Final answer:
The key structural difference is that E.coli lacks a true nucleus and membrane-bound organelles as it is a prokaryotic organism, while Elodea cells have a defined nucleus and organelles, characteristic of eukaryotic cells.
Step-by-step explanation:
The main difference in cell structure between E.coli, a bacterium, and an Elodea cell, a water weed plant, is that E.coli is a prokaryotic organism while Elodea is eukaryotic. Prokaryotic cells like E.coli lack a true nucleus and membrane-bound organelles, whereas Elodea cells, being eukaryotic, have a distinct nucleus housing the DNA and various membrane-bound organelles, such as chloroplasts, which are critical for photosynthesis in plants.