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Reproduction in Unicellular and Multicellular Organisms

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Reproduction in Unicellular and Multicellular Organisms - Lesson Summary

Reproduction
Reproduction is the phenomenon which involves the production of an offspring by particular individual or individuals to propagate their species. Reproduction is done during reproductive phase.  

Reproduction based on the number
Unicellular organisms are single celled forms whereas multicellular organisms are made up of many cells. Depending upon the complexity of the body, reproduction in unicellular organisms is different from that of multicellular organisms. 

  • Unicellular organisms reproduce by asexual means. This asexual mode of reproduction involves single parent to produce their offspring. Different asexual modes of reproduction include binary fission, multiple fission, fragmentation, budding etc.
  • Multicellular organisms can chose both asexual and sexual modes to reproduce their offspring. The mode of reproduction by organisms depends upon the favourable and unfvourable conditions prevailing.

Types of reproduction 
Reproduction can be of two different types, namely, asexual reproduction and sexual reproduction.

  • Asexual mode of reproduction: It is a mode of reproduction in which a single individual is responsible for creating a new generation of species. Reproductive structures are not involved. Vegetative parts of individuls are used for the process of reproduction. Gametes are not formed. Single parent cell gives rise to daughter cell. Offspring formed are exact individuals of the parents
  • Sexual mode of reproduction: It involves the union of two opposite sex cells by the process of fertilisation to give rise to zygote, the single cell. This type of reproduction involves two organisms, the male and the female. Reproductive organs in humans produce gametes - eggs and sperms. Egg is the female gamete produced by female reproductive organ. Sperms are male gametes produced by male reproductive organ. A zygote is the future individual formed by the fusion of an egg and a sperm.


Types of asexual reproduction 
Different asexual modes of reproduction by which offspring can be produced are binary fission, budding, fragmentation, regeneration, vegetative propagation, spore formation etc.

1) Binary fission: It involves the longitudinal or transverse splitting of an organism into two equal halves which develop into two separate individuals.

  • Binary fission is generally seen in unicellular organisms such as amoeba and paramecium falling into the category of protozoa.
  • Amoeba is a simple, unicellular organism which reproduces by transverse binary fission. The division begins with the division of nucleus.
  • Binary fission can also be observed in multicellular animals like sea anemones and planarians. 
  • Binary fission observed in Leishmania is longitudinal binary fission. Longitudinal binary fission is the division occurring in a definite orientation in relation to the whip-like structures located at one end of the cell. 
2) Multiple fission: Multiple fiision is the process by which organisms reproduce under unfavourable conditions. Under unfavourable conditions, organism does not stop reproducing but divides rapidly inside a cyst and forms many individuals inside the cyst, when favourable conditions prevail, organism releases multiple individuals at the same time which are formed by multiple fission
  • Plasmodium is a malarial parasite which reproduces both inside a mosquito and also in human being.
  • It chooses multiple fission, asexual mode of reproduction for producing the cells. 
  • Multiple fission is the division of mother cell into many daughter cells simultaneously. 
  • A multinucleate mass is formed with rich cytoplasm which does not undergo division until certain amount of time. 
  • The specific reason behind the organism reproducing by multiple fission is that, it can divide itself into many cells at the same time inside the cyst during unfavourable conditions in the host.

3) Budding: This is a form of asexual reproduction which involves development of small mass of cells as protuberances on the parental body to give rise to new structures called as buds.

  • These buds separate out from the parental body and develop into new individuals.
  • Two types of budding are external budding and internal budding. 
  • Yeast reproduces by beans of buds. Buds grow in chains or detach from the parent and fall on the substratum. When buds are on the substratum, they grow as new individuals.  

4) Fragmentation: It involves breaking of parent organism into two or many fragments. Each fragment develops into an individual organism. Fragmentation is seen in sea stars which accidentally break their body into fragments. Fragmentation is also observed in annelids, turbellarians and some of poriferans.  Spirogyra reproduces through fragmentation during which each fragment grows into a new individual.  

5) Regeneration:  If the organism is cut up, its pieces can grow into separate individuals.  It occurs in some fully differentiated organisms. Regeneration is also called morphallaxis. e.g.  Hydra, Planaria. Regeneration is referred to tissue repair to normal state. It is restoration of normal structure and function of the organ. It is actually replacement of damaged tissue with same type of cells. Some organisms like lizards have the power of regenerating their tail. 

