Carbon dioxide in photosynthesis
Plants get carbon dioxide from the air through their leaves. The carbon dioxide diffuses through small holes in the underside of the leaf called stomata. (singular: stoma. plural: stomata)
The lower part of the leaf has loose-fitting cells to allow carbon dioxide to reach the other cells in the leaf. That also allows the oxygen produced in photosynthesis to exit the leaves.
Carbon dioxide is present in the air we breathe at very low concentrations. Even though it forms about .04% of the air, it is a needed factor in light-independent photosynthesis.
In higher concentrations, more carbon is incorporated into carbohydrates, therefore increasing the rate of photosynthesis in light-independent reactions.