Class 12 Biology Notes Chapter 1 (Reproduction in organisms) – Biology Book

Alright class, let's dive straight into Chapter 1: Reproduction in Organisms. This chapter lays the groundwork for understanding how life perpetuates itself, a fundamental concept in Biology. Pay close attention, as questions from this section often appear in various government exams, testing your grasp of basic definitions, types, and examples.

Chapter 1: Reproduction in Organisms - Detailed Notes

1. Lifespan:

• Definition: The period from birth to the natural death of an organism.
• Key Characteristics:
  • Highly variable among different species (e.g., Mayfly: 1 day; Parrot: 140 years; Banyan tree: several hundred years).
  • Not necessarily correlated with the size of the organism (e.g., Crow and Parrot are similar in size, but lifespans differ significantly).
  • Regardless of lifespan, death of every individual organism (except single-celled immortals) is a certainty.
• Continuity of life is maintained through Reproduction.

2. Reproduction:

• Definition: A biological process in which an organism gives rise to young ones (offspring) similar to itself.
• Purpose: Enables the continuity of the species, generation after generation.
• Basis: Based on whether there is participation of one organism or two in the process, reproduction is of two types: Asexual and Sexual.

3. Asexual Reproduction:

• Definition: Reproduction involving a single parent, with or without the involvement of gamete formation. Offspring produced are genetically identical to the parent.
• Key Features:
  • Uniparental (single parent involved).
  • No formation or fusion of gametes (usually).
  • Offspring are genetically and morphologically identical to the parent – called Clones.
  • Rapid mode of multiplication.
  • Common in single-celled organisms, plants, and animals with relatively simple organizations.
• Modes of Asexual Reproduction:
  • Fission: Cell division in unicellular organisms.
    • Binary Fission: Parent cell divides into two halves (e.g., Amoeba, Paramecium). Division plane varies (simple, longitudinal, transverse).
    • Multiple Fission: Parent cell divides into many daughter cells simultaneously, usually under unfavourable conditions (e.g., Amoeba during encystation, Plasmodium).
  • Budding: An outgrowth (bud) develops on the parent body, detaches, and grows into a new individual.
    • Examples: Yeast (unicellular fungus), Hydra (multicellular animal). Buds can be exogenous (outside) or endogenous (gemmules in sponges).
  • Spore Formation: Microscopic, motile or non-motile structures formed for dispersal and reproduction.
    • Zoospores: Motile spores, usually flagellated (e.g., Chlamydomonas, other algae).
    • Conidia: Non-motile spores produced exogenously on conidiophores (e.g., Penicillium).
    • Other spores: Chlamydospores, Oidia, Sporangiospores. Common in Fungi and Algae.
  • Fragmentation: Parent body breaks into distinct pieces (fragments), each fragment grows into an adult.
    • Examples: Spirogyra (alga), Hydra, Fungi.
  • Vegetative Propagation (in Plants): Asexual reproduction in plants using vegetative parts (root, stem, leaf). The units used are called vegetative propagules.
    • Natural Methods:
      • Runner: Slender stem growing horizontally (e.g., Grasses, Oxalis).
      • Rhizome: Underground horizontal stem (e.g., Ginger, Turmeric, Banana).
      • Sucker: Arises from the basal underground part of the main stem (e.g., Mint, Chrysanthemum).
      • Tuber: Swollen underground stem tip with buds ('eyes') (e.g., Potato).
      • Offset: Short, thick runner, one internode long (e.g., Water hyacinth - Eichhornia, Pistia). Note: Water hyacinth is known as the 'Terror of Bengal' due to its invasive nature and rapid vegetative propagation.
      • Bulb: Underground condensed stem with fleshy leaves (e.g., Onion, Garlic).
      • Bulbil: Fleshy bud that can form a new plant (e.g., Agave, Oxalis).
      • Leaf Buds: Adventitious buds on leaves (e.g., Bryophyllum).
    • Artificial Methods: Used in agriculture and horticulture (e.g., Cutting, Layering, Grafting, Micropropagation).

