Final answer:
The model of the carbon cycle is missing option 3) carbon dioxide, which is essential for photosynthesis and cellular respiration, and without it, the cycle would be incomplete.
Step-by-step explanation:
If a student's model of the carbon cycle is missing option 3) carbon dioxide, it omits a crucial step in carbon recycling. Photosynthesis and cellular respiration are core processes in the carbon cycle. In photosynthesis, plants and other producers use sunlight to convert carbon dioxide and water into glucose, a type of sugar, which serves as food and energy reservoir for the ecosystem. The general equation for photosynthesis is 6CO2 + 6H2O + energy → C6H12O6 + 602. Conversely, during cellular respiration, consumers, including plants, break down glucose to release energy, water, and carbon dioxide back into the atmosphere, represented by the equation C6H12O6 + 602 → 6CO2 + 6H2O + energy. The continuous exchange of carbon dioxide between the atmosphere and living organisms is essential for maintaining the balance of carbon in the biosphere. Therefore, without carbon dioxide in the model, the cycle would be incomplete, as there would be no way to represent the gas's role in photosynthesis and its release during cellular respiration.