Speaker A
[Music] In this video, we examine the process of digesting our food. We shall see how biological catalysts called enzymes break these food molecules apart and why they're necessary. Think of our food molecules as a train of trucks all coupled together. Digestion disconnects the trucks so that we can reassemble them in our own way. The breaking takes place by hydrolysis, a reaction where water is added and breaks the link. So let's look at the simplified reaction for each of the foods we eat. Starch is a polymer of simple sugars which are joined by a glycosidic link C-O-C. When water is added, the link breaks. Proteins are polymers of amino acids and are joined by the amide link C-O-N-C. Water breaks the chain into separate amino acids. The nucleotides in the DNA double helix are joined by phosphate groups. Water once again breaks the link. In the same way, fats are hydrolyzed into glycerol and fatty acids such as stearic acid used in soap making. Water breaks the link. Once our food is digested into monomers, the molecules are small enough to get through the walls of our gut and into our bloodstream, ready to be used as a fuel—that's the sugars, fats, and oils—or to build up our own proteins and DNA. But there is a problem. These hydrolysis reactions cannot happen by themselves, even in the warmth of our guts. The bonds are strong, making protein and DNA stable. You would need temperatures of several hundred degrees to break these molecules apart. So this is where the enzymes come in. They are made of protein, following instructions from sections of our DNA called genes, folded up into a very specific shape. The shape is just right for each food polymer to fit in. But as the polymer settles into the shape, the bonds get stretched, allowing the water to come in and break the link. All enzymes have names ending in -ase. Amylase splits starch into sugars. Protease splits proteins into amino acids. Lipase splits lipids—that's fats and oils—into fatty acids and glycerol. And nucleases split nucleic acids into nucleotides. Digestion actually starts in the mouth with our saliva, which contains amylase. So if you chew starchy foods long enough before swallowing, you will gradually find them tasting sweet as the starch is broken into sugar. The next stage of digestion takes place in the strongly acidic conditions of the stomach with protease to break the proteins into amino acids. Finally, the pancreas secretes enzymes so that the remainder of the breaking apart can take place in the small intestine, where the broken bits can enter the blood. The human gut is unable to digest cellulose, the fibrous carbohydrate that forms the structures of plants, and we call this roughage. It helps to keep the food flowing through the gut, and it comes out as feces. However, the bacteria that inhabit the gut of ruminant animals such as cows can digest cellulose, and that's how these animals are able to survive on grass. But we can't. So to summarize, digestion is the process of breaking apart large food polymers into their monomers. To enable these reactions to work at body temperatures, biological catalysts called enzymes are needed, with names ending in -ase. The monomers are now able to enter our blood to be used as a fuel or to become built up again into the polymers we need.
