Lesson Video: Enzymes in Digestion Biology

Video Transcript

In this video, we’ll learn how digestive enzymes make it easier for organisms to break down large nutrient molecules, such as carbohydrates, proteins, and lipids or fats in foods. We’ll take a look at some of the enzymes that break these nutrients into smaller molecules so that other enzymes can then either build them back up at the living tissue or break them down even further to extract and store chemical energy. We’ll start with a quick review of enzymes then examine the action of digestive enzymes on nutrients and work a practice problem to apply what we’ve learned.

It’s been said that some eat to live and some live to eat. But either way, food and life are pretty close to inseparable, since even plants, fungi, and single-celled organisms all need similar nutrients, even if they don’t eat. And what the connection really comes down to is that food isn’t just for life; it’s also from past lives.

All organisms contain elements, including carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur, and more. And the atoms of these elements are arranged into small molecules like water or carbon dioxide and larger molecules such as carbohydrates, proteins, fats, and other lipids. And we’re gonna see how enzymes help organisms break down these large nutrient molecules into smaller ones that can be used to build living tissue or to extract energy from.

So here’s a question. If this caterpillar eats this pea plant and then this bird eats the caterpillar, what structures actually transform the molecules of one being into those of another? Right, enzymes! Next, let’s go through a brief review of enzyme structure and function.

Enzymes speed up the incredible number of chemical reactions needed to sustain life. And the sort of chemical reaction we’ll be looking at in digestion is when you start with a large molecule as the reactant and it’s broken down into small molecules as products. To break apart small molecules from a large molecule, an input of energy is required; otherwise larger molecules would just fall apart too easily all on their own. Once enough energy is transferred to the large reactant, the bonds rearrange, which is another way of saying that the chemical reaction occurs, releasing energy to the amount contained in the products.

Enzymes reduce the amount of energy necessary to start the reaction, allowing them to happen frequently enough to sustain life. Without an enzyme, this reaction will need a higher energy input, such as heat, before the smaller molecule products can be formed. But with the right kind of enzyme, less energy is needed to get the reaction going.

The place on the enzyme where the reactant or substrate, as it’s called in an enzyme-controlled reaction, bonds is called the active site. An active site has a complimentary shape that fits the substrate. Once the enzyme and the substrate bond together, the energy requirement for the reaction decreases, allowing the reaction rate to proceed much more quickly than it otherwise would. And after each enzyme catalyzes or speeds up one reaction, it’s active and open and ready to catalyze another.

So enzymes catalyze the same reaction over and over and continue to break the substrate down until the large substrate has been completely converted to smaller products. Next, we’ll take a look at the enzymes that break down our large nutrient molecules in our body.

On this side of the screen, we’ll list names of the large molecule nutrient types, the enzyme group which catalyzes the breakdown of each group, and the type of molecules that result. But before we start, we need to add some vocabulary terms. See if you already know which term goes on which of the four blue lines above. Two of the terms describe large molecules, and two describe small ones.

The term monomer refers to the small, similar molecules that the larger molecules are broken down into. The prefix mono- means one, and -mer means part, so monomer means one part. The term polymer refers to the large molecules that are made out of many connected monomers. The prefix poly- means many, and -mer again means parts, so polymer means many parts.

Chemical reactions catalyzed by enzymes start with one or more substrates. In our example, these are the large molecules. And the result is one or more different chemicals called products. And over here, we’ll add to this diagram that will show us where some of these enzymes are produced in our bodies.

Let’s start with carbohydrates, a nutrient we typically use as an energy source. Carbohydrates are broken down by a group of enzymes known as carbohydrases into the smaller monomer products called sugars. An example of a carbohydrase is the enzyme amylase, which is produced in our salivary glands and it catalyzes the breakdown of the carbohydrate starch into glucose sugars.

Let’s take a look at the types of digestive enzymes that break down proteins next. These are called proteases, and they catalyze the breakdown of proteins into the monomer units of amino acids. The stomach is one organ that produces a protease to start the digestion of proteins. Have you noticed by now that all these enzymes seem to end with -ase? Well, it’s usually true, and it’s a pretty good way to identify enzyme names.

So what do you think the types of enzymes are called that help to break down lipids, such as fats. They’re called lipases, and they break down fats into the monomer units of both glycerol and fatty acids. The pancreas produces lipases as well as other types of enzymes.

