Carbon dioxide is made all the time by plants. Every cell in a plant needs energy in the form of ATP. Plants get the energy to make ATP by breaking down sugar (or similar molecules) into carbon dioxide and water. This requires oxygen. This process is called cellular respiration. It’s the same thing that we animals do. Fungi and many one-celled organisms do it too. We get our sugar and other foods by eating.

Plants get their sugar through photosynthesis. The first stage of photosynthesis has to happen in the light. Plants use light energy to make ATP and another energy-carrying molecule called NADPH. They can “spend” the energy in these molecules to put CO₂ together (along with the hydrogen from water) to make sugar. Oxygen is a waste product.

Now think about a plant. How much of the plant can do photosynthesis? Not the roots; they don’t get any light. Not the branches; they don’t have the green pigment that is needed for photosynthesis. Not the cells deep inside the plant;they don’t get any light either. In most plants, photosynthesis happens mostly in the leaves. The sugars made in the leaves travel through the plant to all of the other cells. The sugar water (sap) travels in tubes called phloem.

So cells all over the plant are producing carbon dioxide all the time. Some of this may be used in photosynthesis, but that’s not happening in many areas of the plant. The CO₂ just diffuses out through the cell membrane and holes in the plant cell walls.

There are actually some plants that only open their stomata at night. Why do you think they do this?