How do Organisms Reproduce?

Introduction

• Reproduction is not essential for an individual organism’s survival, unlike processes like nutrition or respiration, but it consumes significant energy.
• Organisms are noticeable due to their large populations, resulting from reproduction, creating individuals similar to themselves.
• Species Identification: Organisms are grouped into species based on similar appearance, reflecting similar body designs.
• This chapter explores why and how organisms reproduce, focusing on the mechanisms and their significance.

7.1 Do Organisms Create Exact Copies of Themselves?

• Similar body designs in organisms result from similar blueprints, stored in DNA within cell nuclei.
• DNA’s Role:
  • Contains inheritance information, coding for proteins that determine body design.
  • Changed DNA leads to different proteins, altering body designs.
• Reproduction Process:
  • Involves creating a DNA copy via chemical reactions, followed by cell division to form two cells, each with its own DNA and cellular apparatus.
  • DNA copying is not perfectly accurate, introducing variations.
  • Variations mean daughter cells are similar but not identical, forming the basis for evolution.
• Questions:
  1. Importance of DNA Copying: Ensures inheritance of features, maintaining body design for species stability, with variations driving evolution.
  2. Variation’s Benefit: Variations enhance species survival in changing environments (e.g., heat-resistant bacteria survive warming waters), but individuals may not benefit if variations are harmful.

7.1.1 The Importance of Variation

• Reproduction maintains body design features, allowing organisms to occupy specific ecological niches.
• Environmental Changes: Variations in populations ensure some individuals survive drastic niche changes (e.g., temperature shifts, meteorite impacts).
• Variation is crucial for species survival over time, not just individual survival.

7.2 Modes of Reproduction Used by Single Organisms

• Single organisms use asexual reproduction, creating new individuals from one parent, depending on their body design.
• Activity 7.1 (Yeast Growth):
  • Mix sugar solution with yeast, observe under a microscope after 1-2 hours to see budding (small outgrowths forming new yeast cells).
• Activity 7.2 (Mould Growth):
  • Keep wet bread in a cool, moist, dark place; observe mould hyphae and sporangia over a week using a magnifying glass.
  • Comparison: Yeast grows via budding (single cells), while mould spreads via hyphae and reproduces via spores.

7.2.1 Fission

• Unicellular organisms reproduce by cell division (fission), creating new individuals.
• Types:
  • Binary Fission: Splits into two equal halves (e.g., Amoeba, Fig. 7.1a; Leishmania, Fig. 7.1b, with oriented division due to whip-like structure).
  • Multiple Fission: Divides into many daughter cells (e.g., Plasmodium, Fig. 7.2).
• Activity 7.3: Observe Amoeba slides (normal and binary fission) to compare structure and division process.
• Fission’s simplicity suits unicellular organisms’ basic body design.

7.2.2 Fragmentation

• Simple multicellular organisms (e.g., Spirogyra) break into fragments, each growing into a new individual.
• Activity 7.4: Observe Spirogyra filaments under a microscope; note lack of specialized tissues, enabling fragmentation.
• Fragmentation is impractical for complex organisms with specialized tissues and organs.

7.2.3 Regeneration

• Some organisms (e.g., Hydra, Planaria, Fig. 7.3) regenerate complete individuals from body parts using specialized cells that proliferate and differentiate.
• Not equivalent to reproduction, as organisms typically don’t rely on being cut to reproduce.
• Regeneration showcases the developmental potential of specialized cells.

7.2.4 Budding

• Organisms like Hydra form buds via repeated cell division at a specific site, which mature and detach as new individuals (Fig. 7.4).
• Budding leverages regenerative cells for asexual reproduction.

7.2.5 Vegetative Propagation

• Plants use roots, stems, or leaves to produce new plants (e.g., sugarcane, roses).
• Advantages:
  • Faster flowering/fruiting than seed-grown plants.
  • Propagates seedless plants (e.g., banana, jasmine).
  • Produces genetically identical plants, preserving desirable traits.
• Activity 7.5 (Potato):
  • Cut potato pieces, some with buds, place on moist cotton, and observe; only pieces with buds grow shoots and roots.
• Activity 7.6 (Money Plant):
  • Cut money plant pieces with and without leaves, place in water; only leaf-containing pieces grow new leaves, showing leaf-based propagation.
• Example: Bryophyllum leaves produce buds in notches, developing into new plants (Fig. 7.5).
• Tissue Culture: Grows plants from cells in artificial media, forming callus, then plantlets; used for disease-free ornamental plants.
• Vegetative propagation enhances agricultural efficiency.

7.2.6 Spore Formation

• Simple multicellular organisms (e.g., Rhizopus) form sporangia containing spores with thick protective walls (Fig. 7.6).
• Spores germinate on moist surfaces, forming new individuals.
• Spores ensure survival in harsh conditions, aiding reproduction.

7.3 Sexual Reproduction

• Sexual reproduction involves two individuals combining DNA, increasing variation compared to asexual modes.
• Questions:
  1. Binary vs. Multiple Fission: Binary produces two cells (e.g., Amoeba); multiple produces many (e.g., Plasmodium).
  2. Spore Benefits: Spores resist harsh conditions, disperse widely, and require minimal resources.
  3. Regeneration Limits: Complex organisms have specialized tissues, making cell-by-cell regeneration impractical.
  4. Vegetative Propagation: Used for faster growth, seedless plants, and genetic uniformity.
  5. DNA Copying: Essential for inheriting traits and introducing variations.

