If it's a simple explanation of why some trees' leaves turn color each fall, check out Why Leaves Change Color from the College of Environmental Science and Forestry of the State University of New York. For a more detailed explanation (including an "I Can Read" version written just for young students), and for a handful of hands-on science activities that teach about fall's colors, see Simply Science's Why Do Leaves Change Color in the Fall? One really cool experiment at this site involves the use of rubbing alcohol, so it might be appropriate only for older students. Also, check out the Fun Fact question (What do autumn leaves and ripening bananas have in common?) , which will help teachers of all grades to teach an important leaf-color concept.
Photosynthesis is when plants use sunlight to create glucose (sugar). There are a lot of steps that are involved in this process. First, sunlight is absorbed by chlorophyll, which then splits water into hydrogen and oxygen. Energy is then released and put into the Calvin cycle and stored as ATP and NAD PH2 molecules. Next, carbon dioxide turns into carbohydrates. Out of the Calvin cycle comes sugars and everything kind of starts over. That was a super simplified version. This is a complicated process to explain, since there is a lot of information, so I will link a helpful video and some websites.
Although all cells in the green parts of a plant have chloroplasts, the majority of those are found in specially adapted structures called leaves . Certain species adapted to conditions of strong sunlight and aridity , such as many Euphorbia and cactus species, have their main photosynthetic organs in their stems. The cells in the interior tissues of a leaf, called the mesophyll , can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is coated with a water-resistant waxy cuticle that protects the leaf from excessive evaporation of water and decreases the absorption of ultraviolet or blue light to reduce heating . The transparent epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.