Video Transcript
Which of the following is not an
adaptation of the stomach for digestion? (A) The stomach has a low pH that
is optimum for the activation of pepsin. (B) The stomach produces
hydrochloric acid that helps break down food and pathogens. (C) The epithelial cells of the
stomach produce a mucus lining that acts as a protective layer. (D) The stomach produces and stores
bile to aid the breakdown of fats. Or (E) the body of the stomach is
composed of muscle that allows the stomach to mechanically churn food.
This question is asking about how
the stomach is adapted for its job and to select the answer choice which is not a
correct adaptation. To answer this question, we need to
go over some key facts about digestion and the stomach’s role in this process. To do this, let’s remove the answer
choices for now so we’ve got some more space to work with.
Digestion is the process by which
we break down large molecules into smaller ones that can be absorbed and used by the
body. There are two main types of
digestion: mechanical digestion and chemical digestion.
Mechanical digestion is the
physical act of breaking down food by nonchemical means, for example, using muscles
like the tongue or hard structures like the teeth to chew food into smaller chunks
with a comparatively larger surface area.
Chemical digestion is when food
molecules are broken down by chemicals, usually enzymes, into their constituent
subunits. Enzymes are released at various
points along the digestive tract, including in the stomach. And mechanical digestion can help
this process as it provides the enzymes with a larger surface area upon which they
can act.
With this knowledge of the
different types of digestion, let’s return to the functions and adaptations of the
stomach specifically.
The stomach receives food from the
mouth, and this food may contain bacteria and other potentially dangerous
pathogens. Hydrochloric acid, or HCl for
short, is secreted from cells in the stomach lining for two main purposes, one of
which is to try and kill these pathogens to prevent them from entering the blood or
moving further through the digestive tract into the intestines. This acid is also important for
providing the optimum low pH for the enzymes which are secreted for chemical
digestion of proteins in food to occur.
One of the enzymes that’s involved
in chemical digestion in the stomach is a type of protease secreted in an inactive
form called pepsinogen, which is converted to its active form pepsin in the presence
of hydrochloric acid. Pepsin can then break down protein
molecules in food.
Cells in the body are partly made
of protein. So the stomach wall also secretes a
mucus layer to protect the cells of the stomach wall from being digested by
pepsin. The stomach wall is muscular. This helps with mechanical
digestion as the muscles contract and churn the chyme contained within it, which is
a mixture of food and gastric, or stomach, juices.
Now we have reviewed some key facts
about digestion in the stomach, let’s bring back the answer choices and return to
the question. Which choice is not an adaptation
of the stomach for digestion?
First, we can rule out answer
options (A) and (B), as we know that hydrochloric acid is secreted to kill pathogens
and convert inactive pepsinogen into the active enzyme pepsin. We can also rule out option (C), as
mucus needs to be secreted to protect the cells of the stomach from being digested
by pepsin. We can eliminate option (E) because
we now know that the muscular walls are used to mechanically churn the chyme in the
stomach.
This leaves option (D), which
states that the stomach produces and stores bile to aid the breakdown of fats. This is not a role of the stomach,
but rather of two other digestive organs, respectively, the liver and the
gallbladder. This means the option which is not
an adaptation of the stomach for digestion is (D). The stomach produces and stores
bile to aid the breakdown of fats.
