The size of the living.

LIVINGS

In

BIOLOGY

FOR

GENERAL SECONDARY CERTIFICATE

Unit (I)

Chapter (8)

Reproduction in Livings

Reproduction in livings.

All livings depend on various sources for supplying them with energy that is necessary for their life. However, they stay living on this Earth for a limited period which eventually ends by death. So, they should perform the function of nutrition, respiration, excretion, and sensation in order to succeed in staying alive even shortly on Earth.

Importance of the reproductive function:

Any non breeding organism can maintain life naturally. Moreover, the removal of genital organs from some organisms didn’t affect their normal life. Hence, the reproductive function is less important to the individual life than the above mentioned functions. If any of these functions was impeded, the individual perishes immediately. Accordingly, reproduction depends on all the other functions, and not the reverse. However, it is the function that assures the continuity of species on Earth after death of the individuals. If it is impeded at the population level, extinction of the species occurs. All the organisms start life acting to secure their existence as individuals, they save the necessary energy for growing up to certain stage. Then, they begin acting for maintenance of species through reproduction, to which most of their energy and behavior become directed.

The reproductive capacities among livings.

The reproductive capacities differ among organisms due to:

The various ambient.

Prevalent hardships.

Life nature.

Life duration (their ages).

The size of the living.

So, aquatic organisms produce much more progeny than their land relatives. Also, parasites produce much more progeny than other organisms in order to compensate their loss. Similarly, the production of primitive or short-aged organisms’ progeny’s is much more than higher or long-aged ones since the production of the latter is less endangered, due to the provided care and protection. The species and individuals seen around at present, express the success of their ancestors in reproduction and in overcoming the hardships they face throughout their consecutive generations. Conversely, many of the extinct forms had failed to continue up till now, from these we recall the dinosaurs and other giant reptiles that did not continue in reproduction, and so, became in the record of geological history. Similar fossil forms are known in the animal and plant kingdoms.

Methods of reproduction in livings.

Livings reproduce by many ways and modes in order for species to continue. These modes can be grouped into two main methods:

First: Asexual reproduction:

The isolation of a body part either a spore cell, or many cells, or tissues and their growth into a new organism that fully resembles the original from which the above part has been isolated. So, the features of the following generations remain the same even if the surrounding conditions change. At any change in the environment most of the offspring become exposed to destruction unless their parents had been adapted for that change. This kind of reproduction is common in the plant kingdom, but is limited to some primitive forms of the animal kingdom. This reproduction depends on mitotic division of cells where cells resulting from this division receive complete copy (2n) of the original cell’s chromosomes.

Types of Asexual reproduction:

1. Binary fission:

In which the nucleus divides by mitosis, then the cell that represents unicellular organisms divides into 2 cells, each becomes a new individual. Many Protozoa such as Amoeba and Paramecium as well as Algae, and Bacteria reproduce by this type under suitable conditions. On advent of unsuitable conditions, Amoeba secretes a chitinous coat around its body for protection; however it usually divides within that coat several times by repeated binary fission. This leads to numerous Amoebas that liberate from the cyst upon improvement of the surrounding conditions.

2. Budding:

ٍٍSome unicellular organisms as well as some multicellular ones reproduce by budding.

In unicellular organisms:

As Yeast fungus, the bud arises as a lateral projection from the original cell, while the nucleus divides mitotically into 2 nuclei. One of them remains in the mother cell, while the other moves towards the bud. It grows gradually, and may remain connected with the mother cell till it full growth. Then it separates or continue in connection with the mother cell forming cellular colonies with other growing buds.

In multicellular organisms:

Such as Sponges, and Hydra the bud grows as a cellular protrusion form one side of the body due to division of the interstitial cells and their differentiation to a bud. This grows gradually to resemble the mother entirely. It usually separate to start its independent life. It is to be mentioned that Sponges and Hydra reproduce also sexually besides their capacity for regeneration.

3. Regeneration:

This method is common in many plants and some animals as Sponges and Hydra, some worms, and sea star (starfish). They are able to regenerate the lost parts of their bodies due to an accident or rupture. If the body of some animals is cut into several parts, each part can grow to a new individual.

The capacity for regeneration decreases in higher animals. In some Crustaceans and Amphibians, it is limited to restoration of the cut parts only. In higher vertebrates regeneration never exceeds healing of wounds especially those located in skin, blood vessels, and muscles.

Regeneration in Planaria:

Planaria is a fresh water flat worm that regenerates even if cut into several transverse pieces or 2 longitudinal parts each will grow into a new individual.

Regeneration in Hydra:

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