Final answer:
On average, around 45% of the mass of green plants or algae is carbon, which comes from carbon dioxide in the atmosphere and is used to build organic molecules in the plant.
Step-by-step explanation:
The percentage of carbon in green plants or algae can vary, but on average, it is around 45% of the plant's mass. This carbon comes from carbon dioxide (CO₂) in the atmosphere, which is converted into glucose (C₆H₁₂O₆) during photosynthesis. The carbon is then used to build other organic molecules in the plant.