Why don't onions contain chlorophyll?

Answer 1

The practical answer to this is that they don't need chloroplasts. Think of where an onion lives and how much light it normally receives. Chloroplasts only function in the presence of light. Because it costs energy to develop chloroplasts, it is not useful for the plant to put develop them if they won't function.

However accurate, this answer is a little simplified compared to what really happens. Chloroplasts are only one type of plastid, along with chromoplasts (causing the red color of peppers and tomatoes) and amyloplasts (storing starch in potatoes and corn). None of these plastids can be assembled from raw materials within the cell of a plant. Rather they all develop from another type of plastid called a proplastid. Proplastids are small and clear. They divide at the same time the cells divide, so every cell in a plant has proplastids. However, they are difficult to see in most plants because of their size.

One piece of evidence that the proplastids exist in onion cells is easily seen. Leave an onion exposed to light for several days or weeks. It will begin to grow new green leaves. Also, some of the outer layers of the bulb (actually leaves, too), will begin to green up, as their proplastids develop into chloroplasts. It is thought that plastids may be found in all living cells of a plant, and probably are present in every cell in the early stages of development. Later they become restricted to certain cells, and are abundant only in those which have specialized functions, such as photosynthesis, storage, and color manifestation. Plastids multiply freely by division, and in this way the large numbers present in some cells are in part secured. Plastids are present in the very young meristematic dells where they are very minute, the smallest being at the limit of [light, not electron] microscopic visibility.

Answer 2

The onion leaves that grow above the ground DO have chloroplasts, the part that grows underground (the part you eat) does not.

Answer 3

The don't need it

Answer 4

no