How do Organisms Reproduce? — Detailed Notes

Chapter 7 | NCERT Class 10 Science

7.1 Do Organisms Create Exact Copies of Themselves?

Reproduction spends energy an individual does not need for its own survival, but populations of similar-looking individuals define species we notice. Similarity means similar body-design blueprints — DNA in chromosomes. DNA is the information source for proteins; changed DNA → different proteins → altered body designs.

Basic reproductive event: create a DNA copy and additional cellular apparatus, then separate into two cells. DNA copying is not perfectly accurate → variations. Extreme faulty copies may die; subtle variations persist. This tendency for variation underlies evolution.

7.1.1 Importance of Variation

Consistent DNA copying preserves niche-adapted body designs (population stability). But niches can change (temperature, water, disasters). Variants may survive when conditions shift (e.g. heat-resistant bacteria in warming water). Variation helps species survival over time, though a given variant may not help every individual.

Better Method / Strategy

Start “why reproduce?” answers with: not for individual survival; for continuity + DNA copy.

Variation answer: species-level insurance against environmental change.

7.2 Modes of Reproduction Used by Single Organisms

Reproduction modes — Asexual vs sexual reproduction map

Mode depends on body design. Methods below create new generations from one parent → asexual reproduction.

7.2.1 Fission

Unicellular organisms divide.

Binary fission: split into two — Amoeba (any plane); Leishmania (definite orientation relative to whip-like structure)
Multiple fission: many daughter cells at once — Plasmodium

(Budding is not a type of fission — see §7.2.4. Yeast buds; it does not “fission” in the binary/multiple sense above.)

Diagram — binary fission in Amoeba:

7.2.2 Fragmentation

Spirogyra filaments break into pieces; each fragment grows into a new individual. Works in simple multicellular organisation. Complex organisms cannot simply divide cell-by-cell because tissues/organs must stay correctly placed — they need specialised reproductive cells/strategies.

7.2.3 Regeneration

Hydra, Planaria: body pieces can grow into complete organisms via specialised cells that proliferate and develop. Regeneration is not the same as normal reproduction — organisms do not usually depend on being cut up to reproduce.

7.2.4 Budding

Hydra: bud forms by repeated division at one site → matures → detaches as new individual.
Yeast: small outgrowth (bud) forms on the parent cell, receives a nucleus, and separates as a new cell — still budding, not fission.

7.2.5 Vegetative Propagation

Roots, stems, leaves of some plants give new plants (layering, grafting — sugarcane, rose, grapes). Advantages:

Flower/fruit earlier than from seeds
Possible for seedless plants (banana, orange, rose, jasmine)
Offspring genetically similar to parent (desired traits kept)

Bryophyllum: buds in leaf-margin notches fall and grow. Potato “eyes” (buds) grow when present on pieces.

Tissue culture (awareness): cells from growing tip → callus on artificial medium → hormone medium → plantlets → soil; many disease-free plants from one parent (ornamentals).

7.2.6 Spore Formation

Rhizopus (bread mould): hyphae are not the reproductive structures; sporangia (“blob-on-a-stick”) contain spores with thick protective walls until they find moist conditions.

Better Method / Strategy

Exam line formula: Mode — definition — one example — one advantage (if asked).

Regeneration vs budding: both in Hydra possible, but regeneration ≠ routine reproduction.

7.3 Sexual Reproduction

Needs two individuals (generally) so germ-cells can combine.

7.3.1 Why Sexual Mode?

DNA copying errors create variation slowly. Combining DNA from two individuals creates new combinations of already-successful variants — more diversity for species survival.

Problem: combining two full DNA sets would double DNA each generation. Solution: germ-cells formed by meiosis have half the chromosomes/DNA; fertilisation restores the number.

As bodies complexify, gametes specialise: male gamete smaller, motile; female gamete larger, with food stores.

7.3.2 Sexual Reproduction in Flowering Plants

Reproductive parts in flower: stamens (male) and pistil (female). Flowers may be unisexual (papaya, watermelon) or bisexual (Hibiscus, mustard).

Stamen → pollen grains (male germ-cells)
Pistil: stigma (sticky tip) + style + ovary (ovules with egg cells)

Pollination: transfer of pollen from anther to stigma — self (same flower) or cross (another flower) via wind, water, animals.

Fertilisation: pollen tube grows through style to ovule; male germ-cell fuses with egg → zygote.

After fertilisation: zygote → embryo; ovule → seed (tough coat); ovary → fruit. Petals/sepals/stamens/style/stigma may fall. Germination: embryo grows into seedling under suitable conditions.

Diagram — LS of flower:

Flower LS — Longitudinal section of a flower

Diagram — pollen on stigma:

7.3.3 Reproduction in Human Beings

Puberty: when general growth slows, reproductive tissues mature. Shared changes: hair in armpits/genital area, oily skin/pimples. Girls: breast development, menstruation. Boys: facial hair, voice crack, occasional erection. These signal sexual maturation for mating/parenting roles.