So why bother with all this breaking down of long polymer substrates into smaller monomer products? Well, if you eat something like peas, carbohydrases, such as amylase from your salivary glands, can start breaking down longer carbohydrate molecules into sugars that can be used for energy or to build up other molecules. The protein in the peas will start to be broken down into amino acid monomer units. And those amino acids can be used to build up things that we need, like enzymes, muscle tissue, antibodies, and more.

And the pancreas actually produces all three types of enzymes that break down the remaining nutrient polymers in the small intestine. So now those small monomer units of the nutrients can be absorbed into the bloodstream through the walls of the small intestine. But any larger molecules such as fiber, which is made out of the carbohydrate cellulose, will move through the intestine without being absorbed. And the nutrient monomers from the peas are transported by the blood to the cells of our bodies, where they can be used to energize or maintain our tissues thanks to our digestive enzymes.

Next, let’s apply what we’ve learned to working a practice problem.

Which of the following best explains how enzymes aid digestion? (A) Enzymes release energy to aid physical processes of digestion such as chewing. (B) Enzymes slow the rate of digestion so it does not require too much energy. (C) Enzymes regulate the pH of the digestive system to ensure it remains at optimum. (D) Enzymes break down large, complex food molecules into smaller ones that can be absorbed. (E) Enzymes are released by the gall bladder to neutralize stomach acids.

Key knowledge required to select the correct option includes a general understanding of enzyme action and, more specifically, how enzymes aid in the digestion of nutrients. We’ll start with the definitions of these two key terms, enzyme — enzymes are biological molecules that speed up chemical reactions — and digestion — which is the breakdown of food into smaller particles that can be absorbed.

So how are chemical reactions related to food? Well, life, which takes place in in between cells, is largely a complex and organized system of chemical reactions that utilize nutrients and matter for energy. So we eat and digest food so we can keep our sustaining chemical reactions underway. Those nutrients include large molecules such as carbohydrates, proteins, and lipids. We need these nutrients to be transported into our cells, but for that to happen, these larger molecules, which are made out of similar repeating subunits, have to be broken down into the smaller monomer subunits so they can be transported across the cell membranes. And that’s the job of the digestive enzymes.

Carbohydrases breakdown larger carbohydrates into their smaller monomer units called sugars. Proteases break down proteins into their smaller monomer units, which are called amino acids. And lipases break down fats into glycerol and their fatty acids. Now these nutrients can be transported from the inside of the small intestine into the bloodstream and to the cells.

And we’re ready to review the solution options to our question. Option (A) says, enzymes release energy to aid physical processes of digestion such as chewing. There are a couple problems with option (A). First of all, enzymes do not release energy. They reduce the amount of energy needed for a chemical reaction to occur. And that’s another problem. Enzymes don’t help with physical processes. They help with chemical reactions.

Option (B) says, enzymes slow the rate of digestion so it does not require too much energy. But that’s the opposite of what we were just saying. Enzymes speed up chemical reactions, and those are involved in digestion. They definitely aren’t there to slow things down.

Option (C) states, enzymes regulate the pH of the digestive system to ensure it remains at optimum. pH is important for enzymes they only function within a certain range. But as we’ve said before, this isn’t the job of enzymes. That’s to speed up the chemical reactions involved in digestion.

Option (D) says, enzymes break down large, complex food molecules into smaller ones that can be absorbed. And that’s just what we’ve been talking about. So option (D) is looking pretty good.

Option (E) says, enzymes are released by the gall bladder to neutralize stomach acids. But there’s a couple problems here. First of all, the gall bladder releases bile. And enzymes do not neutralize stomach acids. They speed up the chemical reactions of digestion.

Therefore, the correct answer to this question, which of the following best explains how enzymes aid digestion, is enzymes break down large, complex food molecules into smaller ones that can be absorbed.

In this video, we learned how food contains large nutrient molecules that are broken down by our digestive enzymes into smaller units called monomers. Those are transported by our bloodstream to all the many, many cells of our body, where they can be used. Carbohydrates are broken down by carbohydrase enzymes into the smaller units called sugars. An example of a carbohydrase is amylase, which breaks down the large starch molecule into the small glucose monomers. Proteases break down proteins into the monomer units of amino acids. And the enzyme lipase helps to break down lipids, such as fats, into two different types of monomer units, both glycerol and fatty acids. The monomer units then can be circulated in the bloodstream to reach all the cells of the body, where they can be transported inside.