Page 126 - Biology_F5

Comparative studies of natural groups of organisms

(c) They change their shapes depending Life cycle of Plasmodium sp.
on the host it inhabits. For instance, The parasite in the form of sporozoites
plasmodium is sickle shaped in enters the blood stream of the human
salivary glands of mosquito, while it is being after a mosquito bite. It then travels
amoeboid in liver cells of human. to the liver and invades it (Figure 3.15).

The sporozoites grow, divide and produce
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Structure of Plasmodium sp. many haploid forms called merozoites or
The Plasmodium is oval shaped, and schizozoites in liver cells. The merozoites
lacks contractile vacuoles and locomotory in the liver cells exit and re-enter blood
organs. The apical end is a bridged cone- stream, where red blood cells are invaded.
shaped projection demarcated by the polar The merozoites multiply in the red blood
rings. A single mitochondrion is generally cells by asexual reproduction. In the red
present at the posterior end and the Golgi blood cells, they develop into schizonts
apparatus is unremarkable. Just beneath (feeding stage), which rupture the cells,
the inner membrane, there is a row of releasing newly formed merozoites which
microtubules, which originate from the then invade other red blood cells. Some
of the merozoites in infected blood cells,
polar end of the apical end and continue to leave a cycle of asexual replication.
the posterior end. Attached to the nucleus, Instead of replicating, the merozoites
there is an endoplasmic reticulum, a in these cells develop into sexual forms
network of membranous tubules within of the parasite called male and female
the cytoplasm (Figure 3.14). Additionally, gametocytes. The cycle in mosquito
there are rhoptries, which are the starts when the mosquito bites an infected
specialised secretory organelles. human and ingests gametocytes. The male
and female gametes fuse to form diploid
zygote, which develops into moving
Golgi apparatus ookinete. The ookinete burrows in the
Nucleus midgut wall of mosquito, forming oocyst
on the other side. Growth and division
Endoplasmic reticulum
of each oocyst produce numerous active
Mitochondrion haploid forms called sporozoites. After

8 to 15 days (depending on the species),
Rhoptries the oocyst bursts, releasing sporozoites
into the body cavity of the mosquito,
Microtubules from where they travel to, and invade
the mosquito salivary glands. The cycle
of human infection re-starts when the

Figure 3.14: Structure of a Plasmodium sp. mosquito taking a blood meal injects the
sporozoites from its salivary glands into
the human blood stream.

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