Explain how a carbon atom in a co2 molecule in the air can become part of a cellulose molecule in a plant

Your carbon atom enters the leaf as CO2 (carbon dioxide) from the atmosphere. In the leaf, chlorophyll uses the energy from the sun to convert CO2 and H2O to the sugar glucose, C6H12O6. Your carbon atom is part of this molecule; it is now one of the carbons in the glucose. 
Next, the glucose carrying your carbon atom is sent down the phloem tissue of the carrot plant to the root, which is the carrot itself. Enzymes in the carrot root connect the glucose molecule with your carbon atom into a chain with other glucose molecules, forming cellulose, or plant starch, and there you go. Your carbon atom went from being part of an atmospheric gas to a starch via photosynthesis and starch formation in the root. 
I hope that helps!