T he above picture shows the International Space Station as it will look when finished. People will live and work only in the relatively small, slender cylinders in the middle of the construction. The rest of the station -- the flat, black things -- are solar panels. These panels convert sunlight energy into energy usable by people inside the station.
Well, those solar panels are doing exactly the main thing done by most leaves on most kinds of green plants: They are capturing energy, in this case, sunlight energy. Energy is something needed by all living things, from bacteria to backyard trees to people living in space. The problem of acquiring energy is always one of the most important considerations any living thing has to deal with.
Therefore, a leaf's main job is usually this:
To present its broad surface to the sun, to capture sunlight energy, so that inside the leaf's' green tissue the almost-magical process of photosynthesis can take place.
Now, during photosynthesis, sunlight energy is used to power chemical reactions that combine water with the air's carbon dioxide to form starchy carbohydrate. In other words, during photosynthesis, sunlight energy is stored in carbohydrate for later use. Here's a simplified chemical formula for photosynthesis
Some carbohydrate remains in the leaf but mostly it's transported elsewhere in the plant, perhaps into the stem, or maybe into special underground storage areas, such as the potato plant's potatoes.
Don't lose sight of the fact that according to that wonderful chemical formula above, during photosynthesis leaves such as the Red Maple leaves at the left take two essentially transparent items -- carbon dioxide gas and water -- and with energy from sunlight combine them into the substance making up the largest part of any plant you look at. Look at the biggest tree and by golly it's mostly carbohydrate and water, and that carbohydrate has been produced from gas and water, with the process being fueled by sunlight.
Surely this is one of the most amazing facts in the Universe, but we seldom think about it.
Also, notice that when we burn wood, basically we're doing the photosynthesis formula in reverse. We're breaking down the carbohydrate and producing carbon dioxide gas and water, plus energy, which, like sunlight, feels hot and looks bright. In a real sense, first sunlight was captured in the carbohydrate, then the carbohydrate was kept for a while in the plant, and now as the plant burns the sunlight's energy is being released again.
Leaves do serve other purposes, including the following:
It's worth reflecting about the fact that if we humans were put into a room filled with nothing but carbon dioxide, we'd die pretty quickly. In the same way, if all photosynthesizing plants were removed from the Earth, before long we oxygen-needing animals would die.
Now consider this: Right now humans are destroying Earth's photosynthesizing plant communities, especially rainforests and algae in the oceans (agricultural herbicides and many kinds of pollution drain into the oceans) as if our lives did not depend on them...
Therefore, when we speak of leaves photosynthesizing, we're referring to something profoundly important and special.