Video Transcript
Which of the following statements
about the pH of the digestive system is correct? (A) The stomach is highly acidic,
and the small intestine is mildly alkaline. (B) The stomach is highly alkaline,
and the large intestine is mildly acidic. (C) All organs of the digestive
system have a neutral pH. Or (D) the small intestine is
highly acidic, and the large intestine is highly alkaline.
This question is asking about the
pH levels found in various organs of the digestive system, specifically the stomach,
small intestine, and large intestine. So that we have a bit more space to
work this out, let’s remove the answer options for now.
Digestion is the process by which
large molecules are broken down into smaller molecules that can be absorbed into the
blood or other transport systems to be moved to the body cells that require
them. Chemical digestion is a specific
type of digestion that involves chemicals called enzymes, which catalyze the
breakdown of these large molecules into their smaller subunits.
Enzymes are proteins, and they have
a very specific shape with an active site that only fits one particular type of
molecule, which is called a substrate. Different enzymes have different
optimum pHs at which their activity is most efficient. And if the pH of their environment
is too far above or below that enzyme’s optimum pH, its active site irreversibly
changes shape and the enzyme cannot catalyze the breakdown of the substrate.
Let’s take a look at the stomach,
small intestine, and large intestine and any enzymes that will be acting there to
see how their optimum pHs differ and therefore what the pH in those different organs
must be for these enzymes to efficiently carry out chemical digestion.
The stomach is the first of the
organs named in the question that food will travel through on its way through the
digestive tract, though it’s not the first organ where enzymes will be acting. Before food reaches the stomach,
amylase enzymes are secreted from the salivary glands into the mouth, where they
help to break down starch into maltose. The optimum pH of amylase enzymes
is fairly neutral, around 7.4. Protease enzymes catalyze the
digestion of proteins into polypeptides and peptides in the stomach. The most common protease is an
enzyme called pepsin, which is secreted in an inactive form called pepsinogen. A very low and acidic pH of between
1.5 and 2.5 is required to convert inactive pepsinogen into the active enzyme
pepsin. This low pH is provided by
hydrochloric acid that is secreted into the stomach.
In the small intestine, proteases
digest polypeptides and peptides into amino acids, carbohydrases digest maltose into
glucose, and lipases digest lipids into fatty acids and glycerol. In the duodenum, which is the first
part of the small intestine where the label is pointing and these enzymes start to
act, the optimum pH is around six to 7.4. This pH is far less acidic than the
pH of the stomach and is achieved by neutralizing secretions of bile from the
gallbladder, originally produced by the liver, and sodium bicarbonate from the
pancreas to make the small intestine mildly alkaline.
Unlike the small intestine, the
large intestine does not produce any enzymes, nor will any enzymes be acting here to
digest food any further. So while the mouth and small
intestine are mildly alkaline, the stomach is highly acidic.
Using this information, we can
eliminate option (B), as we know that the stomach is acidic and not highly
alkaline. We can also rule out option (C), as
the organs of the digestive system do not all have a neutral pH. Some are highly acidic, while
others are slightly alkaline. The small intestine is not one of
the organs that is highly acidic however, so we can also eliminate option (D).
Now we know the correct answer to
the question. The statement that is correct about
the pH of the digestive system is (A). The stomach is highly acidic, and
the small intestine is mildly alkaline.