(a) Male reproductive system

Testes in scrotum (outside abdomen) — sperm need lower than body temperature; also make testosterone (puberty changes + sperm formation regulation). Sperm travel via vas deferens; seminal vesicles and prostate add fluid for transport and nutrition. Urethra common path for urine and sperm. Sperm: genetic material + tail for motility.

Diagram — male reproductive system:

(b) Female reproductive system

Ovaries produce eggs and hormones. From birth, ovaries hold immature eggs; from puberty, typically one egg matures per month → oviduct/fallopian tube → uterus → cervix → vagina.

Fertilisation usually in oviduct. Zygote → embryo implants in thickened uterine lining → foetus. Placenta: disc in uterine wall; embryonic villi + maternal blood spaces → large surface for glucose/O₂ to embryo and wastes to mother. Development ~nine months; birth by rhythmic uterine contractions.

Diagram — female reproductive system:

Female system — Female reproductive system

(c) If egg is not fertilised

Egg lives ~one day. Prepared thick uterine lining breaks down → blood + mucus via vagina ≈ menstruation (about monthly; lasts ~2–8 days).

(d) Reproductive health

Sexual maturity ≠ automatic readiness for sex/parenting. STDs include bacterial (gonorrhoea, syphilis) and viral (warts, HIV-AIDS). Condoms help prevent many infections.

Contraception categories:

Type	Examples	Notes
Barrier	Condoms (penis/vagina)	Also reduce STD spread
Hormonal	Oral pills	Prevent egg release; possible side-effects
IUDs	Loop, copper-T	In uterus; may irritate; no STD protection
Surgical	Block vas deferens / fallopian tubes	Long-term; surgery risks if poorly done

Illegal sex-selective abortion harms sex ratio; prenatal sex determination prohibited by law. Population size vs inequality debates are raised in NCERT for discussion.

Better Method / Strategy

Pollination = pollen transfer; fertilisation = gamete fusion — never confuse.

Copper-T / pills ≠ STD shield; condoms address both pregnancy and many STDs.

Placenta answer: structure (villi) + two-way exchange (nutrients/O₂ vs wastes).

Chapter Closing Strategy

Two maps: asexual modes; sexual plant + human timelines.
Drill differences tables weekly.
One oral run of contraception methods with “STD protection? yes/no”.

Deep Dive Add-On — Asexual Mode Tables (Clear Sorting)

Fix reminder: Budding (yeast / Hydra) is not listed under fission. Fission = cell splitting (binary / multiple). Budding = outgrowth that separates.

Master asexual comparison

Mode	What happens	Classic example(s)	Exam caution
Binary fission	One cell → two	Amoeba, Leishmania	Plane of division may be fixed (Leishmania)
Multiple fission	One cell → many	Plasmodium	Many daughters simultaneously
Fragmentation	Body breaks; pieces grow	Spirogyra	Needs simple organisation
Regeneration	Piece rebuilds whole body	Planaria, Hydra	≠ normal reproductive strategy
Budding	Bud grows then detaches	Hydra, yeast	Do not call yeast fission
Vegetative propagation	New plant from vegetative part	Potato eye, Bryophyllum, rose graft	Keeps parental traits; less variation
Spore formation	Resistant spores disperse	Rhizopus sporangia	Thick walls; wait for moisture

Fission vs budding (anti-confusion)

Point	Fission	Budding
Basic idea	Parent cell divides into daughters	Localised outgrowth becomes new individual
Yeast?	No	Yes
Amoeba?	Yes (binary)	No
Hydra?	No (not fission)	Yes (also can regenerate if cut)

Regeneration vs vegetative propagation

Point	Regeneration	Vegetative propagation
Typical organisms	Animals like Planaria / Hydra	Plants (stem, root, leaf)
Routine reproduction?	Usually no	Yes, for many cultivated plants
Human use	—	Grafting, cutting, tissue culture

Asexual vs sexual (why both exist)

Point	Asexual	Sexual
Parents	One	Generally two (gamete fusion)
Variation	Limited (copying errors)	Higher (new combinations)
Speed / energy	Often faster, less complex	More costly but diversifying
Species survival angle	Good in stable niche	Better insurance if niche changes

Sexual pathway mini-tables

Flowering plant

Step	Event
Pollination	Pollen (anther) → stigma
Pollen tube	Grows through style to ovule
Fertilisation	Male germ-cell + egg → zygote
After	Ovule → seed; ovary → fruit; zygote → embryo

Human

Step	Event
Gametes	Sperm (testes) + egg (ovary)
Fertilisation site	Usually oviduct
Implantation	Embryo in thickened uterus
Exchange	Placenta (nutrients/O₂ in; wastes out)
No fertilisation	Uterine lining shed → menstruation