Video Transcript
Which of the following organs does
not produce any of the digestive enzymes? (A) Liver, (B) pancreas, (C) small
intestine, (D) salivary glands, or (E) stomach.
This question asks about digestive
enzymes. So let’s begin with a quick review
of what enzymes are.
An enzyme is a biological
catalyst. You probably know that the prefix
bio- means life and a catalyst is a substance that speeds up chemical reactions,
which helps us understand that an enzyme is produced by living organisms to speed up
reactions in the body. Enzymes speed up reactions by
lowering the activation energy needed to start the reaction. These two graphs show the
activation energy needed with and without an enzyme. We can see that the activation
energy is higher in the left-hand graph than in the right-hand graph. So more energy is needed to start a
reaction without an enzyme than when an enzyme is present.
You might be wondering how enzymes
lower the activation energy of reactions. Enzymes are globular proteins with
a special area called an active site. The active site is complementary in
shape to a specific substrate. In biology, “complementary” means
two molecules will fit together like pieces of a jigsaw puzzle.
In the first step of a reaction, a
substrate molecule will bind to the enzyme’s active site to form an enzyme–substrate
complex. The reaction can now proceed, and
afterwards the enzyme will release the products. You may have noticed in the diagram
that the enzyme is not changed or used up in the reaction. This is important because it means
a given molecule of an enzyme can be reused again and again.
Digestive enzymes break large
insoluble food molecules down into small soluble ones so they can be absorbed and
used by the body. Some types of digestive enzymes are
proteases, which break down proteins; carbohydrases, which break down carbohydrates;
and lipases, which break down lipids. To answer our question about where
digestive enzymes are made, let’s follow the path food takes through our digestive
system.
Digestion begins in the mouth,
where salivary glands release the digestive enzyme amylase to start breaking down
the carbohydrate starch. This means we can rule out the
salivary glands as a correct answer. Food is then swallowed and travels
into the stomach. Here, the main enzyme released is
pepsin, which breaks down proteins. So we can rule out the stomach as
an answer as well.
As the food moves into the small
intestine, it passes by the liver, which produces bile. Bile neutralizes the acidic mixture
from the stomach and emulsifies lipids. That is, it breaks large fat
droplets down into smaller ones, similar to how dish soap breaks up fat globules on
your greasy cooking pans. Bile is not an enzyme because it
does not decrease activation energy of a reaction to produce a new product. Instead, it reduces lipid droplet
size, which increases their surface area to facilitate their digestion by
lipases.
During the final stages of
digestion, cells of the small intestine and pancreas release carbohydrases,
proteases, and lipases. The small intestine and pancreas
can therefore be ruled out.
This leaves us with the answer
choice (A), the liver. We have learned that the liver
makes bile, which is important for digestion but is not an enzyme. So the organ that does not produce
any digestive enzymes is the liver.