Video Transcript
Watermelons contain fibers and are rich in water. Which of the following components usually gets absorbed into the blood without being
digested? (A) Sucrose, (B) protein, (C) vitamin A in animal-derived foods, or (D) water.
Most food nutrients that are ingested into the body are too large to be absorbed
immediately, so they need to be digested first. This question asks us to identify which of the listed nutrients can be absorbed into
the blood without digestion. Let’s start by exploring what digestion is, before we address the answer choices
directly.
The term “digestion” describes the process by which large molecules can be broken
down into smaller ones so that they can be absorbed and used by the body. Since humans obtain nutrients such as carbohydrates, lipids, proteins, vitamins,
minerals, and even a lot of water from our food, many of which require digestion
first, this is a very important process. The majority of chemical digestion, which involves enzymes breaking down large
molecules into smaller ones and the subsequent absorption of these smaller
molecules, occurs in the small intestine.
As this question concerns a molecule that is not broken down before it is absorbed
into the blood, let’s take a look at each of the answer choices and determine
whether it can be immediately absorbed or whether it requires digestion first.
Sucrose is a carbohydrate, or sugar, that is naturally occurring in many fruits and
is specifically referred to as a disaccharide. The prefix di- means two and the suffix -saccharide means sugar, referring to the
fact that disaccharides like sucrose are made up of two simple sugars linked
together. These individual units are called monosaccharides, as mono- means one. Each sucrose molecule is made up of a monosaccharide called glucose linked to a
monosaccharide called fructose.
Sucrose is broken down into its constituent subunits, fructose and glucose, in the
digestive system before these smaller monosaccharides are absorbed into the
bloodstream. As we are looking for a component that is not digested before it is absorbed, option
(A) sucrose cannot be the correct answer.
Proteins are fairly large molecules made up of polypeptides, each of which is a long
chain of smaller subunits called amino acids. Proteins must be digested into these amino acids before they can be absorbed into the
bloodstream. So, option (B) is also incorrect.
Vitamin A is not soluble in water like glucose and amino acids are, and as a result
is not absorbed directly into the bloodstream from the small intestine, but instead
absorbed by lymphatic vessels called lacteals. These small vessels eventually rejoin with the bloodstream in order to carry these
important nutrients to all of the cells in the body. As vitamin A is not soluble in water and therefore will not be absorbed into the
blood from the small intestine but into the lymph instead, we can also eliminate
answer choice (C).
Once food has passed through the small intestine, most of the nutrients have been
absorbed into either the lymphatic or circulatory system. The remainder of the food then passes into the large intestine, which mainly
functions to absorb water from food, leaving behind solid feces, which will then be
removed from the body by egestion. As water is not digested but absorbed directly into the bloodstream within the large
intestine, we have found the correct answer to this question. The component that usually gets absorbed into the blood without being digested is
answer choice (D) water.
