Final answer:
Radioactive glucose will first appear in the mesophyll cells of the leaf during a pulse-chase experiment with radioactive CO2 because this is where photosynthesis and glucose synthesis take place before being distributed through the phloem.
Step-by-step explanation:
In an experiment where a pulse of radioactive carbon dioxide (CO2) is provided to isolated leaves, the tissue where radioactive glucose would appear first is the mesophyll cells of the leaf. This is because the mesophyll cells are the site of photosynthesis, where CO2 is fixed and used to produce glucose during the Calvin cycle. Following its formation, glucose can then be transported to other parts of the plant via the phloem tissue.