Onion cells lack chloroplasts because the onion is part of the plant that is not involved in photosynthesis. The part of the plant eaten by humans is called the bulb, and it resides at the base of the plant. The bulb’s primary purpose is energy storage and holding the flower for the second growing season. Growing near the ground, the bulb is in poor position to collect sunlight.
Photosynthesis is the process by which plants use sunlight, water, carbon dioxide and a chemical called chlorophyll, to create sugars. Chloroplasts are small organelles, located in some plant cells, that contain chlorophyll and enable photosynthesis. The onion is a photosynthetic plant, and it holds numerous chloroplasts in the leaves, which receive much more sunlight, but very few in other parts of the plant.
Onions are typically harvested at the end of their first growing season, after they have shed their leaves and become dormant. However, if left in the ground, a new stem and flower will erupt from within the onion bulb the following season. This can also happen to harvested onions that have been stored for a long period. Onions have been cultivated by humans for at least 7,000 years, and they are grown in a variety of flavors, colors and sizes.