7.3.1 Why the Sexual Mode of Reproduction?

• DNA copying introduces variations, but the process is slow due to accuracy in copying mechanisms.
• Sexual reproduction combines DNA from two individuals, creating novel variation combinations, enhancing species survival.
• Challenge: Combining DNA doubles chromosome/DNA content, potentially disrupting cellular control.
• Solution: Meiosis halves chromosome number in germ cells, restoring normal DNA content in the zygote.
• Germ Cell Specialization:
  • Male gamete: Small, motile (e.g., sperm).
  • Female gamete: Large, non-motile, stores food (e.g., egg).
• Sexual reproduction’s variation supports adaptation in complex organisms.

7.3.2 Sexual Reproduction in Flowering Plants

• Flower Structure:
  • Reproductive Parts: Stamens (male, produce pollen) and pistil (female, contains ovary).
  • Pistil Parts: Ovary (contains ovules with egg cells), style (elongated), stigma (sticky, receives pollen).
  • Non-Reproductive: Sepals (protect bud), petals (attract pollinators).
  • Types: Unisexual (e.g., papaya, only stamen or pistil) or bisexual (e.g., Hibiscus, both).
• Pollination:
  • Self-Pollination: Pollen transfers within the same flower.
  • Cross-Pollination: Pollen transfers between flowers, aided by wind, water, or animals.
• Fertilization:
  • Pollen germinates on stigma, forming a pollen tube through the style to the ovary (Fig. 7.8).
  • Male germ cell fuses with egg, forming a zygote.
• Post-Fertilization:
  • Zygote forms an embryo; ovule becomes a seed; ovary becomes a fruit.
  • Other parts (petals, sepals) often fall off, though some persist in fruits.
• Germination: Seeds develop into seedlings under suitable conditions (Activity 7.7: Observe Bengal gram seed parts).
• Seed formation protects embryos and aids dispersal.

7.3.3 Reproduction in Human Beings

• Humans use sexual reproduction, with changes during puberty signaling sexual maturation.
• Puberty Changes:
  • Common: Hair growth (armpits, genitals), oily skin, pimples.
  • Girls: Breast enlargement, menstruation.
  • Boys: Facial hair, voice cracking, penile erections.
  • These gradual changes vary in timing and extent among individuals.
• Puberty’s Role: Reproductive tissues mature as general body growth slows, prioritizing germ cell production.

7.3.3 (a) Male Reproductive System

• Components (Fig. 7.10):
  • Testes: Produce sperm in scrotum (lower temperature) and testosterone (drives puberty changes).
  • Vas Deferens: Transports sperm, uniting with urethra (shared with urine).
  • Prostate/Seminal Vesicles: Add fluid for sperm transport and nutrition.
  • Penis: Delivers sperm during intercourse.
• Sperm are motile, carrying genetic material to the egg.

7.3.3 (b) Female Reproductive System

• Components (Fig. 7.11):
  • Ovaries: Produce eggs and hormones (e.g., oestrogen).
  • Fallopian Tubes: Carry eggs to uterus; site of fertilization.
  • Uterus: Nurtures embryo; thickens monthly.
  • Vagina: Receives sperm; birth canal.
• Egg production: Ovaries contain immature eggs at birth; one matures monthly post-puberty.
• The female system supports fertilization and pregnancy.

7.3.3 (c) What Happens When the Egg is Not Fertilized?

• Unfertilized eggs live ~1 day; uterine lining, prepared for pregnancy, sheds as menstruation (blood and mucous) every ~28 days, lasting 2-8 days.
• Menstruation reflects the reproductive cycle’s preparation for potential pregnancy.

7.3.3 (d) Reproductive Health

• Challenges: Sexual maturation doesn’t ensure mental/physical readiness for reproduction; societal pressures (peers, family, government) complicate choices.
• Sexually Transmitted Diseases (STDs):
  • Bacterial: Gonorrhoea, syphilis.
  • Viral: Warts, HIV-AIDS.
  • Prevention: Condoms reduce transmission risk.
• Contraception:
  • Barriers: Condoms, vaginal coverings prevent sperm-egg contact.
  • Hormonal: Oral pills alter hormonal balance to prevent ovulation (side effects possible).
  • Intrauterine Devices: Copper-T, loops prevent implantation (may cause irritation).
  • Surgical: Vasectomy (blocks vas deferens), tubectomy (blocks fallopian tubes); safe long-term but surgery risks infections.
• Population and Ethics:
  • Population growth strains living standards, but inequality is a larger issue.
  • Illegal sex-selective abortions reduce female-male sex ratios, banned to maintain societal balance.
• Reproductive health balances biological, social, and ethical considerations.

Exercises

1. Budding Organism: (b) Yeast.
2. Not Part of Female System: (c) Vas deferens.
3. Anther Contents: (d) Pollen grains.
4. Advantages of Sexual Reproduction:
   • Greater variation via DNA combination, enhancing species survival.
   • Unlike asexual, allows novel trait combinations.
5. Testis Functions:
   • Produce sperm.
   • Secrete testosterone, driving puberty changes.
6. Menstruation Cause: Uterine lining sheds if no fertilization occurs, as it’s no longer needed.
7. Longitudinal Section of Flower:
   • Diagram includes sepals, petals, stamens (anther, filament), pistil (stigma, style, ovary with ovules).
8. Contraception Methods:
   • Barriers (condoms), hormonal (pills), intrauterine devices (copper-T), surgical (vasectomy, tubectomy).
9. Unicellular vs. Multicellular Reproduction:
   • Unicellular: Simple (fission, budding) due to single-cell design.
   • Multicellular: Complex (sexual, vegetative) due to specialized tissues.
10. Reproduction and Population Stability: Maintains species traits via DNA copying, with variations ensuring survival in changing environments.
11. Reasons for Contraception:
    • Prevent STDs, avoid unplanned pregnancies, manage population growth, protect maternal health.