Class 10 Science Notes Chapter 8 (Chapter 8) – Examplar Problems (English) Book

Detailed Notes with MCQs of Chapter 8, "How do Organisms Reproduce?". This is a crucial chapter, not just for your board exams but also frequently tested in various government competitive exams. Pay close attention to the details.

Chapter 8: How do Organisms Reproduce? - Detailed Notes

1. Reproduction: The Fundamental Process

• Definition: Reproduction is the biological process by which new individual organisms – "offspring" – are produced from their "parents".
• Significance: It ensures the continuity of species, generation after generation. It is essential for the existence and propagation of life.
• Basis of Reproduction: The creation of a DNA copy is the fundamental event in reproduction. Cells use chemical reactions to build copies of their DNA. This requires cellular apparatus. DNA copying is accompanied by the creation of an additional cellular apparatus, and then the DNA copies separate, each with its own cellular apparatus, eventually leading to two cells.

2. Variations during Reproduction

• DNA copying mechanisms are not absolutely perfect and result in some variations.
• Importance of Variation: Variations are the basis for evolution. They help species to adapt to changing environmental conditions and ensure survival. While drastic variations can be harmful, minor variations are beneficial.

3. Modes of Reproduction

A. Asexual Reproduction

• Definition: Production of offspring by a single parent without the involvement of gamete formation or fertilization.
• Characteristics: Offspring are genetically identical (or nearly identical) to the parent, often called clones. It's a faster mode of reproduction compared to sexual reproduction. Common in simpler organisms.
• Types of Asexual Reproduction:
  • Fission: The parent cell divides into two or more daughter cells.
    • Binary Fission: Parent divides into two halves. Examples: Amoeba (any plane), Paramecium (transverse), Leishmania (definite orientation - longitudinal).
    • Multiple Fission: Parent cell divides into many daughter cells simultaneously, usually during unfavourable conditions (cyst formation). Example: Plasmodium (malarial parasite).
  • 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.
  • Regeneration: If an individual is cut or broken up into many pieces, many of these pieces grow into separate individuals. This is carried out by specialized cells that proliferate and differentiate. Note: Regeneration is also a mechanism for repair/growth, not solely reproduction. Examples: Hydra, Planaria.
  • Budding: A bud develops as an outgrowth due to cell division at one specific site. These buds develop into tiny individuals and, when fully mature, detach from the parent body to become new independent individuals. Examples: Hydra, Yeast.
  • Vegetative Propagation: A mode of asexual reproduction in plants where vegetative parts like roots, stems, or leaves develop into new plants under appropriate conditions.
    • Natural Methods: Runners/Stolons (grass), Rhizomes (ginger, turmeric), Tubers (potato - 'eyes' are buds), Bulbs (onion), Leaves (Bryophyllum - buds on leaf margins).
    • Artificial Methods: Used in agriculture and horticulture.
      • Cutting: A piece of stem or leaf is cut and planted. (e.g., Rose, Sugarcane).
      • Layering: A branch of the plant is bent and covered with soil while still attached to the parent plant. Roots develop, and then it's detached. (e.g., Jasmine).
      • Grafting: Tissues from one plant (scion) are inserted into those of another (stock) so that the two sets of vascular tissues may join together. Used to combine desirable traits. (e.g., Mango, Apple).
    • Advantages of Vegetative Propagation: Faster growth, propagation of seedless varieties (banana, orange), plants are genetically similar to the parent (maintaining desired traits), earlier flowering and fruiting.
    • Tissue Culture (Micropropagation): Growing plant tissues in an artificial medium under sterile conditions to produce many plantlets.
  • Spore Formation: Common in fungi (like Rhizopus - bread mould) and some bacteria. Parent plant produces hundreds of microscopic reproductive units called spores within a structure called sporangium. When the sporangium bursts, spores are released into the air and germinate on finding suitable moist surfaces. Spores have thick walls to survive unfavourable conditions.

B. Sexual Reproduction

• Definition: Production of offspring by the fusion of specialized male and female reproductive cells called gametes. Usually involves two parents.
• Characteristics: Offspring are genetically different from both parents, leading to variation. Slower process compared to asexual reproduction. Involves gamete formation (meiosis) and fertilization.
• Significance: Promotes genetic diversity, enabling better adaptation and evolution.

