NEET Biology Class 12 Reproduction In Organisms Notes

Reproduction In Organisms Notes

Reproduction is a process in which an organism produces young ones (offspring) similar to itself. The period from birth to the natural death of an organism is known as its lifespan. No individual is immortal, except unicellular organisms. There is no natural death in unicellular organisms.

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Based on the number of participants, reproduction is 2 types: Asexual reproduction and Sexual reproduction.

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Asexual Reproduction In Organisms

It is the production of offspring by a single parent. It is seen in unicellular organisms, simple plants and animals. The offspring are identical to one another and their parent. Such morphologically and genetically similar individuals are known as clones.

Types of asexual reproduction

1. Asexual Reproduction Fission: In this, the parent cell divides (cell division) into two or more individuals. Examples are Protists and Monerans. Fission is 2 types:
   • Binary fission: It is the division of a parent cell into two individuals. Examples are amoeba and paramecium.
   • Multiple fission: It is the division of a parent cell into many individuals. Examples, are Plasmodium and amoeba.
2. Asexual Reproduction Budding: In this, a bud appears and grows in the parent body. After maturation, it is detached from the parent body to form a new individual. Example, Hydra, Sponge, Yeast etc.
3. Asexual Reproduction Vegetative propagation: It is the production of offspring from vegetative propagules in plants. Vegetative propagules are units of vegetative propagation.
   • Examples of vegetative propagules:
     • Buds (‘eyes’) of the potato tuber.
     • Rhizomes of banana and ginger. Buds and Rhizomes arise from the nodes of modified stems. The nodes come in contact with damp soil or water and produce roots and new plants.
     • Adventitious buds of Bryophyllum. They arise from the notches at the margins of leaves.
     • Bulbil of Agave.
     • Offset of water hyacinth.
     • Runner, sucker, tuber, bulb etc.

Class 12 Biology Notes For Neet

Other asexual reproductive structures Examples: are zoospores (microscopic motile structures in some algae and protists), conidia (Penicillium) and gemmules (sponge).

Asexual reproduction is the common method in simple organisms like algae and fungi. During adverse conditions, they can shift to sexual methods. Higher plants reproduce asexually (vegetatively) and sexually. But most of the animals show only sexual reproduction.

Sexual Reproduction In Organisms

It is the reproduction that involves the formation of male and female gametes, either by the same individual or by different individuals of the opposite sex. It results in offspring that are not identical to the parents or amongst themselves. It is an elaborate, complex and slow process as compared to asexual reproduction.

• The period of growth to reach maturity for sexual reproduction is called the juvenile phase. In plants, it is known as the vegetative phase.
• In higher plants, the flowering indicates the end of the vegetative phase (beginning of the reproductive phase).
• Annual and biennial plants show clear-cut vegetative, reproductive and senescent phases. In perennial plants, these phases are very difficult to identify.

Reproduction In Organisms Class 12 Notes

Some plants exhibit unusual flowering. Example,

• Bamboo species flower only once in their lifetime (after 50-100 years), produce a large number of fruits and die.
• Strobilanthus kunthiana flowers once in 12 years.
• In animals, the juvenile phase is followed by morphological and physiological changes before active reproductive behaviour.
• Birds living in nature lay eggs only seasonally. However, birds in captivity (for example, poultry) can be made to lay eggs throughout the year.
• The females of placental mammals exhibit cyclical changes in the ovaries, accessory ducts and hormones during the reproductive phase. It is called the oestrus cycle in non-primates (cows, sheep, rats, deer, dogs, tigers etc.) and the menstrual cycle in primates (monkeys, apes and humans).

Based on breeding season, mammals are of 2 types:

1. Seasonal breeders: The mammals (living in natural conditions) exhibit reproductive cycles only during favourable seasons.
2. Continuous breeders: They are reproductively active throughout their reproductive phase.

Reproduction In Organisms Class 12 Notes

Senescence (old age):

• It is the last phase of the lifespan and the end of the reproductive phase.
• During this, concomitant changes occur in the body. For example, slowing of metabolism etc. It ultimately leads to death.
• In plants and animals, hormones cause a transition between juvenile, reproductive and senescence phases. Interaction between hormones and environmental factors regulates the reproductive processes and the associated behavioural expressions of organisms.

Events In Sexual Reproduction: Pre-fertilisation-Fertilisation-Post-fertilisation

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Pre-fertilisation Events In Sexual Reproduction: These are the events before the fusion of gametes. They include gametogenesis and gamete transfer.

1. Gametogenesis: It is the formation of male and female gametes. Gametes (haploid cells) are 2 types:

1. Homogametes (isogametes): Similar gametes. They cannot be categorised into male and female gametes. E.g. Some algae like Cladophora.
2. Heterogametes: The male and female gametes are distinct types. The male gamete is called an antherozoid (sperm) and the female gamete is called an egg (ovum). Example, Fucus (an alga), Human beings etc.

