NCERT Solutions Of Class 11 Biology-Entrancei

How Do Organisms Reproduce Introduction: • •

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The production of new organisms from the existing organism of the same species is known as reproduction.

Reproduction is the creation of new living beings. (From existing living beings) It involves the transmission of genetic material from the parental generation to the next generation.

Process of reproduction is essential for existence and continuity of a species.

Types of Reproduction There are many ways in which new organisms are produced from their parents. All the different ways of reproduction can be divided into two main groups: (a) Asexual • •

Uniparental/Somatogenic.

Some special structure formed for reproduction other than gametes.

Reproduction

(b) Sexual • Biparental • Gamete formation and fusion take place.

Asexual Reproduction Asexual reproduction is the formation of new individuals without involving the formation and fusion of gametes.

Modes of Asexual Reproduction • Fission • Budding • Spore Formation • Regeneration • Fragmentation • Vegetative propagation

(a) • • • •

Fission Many single-celled organisms like protozoa and bacteria just split (or break) into two identical halves during cell division, leading to the creation of new organisms. This is called fission.

In the process of fission, an unicellular organism splits (or divides) to form two (or more) new organisms.

The reproductive unit is the whole body.

It is seen in bacteria and protozoans.

Fission is of the following types:

§ §

Binary fission Multiple fission

Binary Fission The parent organism divides to form two new organisms.

For example, Amoeba reproduces by binary fission, which takes place not in any specific plane (irregular binary fission).

• Longitudinal binary fission § Euglena undergoes longitudinal binary fission. § Leishmania another protozoan undergoes longitudinal binary fission. § Transverse binary fission § Paramecium undergoes transverse binary fission.

1 Parent Cell

3 Cytoplasm Divides

2 Nucleus Divides

4 Two Daughter Cells

Amoeba showing binary fission

Multiple Fission •

In this, the parent body divides into many daughter cells, simultaneously.

• This usually happens under unfavorable conditions.

• For example in Plasmodium, that causes malaria, the parent cell divides into many daughter cells simultaneously by multiple fission.

• When the favorable conditions arrive, the cyst enclosing many daughter cells breaks open, releasing daughter cells.

• Each daughter cell forms a new organism.

• Other example of organisms showing multiple fission: Algae, sporozoans

(b)

Budding

• One or more unicellular or multi-cellular outgrowths, called buds are formed on or inside the parental body.

• These buds develop into tiny individuals and when fully mature, detach from the parent body and become new independent individuals.

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It is seen in sponges, Hydra and yeast.

Budding in Hydra

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Hydra uses regenerative cells for reproduction in the process of budding. A bud develops as an outgrowth due to repeated cell division at one specific site.

Yeast reproduces by budding. In yeast, first a bud appears on the outside of the cell wall. The nucleus of parent then divides into two parts and one part of the nucleus moves into the bud. The budding in yeast often takes place so fast that the first buds start forming their own buds and all of them remain attached to the parent yeast cell forming a chain of yeast cells.

Single Cell

Cytoplasm Nucleus

Vacuole Reserve Food Bodies

Budding

in

yeast cells

(c) •

• • • • • •

Spore Formation In many simple multicellular organisms, the threadlike structures that develop on the bread are the hyphae of the bread mould (Rhizopus).

At the tip of the hyphae tiny blob-on-a-stick structures are present that are involved in reproduction.

The blobs are sporangia, which contain cells or spores that can eventually develop into new Rhizopus individuals.

The spores are covered by thick walls that protect them until they come into contact with another moist surface and can begin to grow.

Thus, spores are a kind of seeds in plants.

Spore formation method is used by unicellular organisms such as bacteria and also multicellular fungi such as Penicillium, Rhizopus, Mushrooms etc.

Non flowering plants like moss and fern also reproduce by spore formation.

The production of new plants from a small piece of plant tissue (or cells) removed from the growing tips of a plant in suitable growth medium (called culture solution) is called tissue culture.

(d) Regeneration • • • • • • •

It is the phenomenon which brings about repair of the damaged cells/tissues or body parts.

Many fully differentiated organisms have the ability to give rise to new individual organisms from their body parts.

That is, if the individual is somehow cut or broken up into many pieces, many of these pieces grow into separate individuals.

For example, simple animals like Hydra and Planaria can be cut into any number of pieces and each piece grows into a complete organism.

Regeneration is carried out by specialized cells.

These cells proliferate and make large numbers of cells.

From the mass of cells, different cells undergo changes to become various cell types and tissues.

• • •

These changes take place in an organized sequence referred to as development.

However, regeneration is not the same as reproduction, since most organisms would not normally depend on being cut up to be able to reproduce.

In complex, multicellular organisms, specialized cells makes up tissue, organs and organ systems. Since they have a high degree of dissociation in their body, they cannot be reproduced from cut body parts by the process of regeneration.

(e) • • • • • • •

(f)

Fragmentation Fragmentation is seen in multi-cellular organisms with relatively simple body organization.

These pieces or fragments grow into new individuals.

The parental body breaks into two or more fragments by wave action or by death and decay of older parts.

For example, Spirogyra breaks up into smaller pieces upon maturation.

It is also seen in sponges, sea anemones and other coelenterates and flatworms.

In echinoderms like starfish, even one arm with a part of central disc can develop into a whole animal.

The main difference between fission and fragmentation is that in fission, a unicellular organism breaks up to form two (or more) daughter organisms. Whereas, in fragmentation, a multicellular organism breaks up to form two or more daughter organisms.

Vegetative Propagation •

There are many plants in which parts like the root, stem and leaves develop into new plants under appropriate conditions.

• In vegetative propagation, new plants are obtained from the parts of old plants (like stem, roots and leaves), without the help of any reproductive organs.

• Vegetative Propagation of following types- (a) Natural and (b) Artificial

Natural • For example in Bryophyllum, buds produced in the notches along the leaf margin fall on the soil and develop into new plants. Also, the green grass grows in the fields after rains from the dry, old stems Leaf of Bryophyllum with buds of grass plants present in the fields, by the method of vegetative propagation.

• Vegetative propagation can be observed in stem of plants too. For example, potato displays

vegetative propagation in its tuber. A tuber is a thickened, underground stem of a plant which is swollen with stored food. The tuber has number of buds (eyes). Potato tuber is an underground stem of the plant.

Potato tuber can be used for the vegetative reproduction of potato plants. Each potato tuber can reproduce more than one plant. Potato has many buds on its body which act as organ for vegetative reproduction. When this tuber with the bud is planted in the soil, then the various buds of the tuber start growing to form new potato plants.

Potato showing its eyes (bud)

Artificial (a) •

• • • •

Grafting In this method, stems of two different plants (one with roots and the other without roots) are joined together, which then grows as a single plant.

The cut stem with roots is called as stock, which makes the lower part of the plant.

The other cut part of stem that is without roots is called as scion, which forms upper part of the plant.

Cambium layer of scion should be in contact with cambium of stock, for the growth of plant to take place.

Grafting method is mainly used for fruit trees like apple, peach, apricot, pear, mango etc.