Final answer:
Chloroplasts are present in Elodea as it is an aquatic plant with cells adapted for photosynthesis, whereas the onion epidermis does not perform photosynthesis and therefore lacks chloroplasts.
Step-by-step explanation:
The presence of chloroplasts in Elodea but not in onion epidermis cells is due to the different functions and environments of these plant structures. Chloroplasts are the organelles where photosynthesis occurs, and they are typically found in the mesophyll cells of leaves where they can access light. Elodea, being an aquatic plant, has cells in its leaves that are adapted to perform photosynthesis under water and thus have chloroplasts.
Onion epidermal cells, on the other hand, are generally found on the underground parts of the onion that do not perform photosynthesis. These cells are not exposed to light and do not contain chloroplasts as a result. The onion epidermis serves primarily as a protective layer, which is why it lacks these light-capturing organelles.