Final answer:
Plants obtain carbon atoms for their cells through photosynthesis by converting carbon dioxide from the atmosphere into glucose and oxygen
Step-by-step explanation:
The source of carbon atoms in a plant's cells is carbon dioxide (CO₂) from the atmosphere. Through a process called photosynthesis, plants use the energy from sunlight to convert carbon dioxide and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂).
This process occurs in the chloroplasts of plant cells, specifically in the grana of thylakoid membrane. The grana contain chlorophyll, which is responsible for capturing energy from sunlight.
Learn more about Plant cells and photosynthesis