Sexual Reproduction in Flowering Plants (Angiosperms)

• Reproductive Part: Flower.
• Parts of a 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 a filament (stalk) and an anther (produces pollen grains). Pollen grains contain the male gametes.
  • Gynoecium/Pistil: Female reproductive part, located centrally. Composed of carpels. Each carpel has:
    • Stigma: Terminal receptive tip for pollen grains.
    • Style: Stalk connecting stigma to the ovary.
    • Ovary: Swollen basal part containing ovules. Each ovule contains the female gamete (egg cell).
• Types of Flowers:
  • Unisexual: Contains either stamens or carpels (e.g., Papaya, Watermelon).
  • Bisexual: Contains both stamens and carpels (e.g., Hibiscus, Mustard).
• Pollination: 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: Wind, water, insects, animals.
• Fertilization:
  • After landing on a suitable stigma, the pollen grain germinates, forming a pollen tube that grows down through the style to reach the ovule in the ovary.
  • The pollen tube carries two male gametes.
  • One male gamete fuses with the egg cell (syngamy) to form the zygote (diploid).
  • (In Angiosperms, the other male gamete fuses with the central cell to form the primary endosperm nucleus - this is called double fertilization, but focus for Class 10 is mainly on zygote formation).
• Post-Fertilization Changes:
  • Zygote develops into the embryo.
  • Ovule develops into the seed.
  • Ovary develops into the fruit.
  • Other parts like petals, sepals, stamens usually wither and fall off (though sepals may persist in some cases like brinjal).
• Seed: Contains the embryo (future plant) and stored food (in cotyledons or endosperm), protected by a seed coat.
• Germination: The process by which the embryo in the seed resumes growth under favourable conditions (water, oxygen, suitable temperature) to develop into a seedling.

Sexual Reproduction in Human Beings

• Puberty: The age at which reproductive organs become functional, gametes and sex hormones begin to be produced, and secondary sexual characteristics develop. Generally occurs earlier in girls (10-12 years) than boys (13-14 years).
  • Changes in Boys: Growth of hair (pubic, underarm, facial), voice deepens, penis and testes enlarge, muscle development.
  • Changes in Girls: Growth of hair (pubic, underarm), breast development, onset of menstruation, hip widening.
• Male Reproductive System:
  • Testes (singular: testis): Located outside the abdominal cavity in the scrotum. Scrotum provides a lower temperature (2-2.5 °C lower than body temp) necessary for sperm production. Functions: Produce sperm (male gametes) and testosterone (male sex hormone).
  • Epididymis: Coiled tube where sperm mature and are stored.
  • Vas Deferens (Sperm Duct): Tube that carries sperm from the epididymis towards the urethra.
  • Seminal Vesicles & Prostate Gland: Add secretions (forming semen) that provide fluid medium, nourishment, and aid sperm motility.
  • Urethra: Common passage for both urine and semen, runs through the penis.
  • Penis: External organ for transferring sperm into the female reproductive tract.
• Female Reproductive System:
  • Ovaries (singular: ovary): Located in the abdominal cavity. Functions: Produce ova/eggs (female gametes) and female sex hormones (estrogen and progesterone). A girl is born with thousands of immature eggs.
  • Fallopian Tubes (Oviducts): Tubes extending from near the ovaries to the uterus. Site of fertilization. Funnel-shaped opening near the ovary picks up the released egg.
  • 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: If intercourse occurs around the time of ovulation, sperm travel up the vagina, uterus, and into the fallopian tubes. If an egg is present in the fallopian tube, one sperm may fuse with it to form a zygote.
• Implantation: The zygote undergoes repeated cell divisions to form an embryo, which travels down the fallopian tube and embeds itself in the thick, vascular lining of the uterus.
• Placenta: A special disc-like tissue embedded in the uterine wall, connecting the embryo/fetus to the mother. Functions: Provides nutrients and oxygen from mother's blood to the fetus, removes waste products from fetal blood to mother's blood, acts as an endocrine gland (produces hormones). Connected to the fetus by the umbilical cord.
• Gestation: The period of development of the embryo/fetus inside the uterus (approx. 9 months or 280 days in humans).
• Parturition: The process of childbirth, involving rhythmic contractions of the uterine muscles.
• Menstrual Cycle: A recurring cycle of events (approx. 28 days) in the female reproductive system (ovaries and uterus) from puberty to menopause, regulated by hormones.
  • Menstruation: If the egg is not fertilized, the thickened uterine lining breaks down and is discharged along with blood and the unfertilized egg through the vagina (lasts 3-5 days).
  • Ovulation: Release of a mature egg from one of the ovaries, usually around the 14th day of the cycle.
  • The cycle involves changes in hormone levels (estrogen, progesterone, LH, FSH) that regulate the maturation of the egg and the preparation of the uterine lining for potential implantation.

