For example, rice is low in isoleucine and lysine. However, different vegetable sources are deficient in different amino acids - by combining different foods you can get all of the essential amino acids throughout the course of the day. Some vegetable sources contain quite a bit of protein - things like nuts, beans, soybeans, etc. are all high in protein. By combining them you can get complete coverage of all essential amino acids. The digestive system breaks all proteins down into their amino acids so that they can enter the bloodstream.
Cells then use the amino acids as building blocks.
From this discussion you can see that your body cannot survive strictly on carbohydrates. You must have protein. So a 150 pound person needs 54 grams of protein per day. On the right is the Nutritional Facts label from a can of tuna. A glass of milk contains about 8 grams of protein.