identify the molecules involved in these processes. ATP and NADPH are produced in the light reactions and consumed in the Calvin cycle, but they are recycled between these stages. Ribulose-1,5-bisphosphate (RuBP) is a molecule regenerated during the Calvin cycle.
3. Recognize that carbon dioxide (CO\_2) is the molecule that enters the Calvin cycle from the atmosphere and is fixed into organic molecules. Unlike ATP, NADP\^+, and RuBP, CO\_2 is not regenerated within the chloroplast; it is consumed and must be continually supplied from outside.
4. Conclude that the component consumed and not recycled in the photosynthetic process, requiring continuous external supply, is carbon dioxide (CO\_2).
5. Summarize that ATP, NADP\^+, and RuBP are recycled within the chloroplast, whereas CO\_2 is the input molecule that must be replenished for photosynthesis to proceed.