Class 12 Biology: Sexual Reproduction

Overview of the Session

• The class is a live session focused on covering three chapters from Unit 1 of Class 12 Biology: Reproduction in Flowering Plants, Human Reproduction, and Reproductive Health.
• The session includes a 3-day challenge where the teacher will be live with the students for most of the day to work together.
• A separate Telegram group was created for the 3-day challenge to share links and coordinate.
• The teacher aims to complete the unit by 9 PM, dedicating approximately 6 hours to the session.

Sexual Reproduction in Flowering Plants (Angiosperms)

• The chapter starts with the statement that we are lucky that plants reproduce sexually
• The importance of flowers for humans is mentioned in various aspects like aesthetics, ornaments, social & religious events, and expressing emotions.

Flower Structure and Parts

• Pistil/Carpel: The female part of the flower.
• Stamen: The male part of the flower.
• Petals (Corolla): The colorful parts of the flower that attract pollinators.
• Sepals (Calyx): The green parts that protect the developing flower bud.
• Thalamus: The base of the flower to which all parts are attached.
• Pedicel: The stalk of the flower.
• The focus shifts to understanding the structure of male and female reproductive parts in flowers, pollination and fertilization processes, and the formation of seeds and fruits.

Male Reproductive Part (Stamen)

• Stamen consists of two parts:
  • Anther: The bilobed structure at the top of the stamen containing pollen sacs.
  • Filament: A slender stalk attaching the anther to the thalamus or petal.
• Each lobe of the anther has two theca, making it a dithecous structure.
• Microsporangia: Each anther has four microsporangia located at the corners, two in each lobe.
• Tetrasporangiate: The condition of having four microsporangia in the anther.
• The microsporangium develops into a pollen sac containing pollen grains.
• Microspores: Located inside microsporangium
• Pollen sac: formed when microsporangium enlarges
• Pollen grain: formed when microspores enlarge

Microsporangium Structure

• The microsporangium is surrounded by four wall layers:
  • Epidermis: The outermost protective layer.
  • Endothecium: The layer below the epidermis, aiding in anther dehiscence.
  • Middle Layers: Several layers between the endothecium and tapetum.
  • Tapetum: The innermost nutritive layer providing nourishment to developing pollen grains.
• Sporogenous Tissue: A mass of homogenous cells found in the center of each microsporangium, which undergoes meiosis to produce microspores.
• The cells of the tapetum have dense cytoplasm and generally possess more than one nucleus.
• The young anther contains homogenous sporogenous tissue.

Microsporogenesis

• Microspore Mother Cell (Pollen Mother Cell): A cell within the sporogenous tissue capable of undergoing meiosis to form microspores.
• Plloidy of the sporogenous tissue is 2n, and it forms microspores through meiosis.
• Meiosis: The microspore mother cell divides meiotically to produce a tetrad of four haploid microspores.
• Tetrad: cluster of four haploid microspores
• As the anther matures, the microspores dissociate and develop into pollen grains.

Pollen Grain

• Represents the male gametophyte.
• Generally spherical, measuring about 25-50 micrometers in diameter.
• Has a two-layered wall:
  • Exine: The outer, tough layer made up of sporopollenin, one of the most resistant organic materials known.
  • Intine: The inner, thin and continuous layer made of cellulose and pectin.
• Germ Pores: Apertures on the exine where sporopollenin is absent, allowing the pollen tube to emerge during germination.
• Pollen grains contain two cells:
  • Vegetative Cell: Larger cell with abundant food reserve and a large, irregularly shaped nucleus.
  • Generative Cell: Small, spindle-shaped cell floating in the cytoplasm of the vegetative cell, which divides to form two male gametes.
• Two celled state: the pollen is shed at this stage in 60% of angiosperms
• Three celled stage: The generative cell divides and the pollen is shed, in 40% of angiosperms
• Pollen Viability: Varies from species to species; some lose viability in 30 minutes (e.g., rice, wheat), while others maintain viability for months (e.g., members of Rosaceae, Leguminosae, and Solanaceae). Storage of pollen can be done in liquid nitrogen (-196°C).
• Pollen Allergy: Causes allergies and bronchial ailments; Parthenium (carrot grass) is a major culprit.
• Pollen Products: Rich in nutrients, pollen tablets and syrups enhance performance, widely used as supplements.

Past Year Questions (Male Reproductive System)

• Proximal end of stamen filament is attached to the thalamus or petal.
• The four microspores of tetrad are covered by a layer made of callose.
• Correct statement: Tapetum nourishes pollen grains.
• In angiosperms, microsporogenesis involves meiosis.
• Pollen grains remain as fossils due to sporopollenin.
• Sporopollenin can withstand high temperatures and strong acids.
• Incorrect Statement: Tapetum helps in dehiscence of anther.
• In angiosperms, the male gametophyte is shed at three-celled stage.
• Male gametes are formed from the generative cell.
• Pollen tablet is used for supplementing food.
• Pollen grains are viable in a long time for some members of solanaceae.

Female Reproductive Part (Pistil or Carpel)

• Consists of:
  • Stigma: The receptive surface for pollen grains.
  • Style: The elongated, slender part connecting the stigma to the ovary.
  • Ovary: The enlarged basal part containing one or more ovules.
• Carpellary terminology: -One pistil - Monocarpellary -More than one pistil - Multicarpellary >Apocarpous: if carpels are free >Syncarpous: if carpels are fused
• Ovarian Cavity(Locule): the chamber inside the ovary
• Ovule: Each chamber has one to many ovules, attached to the placenta. Examples: Rice, wheat have a few; Papaya, watermelon have so many

Ovule Structure

• Funicule: The stalk by which the ovule is attached to the placenta.
• Hilum: The point of attachment between the ovule and funicle.
• Integuments: Protective envelopes surrounding the nucellus, leaving a small opening called the micropyle. -Protective - 2 Layers - Integuments
• Nucellus: The main body of the ovule, enclosed within the integuments, containing reserve food material.
• Embryo Sac (Female Gemetophyte): Located inside the nucellus, containing the egg apparatus and other cells,

Megasporogenesis

• Megaspore Mother Cell (MMC): A diploid cell (2n) within the nucellus that undergoes meiosis to form four haploid megaspores.
• Meiosis: Division of megaspore mother cell produces four megaspores. From a tetrad of megaspores, only one survives as functional megaspore, three degenerate -Functional megaspore develops into embryo sac >Micropylar End: Towards base, close to where spores are >Chalazal End: Upper side, away from pores

Embryo Sac Development

• The functional megaspore undergoes three successive mitotic divisions to form an eight-nucleate embryo sac, free nuclear divisions.

• Cell Walls formation after division results in a 7-celled condition in the embryo sac:

• Organization of nuclei: >Egg cell one egg towards micropylar end. has two cells called (Synergids) that has cellular thickening called **Filiform Apparatus

    >**Antipodal Cells** Towards the chalazal end, three antipodal cells
    >**Polar Nuclei** remaining 2 nuclei, situated below egg apparatus, referred as Polar Nuclei
  

• At maturity the typical embryo sac consists of 8-nucleate,7-celled