Where does carbon dioxide enter the leaf?
Through the 'stomata' (usually on the underside) of the leaf.
Guard cells allows the exchange of oxygen and carbon dioxide in and out of the leaf. This is known as leaf exchange. When guard cells are full of water they swell up and close meaning no oxygen can leave or carbon dioxide can enter, but when the water is taken away from the cell they open up to allow oxygen to exit and carbon dioxide to enter.