What part of a leaf contains the most chloroplasts?

The mesophyll contains the most chloroplasts in a typical leaf. It is the internal tissue between the upper and lower epidermis, and its cells are specialized for photosynthesis because they receive light and have access to carbon dioxide entering through the stomata.

Mesophyll is usually divided into palisade mesophyll and spongy mesophyll. Palisade cells, located just beneath the upper epidermis, are often especially rich in chloroplasts and capture much of the incoming light. Spongy mesophyll has more air spaces, allowing gases to diffuse efficiently between stomata and photosynthetic cells.

A common mix-up is to choose the epidermis or stomata. These outer structures help protect the leaf or regulate gas exchange, but most epidermal cells lack chloroplasts, and guard cells contain far fewer chloroplasts than typical mesophyll cells. Chloroplasts contain chlorophyll and the internal membranes where the light-dependent reactions occur.