Video Transcript
Which of the following statements
would correctly replace X in the Venn diagram? (A) Are exclusively
heterotrophic. (B) Have genetic material that is
mainly stored in plasmids. (C) Have a cell membrane that
contains peptidoglycan. Or (D) have cell walls that contain
silicon dioxide.
A scientist named Whittaker
classified all life into five main kingdoms: Plantae, Animalia, Fungi, Monera, and
Protista. Like the other kingdoms, kingdom
Protista can be divided into several different phyla, including Protozoa,
Euglenophyta, Chrysophyta, and Pyrrophyta. This question shows us a Venn
diagram displaying some similarities and differences between the phyla Chrysophyta
and Pyrrophyta. It asks about one difference in
particular that is unique to Chrysophyta. So let’s talk about these organisms
and what makes them different.
Protists, as well as plants,
animals, and fungi, are all eukaryotic. This means that the majority of
their genetic material is stored in a membrane-bound nucleus within their cells,
which often contain other membrane-bound organelles too. In contrast, prokaryotic cells,
which are exclusively found in kingdom Monera, do not have a nucleus or other
membrane-bound organelles. Instead, their genetic material is
found in the cytosol in the form of circular chromosomes and smaller loops called
plasmids.
Option (B) suggests that the
genetic material of organisms belonging to the Chrysophyta phylum is mainly stored
in plasmids. With the information we have
learned, we can eliminate this option as these organisms are eukaryotic. So the majority of their genetic
material is not stored in plasmids.
As we can see in the Venn diagram,
the organisms that are classified into both of these phyla have cell walls. However, organisms of the kingdom
Protista do not have cell walls made of peptidoglycan. This is another property unique to
some of the organisms in kingdom Monera. Protists certainly do not have cell
membranes containing peptidoglycan. So we can exclude option (C): have
a cell membrane that contains peptidoglycan.
Now that we have excluded two
answers just by checking the characteristics of protists, we can take a closer look
at the differences between Pyrrophyta and Chrysophyta. Organisms belonging to the
Pyrrophyta phylum tend to have photosynthetic pigments, such as chlorophyll, which
likens them to plants, even though they are classified in an entirely different
kingdom. As a result of these pigments, many
Pyrrophyta are often referred to as phytoplankton. This term derives from the Greek
words meaning plant wanderer, as they drift through their watery environment
photosynthesizing much like a plant would.
Another feature that likens these
organisms to plants is that they have cellulose in their cell walls. Many of the organisms that belong
to this phylum are commonly known as dinoflagellates. The suffix -flagellate refers to
the fact that they are motile organisms that use their two flagella for
locomotion. Organisms such as diatoms that are
classified within the phylum Chrysophyta are also usually photosynthetic. As they are autotrophic organisms
like those belonging to the Pyrrophyta phylum, they too are able to make their own
food from simple inorganic materials. Knowing this, we can eliminate
option (A), which proposes that chrysophytes are exclusively heterotrophic. As diatoms are autotrophic
Chrysophyta, the phylum Chrysophyta cannot be exclusively heterotrophic.
Unlike the cellulose cell walls of
pyrophytes, the cell walls of chrysophytes are made up of silicon dioxide and an
organic matrix that allows them to withstand great pressure. With this information, we can
deduce the correct answer to this question. A feature that distinguishes
organisms in the Chrysophyta phylum from those in the Pyrrophyta phylum is (D). Chrysophyta have cell walls that
contain silicon dioxide.
