Concept 4: Structure of a Leaf. Leaves are a plant's main photosynthetic organs. Leaf structure is closely associated with its photosynthetic function. Leaves must permit carbon dioxide access to the photosynthetic cells but impede water from diffusing out.he oxygen that is a waste product of photosynthesis must be allowed to escape from the leaf. Demonstrate your knowledge of leaf structure by placing the labels in their correct locations. NOTE: Answers to questions may be lost when you proceed to the next screen. Print this screen if you need a record of your answers.