What gas do plants absorb from the air for photosynthesis?

Photosynthesis is essentially a way of building sugar out of air and water using sunlight as the energy source. The carbon that ends up in a leaf, a trunk or a potato comes from carbon dioxide drawn in through tiny pores on the leaf surface called stomata, each flanked by guard cells that open and close them. Inside the chloroplasts, chlorophyll captures light energy, water molecules are split, and the carbon dioxide is combined into carbohydrate. Oxygen is left over and released back into the air, which is the reverse of what animals do when breathing. This means the bulk of a tree's dry mass did not come from the soil at all but from the atmosphere, an idea that surprises most people the first time they hear it. The soil supplies water and dissolved minerals such as nitrogen and magnesium, which the plant needs to build proteins and chlorophyll itself, but not the carbon backbone of its tissue.

Plants respire around the clock too, so at night they consume oxygen and give off carbon dioxide.