4.1 Sexual and Asexual Reproduction
Reproduction is the biological process by which living organisms produce new individuals (offspring) to ensure the survival of their species and prevent extinction.
Comparison Between Sexual and Asexual Reproduction
- Sexual Reproduction: Involves two parents of opposite sexes, production of gametes (sperm and ovum), and fertilization. Offspring possess genetic variations.
- Asexual Reproduction: Involves only one parent, no gamete formation, and no fertilization. Offspring are genetically identical clones of the parent.
Types of Asexual Reproduction
- Binary Fission: Division of a single parent cell into two identical daughter cells (e.g., Amoeba, Paramecium, bacteria).
- Budding: Formation of a small outgrowth (bud) on the parent body that eventually detaches (e.g., Hydra, yeast).
- Spore Formation: Production of microscopic dormant spores in a sporangium that germinate under favorable conditions (e.g., Mucor, ferns).
- Vegetative Reproduction: Producing new plants from vegetative structures like roots, stems, or leaves (e.g., onion, potato, ginger, Bryophyllum).
- Regeneration: Ability of an organism to grow a complete new body from a severed fragment (e.g., Planaria, starfish).
4.2 Human Male and Female Reproductive Systems
Male Reproductive System
- Testes: Produce male gametes (sperm) and testosterone hormone.
- Scrotum: Holds and protects the testes outside the body cavity at a temperature lower than normal body temperature for sperm development.
- Sperm Duct (Vas Deferens): Transports sperm from the testes to the urethra.
- Seminal Vesicle & Prostate Gland: Secrete nutrient-rich fluid that nourishes sperm and aids mobility.
- Urethra: Channel through which semen and urine exit the body.
- Penis: Transfers sperm into the female vagina during copulation.
Female Reproductive System
- Ovaries: Produce female gametes (ova/eggs) and female hormones (estrogen and progesterone).
- Fallopian Tube (Oviduct): Channel connecting ovary to uterus; site where fertilization typically occurs.
- Uterus (Womb): Muscular structure where a fertilized embryo implants and develops during pregnancy.
- Cervix: Narrow neck of the uterus that opens into the vagina.
- Vagina: Receives sperm during copulation and serves as the birth canal.
4.3 The Menstrual Cycle
The menstrual cycle is a monthly cycle of physiological changes in the female reproductive system involving ovulation and breakdown of the uterine lining, lasting about 28 days on average.
Phases of the Menstrual Cycle
- Menstruation Phase (Days 1–5): Breakdown of uterine lining and blood vessels, discharged through the vagina as menstrual flow due to low hormone levels.
- Repair Phase (Days 6–11): Uterine lining begins to repair and thicken with blood vessels.
- Fertile Phase / Ovulation (Days 12–17): An egg is released from the ovary into the Fallopian tube (Ovulation occurs around Day 14). High probability of fertilization if sperm is present.
- Premenstrual Phase (Days 18–28): Uterine lining becomes fully thickened and vascularized for embryo implantation. If no fertilization occurs, hormone levels drop, triggering menstruation.
4.4 Development of Human Fetus and Birth
Fertilization occurs when a single sperm fuses with an egg in the Fallopian tube to form a zygote.
$$\text{Sperm } (n) + \text{Ovum } (n) \longrightarrow \text{Zygote } (2n) \longrightarrow \text{Embryo} \longrightarrow \text{Fetus}$$
- Zygote: Fertilized egg undergoes cell division to form a ball of cells called an embryo.
- Implantation: The embryo attaches to the thickened uterine wall.
- Placenta: Organ formed between fetus and uterine wall; allows exchange of oxygen, nutrients, and antibodies from mother to fetus, and removal of waste products (urea, carbon dioxide). Maternal and fetal blood do not mix directly.
- Amniotic Sac & Fluid: Surrounds the developing fetus, acting as a cushion against mechanical shock.
- Umbilical Cord: Connects fetus to placenta, containing blood vessels for transport.
4.5 Sexual Reproduction in Flowering Plants
Structure of a Flower
- Male Structure (Stamen): Consists of Anther (produces pollen grains containing male gametes) and Filament.
- Female Structure (Pistil/Carpel): Consists of Stigma (sticky surface for pollen landing), Style, Ovary, and Ovule (contains female gamete).
- Petals: Brightly colored structures to attract pollinators.
- Sepals: Protect the flower during the bud stage.
Pollination and Fertilization in Plants
Pollination is the transfer of pollen grains from the anther to the stigma.
- Self-pollination: Transfer of pollen within the same flower or another flower on the same plant.
- Cross-pollination: Transfer of pollen from anther of one plant to stigma of another plant of the same species (increases genetic diversity and quality of seeds/fruits).
- Fertilization: Pollen tube grows down the style into the ovary. Male gamete fuses with egg cell inside the ovule to form a zygote.
- Post-fertilization: The ovary develops into a fruit, and the ovule becomes a seed.