The picture shows the cells of an
organism belonging to the kingdom Plantae under a microscope. The walls surrounding the cells are
visible. What is the main component of these
walls? (A) Glucose, (B) chitin, (C)
cellulose, or (D) peptidoglycan.
This question is asking us about
the structure of plant cell walls. So let’s start by reviewing some of
the facts about the kingdom Plantae. The kingdom Plantae is one of the
five kingdoms that organisms can be classified into. Nearly 400000 different species of
plants have been identified. But they are unified by having
certain characteristics in common. Organisms in the kingdom Plantae
are eukaryotes, which means their cells contain a membrane-bound nucleus and other
membrane-bound organelles. They also have a cell wall around
the cells, which is utilized for structure and support.
So what are these cell walls made
of if they can perform these functions? Let’s have a look. The Plantae kingdom is not the only
kingdom that utilizes cell walls. Fungi, most species of Monera, and
some protists also have these structures. However, they are made of different
materials. Fungi and fungi-like protists have
cell walls made of a very strong polysaccharide called chitin, the same substance
that is found in the exoskeleton of insects. Monera have cell walls made of
peptidoglycan, a substance made of sugars and amino acids. The cell walls of plants and the
plant-like protists are made up of a strong polysaccharide called cellulose.
Poly- means many. So cellulose is made up of many
single sugar molecules, linked together to form fibers. These fibers then bind together to
form a tough cell wall. This provides structure and
Let’s therefore return to our
question which is asking what is the main component of plant cell walls. We now know that chitin is found in
the cell walls of fungi and peptidoglycan is found in the cell walls of Monera. So answers (B) and (D) are
Next, let’s discuss option (A). Glucose is a monosaccharide, which
is made during the process of photosynthesis. It is then used by plants for
respiration or to make other substances, such as fructose, the sugar found in
fruits; sucrose; and cellulose, to name just a few. As it is a simple, soluble sugar,
it cannot be used for structural purposes, so we can eliminate this option.
We now know that the correct answer
is (C). The main component of cell walls of
organisms belonging to the kingdom Plantae is cellulose.