Lesson Explainer: Enzymes in Digestion Biology • Second Year of Secondary School

In this explainer, we will learn how to explain the action and importance of digestive enzymes.

Enzymes are protein molecules that have a huge range of helpful functions in living organisms. We get some enzymes from the food we eat, but many of the enzymes we need are produced within our bodies themselves. Scientists have been inspired by digestive enzymes within living things to produce a whole range of products, such as detergents, which make use of these fantastic proteins. If you see the word bio on your laundry detergent, this means it has enzymes in it that break down the fats and oils on your clothes to clean them.

Enzymes are biological catalysts that speed up the rate of biological reactions without being used up themselves. They do this by lowering the activation energy required for a reaction to occur. This means that more chemical reactions can occur over a set amount of time than without the enzyme, increasing the rate of reaction. You can see in Figure 1 how the energy that must be supplied for a reaction to occur is much higher without an enzyme than with the enzyme. Imagine this one reaction occurring thousands of times over, and you will be able to see why it is cheaper energetically to use an enzyme in biological reactions.

Each enzyme has a different specifically shaped active site. This is because each type of enzyme is complementary to one particular molecule that binds to it, which is called the substrate, which you can see demonstrated in Figure 2 below. This is often called the lock-and-key model, as there is a specific substrate complementary to a particular enzyme’s active site, much like how a specific key is complementary to a particular lock.

As we can see in Figure 2, the substrate binds to the enzyme’s active site, at which point the whole structure is called an enzyme–substrate complex. After the enzyme finishes its job, it releases the molecules (which are now called the products) from its active site. Therefore, the active site becomes free to have more substrate molecules bind to it. In chemical reactions, enzymes are not used up, meaning that they can continue catalyzing reactions even after several reactions have occurred.

Depending on their type, enzymes can either break down large molecules, in a process called digestion, or build up larger biological molecules from smaller subunits. Digestion is essential in the human body, as it is how we transform the large molecules in the food we eat into a small enough form to be absorbed into the bloodstream. The blood then carries these smaller nutrients to the body cells, which use them to build up a diverse range of carbohydrates, lipids, and proteins.

Since our main focus in this explainer is on digestive enzymes, we are going to look first at the composition of the different large biological molecules involved in digestion in Figure 3, before seeing which enzymes can be used to break them down. The enzymes involved in digestion in humans are used to convert large biological macromolecules, shown on the right in Figure 3, into smaller subunits, shown on the left.

The term polymer is a broad term referring to a long-chained molecule, such as the biological molecules shown on the right of Figure 3. These molecules are generally made up of smaller ones called monomers, joined together by chemical bonds.

You can see in Figure 3 that carbohydrate polymers are long chains of simple sugar monomers. Starch is an example of a carbohydrate polymer that is made up of lots of glucose molecules joined together. Bananas are full of starch, and they become sweeter the more ripe they are because the starch molecules within them break down into smaller sugars.

Proteins are polymers that are made up of different amino acid monomers bonded together by peptide bonds. There are about 20 standard amino acids that make up proteins in the human body, and they can be arranged in different combinations to form a massive range of different protein polymers.

Lipids are macromolecules that are made up of glycerol and fatty acid subunits. An example of a fat is the triglyceride molecule shown in Figure 3, which consists of one glycerol molecule and three fatty acid tails. Just like carbohydrates and proteins, there are many different fats with different compositions of glycerol and fatty acids.

A useful way of spotting whether something is an enzyme is that enzymes or groups of enzymes often end in -ase. You can sometimes identify which polymer these enzymes break down from the start of the word. For example, carbohydrases break down carbohydrates, proteases break down proteins, and lipases break down lipids.

Be careful, though, as not all enzymes end in -ase and not all enzymes have the substrate they act on in their name either. For example, the amylase enzyme, shown in Figure 4, breaks down starch into maltose, which is then broken down into glucose by another enzyme called maltase. Figure 4 shows an amylase, protease, and lipase working to break down their specific polymer substrates.

Example 2: Explaining How Enzymes Aid Digestion

Which of the following best explains how enzymes aid digestion?

1. Enzymes are released by the gallbladder to neutralize stomach acid.
2. Enzymes regulate the pH of the digestive system to ensure it remains at the optimum.
3. Enzymes slow down the rate of digestion so it does not require too much energy.
4. Enzymes break down large, complex food molecules into smaller ones that can be absorbed.
5. Enzymes release energy to aid the physical processes of digestion, such as chewing.

Answer

We need to take care when answering questions asking for the best explanation. This is because although they are multiple-choice questions, they are not easy as more than one answer may technically be correct.

Enzymes are biological catalysts that lower the activation energy needed for a reaction to occur. This means that more reactions can occur over a set amount time, so enzymes increase the rate of reaction. A typical role that enzymes play is breaking down large polymers into smaller monomers. There are many examples of this in the human digestive system. For example, protease enzymes break down the proteins in food into smaller amino acids. This is helpful because smaller molecules can more easily be absorbed from the digestive tract into the bloodstream. The blood then delivers these smaller molecules to the body cells requiring them.

