The green cells, which contain chlorophyll for photosynthesis, are primarily located in the leaves of an onion plant.

Here's why:

* Leaves: These are the primary photosynthetic organs. They are exposed to sunlight, allowing the chlorophyll in their cells to capture energy and convert it into sugars.

* Bulb: The bulb is primarily for storing food. While it contains some cells, these are mainly for storage and not actively involved in photosynthesis.

* Roots: Roots are for absorbing water and nutrients from the soil. They don't contain chlorophyll and are not photosynthetic.

So, while the entire onion plant is green, the green cells responsible for photosynthesis are concentrated in the leaves.