Explanation:

The primary producers, plants (specifically their chloroplasts), are responsible for the production of glucose through the process of photosynthesis. Glucose, the simplest carbohydrate, serves as the building block for more complex carbohydrates, such as starch and cellulose. These, in turn, form the backbone of more intricate carbohydrate structures found in plants. Therefore, the carbon atoms that make up the carbohydrate backbone ultimately originate from plants.