Mechanical digestion is the process in which food is physically crushed into smaller parts to increase the surface area available for the enzymes to work on. This means that these enzymes can digest the nutrients within food more efficiently. Mechanical digestion is carried out by organs such as the teeth and stomach pushing on the food. Enzymes have no effect on regulating pH or neutralizing acids, although they do have specific optimum pH values and temperatures at which each enzyme works most effectively.

Our correct answer is, therefore, that enzymes break down large, complex food molecules into smaller ones that can be absorbed.

Example 3: Identifying Enzymes and Their Substrates and Products

Complete the table to state the correct enzyme, substrate, and product(s).

Enzyme	Substrate	Product
Protease	Proteins	1
2	Starch	Glucose
Lipase	3	Glycerol and fatty acids

Answer

Enzymes are biological catalysts that lower the activation energy needed for a reaction to occur. This means that more reactions can occur over a set amount of time, so enzymes increase the rate of reaction.

Each enzyme has a different specifically shaped active site. This is because each type of enzyme is complementary to one particular molecule that binds to it, which is called the substrate. When the enzyme catalyzes the reaction, it releases products from its active site, as you can see in the image below.

A role that some enzymes can play is breaking down large macromolecule substrates into smaller subunits. There are many examples of this in the human digestive system, where the large macromolecules from our food bind to the specific, complementary active sites of their specific enzyme. The smaller subunit products that are produced are then released from the enzyme’s active site to be absorbed into the bloodstream.

Protease enzymes break down proteins in food into smaller amino acids. Lipase enzymes break down lipids into glycerol and fatty acids. Amylase enzymes break down starch into maltose, which is then broken down into glucose monomers.

Our correct answers are, therefore, as follows:

1. Amino acids
2. Amylase
3. Lipids

Let’s see where these different enzymes are produced and where they act.

Most of the chemical digestion of molecules using enzymes occurs in the small intestine, and the majority of enzymes involved in the process of digestion in the human body are produced by the pancreas. The pancreas is connected to the small intestine, into which it secretes amylases, lipases, and a protease enzyme called trypsinogen. Trypsinogen is transformed into its active form, trypsin, by another enzyme called enterokinase, which is secreted by the small intestine. The pancreas is called an accessory organ because even though it is vital to digestion, the food does not actually pass through it; it sits just behind the stomach.

Figure 5 shows the positions of the main organs of the digestive system, with the sites of production and action of each enzyme color coded.

Amylase, which breaks down starch into maltose (which is then broken down into glucose), is produced by the salivary glands and pancreas. It acts in the mouth, which is connected to the salivary glands, and in the small intestine. This is shown in green in Figure 5.

Lipases, which break down lipids into glycerol and fatty acids, are produced in the pancreas and act in the small intestine. Lipases are shown in orange in Figure 5.

Proteases, which break down proteins into polypeptides, peptides, or amino acids, are produced in the pancreas, small intestine, or stomach and act in the small intestine and stomach. This is shown in pink in Figure 5.

An example of a protease, pepsin, works in the stomach to break down proteins into smaller units called peptides. Pepsin is one of the very few enzymes that can survive in the stomach, which is full of hydrochloric acid secreted from the stomach lining as part of the gastric juice to protect the rest of the digestive system from disease-causing microorganisms. Therefore, pepsin has a very low optimum pH (1.5–2.5) to be able to work effectively under these acidic conditions. In fact, pepsinogen is secreted from the stomach lining and is only activated and converted into pepsin under the acidic conditions that are provided by the hydrochloric acid.

Another example of a protease is the enzyme trypsin, which, as previously mentioned, is produced via the conversion of trypsinogen by the enterokinase enzyme in the small intestine. Trypsin continues the work of pepsin, by breaking down peptides and polypeptides into amino acid subunits. It has a comparatively higher optimum pH (9) than that of pepsin. These alkaline conditions are provided in the small intestine by pancreatic juice secretions, which contain sodium bicarbonate and neutralize the acidity of any material entering the small intestine from the stomach. Table 1 compares these two protease enzymes.

Table 2 below provides a summary of the substrates, products, and locations of production and action of each enzyme group.

Example 4: Identifying the Source of Digestive Enzymes

The diagram given shows the basic outline of the human digestive system. The pancreas produces and releases a large amount of digestive enzymes. Which number points to the pancreas on the diagram?

Answer

As food passes through the digestive system from the mouth and esophagus, it moves into the stomach first (label 1) and then into the small intestine (label 5), before passing to the large intestine (label 3) and then out of the body.

Digestive enzymes include proteases, lipases, and amylase (a type of carbohydrase). All three of these enzymes are produced by the pancreas. Food does not pass through the pancreas. Therefore, the pancreas is described as an accessory organ and is easier to spot as one of the organs that food does not directly pass through but is still essential to the digestive process. It sits just behind the stomach (label 2).

The pancreas injects the three main digestive enzymes into the top of the small intestine, where they can act to break down large food molecules into smaller nutrients to be absorbed into the bloodstream.

The liver is also an accessory organ but is considerably larger than the pancreas and has another small organ (the gallbladder) attached to it (label 4).

The diagram below shows the organs labeled correctly.

Therefore, the number pointing to the pancreas on the original diagram is 2.

Let’s recap some of the key points that we have covered in this explainer.