Class 10 Notes

Class 10 Science Notes Chapter 8 (How do organisms reproduce?) – Science Book

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Philoid

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Science
Detailed Notes with MCQs of Chapter 8, 'How do organisms reproduce?'. This is a fundamental chapter in biology, crucial not only for understanding life itself but also frequently tested in various government examinations. Pay close attention to the concepts and examples.

Chapter 8: How do Organisms Reproduce?

1. Introduction to Reproduction

  • Definition: Reproduction is the biological process by which new individual organisms – "offspring" – are produced from their "parents".
  • Importance: It is essential for the continuation of species, generation after generation. It ensures that life persists on Earth.
  • Basis of Reproduction: The creation of a new individual involves the copying of DNA (Deoxyribonucleic Acid), the cell's genetic material. This process is accompanied by the creation of additional cellular apparatus. DNA copying mechanisms are not absolutely perfect, leading to variations, which are the basis for evolution.

2. Types of Reproduction

There are two main types:

  • Asexual Reproduction
  • Sexual Reproduction

3. Asexual Reproduction

  • Definition: A mode of reproduction in which a new offspring is produced by a single parent.

  • Characteristics:

    • Involves only one parent.
    • No formation or fusion of gametes (sex cells).
    • Offspring are genetically identical or very similar to the parent (clones).
    • Generally faster than sexual reproduction.
    • Common in simpler organisms (unicellular) and some multicellular organisms (plants, simple animals).

  • Modes of Asexual Reproduction:

    • a) Fission: The parent cell divides into two or more daughter cells.

      • Binary Fission: Parent divides into two equal halves. Seen in unicellular organisms like Amoeba (division in any plane), Leishmania (causes kala-azar; division occurs in a definite longitudinal plane due to whip-like structure), Bacteria.
      • Multiple Fission: Parent cell divides simultaneously into many daughter individuals. Seen in Plasmodium (malarial parasite) under unfavourable conditions (cyst formation) or as part of its life cycle.

    • b) Fragmentation: The body of a simple multicellular organism breaks up into smaller pieces (fragments) upon maturation, and each fragment grows into a new individual. Example: Spirogyra (an alga).

    • c) Regeneration: The ability of some organisms to regrow lost body parts. If the organism is cut into pieces, some species can regenerate a whole new individual from each piece. This requires specialized cells that can proliferate and differentiate. Example: Planaria (flatworm), Hydra. (Note: Regeneration is also a mechanism for repair and growth, not solely reproduction in all cases).

    • d) Budding: A bud develops as an outgrowth on the parent body due to repeated cell division at one specific site. This bud detaches (or sometimes remains attached) and develops into a new individual. Example: Hydra, Yeast (unicellular fungus).

    • e) Vegetative Propagation: A type of asexual reproduction in plants where new plants are produced from vegetative parts like roots, stems, or leaves.

      • Natural Methods:
        • Roots: Sweet Potato, Dahlia
        • Stems: Potato (eyes are buds on tuber), Ginger (rhizome), Turmeric (rhizome), Onion (bulb), Runners/Stolons (grass, strawberry)
        • Leaves: Bryophyllum (buds arise from notches along the leaf margin).
      • Artificial Methods (used in agriculture/horticulture):
        • Cutting: A piece of stem or leaf is cut and planted (e.g., Rose, Sugarcane).
        • Layering: A branch is bent towards the ground and covered with soil; roots develop, then it's detached (e.g., Jasmine).
        • Grafting: A stem part (scion) of a desired plant is attached to the rooted stem (stock) of another plant (e.g., Mango varieties, Roses).
      • Advantages of Vegetative Propagation: Faster growth, plants bear fruits/flowers earlier, preservation of desirable parental characteristics, propagation of seedless varieties (e.g., Banana, Grapes).
      • Tissue Culture (Micropropagation): Growing plant tissues or cells in an artificial nutrient medium under sterile conditions to produce many plantlets. Used for ornamental plants, disease-free plants.

    • f) Spore Formation: Common in fungi (like Rhizopus - bread mould), mosses, and ferns. The parent plant produces hundreds of microscopic reproductive units called spores within a structure called a sporangium. When the sporangium bursts, spores are released and dispersed by air, water, or animals. Under favourable conditions (moisture, suitable temperature), each spore germinates into a new individual. Spores have thick walls to survive unfavourable conditions.

4. Sexual Reproduction

  • Definition: A mode of reproduction involving two parents (usually male and female) contributing genetic material to produce offspring.

  • Characteristics:

    • Involves the formation and fusion of specialized sex cells called gametes (male gamete and female gamete).
    • Fusion of gametes is called fertilization, resulting in a zygote.
    • The zygote develops into an embryo and then a new individual.
    • Offspring are genetically different from both parents, leading to variation. Variation is crucial for adaptation and evolution.
    • Generally slower and more complex than asexual reproduction.

