Q.When Plasmodium sp. (malarial parasite) enters the human body, it reproduces asexually and increases its number. During this course, the affected person suffers from malaria. After a number of asexual cycles, the parasite, in order to complete its life cycle, enters the sexual phase of its life. Trace the sexual stages in the life cycle of Plasmodium in a female Anopheles mosquito, when it bites an infected person, up to the formation of mature infective stages of the parasite.

The key idea is the sexual cycle of Plasmodium inside the female Anopheles mosquito, which begins when the mosquito takes a blood meal from an infected human.

1. When the mosquito bites an infected person, it ingests gametocytes (the sexual forms: microgametocytes and macrogametocytes) from the human blood.
2. In the mosquito’s gut, the gametocytes mature: microgametocyte produces male gametes (exflagellation), and macrogametocyte forms the female gamete. Fertilisation occurs, forming a zygote.
3. The zygote becomes a motile ookinete, which penetrates the mosquito’s gut wall and forms an oocyst on the outer surface. …

The key to understanding this part of the Plasmodium life cycle is to remember that the mosquito is the definitive host — the place where sexual reproduction occurs. The human is just the intermediate host where asexual multiplication happens. So when the mosquito takes a blood meal from an infected person, it isn't just getting a meal; it's picking up the parasite's sexual forms that have been waiting in the human blood.

Let's trace exactly what happens, step by step, inside the mosquito.

1. Ingestion of gametocytes. When a female Anopheles mosquito bites a person suffering from malaria, it sucks up blood that contains the asexual stages of Plasmodium. But critically, it also ingests the gametocytes — the sexual forms. These are of two types: microgametocytes (male) and macrogametocytes (female). These gametocytes are the only stages that can survive and develop inside the mosquito's gut.

2. Formation of gametes in the mosquito's gut. Inside the mosquito's stomach (midgut), the change in environment — a drop in temperature and a rise in pH — triggers the gametocytes to mature. The macrogametocyte develops into a single, large macrogamete (the female gamete). The microgametocyte undergoes a process called exflagellation, where its nucleus divides rapidly and it produces 4–8 flagellated microgametes (male gametes). These microgametes are motile and swim towards the macrogamete.

3. Fertilization — formation of the zygote. One microgamete fuses with the macrogamete inside the mosquito's gut. This fusion produces a zygote — a diploid cell. This is the only diploid stage in the entire Plasmodium life cycle. The zygote is non-motile at first.

4. Development into the ookinete. The zygote soon elongates and becomes motile, transforming into a ookinete (pronounced "oh-oh-kin-eet"). The ookinete is a worm-like, motile form that penetrates the wall of the mosquito's midgut. It wriggles through the epithelial cells and comes to rest on the outer surface of the gut wall, just beneath the basement membrane.

5. Formation of the oocyst. Once settled under the gut wall, the ookinete rounds up and secretes a tough, protective cyst wall around itself. This structure is now called an oocyst. Inside the oocyst, the parasite undergoes sporogony — a type of asexual multiplication. The nucleus divides repeatedly, and the cytoplasm segments, producing thousands of tiny, spindle-shaped cells. …