Have you ever wondered how you can eat as many as 5 stalks of celery and still be intaking only about 30 calories? Well, our bodies work in an interesting way and the reason behind this question is a part of why humans are unique. The human body is made up of many molecules and has many parts that enable it to be the masterpiece it is. There are four main categories of molecules that are important to all living things: lipids, proteins, nucleic acids, and carbohydrates. As soon as we intake foods that contain lipids, proteins, or carbohydrates, our body starts its work. Through the process of digestion, our body works to break these molecules of food into a form that can be used for energy. Our cells’ preferred energy source is carbohydrates. This is because we have enzymes that can break down the polysaccharides that we consume in a process called hydrolysis(1).
Enzymes have many functions in our body– one essential one is their role of breaking down our food. The human body has about 75,000 enzymes and the enzymes that a body contains determines what foods an individual can digest(2).
That brings us back to our original question. The reason that our bodies intake a small number of calories from plants like celery is that plant cells are enclosed by the organic compound cellulose. This compound is simply a polymer of glucose that is an important ingredient to the strong walls of plant cells(3). Plants like celery and spinach contain a lot of cellulose. Since our bodies do not have the enzyme cellulase, they cannot break down the polysaccharides and intake a lot of calories. Unlike us, animals like cows can eat plants like grass and intake a lot of calories from it. Overall, it is the lack of an enzyme that has made vegetables the viable option for those who do not want to intake lots of calories.
Enzymes are an interesting thing. They are the reason that vegetables are so calorie sparse for humans but one of the main sources of energy for big animals like cows. Enzymes are also the reason why some individuals are lactose-intolerant. Without the enzyme lactase, people cannot break down the lactose found in dairy products. All of these things prove that there is a lot of complexity to our bodies.
References and Footnotes
- Hydrolysis is the process where a polymer is broken down or separated by a molecule of water. It reverses the process of dehydration synthesis, which is when two monomers are bonded in a reaction that takes away a molecule of water.
- Svarney, Patricia Barnes, and Thomas E Svarney. “How Many Enzymes Are in the Human Body? – The Handy Biology Answer Book.” Why Were Spain’s King Ferdinand and Queen Isabella so Powerful? – The Handy History Answer Book, www.papertrell.com/apps/preview/The-Handy-Biology-Answer-Book/handy%20answer%20book/How-many-enzymes-are-in-the-human-body/001137031/content/SC/52caff2582fad14abfa5c2e0_default.html.
- Reece, Jane B., et al. Campbell Biology: Concepts and Connections. Pearson, 2015.