4. Sexual Reproduction:

• Definition: Reproduction involving the formation and fusion (fertilization) of male and female gametes to form a zygote, which develops into a new organism.
• Key Features:
  • Usually Biparental (involves two parents of opposite sex). Can be Uniparental in bisexual organisms.
  • Involves formation of haploid (n) gametes through meiosis.
  • Involves fusion of gametes (syngamy or fertilization) to form a diploid (2n) zygote.
  • Offspring are genetically different from parents and from each other, leading to variation.
  • A more complex and slower process compared to asexual reproduction.
• Phases in Life Cycle:
  • Juvenile Phase: Period of growth before reproductive maturity. Called 'vegetative phase' in plants. Variable duration.
  • Reproductive Phase: Organism is capable of reproduction. Marked by specific events (e.g., flowering in plants, hormonal changes in animals).
    • Plants: Annual, Biennial, Perennial. Some show unusual flowering phenomena (e.g., Bamboo species flower once in a lifetime, Strobilanthus kunthiana (Neelakurinji) flowers once in 12 years).
    • Animals: Seasonal breeders vs. Continuous breeders. Cycles in placental mammals: Oestrous cycle (non-primates like cows, sheep, rats) vs. Menstrual cycle (primates like monkeys, apes, humans).
  • Senescent Phase: End of reproductive phase; period of aging leading to death. Concomitant changes like slowing metabolism occur.
• Events in Sexual Reproduction: Sequential and grouped into three stages:
  • I. Pre-fertilization Events: Processes occurring before the fusion of gametes.
    • (a) Gametogenesis: Process of formation of gametes (male and female).
      • Types of Gametes:
        • Homogametes (Isogametes): Morphologically similar gametes (e.g., some algae like Cladophora).
        • Heterogametes (Anisogametes): Morphologically distinct gametes - male (sperm/antherozoid) and female (egg/ovum) (e.g., Fucus (alga), Humans, most sexually reproducing organisms).
      • Sexuality in Organisms:
        • Bisexual/Monoecious/Hermaphrodite: Both male and female reproductive structures present in the same individual (e.g., Earthworm, Tapeworm, Leech; Plants like Chara, Cucurbits, Coconut).
        • Unisexual/Dioecious: Male and female reproductive structures present on different individuals (e.g., Cockroach, Humans; Plants like Papaya, Date Palm, Marchantia).
        • Fungi: Homothallic (bisexual condition) vs. Heterothallic (unisexual condition).
      • Cell Division during Gamete Formation:
        • If parent body is Haploid (n) (e.g., Monera, Fungi, Algae, Bryophytes), gametes are formed by Mitosis.
        • If parent body is Diploid (2n) (e.g., Pteridophytes, Gymnosperms, Angiosperms, most Animals), gametes are formed by Meiosis (reductional division) in specialized cells called Meiocytes (gamete mother cells). Note: The chromosome number table (e.g., Human meiocyte 46, gamete 23; Fruit fly meiocyte 8, gamete 4) is important.
    • (b) Gamete Transfer: Bringing male and female gametes together for fertilization.
      • In most organisms, the male gamete is motile, and the female gamete is stationary.
      • A medium is required for transfer (Water in algae, bryophytes, pteridophytes). Large number of male gametes produced to compensate for loss.
      • In seed plants (Gymnosperms, Angiosperms), pollen grains carry male gametes. Pollination is the transfer of pollen to the stigma. Fertilization occurs internally.
      • In dioecious animals, specialized mechanisms for gamete transfer (e.g., copulation) are needed.
  • II. Fertilization (Syngamy): The fusion of male and female gametes to form a diploid zygote. This is the most vital event.
    • External Fertilization: Syngamy occurs outside the body, in an external medium (usually water).
      • Examples: Most aquatic organisms like algae, fishes, amphibians.
      • Requires synchrony in gamete release. Offspring vulnerable to predators.
    • Internal Fertilization: Syngamy occurs inside the body of the female organism.
      • Examples: Fungi, Reptiles, Birds, Mammals, Bryophytes, Pteridophytes, Gymnosperms, Angiosperms.
      • Male gamete is usually motile and reaches the egg. Sperm numbers are large, egg numbers are low. Ensures better protection.
    • Parthenogenesis: Development of the female gamete (ovum) into a full organism without fertilization (a modification).
      • Examples: Rotifers, Honeybees (drones develop this way), some lizards, Turkey (bird).
  • III. Post-fertilization Events: Events occurring after the formation of the zygote.
    • (a) The Zygote:
      • The first diploid cell formed after fertilization. It's the vital link ensuring continuity between generations.
      • Development depends on the organism's life cycle and environment.
      • In fungi and algae, the zygote may develop a thick wall (zygospore) to resist desiccation and damage, undergoing a rest period before germination.
      • In organisms with a haplontic life cycle, the zygote undergoes meiosis. In diplontic and haplodiplontic life cycles, it undergoes mitosis.
    • (b) Embryogenesis: The process of development of the embryo from the zygote.
      • Involves:
        • Cell Division (Mitosis): Increases the number of cells in the developing embryo.
        • Cell Differentiation: Cells undergo modifications to form specialized tissues and organs.
      • Based on Zygote Development Location:
        • Oviparous: Animals that lay fertilized or unfertilized eggs. Zygote development occurs outside the female parent's body. Eggs often covered by a hard calcareous shell (e.g., Reptiles, Birds). Young ones are vulnerable.
        • Viviparous: Animals where the zygote develops inside the female parent's body, and they give birth to young ones (e.g., Majority of mammals, including humans). Embryo gets nourishment and better protection, increasing survival chances.
      • In Flowering Plants:
        • Zygote develops into the Embryo.
        • Ovules develop into the Seed.
        • Ovary develops into the Fruit.
        • The wall of the ovary develops into the Pericarp (protective fruit wall).
        • Seeds germinate under favourable conditions to produce new plants after dispersal.

