Which substance is a reactant in photosynthesis and also a product of cellular respiration in the classic summary equations of these processes?

Introduction / Context:
Cellular respiration and photosynthesis are complementary. Recognizing which molecules switch roles between these processes helps you understand ecosystem-level cycles of matter and energy. This item focuses on the classic balanced equations used in textbooks.

Given Data / Assumptions:

• Photosynthesis (overall): 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2.
• Respiration (overall): C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP + heat).
• We are interested in a molecule that is a reactant for photosynthesis and a product of respiration.

Concept / Approach:
From the canonical equations, CO2 is consumed (reactant) in photosynthesis and produced (product) in cellular respiration. Oxygen is the reverse: produced by photosynthesis and consumed in respiration. Water is more nuanced (reactant in photosynthesis, product in respiration), but among the given choices, CO2 most directly matches the prompt and the standard framing of these paired processes.

Step-by-Step Solution:

Verification / Alternative check:
Isotopic labeling (e.g., 14C-CO2) demonstrates carbon incorporation into plant carbohydrates, while breath analysis shows CO2 increase during respiration in animals and microbes.

Why Other Options Are Wrong:

• O2: product of photosynthesis and reactant of respiration (opposite of what is asked).
• Sunlight: an energy input to photosynthesis, not a product of respiration.
• ATP: produced in respiration and used within the Calvin cycle, but it is not the canonical cross-process molecule highlighted in the overall equations.
• H2O: also fits reactant/product roles, but the question’s standard single best answer in classical teaching is CO2, which is unambiguous across both equations.

Common Pitfalls:
Overthinking ATP’s role across pathways. The question refers to the classic equations most students memorize, for which CO2 is the clean match.

Final Answer:
CO2.