The leaf is VERY important in photosynthesis. Lets examine some of the features which make it suitable for photosynthesis. Use the diagram as a reference.

The waxy cuticle at the top of the leaf is transparent, allowing light to enter for photosynthesis. It also stops transpiration for happening throught the leaf, other than in the stomata.

The upper epidermal cells are also transparent to allow light in to reach the chloroplasts for photosynthesis.

The lower epidermis has stomata, which allow gaseous exchange to occur. Gasious exchange being the intake of CO₂ and the release of O₂, which is essential for photosynthesis.

The leaf is thin, and has a very large surface area - making it ideal for diffusion and absorbsion. The larger the surface area, the more sunlight can be absorbed. The leaf is also thin so that the mesophyll cells are closer to the surface - reducing the diffusion distance of CO₂ from the surroundings to the mesophyll cells.

Vascular Tissue
This is to do with the xylem and the phloem. Remember that the xylem is always at the top of the leaf, and the phloem at the bottom. This is because the xylem transports water(needed for photosynthesis) to the leaf cells, and diffuses into the chloroplasts.
The phloem transports food from the mesophyll cells to the rest of the plant. The xylem and phloem are vascular tissue that are situated in the leaf vein. The vein is supported by fibres(sclerenchyma), which keep the shape of the leaf, flat.

Mesophyll
There are two main types of Mesophyll cells, the palisade and spongy cells.
Palisade - contain the most chloroplast, and are at the top of the leaf. Are closely packed - allowing more cells and therefore more chloroplasts to be near the surface. There are small intercellular spaces inbetween them which allows gaseous exchange to occur.
Spongy - have less chloroplasts, and larger intercellular spaces. They also store carbohydrates made by photosynthesis. Dissolved carbohydrates diffuse into the phloem, to be transported to the rest of the plant.

Chloroplasts
These are the structures which contain chlorophyll, and this is where the photosynthesis actually happens! They have a double membrane, which is selectively permiable. This allows H₂O and CO₂ to diffuse into it, and O₂ to diffuse out of it.