HOW DO ORGANISM REPRODUCE

 ORGANISMS :-

An organism refers to a living thing that has an organized structure, can react to stimuli, reproduce, grow, adapt, and maintain homeostasis. An organism would, therefore, be any animal, plant, fungus, protist, bacterium, or archaeon on earth. These organisms may be classified in various ways. One of the ways is by basing upon the number of cells that make it up. The two major groups are the single-celled (e.g. bacteria, archaea, and protists) and the multicellular (animals and plants). Organisms can also be classified according to their subcellular structures. Those with a well-defined nucleus are referred to as eukaryotes whereas those without are called prokaryotes. Both of them possess a genetic material but the location differs. In eukaryotes, the genetic material is found inside the nucleus whereas, in prokaryotes, it is located in a special region called a nucleoid.

Asexual Reproduction

• It involves only one parent.
• There is no formation and fusion of gametes.
• The young ones formed are almost identical to each other as well as to the parent cell.
• Asexual reproduction generally occurs during favourable environmental conditions and when there is an abundance of food.
• It is a faster method of reproduction.            
  
  
  

Types of Asexual Reproduction is Unicellular Organism
(i) Binary Fission: Seen in bacteria, protozoa like Amoeba, Paramecium. (In these first pseudopodia withdrawn (karyokinesis) the nucleus of the parent cell divides and then the cytoplasm divides (cytokinesis) resulting in the formation of two daughter cells). It occurs during highly favourable conditions. The cell division can occur in any plane as in case of Amoeba. However, organisms like Leishmania. (cause Kala-azar), which have a whip like flagella at one end, binary fission occurs in a definite orientation in relation to the flagellum.

Cytokinesis: Division of cytoplasm.
Karyokinesis: Division of Nucleus.

(ii) Multiple Fission: Seen in Plasmodium, (a malarial parasite). In this during unfavourable conditions, the parent cell develops a thick resistant wall around itself forming a cyst. Within the wall, the cytoplasm divides many times to form many plasmodia. When conditions become favourable, the cyst wall breaks and the Plasmodium are released.

(iii) Budding: Seen in Yeast (a fungus). The parent yeast cell develops a protrusion or an outgrowth at its upper end. The nucleus of the parent cell divides and one of them moves into the outgrowth which grows bigger and finally separates from the parent cell to lead an independent existence. Very often if the conditions are highly favourable, a chain of buds is formed.

Types of Asexual Reproduction in Multicellular Organisms :
(i) Fragmentation: Seen in multicellular organisms which have a relatively simple body organisation like Spirogyra. Spirogyra has a filamentous body. (If it breaks into smaller pieces or fragments). Each fragment has the capacity to form a new individual.
However, all multicellular organisms cannot show cell-by-cell division as cells from tissues which form organs. These organs are placed at definite positions in the body. Hence, they need to use more complex methods of reproduction.

(ii) Regeneration: It is the ability of organisms to develop their lost parts. Some organisms show have high regenerative capacity it is also a means of reproduction for example; Planaria. (Regeneration is carried out by specialized cells which redivide to form a mass of cells from which different cells undergo changes to become different cell types and tissues. These changes occur in an organized sequence known as development).

(iii) Budding: Seen in Hydra. Parent Hydra develops a bud at its lower end. This grows in size and finally breaks off to live independently.

(iv) Spore Formation: Seen in Rhizopus (a fungus). Rhizopus body is made up of thread-like structures called hyphae. The erect hyphae bear sporangia inside which reproductive structures called spores are formed. Spores are asexually reproducing bodies having a thick protective wall. They are produced during unfavourable times and help to tide over the unfavourable environmental conditions. When the spores fall on a suitable medium, each one forms a new individual.

(v) Vegetative Propagation: Method by which plants reproduce by their vegetative parts such as roots, stems, and leaves.

Types of Vegetative Propagation: It is two types

• Natural vegetative propagation.
• Artificial vegetative propagation (Tissue culture).                                                              
  
  
  

Mint reproduces naturally by roots. Sugarcane, jasmine by stems and Biyophyllum by leaves. In biyophyllum buds are produced in the notches along the leaf margins and when they fall on the soil, they develop into new plants.

