Proteins from plants, such as beans, legumes and grains, are incomplete proteins. That means that in order to get everything you need from a vegetarian protein source, you need to eat a combination of them in order to create a complete one.roteins from plants, such as beans, legumes and grains, are incomplete proteins. That means that in order to get everything you need from a vegetarian protein source, you need to eat a combination of them in order to create a complete one.