In the context of the Calvin cycle and overall photosynthesis, most of a plant's biomass (dry mass) ultimately comes from which source?

• A. Oxygen produced during the light reactions and incorporated into carbohydrates
• B. Water absorbed by roots that is converted directly into sugars during the Calvin cycle
• C. Carbon dioxide fixed from the atmosphere into organic molecules
• D. Minerals absorbed from the soil and incorporated as the main structural material

Answer: (C)

Verified step by step guidance

1. Understand that the Calvin cycle is the set of biochemical reactions in photosynthesis where carbon fixation occurs, converting inorganic carbon into organic molecules.
2. Recognize that during the Calvin cycle, the enzyme RuBisCO incorporates carbon dioxide (CO\_2) from the atmosphere into a 5-carbon sugar, initiating the synthesis of carbohydrates.
3. Recall that the oxygen produced during the light reactions comes from splitting water molecules, but this oxygen is released into the atmosphere and is not the source of the plant's biomass.
4. Note that water absorbed by roots provides hydrogen and oxygen atoms but is not directly converted into sugars; instead, it supports the photosynthetic process and cellular functions.
5. Conclude that the majority of a plant's dry mass (biomass) is derived from carbon atoms fixed from atmospheric CO\_2, which are assembled into organic molecules like glucose during the Calvin cycle.