The organelle that was important in classifying cells as either a plant or an animal cell is the chloroplast.

Here's why:

* Chloroplasts are the sites of photosynthesis in plant cells. They contain chlorophyll, the green pigment that absorbs light energy to convert it into chemical energy in the form of sugars.

* Animal cells do not have chloroplasts and therefore cannot perform photosynthesis.

Therefore, the presence of chloroplasts is a key characteristic that distinguishes plant cells from animal cells.