Importance of Vegetative Propagation

• Plants can bear flowers and fruits earlier.
• Plants which have lost the ability to produce viable seeds can also reproduce by vegetative propagation.
• All plants are genetically almost similar to the parent plant.
• Seedless varieties can be obtained.
• The property of vegetative propagation is used by horticulturists in developing methods like layering, grafting to grow many plants like sugarcane, roses, or grapes.

Tissue Culture: The technique of developing new plants from a cell or tissue in a nutrient medium under aseptic conditions. The cell or tissue is placed in a nutrient medium where it forms a mass of cells called callus. This callus is then transferred to another nutrient medium where it differentiates and forms a new plant.

Sexual Reproduction: Sexual reproduction in plants, Sexual reproduction in human beings. The mode of reproduction that takes place with the involvement of two individuals of two different sexes i.e. male and female.
During sexual reproduction, male organism having male sex organs produces male gametes i.e. sperms which are small and motile and the female organism having female sex organs produces ova which are generally large and store food. Male and female gametes fuse to form a zygote that grows into a new organism.

Significance of Sexual Reproduction :

• Sexual reproduction involves DNA as well as cellular apparatus of two different organisms which promotes diversity of characters in the offspring.
• Since gametes are derived from two different organisms, it results in a new combination of genes which increases the chances of genetic variations.
• Sexual reproduction results in the origin of. new species.
• Sexual reproduction involves division in the sex organs that reduces the DNA matter to half so that the zygote formed after fusion has the same amount of DNA as the parents it maintains DNA in a species.

Limitation of Sexual Reproduction: Sexual reproduction involves the process of combining DNA from two different organisms which may bring some undesirable features also.

Sexual reproduction in flowering plants

• The reproductive parts are present in the flower.
• The parts of the flower are sepals, petals, stamens and carpels.
• Sepals are green structures that protect the inner parts when the flower is in bud stage.
• Petals are colourful and attract the insects for pollination.
• Stamens are male reproductive parts and produce pollen grains that contain male gametes. Each stamen has two parts—
• Filament i.e. stalk and Anther i.e. swollen top part which has large number of pollen grains.

The carpel is the female reproductive part and produces ovules that contain female gametes. It has three parts—Stigma which is top sticky part and receives pollen grains during pollination. Style which is the middle long part and ovary which is the swollen part and contains ovules. Each ovule has an egg cell i.e. female gamete.

The flowers may be bisexual i.e. having both stamens and carpels for example; Mustard China Rose (Hibiscus).
The flower may be unisexual i.e. paving either stamens or carpels for example; Papaya, Watermelon.

Pollination: The process of transfer of pollen grains from an anther to the stigma of the flower is pollination. Two types of pollination are:

(i) Self-pollination: The transfer of pollen grains from the anther to the stigma of the same flower or another flower of the same plant.
(ii) Cross-pollination: The transfer of pollen grains from the anther to the stigma of another flower or another flower of a different plant of the same species. It generally takes place with the help of some agents like insects, birds, wind and water.

Fertilization: Fertilization is the process of fusion of male and female gamete to form a zygote during sexual reproduction. Pollination is followed by fertilisation in plants. The events are
Pollen grains land on the stigma of the ovary.
Pollen tubes grow out of the pollen grains, travel through the style and reach the ovary, through micro pyle.
Pollen tube has two male germ cells. Each ovule has two polar nuclei and a female germ cell (egg).

Pollen tube releases two male germ cells inside the ovule, one of them fuses with female germ cell and forms a zygote which grows into the baby plant i.e. embryo, the fusion is known as syngamy. The other male germ cell fuses with two polar nuclei, the process is known as triple fusion. So in flowering plants two fusions take place during fertilisation. It is called double fertilisation.

Post-fertilisation changes: After fertilisation the following changes takes place in the flower.
Zygote divides several times and forms an embryo inside the ovule.

• The ovule develops a tough coat and changes into the seed.
• The ovary grows rapidly and ripens to form a fruit.
• Petals, sepals, stamens, style and stigma shrivel and fall off.

Seed and its parts: The advantage of seed is that it protects the future plant i.e. embryo.

Seed has two parts: Cotyledons and Embryo Cotyledons store food for the future plant.

Embryo has two parts: plumule and radicle. Plumule develops into shoot and radicle develops into root.

The process of development of a seedling from the embryo under appropriate conditions is known as germination.