Chapter 7: How Do Organisms Reproduce?

1. Introduction

All living organisms have a fixed life span. To continue their species, they must produce new individuals — this process is called reproduction.
 Reproduction helps in:

  • Continuation of species
  • Transfer of genetic information
  • Evolution through variation

Organisms reproduce mainly in two ways:

  1. Asexual Reproduction
  2. Sexual Reproduction

2. Asexual Reproduction

Asexual reproduction involves only one parent.
 There is no fusion of gametes, and the offspring are identical to the parent.
 It usually occurs in unicellular organisms and some plants and animals.

2.1. Types of Asexual Reproduction

(a) Fission

The parent cell divides into two or more individuals.

      • Binary Fission – The cell divides into two equal halves.
        Example: Amoeba, Paramecium, Euglena
         → Amoeba → divides → 2 Amoebae
    • Multiple Fission – The parent cell divides into many daughter cells.
       Example: Plasmodium (causes malaria)
       → Plasmodium → divides → many daughter cells

(b) Budding

A small outgrowth or bud develops on the parent’s body.
 This bud grows, detaches, and becomes a new individual.
 Example: Hydra, Yeast

→ Yeast → bud formation → new yeast cell

(c) Fragmentation

The body of the organism breaks into fragments, and each fragment develops into a new individual.
 Example: Spirogyra (a green alga)
 → Spirogyra filament → fragments → new filaments

(d) Regeneration

Some organisms can regenerate lost body parts, and each part can grow into a new organism.
 Example: Planaria, Starfish

→ Planaria → cut pieces → each grows into new Planaria

(e) Spore Formation

Organisms produce tiny spores that grow into new individuals when conditions are favorable. Example: Rhizopus (bread mould)


 → Rhizopus → spores in sporangia → released → grow into new fungus

(f) Vegetative Propagation

A form of asexual reproduction in plants where new plants grow from roots, stems, or leaves instead of seeds.

Plant Part

Example

Root

Sweet potato

Stem

Potato (tuber), Ginger (rhizome), Onion (bulb)

Leaf

Bryophyllum (buds on leaf margins)

Advantages:

  • Plants grow faster than from seeds.
  • Useful for growing plants with desired qualities.
  • Helps in producing seedless plants like banana and sugarcane.

3. Sexual Reproduction

Sexual reproduction involves two parents — a male and a female.
 It requires the fusion of male and female gametes to form a zygote, which grows into a new organism.

This method leads to variation, making offspring genetically different from parents.

Steps of Sexual Reproduction

  1. Formation of Gametes – Male and female gametes are produced.
  2. Fusion (Fertilization) – Male and female gametes unite to form a zygote.
  3. Development – The zygote divides and grows into a new organism.

3.1. Sexual Reproduction in Flowering Plants

The flower is the reproductive organ of the plant.

Structure of a Flower

Part

Function

Stamen (Male part)

Produces pollen grains (male gametes)

Carpel/Pistil (Female part)

Contains ovary with ovules (female gametes)

Petals

Attract insects for pollination

Sepals

Protect the flower in bud stage

Pollination

The transfer of pollen grains from the anther to the stigma of a flower.

Types:

  1. Self-pollination: Pollen from the same flower or same plant.
  2. Cross-pollination: Pollen from one plant to another.

Agents of Pollination: Wind, water, insects, birds.

Fertilization in Plants

After pollination:

  • Pollen grain lands on stigma → grows a pollen tube → reaches ovule inside ovary.
  • Male nucleus fuses with female nucleus → zygote is formed.
  • Ovule → seed; Ovary → fruit.

Seed Germination

When a seed gets water, oxygen, and warmth, it grows into a new plant.

3.2. Sexual Reproduction in Humans

Humans reproduce through the sexual method involving male and female reproductive systems.

3.2.1. Male Reproductive System

Main Organs:

  1. Testes – Produce sperms and male hormone testosterone.
  2. Scrotum – Sac that protects testes and keeps temperature low for sperm production.
  3. Vas deferens (sperm duct) – Carries sperms to the urethra.
  4. Seminal vesicle & Prostate gland – Add fluids to sperms to form semen.
  5. Penis – Helps in transferring sperms into the female body.

3.2.2. Female Reproductive System

Main Organs:

  1. Ovaries – Produce eggs (ova) and female hormones estrogen and progesterone.
  2. Oviducts (Fallopian tubes) – Site of fertilization; carries egg to the uterus.
  3. Uterus – Where the zygote develops into an embryo.
  4. Cervix – Opening between uterus and vagina.
  5. Vagina – Receives sperm during reproduction.

3.2.3. Fertilization in Humans

  • Internal fertilization occurs inside the female body.
  • Sperm (n) + Ovum (n) → Zygote (2n)
  • The zygote divides to form an embryo, which attaches to the uterine wall and grows.

3.2.4. Development of the Embryo

  • The embryo develops a placenta, which provides oxygen and nutrients from the mother’s blood.
  • After about 9 months (gestation period), the baby is born.

4. Reproductive Health

Reproductive health means physical, mental, and social well-being related to the reproductive system.

4.1. Puberty

The stage when a person becomes sexually mature.

Changes in Boys:

    • Growth of beard, mustache, body hair
    • Deep voice
    • Enlargement of testes and penis

Changes in Girls:

    • Development of breasts
    • Menstruation begins
    • Hips widen

Age: 11–14 years (varies in individuals)

4.2. Menstrual Cycle

A monthly cycle in females (≈28 days).

Stage

Description

1–5 days

Menstruation – unfertilized egg and uterus lining come out

6–14 days

Egg development in ovary

14th day

Ovulation – release of egg

15–28 days

Uterus prepares for fertilized egg

If fertilization does not occur, menstruation starts again.

4.3. Contraception (Birth Control)

Used to avoid unwanted pregnancy and maintain reproductive health.

Methods:

Type

Example

Function

Barrier

Condom

Prevents sperm entry into vagina

Hormonal

Pills

Prevent ovulation

Intrauterine Device (IUD)

Copper-T

Prevents implantation

Surgical

Vasectomy (male), Tubectomy (female)

Blocks gamete transport

4.4. Sexually Transmitted Diseases (STDs)

Diseases spread through sexual contact.

Examples:

      • AIDS (caused by HIV)
      • Gonorrhea
      • Syphilis

Prevention:

      • Use protection (condoms)
      • Maintain hygiene
      • Get regular medical checkups

4.5. Importance of Reproductive Health

  • Prevents early pregnancy and STDs
  • Promotes family planning
  • Encourages healthy lifestyle and awareness

5. Importance of Variation

Variations occur during sexual reproduction due to genetic recombination.
 They help species adapt to changes and improve chances of survival.

Example:
 Bacteria that show variation may survive antibiotics better.

6. Summary Table

Mode

Parent(s)

Gametes

Example

Asexual

One

Not involved

Amoeba, Yeast

Sexual

Two

Male + Female

Humans, Plants

7. Key Terms

  • Reproduction: Process of producing new organisms
  • Fertilization: Fusion of male and female gametes
  • Zygote: Single cell formed after fertilization
  • Pollination: Transfer of pollen to stigma
  • Embryo: Early stage of developing baby
  • Menstruation: Monthly shedding of uterus lining
  • Contraception: Prevention of pregnancy

8. Quick Revision Points

  • Asexual reproduction – one parent, no variation
  • Sexual reproduction – two parents, variation present
  • Fertilization forms zygote → embryo → baby
  • Puberty brings reproductive maturity
  • Maintain reproductive health and hygiene

 

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