  • A. Sexual Reproduction in Flowering Plants (Angiosperms)

    • Reproductive Part: The Flower.
    • Parts of a typical Bisexual Flower:
      • Calyx: Outermost whorl, composed of sepals (usually green, protect the bud).
      • Corolla: Composed of petals (usually brightly coloured to attract pollinators).
      • Androecium: Male reproductive part, composed of stamens. Each stamen has:
        • Anther: Produces pollen grains, which contain the male gametes.
        • Filament: Stalk that supports the anther.
      • Gynoecium/Pistil: Female reproductive part, located centrally, composed of carpels. Each carpel (or fused carpels) has:
        • Stigma: Sticky tip that receives pollen grains.
        • Style: Stalk connecting stigma to the ovary.
        • Ovary: Swollen base containing ovules. Each ovule contains the female gamete (egg cell).
    • Types of Flowers:
      • Unisexual: Contains either stamens (male/staminate flower) or carpels (female/pistillate flower). Examples: Papaya, Watermelon.
      • Bisexual: Contains both stamens and carpels. Examples: Hibiscus, Mustard.
    • Pollination: The transfer of pollen grains from the anther to the stigma.
      • Self-Pollination: Transfer within the same flower or to another flower on the same plant.
      • Cross-Pollination: Transfer from the anther of one flower to the stigma of a flower on a different plant of the same species.
      • Agents of Pollination: Wind, water, insects, birds, animals.
    • Fertilization:
      • After landing on a suitable stigma, a pollen grain germinates, growing a pollen tube down through the style to reach the ovule inside the ovary.
      • The male gamete travels down the pollen tube.
      • Fusion of the male gamete with the female gamete (egg cell) inside the ovule occurs. This fusion is fertilization.
      • The product of fertilization is the zygote.
    • Post-Fertilization Changes:
      • Zygote develops into the embryo.
      • Ovule develops into the seed. The seed contains the embryo and stored food (in cotyledons or endosperm), protected by a tough seed coat.
      • Ovary develops into the fruit. The fruit protects the seeds. Other floral parts (sepals, petals, etc.) usually wither and fall off.
    • Germination: The process by which the embryo within the seed resumes growth and develops into a seedling under favourable conditions (water, oxygen, suitable temperature).

  • B. Sexual Reproduction in Human Beings

    • Puberty: The age at which reproductive organs become functional, and secondary sexual characteristics develop. Triggered by hormones.
      • In Boys (approx. 13-14 yrs): Growth of hair (pubic, underarm, facial), deepening of voice, broadening shoulders, muscle development, enlargement of penis and testes, start of sperm production. Hormone: Testosterone.
      • In Girls (approx. 10-12 yrs): Growth of hair (pubic, underarm), breast development, widening of hips, onset of menstruation (menarche), start of egg maturation. Hormones: Estrogen and Progesterone.
    • Male Reproductive System:
      • Testes (singular: Testis): Primary reproductive organs. Produce sperm (male gametes) and testosterone. Located outside the abdominal cavity in the scrotum (maintains a temperature 2-3°C lower than body temp, essential for sperm production).
      • Epididymis: Coiled tube where sperm mature (often simplified in NCERT).
      • Vas Deferens (Sperm Duct): Tube that carries sperm from testis towards the urethra.
      • Seminal Vesicles & Prostate Gland: Glands that add fluids to the sperm. This mixture (semen) provides nourishment and aids sperm transport.
      • Urethra: Common passage for both urine and semen, running through the penis.
      • Penis: External organ for transferring sperm into the female reproductive tract.
    • Female Reproductive System:
      • Ovaries (singular: Ovary): Primary reproductive organs. Produce ova or eggs (female gametes) and hormones (estrogen, progesterone). Located in the abdominal cavity. A girl is born with thousands of immature eggs. Usually, one egg matures and is released each month after puberty.
      • Oviduct (Fallopian Tube): Tube that carries the egg from the ovary towards the uterus. Site of fertilization.
      • Uterus (Womb): Muscular, pear-shaped organ where the fertilized egg implants and the fetus develops.
      • Cervix: Narrow lower part of the uterus that opens into the vagina.
      • Vagina: Muscular tube connecting the cervix to the outside; receives sperm during intercourse and serves as the birth canal.
    • Fertilization in Humans:
      • During intercourse, semen is deposited in the vagina.
      • Sperm travel up through the cervix and uterus into the oviducts.
      • If an egg is present in the oviduct (released during ovulation), one sperm may fuse with it – fertilization.
      • The fertilized egg (zygote) starts dividing and travels down the oviduct to the uterus.
    • Implantation and Development:
      • The developing embryo embeds itself in the thick lining of the uterus (implantation).
      • Placenta: A special disc-like tissue develops between the uterine wall and the embryo. It facilitates the exchange of nutrients, oxygen, and waste products between the mother and the fetus via the umbilical cord. It also acts as an endocrine gland, producing hormones necessary for pregnancy.
      • Gestation Period: The duration of pregnancy (approx. 9 months or 40 weeks in humans).
      • Parturition: The process of childbirth.
    • Menstrual Cycle:
      • If the egg is not fertilized, the thickened uterine lining is not needed.
      • The lining breaks down and is discharged along with the unfertilized egg, blood, and mucus through the vagina. This is called menstruation.
      • It occurs approximately every 28 days. The entire cycle of egg maturation, release (ovulation), thickening of the uterine lining, and its shedding (if no fertilization) is the menstrual cycle.