4. Reproductive Health

• Definition: A state of complete physical, mental, and social well-being in all matters relating to the reproductive system and its functions and processes.
• Sexually Transmitted Diseases (STDs) / Venereal Diseases (VDs): Infections transmitted through sexual contact.
  • Bacterial: Gonorrhoea, Syphilis (curable with antibiotics if detected early).
  • Viral: HIV-AIDS, Genital Warts (caused by HPV). Viral STDs are generally not fully curable but can be managed.
  • Prevention: Using condoms during intercourse, avoiding multiple/unknown partners, maintaining hygiene.
• Contraception (Birth Control): Methods to prevent pregnancy.
  • Need: Population control, preventing frequent pregnancies (affecting mother's health), preventing STDs.
  • Methods:
    • Barrier Methods: Prevent physical meeting of sperm and egg. E.g., Condoms (male and female - also prevent STDs), Diaphragm, Cervical Cap.
    • Chemical Methods: Hormonal pills (oral contraceptives) prevent ovulation or implantation. Need medical advice. Side effects possible.
    • Intra-Uterine Contraceptive Devices (IUCDs): Devices like Copper-T inserted into the uterus by medical professionals. Prevent implantation, Copper ions suppress 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: Illegal practice of aborting female fetuses, often due to societal preference for male children. Leads to skewed sex ratio and social problems. Requires awareness and strict law enforcement.

Multiple Choice Questions (MCQs)

1. In Leishmania, binary fission occurs in:
   a) Any plane
   b) A transverse plane
   c) A longitudinal plane
   d) Multiple planes simultaneously

2. Which of the following is NOT a part of the female reproductive system in human beings?
   a) Ovary
   b) Uterus
   c) Vas deferens
   d) Fallopian tube

3. The ability of a cell to divide into several cells during reproduction in Plasmodium is called:
   a) Budding
   b) Reduction division
   c) Binary fission
   d) Multiple fission

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

5. During adolescence, several changes occur in the human body. Mark one change associated with sexual maturation in boys:
   a) Loss of milk teeth
   b) Increase in height
   c) Cracking of voice
   d) Weight gain

6. In human males, the testes are located in the scrotum because:
   a) It provides more space for testes development.
   b) It helps in the easy transfer of gametes.
   c) It requires a temperature lower than the body temperature for sperm formation.
   d) It protects the testes from physical shocks.

7. Which among the following is a unisexual flower?
   a) Mustard
   b) Hibiscus
   c) Papaya
   d) Rose

8. Following fertilization, which part of the flower develops into the fruit?
   a) Ovule
   b) Ovary
   c) Stigma
   d) Petals

9. Which of the following is a contraceptive method that also provides protection from sexually transmitted diseases?
   a) Copper-T
   b) Oral pills
   c) Condom
   d) Tubectomy

10. The process where the embryo gets embedded in the wall of the uterus is known as:
    a) Fertilization
    b) Ovulation
    c) Menstruation
    d) Implantation

Answer Key for MCQs:

1. c) A longitudinal plane
2. c) Vas deferens
3. d) Multiple fission
4. b) Stem, roots, leaves
5. c) Cracking of voice
6. c) It requires a temperature lower than the body temperature for sperm formation.
7. c) Papaya
8. b) Ovary
9. c) Condom
10. d) Implantation

Study these notes thoroughly. Understand the processes, functions of different parts, and the examples given. This will build a strong foundation for your exams. Let me know if any part needs further clarification.