Sexuality (bisexual or unisexual) in organisms:

• Bisexual In Organisms: Male and female reproductive structures present in the same individual.
  • Bisexual plants: For example, Hibiscus, and Pisum. In flowering plants, the male flower is staminate (bears stamens) and the female flower is pistillate (bears pistils). If male and female flowers are present on the same plant, it is called monoecious. Examples are cucurbits and coconuts.
  • Bisexual animals (hermaphrodites): For example, Earthworms, leeches, sponges, tapeworms, etc.
• Unisexual In Organisms: Male and female reproductive structures are present on different individuals. If male and female flowers are present on different plants, it is called dioecious. For example, papaya and date palm.
  • Unisexual animals: For example, cockroaches, higher animals etc. Fungi may be homothallic (bisexual) or heterothallic (unisexual).

Reproduction In Organisms Class 12 Notes

Cell division during gamete formation:

• Many monerans, fungi, algae and bryophytes have haploid parental bodies. They produce haploid gametes by mitosis.
• Pteridophytes, gymnosperms, angiosperms and animals have diploid parental bodies. They produce haploid gametes by meiosis of meiocytes (gamete mother cell).

 

 

 

 

 

 

 

 

 

Biology Reproduction In Organisms Class 12

2. Gamete Transfer: Male gametes need a medium to move towards female gametes for fertilisation. In most organisms, the male gamete is motile and the female gamete is stationary. In some fungi and algae, both types of gametes are motile.

• In simple plants (algae, bryophytes and pteridophytes), gamete transfer takes place through water medium. To compensate for the loss of male gametes during transport, a large number of male gametes is produced.
• In seed plants, pollen grains (in anthers) carry male gametes and the ovule carries the egg. Pollen grains are transferred to the stigma.
• In bisexual self-fertilizing plants (for example, peas), anthers and stigma are closely located. So transfer of pollen grains is easy.
• In cross-pollinating plants (including dioecious plants), pollination helps in the transfer of pollen grains. Pollen grains germinate on the stigma and the pollen tubes carrying the male gametes reach the ovule and discharge male gametes near the egg.
• In dioecious animals, fertilisation helps for the successful transfer and coming together of gametes.

Fertilisation (syngamy) In Sexual Reproduction: It is the fusion of gametes to form a diploid zygote. In rotifers, honeybees, some lizards, birds (turkey) etc., female gamete develops to new organisms without fertilisation. This is called parthenogenesis.

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Types of fertilization:

1. External fertilisation: Syngamy occurs in the external medium (water), i.e. zygote is formed outside the body. For example, most aquatic organisms (many algae, bony fishes etc.) and amphibians. Such organisms show synchrony between the sexes and release a large number of gametes into the surrounding medium to ensure syngamy.
   • External fertilisation Disadvantage: The offspring are extremely vulnerable to predators threatening their survival up to adulthood.
2. Internal fertilisation: Syngamy occurs inside the body of the organism. Examples are terrestrial organisms, belonging to fungi, animals (reptiles, birds, mammals) and plants (bryophytes, pteridophytes, gymnosperms and angiosperms).
   • In this, the non-motile egg is formed inside the female body to where the motile male gamete reaches and fuses.
   • In seed plants, the non-motile male gametes are carried to the female gamete by pollen tubes.
   • There is a large number of sperm produced but the number of eggs is very low.

Biology Reproduction In Organisms Class 12

Post-fertilisation Events In Sexual Reproduction: These are the events after the formation of the zygote.

• Zygote
  • Development of the zygote depends on the type of life cycle of the organism and the nature of the environment.
  • In fungi and algae, the zygote develops a thick wall that is resistant to desiccation and damage. It undergoes a period of rest before germination.
  • In organisms with a haplontic life cycle, zygote divides by meiosis into haploid spores that grow into haploid individuals.
  • Sexually reproducing organisms begin life as a zygote.
  • The zygote is the vital link between organisms of one generation and the next.
• Embryogenesis
  • It is the development of an embryo from the zygote.
  • During embryogenesis, the zygote undergoes cell division (mitosis) and cell differentiation.
  • Cell divisions increase the number of cells in the embryo. Cell differentiation causes the modifications of groups of cells into various tissues and organs to form an organism.

Biology Reproduction In Organisms Class 12

Based on the place of zygote development, animals are of 2 types:

1. Oviparous: Here, animals lay fertilized/unfertilized eggs. For example, In reptiles and birds, the fertilized eggs covered by hard calcareous shells are laid in a safe place. After incubation, young ones hatch out.
2. Viviparous: Here, the zygote develops into a young one inside the female body. Later, the young ones are delivered out of the body. For example, most of the mammals. Because of proper care and protection, the chances of survival of young ones are greater in viviparous animals.