Video Transcript
The protist 𝑃𝑙𝑎𝑠𝑚𝑜𝑑𝑖𝑢𝑚 is
a parasite that lives in two hosts, humans and mosquitoes, as shown in the
figure. An alternation of generations can
also be seen in its life cycle. At which stage in the
𝑃𝑙𝑎𝑠𝑚𝑜𝑑𝑖𝑢𝑚 life cycle does fertilization occur? (A) Stage two, (B) stage three, (C)
stage four, (D) stage five, or (E) stage six.
𝑃𝑙𝑎𝑠𝑚𝑜𝑑𝑖𝑢𝑚 is a
unicellular parasitic genus of protists that can infect humans and cause a disease
called malaria. Its life cycle is an example of an
interesting phenomenon called alternation of generations, which is a pattern of
reproduction that involves alternation between a sexual phase and an asexual
phase. Fertilization will occur in the
sexual phase, as it involves the fusion of two haploid gametes to form a diploid
zygote. Let’s take a closer look at the
life cycle of 𝑃𝑙𝑎𝑠𝑚𝑜𝑑𝑖𝑢𝑚 so we can work out when the sexual phase, and
therefore fertilization, occurs.
If we look at stage one of the
diagram provided to us by the question, the first infected mosquito has haploid
𝑃𝑙𝑎𝑠𝑚𝑜𝑑𝑖𝑢𝑚 cells called sporozoites in its salivary glands. When the infected mosquito bites a
human, these sporozoites are transferred to the human host, as shown in stage two of
the diagram.
In stage three, we can see that the
sporozoites infect the human liver cells. There, the sporozoites multiply
asexually through a process called sporogony and develop into haploid
𝑃𝑙𝑎𝑠𝑚𝑜𝑑𝑖𝑢𝑚 cells called merozoites. These merozoites eventually burst
out of the liver cells and infect the human host’s red blood cells, causing the
common symptoms of malaria, which include fever, chills, and sweating. This occurs in stage four of the
diagram.
In the red blood cells, the
merozoites differentiate into female and male haploid gametocytes. The gametocytes are picked up when
a second mosquito bites the infected human and quickly develop into sexual stage
male and female gametes. Once the blood from the bitten
human host enters the second mosquito’s midgut, fertilization of the
𝑃𝑙𝑎𝑠𝑚𝑜𝑑𝑖𝑢𝑚 gametes can occur. This occurs in stage five of the
diagram.
The diploid zygote that is produced
through fertilization of these haploid gametes develops within the mosquito’s midgut
and eventually produces the sporozoites that will migrate to the second mosquito’s
salivary glands. When the second infected mosquito
bites another human, the cycle will begin again, as shown in stage six of the
diagram.
Now we know the stage in the
diagram of the 𝑃𝑙𝑎𝑠𝑚𝑜𝑑𝑖𝑢𝑚 life cycle at which fertilization occurs. It occurs at (D), stage five.