6) Spore formation:  Spore formation is one form of asexual reproduction. Spore formation is the method of developing new individuals by forming reproductive structures called spores.

  • A spore is a small spherical or oval structure which protects the future individual in a thick protective covering.
  • Spore germinates on a substratum under favourable conditions.
  • Some organisms like ferns, some groups of fungi reproduce by spore formation.
  •  Fungus reproduces by means of spores. Fungus like bread mould produce spores which germinate on moist organic surfaces. The cottony white mass on bread formed by fungus after spore germination is called a mould. Spores can survive in extreme conditions because of the protective hard coat.

7) Vegetative propagation:  Vegetative propagation is one mode of asexual reproduction in plants.

  • Vegetative propagation  is the production of new plants from the vegetative parts of the plant.
  • Roots, stems and leaves are called the vegetative parts of a plant. Any of these parts serve as vegetative propagules.
  • Vegetative propagation takes place by different methods like Grafting, Leaf propagation, Root propagation, underground stem propagation etc.       
  • Though vegetative reproduction results in the production of varieties of plants, offspring propagated through vegetative propagation are more uniform to parents.

Vegetative propagules
Vegetative propagules are the structures used to raise new plants. These are the vegetative structures from the plant which are used to develop new individuals. These can be like rhizome (ginger), runner(grass), bulbs(onion), tuber (potato), leaves(bryophyllum).

Advantages of vegetative propagation

  • Vegetative propagation is an easier method than growing plants from seeds. 
  • Vegetative propagation is used in plants which do not produce seeds.
  • Plants propagated through vegetative propagation require less time to grow.
  • Plants propagated through vegetative propagation are more uniform to parents.
  • Vegetative propagation helps in producing varieties in the plants.
Disadvantages of vegetative propagation
  • Plants produced by asexual reproduction are short lived than the plants produced by sexual reproduction.
  • The wood of vegetatively produced plants are small in size hence is not costly.
  • Vegetatively propagated plants cannot be cultivated on a large scale.
  • There os a reduction in plant vigour.
  • Parent plants with diseases transfer their diseases to the offspring.
Natural methods of vegetative propagation
a) Leaf propagation: Bryophyllum propagates vegetatively by the formation of leaf buds on the margins of a leaf. When the buds come in contact with moist soil, each bud is capable of growing into a new plant.
b) Root propagation: Sweet potato and Dhalia are capable of producing young ones from their roots.
c) Stem propagation: Potato giving rise to new plants is an example of stem propagation. 

Artificial methods of vegetative propagation
Many artificial methods are developed to propagate plants vegetatively. Layering and grafting are some artificial methods of vegetative propagation. 

a) Layering: This involves bending of a young stem towards the ground and burying it under the soil for development of roots . After a period of time, as the roots develop, the bent stem is cut off from the parent plant. This acts as a new plant. e.g. Jasmine. 

b) Cutting: It involves planting a young cutting of the stem with buds into moist soil. This develops roots which absorb nutrition from the soil and help in the growth of new plant. e.g. Bougainvellia.

c) Grafting: It involves fusion of tissues of one plant with those of another plant. Grafting is a vegetative method of propagation for apples and roses.
      
8)Tissue culture:  Plant tissue culture is a method used to propagate exact copies of plants under hygienic conditions. Tissue culture is the process of culturing single line cells under sterile, controlled conditions, in vitro by exposing them to nutrients, hormones and specific amount of sunlight. There are different steps involved in the tissue culture process. 

  • Step 1: Material is selected and sterilized for propagation of cells, in vitro initiating the process of culturing the cells.
  • Step 2: The plant material is re-divided and placed in separate plates with medium rich in plant growth regulators. These growth regulators induce shoot formation. This process can be repeated till the desired number of plants is obtained.
  • Step 3: The hormones which induce the formation of root are introduced into the medium. Now, complete plantlets are formed.   These plantlets are transferred from lab into the green house for further growth.
9) Cloning
It is a type of asexual reproduction which involves the creation of an exact copy of a biological entity.
  • Cloning involves the process of forming a cell or a complete individual from a body cell. 
  • A clone is created by inserting the complete genetic material of a regular body cell from a donor into a recipient. 
  • Cloning is done in many animals including sheep, cattle, pigs etc. But for the first time cloning of sheep was successful in producing a clone of the parent sheep. 

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