5. Reproductive Health

  • Definition: A state of complete physical, mental, and social well-being in all matters relating to the reproductive system and its functions.
  • Sexually Transmitted Diseases (STDs) / Sexually Transmitted Infections (STIs): Infections spread primarily through sexual contact.
    • Bacterial: Gonorrhoea, Syphilis (treatable with antibiotics).
    • Viral: HIV-AIDS (Human Immunodeficiency Virus - Acquired Immuno Deficiency Syndrome), Genital Warts (Human Papillomavirus - HPV) (Viral STDs are generally not curable but can be managed).
    • Prevention: Using condoms during intercourse, avoiding multiple partners, maintaining hygiene.
  • Contraception (Birth Control): Methods used to prevent pregnancy.
    • Need: Preventing unwanted pregnancies, controlling population growth, spacing between children, preventing STDs (barrier methods).
    • Methods:
      • Barrier Methods: Prevent physical meeting of sperm and egg.
        • Condoms: Sheath worn over the penis (male) or inside the vagina (female). Also prevent transmission of STDs.
        • Diaphragm/Cervical Cap: Placed in the vagina to cover the cervix.
      • Chemical Methods:
        • Oral Contraceptive Pills: Hormonal pills taken by females that prevent ovulation (release of egg).
        • Vaginal Pills/Creams: Spermicidal chemicals.
      • Intra-Uterine Devices (IUDs): Devices like Copper-T inserted into the uterus by medical professionals. They prevent implantation or affect sperm motility.
      • Surgical Methods (Sterilization): Permanent methods.
        • Vasectomy (Males): Vas deferens is cut and tied, preventing sperm release.
        • Tubectomy (Females): Fallopian tubes are cut and tied, preventing the egg from reaching the uterus and sperm from reaching the egg.
  • Female Foeticide: The practice of aborting female fetuses. A major social issue linked to misuse of pre-natal sex determination techniques. Leads to skewed sex ratio. Illegal and unethical.

Multiple Choice Questions (MCQs)

  1. Which of the following is NOT a mode of asexual reproduction?
    (a) Budding
    (b) Fertilization
    (c) Fission
    (d) Fragmentation

  2. In Spirogyra, asexual reproduction takes place by:
    (a) Breaking up of filaments into smaller bits (Fragmentation)
    (b) Division of a cell into two cells (Binary Fission)
    (c) Formation of a large number of buds
    (d) Division of a cell into many cells (Multiple Fission)

  3. The ability of an organism to develop its lost body parts is called:
    (a) Budding
    (b) Fragmentation
    (c) Regeneration
    (d) Fission

  4. Vegetative propagation refers to the formation of new plants from:
    (a) Stem, roots, and flowers
    (b) Stem, roots, and leaves
    (c) Stem, flowers, and fruits
    (d) Stem, leaves, and flowers

  5. In a flower, the parts that produce male and female gametes are, respectively:
    (a) Sepal and anther
    (b) Filament and stigma
    (c) Anther and ovary
    (d) Petal and ovule

  6. Which of the following is a post-fertilization event in flowering plants?
    (a) Pollination
    (b) Germination of pollen
    (c) Development of embryo from zygote
    (d) Formation of gametes

  7. In human males, the testes are located in the scrotum because:
    (a) It provides more space for testes development.
    (b) It protects the testes from mechanical shocks.
    (c) Scrotum helps in maintaining a lower temperature required for sperm formation.
    (d) It facilitates the easy transfer of sperm to the vas deferens.

  8. The structure that provides vascular connection between the fetus and the uterus is:
    (a) Oviduct
    (b) Amniotic sac
    (c) Placenta
    (d) Vas deferens

  9. Which of the following is a Sexually Transmitted Disease caused by bacteria?
    (a) AIDS
    (b) Genital Warts
    (c) Syphilis
    (d) Hepatitis B

  10. Which contraceptive method also provides protection from acquiring STDs?
    (a) Oral pills
    (b) Copper-T
    (c) Condoms
    (d) Tubectomy

Answer Key for MCQs:

  1. (b) Fertilization
  2. (a) Breaking up of filaments into smaller bits (Fragmentation)
  3. (c) Regeneration
  4. (b) Stem, roots, and leaves
  5. (c) Anther and ovary
  6. (c) Development of embryo from zygote
  7. (c) Scrotum helps in maintaining a lower temperature required for sperm formation.
  8. (c) Placenta
  9. (c) Syphilis
  10. (c) Condoms

Remember to correlate these notes with the NCERT textbook diagrams for better understanding, especially the parts of a flower and the human reproductive systems. Good luck with your preparation!

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