What food component does cooking break down to make it more digestible?

Cooking food breaks down molecular bonds that makes it easier to digest.

Your body is essentially expending less energy digesting cooked food than raw food, because the process of adding heat to your food has done some of the work for you. Think about eating something like celery. When you eat raw celery, it is very fibrous and hard break down without a lot of chewing. Now, when you eat cooked celery in something like soup, the same celery stalk require less work to be digested. This is because the soup-making process has broken down a lot of that fibrous material that required you to chew so much when the celery was raw. Other foods are known to have a lot of natural digestive enzymes within them. By cooking these foods, you are denaturing these enzymes and losing the inherent digestive properties of the food itself.