Conclusion: Reproduction, whether asexual or sexual, is essential for the existence and continuity of species on Earth. While asexual reproduction allows for rapid multiplication, sexual reproduction introduces variations crucial for adaptation and evolution. Understanding these fundamental processes is key to comprehending the diversity and resilience of life.

Multiple Choice Questions (MCQs)

1. Which of the following organisms exhibits binary fission as a primary mode of asexual reproduction?
   a) Yeast
   b) Hydra
   c) Amoeba
   d) Penicillium

2. The vegetative propagule 'offset' is characteristic of which aquatic plant known as the 'Terror of Bengal'?
   a) Pistia
   b) Eichhornia (Water Hyacinth)
   c) Hydrilla
   d) Vallisneria

3. Offspring formed by asexual reproduction are referred to as clones because they are:
   a) Morphologically similar but genetically different from the parent
   b) Genetically similar but morphologically different from the parent
   c) Morphologically and genetically identical to the parent
   d) Morphologically and genetically different from the parent

4. In organisms with a haplontic life cycle, gametes are produced by:
   a) Meiosis in diploid cells
   b) Mitosis in haploid cells
   c) Meiosis in haploid cells
   d) Mitosis in diploid cells

5. Which of the following plant groups requires water as a medium for the transfer of male gametes?
   a) Gymnosperms and Angiosperms
   b) Algae, Bryophytes, and Pteridophytes
   c) Bryophytes and Gymnosperms only
   d) Fungi and Angiosperms

6. The fusion of male and female gametes is termed:
   a) Gametogenesis
   b) Embryogenesis
   c) Syngamy
   d) Parthenogenesis

7. Which of the following is an example of a monoecious plant?
   a) Papaya
   b) Date Palm
   c) Marchantia
   d) Coconut

8. Development of an embryo from the zygote inside the female parent's body, leading to the birth of young ones, is characteristic of:
   a) Oviparous animals
   b) Viviparous animals
   c) Parthenogenetic animals
   d) Animals showing external fertilization

9. In flowering plants, after fertilization, the ovary develops into the:
   a) Seed
   b) Embryo
   c) Fruit
   d) Pericarp

10. The phenomenon where the female gamete develops into a new organism without fertilization is called:
    a) Syngamy
    b) Budding
    c) Parthenogenesis
    d) Embryogenesis

Answer Key:

1. c) Amoeba
2. b) Eichhornia (Water Hyacinth)
3. c) Morphologically and genetically identical to the parent
4. b) Mitosis in haploid cells
5. b) Algae, Bryophytes, and Pteridophytes
6. c) Syngamy
7. d) Coconut
8. b) Viviparous animals
9. c) Fruit
10. c) Parthenogenesis

Make sure you revise these points thoroughly, focusing on the definitions, differences, and especially the examples provided in the NCERT textbook. Good luck with your preparation!