Leaves are very important for trees – they provide food for the whole tree (or plant)! How do they do it? Leaves use a very special process called photosynthesis (say: foto-SIN-thuh-sis) to convert energy from sunlight into sugars and starches that a tree uses as food.
Leaves have an important chemical inside of them called chlorophyll (say: KLOR-uh-fil), which is what makes them green, and is also what allows them to do photosynthesis. You might remember learning about photosynthesis in the newsletter about flowers a few months ago. If you missed that issue, you can read it here.
Different chemicals found in leaves are what cause them to change colors in the fall. As we already learned, chlorophyll is the chemical that gives leaves their green color while they make food through photosynthesis during the spring and summer months. When it starts to get cooler outside and the sun does not shine for as long each day, trees know it is time to start storing up food for winter.
To do that, the chlorophyll in the leaves starts to break down and the food that the leaves have been making is stored inside the tree instead of in the leaves. Now that the chlorophyll is gone, instead of being green, the leaves become all the pretty colors of fall, like orange, yellow, red, or even purple!
These colors have actually been in the leaves all summer long, we just couldn’t see them because the green from the chlorophyll was blocking them. Chlorophyll and the other chemicals that cause the colors in leaves are called pigments. Pigments are also used to dye thread and fiber that clothes are made out of.
The other leaf pigments besides green come from chemicals called carotenoids (say: kuh-ROT-in-oidz) and anthocyanins (say: an-thuh-SYE-an-inz). Carotenoids make leaves yellow, orange, and brown and are always in leaves, just like chlorophyll. Not all leaves have anthocyanins, which cause colors like red and purple to appear.
Anthocyanin is formed when sugar gets trapped in a leaf after the chlorophyll is gone. Then, when the leaf is exposed to sunlight, the anthocyanin turns leaves red and purple! The leaves in the picture have a little of each of the pigments in them.
As the weather gets cooler and the days get shorter in the fall, trees start to prepare for winter. Trees use sunlight to make a special layer or seal between each leaf and the branch it is connected to. Then the leaves fall easily to the ground, leaving the branches of the tree protected from the cold that will come in the winter and also helping the tree store up food!