Biology Reproduction in living organisms 3 ثانوية عامة

Biology | Reproduction in living organisms 3
Alternation of generations

- The life cycle of the living organisms includes two generations, one as the result of sexual reproduction while the other as the result of asexual reproduction as some plants and animals species.

- They gain from both methods their advantages of rapid production and genetic variation.

- These enable them to spread widely and to adapted with the change in the environmental conditions.

Examples of Alternation of generations

1. Life cycle of Plasmodium (Malaria parasite)
2. The life cycle of Schistosoma (Bilharzia worms).
3. The life cycle of ferns as Polypodium & Adiantum



1- Life cycle of Plasmodium (Malaria parasite)

- Plasmodium is Protozoan parasite infects both man and female Anopheles mosquito.

- Its life cycle starts when the infected mosquito bites human skin, it pours Sporozoites (small spindle shape) in his blood.

- Sporozoites move towards, the liver where they spend an incubation period during which nuclei divide by Schizogony giving several Merozoites by 2 cycles of asexual reproduction.

- Merozoites migrate to the blood infecting the red cells where they divided many times producing huge numbers of Merozoites that are released together every 2 days with the destroyed red blood cells causing the symptoms of Malaria fever appear on the patient as heat, chill and sweating.




- Some of Merozoites change into inactive gametocytes in the human blood.

- When the another mosquito bites the patient skin, gametocytes migrate to the mosquito with patient’s blood where they activated and develop to gametes in mosquito’s stomach.

- Gametes fuse into a zygote (2N) that changes into Ookinete(2N), which penetrates into stomach wall ÷d by meiosis (meiotically) producing, Oocyst(1N).

- The nucleus of Oocyst is divided mitotically producing great number of Sporozoites ( asexually by Sporogony).

- Sporozoites liberate and move towards the mosquito’s salivary glands to be ready for human infection



2- Life cycle of a fern plant

From the common ferns:
1- Polypodium is an ornamental plant.
2- Adiantum which grows on well edges and shaded streams.


Biology | Reproduction in living organisms 3

The life cycle of Polypodium plant
- It starts by the “sporophyte” which carries the leaves.

- the lower surfaces of the leaves have the sori, which contain great number of spore mother cells (2N), which divide by meiosis giving the spores (N).

- The mature spores are released by winds to far distances until reach a wet soil, the spore germinates forming several cells that develop to a flat heart-shaped body called “gametophyte” that grows over the soil.





-The lower surface of the gametophyte produces rhizoid processes that penetrate into the soil to absorb water and salts.

-The anterior region of gametophyte carries the genital organs,

-The male organs are Antheridia while the female organs are Archegonia.

- Antheridia produce ciliated sperms (male gametes) liberate and swim over soil water to reach the mature Archegonia for fertilizing its egg forming the zygote (2N).

-The zygote divides and differentiates into a new sporophyte that grows over the gametophyte and depends on it until developing its own roots, stem and leaves.





- The gametophyte then degenerates while the sporophyte grows to repeat the life cycle. reproduces asexually

- So the sporophyte (2N) that alternate with the gametophyte (N) that reproduces sexually in the life cycle of ferns.

- Polypodium represents a typical example of alternation of generations in the living organisms.

3. Life cycle of Schistosoma (Bilharzia).

- The life cycle of Schistosoma is an example of alternation of generations in the animals



- The worms reproduce sexually in man and asexual in the snails as intermediate host the two methods of reproduction enable the parasite to face the life of parasitism.