Grains, seeds, nuts and root vegetables are examples of carbohydrates and they take a amount of time to break down, allowing one to feel full longer.

The words you are looking for are complex and large.

Complex carbohydrates like those of these plants usually belong to two types; Starch and Fiber. These complex carbohydrates are meant to store energy for the plant in case of periods of little production of simple energy forming carbohydrates like Sucrose and Fructose. Because of the complex nature of these molecules, it takes a while to break them down (Fiber is especially difficult as humans do not have the specific stomach flora and enzymes to break it down).