1. Palisade mesophyll cells: These cells are elongated and tightly packed, forming a layer just below the upper epidermis of the leaf. They are the primary site of photosynthesis due to their high density of chloroplasts.

2. Spongy mesophyll cells: These cells are irregularly shaped with large air spaces between them. They also contain chloroplasts, but in a lower density compared to palisade mesophyll cells. The air spaces allow for gas exchange (CO2 and O2) during photosynthesis.

3. Guard cells: These specialized cells surround stomata, which are small pores on the leaf surface that regulate gas exchange. While their primary function is controlling the opening and closing of stomata, guard cells also contain chloroplasts and can contribute to photosynthesis to a lesser extent.

Therefore, all three cell types contribute to photosynthesis in leaves, but palisade mesophyll cells are the most specialized and efficient for this process.