unit 1 . bio

Introduction to Biology and Reproduction

• Biology is essentially the story of life on Earth.

• Individual organisms die, but species persist unless facing extinction.

• Reproduction is vital for species survival; individuals reproduce asexually or sexually.

  • Sexual reproduction creates new variants, enhancing survival advantages.

• This unit covers reproductive processes in flowering plants and humans.

  • Chapters Include:

    • Chapter 1: Sexual Reproduction in Flowering Plants

    • Chapter 2: Human Reproduction

    • Chapter 3: Reproductive Health

Panchanan Maheshwari: Key Figure in Botany

• Life & Legacy

  • Born: November 1904, Jaipur, Rajasthan.

  • Education: D.Sc. from Allahabad.

  • Inspired by Dr. W. Dudgeon to pursue botany, especially morphology.

  • Established the Department of Botany, University of Delhi, promoting research in embryology and tissue culture.

• Contributions:

  • Pioneered test tube fertilisation and intra-ovarian pollination.

  • Authored the first Biology textbooks for Higher Secondary Schools (NCERT, 1964).

  • Received numerous accolades, including a fellowship with the Royal Society of London (FRS).

Sexual Reproduction in Flowering Plants

• Importance of Flowers

  • Flowers are integral to sexual reproduction in angiosperms.

  • They contribute to beauty, scents, and pollination necessary for reproduction.

  • Diversity in flower structures aids in forming fruits and seeds.

• Chapter Overview:

  • 1.1 Flower Structure

  • 1.2 Pre-fertilisation Structures and Events

  • 1.3 Double Fertilisation

  • 1.4 Post-fertilisation Structures and Events

  • 1.5 Apomixis and Polyembryony

1.1 Flower: A Fascinating Organ of Angiosperms

• Flowers have aesthetic, ornamental, and cultural significance.

• Essential parts of flowers include:

  • Androecium (male reproductive organ)

  • Gynoecium (female reproductive organ)

  • Most flowering plants reproduce sexually.

1.2 Pre-fertilisation: Structures and Events

• Flowering begins with hormonal changes leading to floral primordium development.

• Androecium includes stamens (male structures) and Gynoecium includes the pistil (female structure).

• Stamen Structure:

  • Consists of filament (stalk) and anther (pollen-producing part).

  • The number and length of stamens vary among species.

Stamen, Microsporangium, and Pollen Grain

• A typical anther is bilobed and tetrasporangiate.

• Microsporangium contains sporogenous tissue that undergoes meiotic division to form microspore tetrads.

• Microsporogenesis: Formation of microspores from pollen mother cell through meiosis; results in a cluster of four microspores (tetrad).

• Mature microspores develop into pollen grains.

1.3 Double Fertilisation

• Double fertilisation occurs when:

  • One male gamete fuses with the egg (forming the zygote).

  • The other male gamete fuses with polar nuclei (forming the triploid primary endosperm nucleus, PEN).

• This process is unique to flowering plants and crucial for seed development.

1.4 Post-fertilisation: Structures and Events

• The development of endosperm precedes embryo development, providing nutrition to the embryo.

• Endosperm Development:

  • Initially free-nuclear (many nuclei), then cellular.

  • May completely or partially persist at seed maturity.

• Embryo Formation:

  • Zygote develops into the embryo with characteristics of cotyledons and embryonal axis.

1.5 Apomixis and Polyembryony

• Apomixis: Formation of seeds without fertilisation.

  • A form of asexual reproduction mimicking sexual processes.

• Polyembryony: Occurrence of multiple embryos from a single ovule.

  • Can be observed in several flowering plants, useful in horticulture.

Summary of Key Concepts

• Flower Functions: Site of sexual reproduction; includes male (androecium) and female (gynoecium) structures.

• Pollen Grain Structure: Two-layered wall (exine and intine); developed in microsporangia.

• Egg Formation: In embryo sac, formed from a functional megaspore.

• Pollination: Transfer of pollen to stigma is crucial for fertilisation; includes autogamy, geitonogamy, and xenogamy.

• Double Fertilisation: Leads to zygote and primary endosperm.

• Seed Formation: Result of fertilised ovules; significant for agriculture, seeds have dormant phases allowing for extended survival.

Exercises for Review

1. Identify parts of angiosperm flowers involved in gametophyte development.

2. Differentiate between microsporogenesis and megasporogenesis.

3. Describe the developmental sequence leading to pollen grain formation.

4. Explain the importance of apomixis in agriculture.