When chemical bonds are broken by animals the energy is released in the form of work and heat. This is another way of stating that chemical reactions are usually one directional without the input of energy. For example, we can breakdown glucose to carbon dioxide and water because these compounds have lower potential energies than glucose. However, organisms cannot put carbon dioxide and water together to form glucose without the input of energy. This law also gives rise to the idea of entropy , which is very important in biology.
High entropy means low potential energy.
To fight entropy, to maintain order, one must expend energy, which is what you do every time you clean your dorm room. Remember, organisms are highly organized systems of cells, tissues and organs. There is a lot of potential energy to be lost. The second law of thermodynamics says that this order should be lost without the input of energy. Typically, most organisms spend more energy maintaining themselves than they do to grow and reproduce.