The leaf is the organ in a plant specially adapted for photosynthesis.  You need to understand the structure of the tissues in a leaf together with their functions.

Upper Epidermis:  this is the tissue on the upper surface of the leaf.  It produces a waxy layer, called the cuticle, which is not made of cells but is a waterproof barrier to prevent excessive evaporation through the hot upper surface of the leaf.  The upper epidermis cells have no chloroplasts so light passes through them easily.

Palisade Mesophyll:  this tissue is where 80% of the photosynthesis takes place in the leaf.  The palisade cells have many chloroplasts in their cytoplasm and the box-like shape and arrangement of these cells ensures they are packed tightly together.

Spongy Mesophyll: this tissue contains large air spaces which are linked to the atmosphere outside the leaf through microscopic pores called stomata on the lower surface.  Spongy mesophyll cells also contain chloroplasts and photosynthesis occurs here too.  The air spaces reduce the distance carbon dioxide has to diffuse to get into the mesophyll cells and the fact that these cells have fairly thin cell walls which are coated with a film of water together means that gas exchange between air space and mesophyll is speeded up.

Lower Epidermis is the most dull tissue in the leaf.  The only interesting thing about it is that it contains specialised cells called guard cells which enclose a pore called a stoma.  Carbon dioxide can diffuse into the leaf through the stomata when they are open (usually at day time) and water evaporates out of the stomata in a process called transpiration.

Adaptations of a Leaf for Photosynthesis

• Large Surface Area – to maximise light harvesting
• Thin – to reduce distance for carbon dioxide to diffuse through the leaf and to ensure light penetrates into the middle of the leaf
• Air Spaces – to reduce distance for carbon dioxide to diffuse and to increase the surface area of the gas exchange surface inside the leaf
• Stomata – pores to allow carbon dioxide to diffuse into the leaf and water to evaporate out (transpiration)
• Presence of Veins – veins contain xylem tissue (carries water and minerals to the leaf from the roots) and phloem (transports sugars and amino acids away from the leaf)
• Chloroplasts – mesophyll cells and guard cells contain many chloroplasts.  These organelles contain the light harvesting pigment chlorophyll and are where all the reactions of photosynthesis occur