Humans and other living organisms that consume food for energy need to break down the molecules of food into smaller molecules that the cells can use for various chemical reactions. Your body, and that of any other food-consuming organism, uses digestive enzymes to break down food. Some enzymes specialize in fat breakdown, while others specialize in cellulose breakdown, and still others specialize in breakdown of other types of food molecules.
Enzymes
An enzyme is a protein that functions as a biological catalyst, where a catalyst is a chemical that makes a reaction take place faster than it otherwise would. Humans and other living organisms use enzymes to help digest food molecules in a timely manner. Your body, for instance, secretes various digestive enzymes including amylases, which digest starch, proteases, which digest protein, and lipases, which digest fat.
Process
Like most biologically active protein molecules, enzymes interact with chemicals in a very specific fashion. Drs. Reginald Garrett and Charles Grisham explain in their book "Biochemistry" that each type of enzyme reacts with and breaks down only one kind of food molecule. Amylases, for instance, break down starch but can't react with proteins, fats or other molecule types. You, like other living organisms, can only digest the molecule types for which you possess the enzymes.
Function
Interestingly enough, enzymes function so specifically that it takes a particular enzyme to digest a particular type of carbohydrate or protein. For instance, both starch and cellulose--plant fiber--are made up of long chains of glucose molecules. The difference between starch and cellulose is in the shape of the glucose linkages. Despite the chemical similarities between the two, the chemical differences between starch and cellulose are significant enough that it takes two different enzymes to digest the two. Amylase digests starch, but the enzyme cellulase digests cellulose.
Considerations
While you, like many other living organisms, produce amylase and can digest starch, your body can't digest cellulose. Humans don't produce cellulase, and neither do other animals. Many fungi and bacteria produce cellulase and can digest fiber, but humans can't. This is the reason that cellulose, which is often called dietary fiber, is indigestible and doesn't provide any calories when consumed by humans.
Expert Insight
Your digestive tract does produce lipase, which is why you're able to digest fat. One problem associated with fat digestion is that fat is not water-soluble, but lipase enzymes are. As a result, the gallbladder has to secrete bile salts into the intestine for lipases to be able to function. Bile salts combine with fat and emulsify it, or pull it into water solution. This allows lipases to contact, and digest, the fat you consume.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
