The exchange of oxygen and carbon dioxide in the leaf (as well as the loss of water vapor in transpiration) occurs through pores called stomata (singular = stoma). Normally stomata open when the light strikes the leaf in the morning and close during the night.
Stomata (the receptors of gas exchange process) are tiny holes found in the underside of leaves. They control water loss and gas exchange by opening and closing. They allow water vapour and oxygen out of the leaf and carbon dioxide into the leaf. In low light the guard cells lose water and become flaccid, causing the stomata to close.