Video Transcript
Undigested food is passed to the
large intestine. What is the main function of the
large intestine? (A) To absorb water and salts from
undigested food. (B) To absorb digested food into
the bloodstream. (C) To break down carbohydrates and
proteins into their monomers. (D) To emulsify fats. Or (E) to release the digestive
enzymes.
The answer options give us a lot of
different functions of various components of the digestive system. However, the question is asking us
specifically about the function of the large intestine. So, let’s have a look at the
different functions of various parts of the digestive system to see which one is
correctly identifying the large intestine. This diagram shows us a simplified
view of the human digestive system, with the large intestine, our organ of interest,
shown in pink.
Food initially enters the mouth,
into which digestive enzymes are secreted. These enzymes start to catalyze the
breakdown of the large nutrients in food. In the mouth specifically, this
will include breaking down large carbohydrates into smaller molecules. Food then passes down the esophagus
and into the stomach. Different enzymes are secreted into
the stomach to those that acted in the mouth. The enzymes in the stomach are
responsible for starting the breakdown of proteins in food. The food then passes from the
stomach into the first section of the small intestine. At this point, even more enzymes
are secreted from an organ called the pancreas into the small intestine. These enzymes secreted by the
pancreas can break down proteins, carbohydrates, and lipids in the small
intestine.
Let’s label the other structures
that we’ve mentioned so far. By this point in digestion, all of
the enzymes that will be acting to break down food will have already been released,
but we haven’t yet reached the large intestine. This is because by the time food
reaches the end of the small intestine, all of the chemical digestion, which is that
using enzymes, has already occurred. So, we can eliminate one of our
answer options as we know that the large intestine is not responsible for releasing
digestive enzymes, as none of them will be acting there. We can also therefore tell that it
is not the job of the large intestine to break down carbohydrates and proteins into
their monomers.
There are still a couple of organs
in our diagram that we haven’t explored yet. This organ is called the liver. One of the functions of the liver
is to produce a substance called bile, which is secreted into the small
intestine. The role of bile is to emulsify
fats, which makes them easier for digestive enzymes to break down. This process will be occurring in
the small intestine, but not in the large intestine. So, we can eliminate another one of
our answer options.
We’ve already learned how one of
the roles of the small intestine is to break down nutrients using digestive
enzymes. Another role of the small intestine
is to absorb the subunits that are formed as a product of this digestion of large
nutrients. This digested food is absorbed into
the bloodstream. By the time food reaches the large
intestine, most of the nutrients have already been absorbed into the
bloodstream. Therefore, we can deduce that this
is not the main function of the large intestine.
Instead, the role of the large
intestine is to absorb water and salts from undigested food into the blood. It’s also responsible for the
defecation of any undigested food that remains after the food has passed through the
large intestine. Therefore, we’ve worked out that
the main function of the large intestine is (A), to absorb water and salts from
undigested food.
