The green pigment in a plant's leaves-chlorophyll-uses light to produce glucose, which plants use as fuel. During this process, which is called photosynthesis, water combines with carbon dioxide from the air, releasing oxygen back into the atmosphere. The gases are exchanged through pores called stomata on the underside of its leaves. At night the process is reversed and carbon dioxide is given off.
Traditionally, plants were removed from bedrooms at night because of this, particularly if someone was ill, but the amount of carbon dioxide given off by a few plants is much too small to have any significant a effect on anyone.