Parts of a leaf 

Upper epidermis: A single layer of cells that are transparent and thin to allow light to enter the leaf and reach palisade cells.

Palisade mesophyll: Contain lots of chlorophyll to carry out photosynthesis. They have a long shape to maximise light absorption.

Spongy mesophyll: More rounded cells with lots of air spaces between them; gas exchange occurs here. The cells are packed loosely for maximum gas exchange.

Lower epidermis: No thick cuticle. Has lots of stomata to allow for gas exchange.