Final answer:
Through the process of photosynthesis, plants convert atmospheric carbon dioxide into glucose. This carbon is then used to construct complex macromolecules like proteins, with amino acids being the building blocks that contain carbon.
Step-by-step explanation:
A carbon atom from a CO2 molecule in the air can end up as a carbon atom in a protein in a plant cell through a series of biological processes. Initially, plants capture the carbon dioxide from the atmosphere through the process of photosynthesis. The carbon atom is then fixed into glucose, a simple sugar with a backbone of carbon atoms. Subsequently, glucose can be used by the plant to build larger macromolecules like proteins, during which the carbon atoms are reorganized to form the protein's structure.
Proteins, being one of the many complex organic molecules present in cells, contain carbon atoms that are essential to the molecule's structure and function. The protein synthesis process in plants uses amino acids, which contain carbon, to form the polypeptide chains that make up proteins. An example of such an amino acid is glycine, which is a component of proteins and consists of carbon, nitrogen, oxygen, and hydrogen atoms.
Through these processes, a carbon atom can transition from being part of an inorganic molecule in the gaseous state to becoming an integral part of an organic protein within a living plant cell, demonstrating the dynamic movement of carbon within the earth's carbon cycle.