The plant cell structure that appears to be missing from the cells of an onion bulb is chloroplasts.

Here's why:

* Onion bulbs are underground storage structures. They are not exposed to sunlight, which is essential for photosynthesis.

* Chloroplasts are the organelles responsible for photosynthesis. They contain chlorophyll, the pigment that absorbs light energy.

* Onion cells primarily focus on storing nutrients (sugars) and water. They do not need to perform photosynthesis.

While onion cells lack chloroplasts, they still have the other typical plant cell structures like a cell wall, a large central vacuole, a nucleus, and other organelles.