This is the second of a five-part series on Metabolism.
The purpose of your entire digestive system is to reverse the process of photosynthesis and liberate the energy locked within the glucose molecules But first it must digest or break down the plants components into small enough pieces to be absorbed.
Digestion begins in the mouth with a carbohydrate-digesting enzymes in your saliva. As you chew a piece of bread you will notice that it starts out savory but the longer you chew the sweeter it becomes. These are the sugar molecules being snapped off the larger starch molecule by enzymes–hence the sweet taste. Digestive enzymes snap larger molecules into two. Different digestive enzymes catalyze different types of macronutrients: proteases digests protein, lipases digests lipid or fat, and amylases digest carbohydrate. All of the digestive enzymes have names that end with “ase”.
As we saw in the last chapter, your digestive system is basically a long tube with muscular walls. From the perspective of your cells, it functions like an assembly line. They remain stationary while the meal moves slowly through the tube in front of them. Some of the cells work in another department, in the pancreas for example, and sends the products they make down a chute, the common bile duct for example which pours the product onto the assembly line as it moves by. Each type of cell does the same job over and over. The food is propelled along our assembly line by a series of wave-like muscle contractions called peristalsis. At each stop along the line the chewed meal is subjected to some process until it reaches the end and exits looking nothing like when it entered.