Video Transcript
What is the primary purpose of
digestion in the human body? (A) To transport essential
nutrients and oxygen around the body. (B) To coordinate and control the
body’s responses to internal and external stimuli. (C) To regulate cellular metabolism
and respiration. (D) To break down large molecules
into smaller, soluble ones that can be absorbed by the body. Or (E) to maintain a constant
internal environment.
Let’s take a look at why digestion
is so important to the human body so that we can answer this question correctly.
Animals like humans obtain all
vital nutrients by ingesting them as a part of the food that makes up our diet. However, many of these nutrients
are far too large to be absorbed immediately in a form that can be transported
around our bodies and used by our cells. First, they need to be broken down
into a smaller form. This is the main function carried
out by our digestive system.
For example, food first enters the
mouth, where enzymes will break down large carbohydrates like starch in our food
into smaller sugars like maltose, while our teeth and tongue mash up the food making
it easier for these enzymes to act upon. Food then travels to the stomach,
where different enzymes break down large proteins in our food into smaller
polypeptides. Food then moves into the small
intestine, where various enzymes break down the sugars like maltose into even
smaller ones like glucose and break down the polypeptides into amino acids.
Now, these molecules are small
enough to be absorbed across the wall of the small intestine and into the
bloodstream. The blood can then transport the
sugars and amino acids to the body cells that require them, where they can be
reconstructed into a huge range of different carbohydrates and proteins for our
various cellular functions.
Sugars and amino acids are water
soluble, which means that they are easy to transport in blood plasma as 90 percent
of it is water. However, it is important to note
that not all of the products of digestion of the large molecules in food are water
soluble.
Lipids, for example, are broken
down in the small intestine into fatty acids and glycerol, which are much smaller
than lipids but are still not water soluble. For this reason, these products,
some undigested lipids, and some fat-soluble vitamins are not absorbed directly into
the bloodstream from the small intestine but instead into the lymphatic system,
which empties them into the bloodstream at a larger junction.
By looking at these examples, we
can see that digestion is the process by which large molecules in our food are
broken down into smaller molecules that can be absorbed and used by the body
cells. This suggests that the primary
purpose of digestion in the human body, and therefore the best answer to this
question, is (D): to break down large molecules into smaller, soluble ones that can
be absorbed by the body.
