When you're making your lemon smoothie, you do not want to add the entire lemon. Adding the whole lemon will add the seeds and rind to your smoothie, and neither appetizing. Instead, you want to use the best parts of the lemon which include the zest, and the juice. First, zest the lemon using a zester, then cut your lemon in half and juice using a citrus juicer that removes the seeds.