How Do Organisms Reproduce? – Class 10 Science Notes

How Do Organisms Reproduce?

Topics covered

Why Do Organisms Reproduce?
DNA Copying & Variations
Asexual Reproduction

Fission
Fragmentation
Regeneration
Budding
Vegetative Propagation
Spore Formation

Sexual Reproduction

Sexual Reproduction in Plants
Sexual Reproduction in Humans

Male Reproductive System
Female Reproductive System
Menstruation
Reproductive Health

Why Do Organisms Reproduce?

Reproduction is the biological process by which living organisms produce new individuals of their own kind, ensuring the continuation of their species.

Importance of reproduction:

Maintains continuity of species
Increases population
Transfer of characters from parent to offspring
Variation helps in survival & evolution

Role of DNA in Reproduction

DNA is the genetic material present in chromosomes.
It stores information to control body structure and functions.
Reproduction involves making a copy of DNA.
Cell divides → DNA copies separate → two similar cells are formed.

Is Copying Perfect?

No biochemical reaction is 100% perfect.
During DNA copying, small changes (variations) occur.
Some harmful variations → cell dies
Useful variations → help in survival → basis of evolution

Importance of Variation

Environment keeps changing (climate, food, water etc.)
Variations help some individuals survive when conditions change.
Example: few heat-resistant bacteria survive global warming.
Conclusion: Variation = survival of species

📌 NCERT Questions

Importance of DNA copying → to pass genetic information to next generation & maintain body design.
Variation benefits species → helps survival during environmental changes.

DNA Copying and Variation

DNA (Deoxyribonucleic acid) is a molecule in the nucleus of cell which carries genetic information and controls the transfer of characters from parents to offspring.
During reproduction, DNA is copied → new cell gets same blueprint
Copying is not 100% accurate → small changes occur called variations
Variations help species survive in changing environment

Asexual Reproduction

New organism formed from one parent only, no gametes involved.

1. Fission:

The process of reproduction in unicellular organisms in which a parent cell divides into two or more new individuals.

Binary fission:
Binary fission is a type of asexual reproduction in which a unicellular parent organism divides into two daughter cells.

Process:

The nucleus divides first, followed by the division of cytoplasm (cytokinesis).
Each daughter cell receives a copy of DNA and develops into a complete new organism.

Example,

Amoeba — Splitting of the cell can take place in any plane. Parent cell divides into two daughter cells
Leishmania – Unicellular organism has a whip-like structure at one end of the cell. Binary fission occurs in a definite orientation in relation to the structure. It causes Kala-azar disease.

Multiple fission: It is a type of asexual reproduction in which single parent cell divides to form many daughter cells. Example, Plasmodium (malaria causing parasite).

Process:

First, the nucleus divides repeatedly to produce several nuclei.
Then, the cytoplasm divides, forming many daughter cells inside the parent cell.
Under unfavourable conditions, a cyst wall forms around the parent cell for protection.
When conditions become favourable, the cyst wall breaks, releasing all the daughter cells.

2. Fragmentation

Organism breaks into two or more fragments and each fragment becomes new organism. It can be seen in simple multicellular organisms.
Occurs when the organism matures.
Happens in favourable conditions — enough water, nutrients, and sunlight.
In complex multicellular organisms, this type of reproduction is not possible because their cells are organised into tissues and organs.
Example: Spirogyra

3. Regeneration

If body is cut into pieces, each piece forms a complete organism.
Example: Planaria, Hydra
Done by special regenerative cells
Regeneration occurs only in simple organisms because they have simple body design and less cell specialization. Their cells can divide and form all body parts.
In complex organisms, cells are highly specialized, so they can only repair tissues, not regenerate the whole body.

Process:

These specialized cells proliferate (multiply) very rapidly.
This causes them to form a large, unorganized group or mass of cells.
These cells develop into various cell types and tissues.
The changes occur in an organised sequence called development.

4. Budding

Budding is a type of asexual reproduction where a new individual grows directly from the parent.

A small bump or "bud" appears on the parent body.
This bud grows larger using cell division.
Once fully developed, the bud breaks off and lives as a separate, new organism.

Examples: Yeast, Hydra

5. Vegetative Propagation

A type of asexual reproduction in which roots, stems, or leaves of a plant develop into a new plant under suitable conditions.

Parts of a plant:

Reproductive part: This part of the plant (flower) which is involved in the process of sexual reproduction.
Vegetative part: These parts of the plant (stem, leaf, root) are not involved in the process of sexual reproduction.

Modes of Vegetative Propagation

Mode of Vegetative Propagation	Organism / Example
By Underground Stem (Tubers)	Potato
By Roots (Tuberous Roots)	Sweet Potato, Dahlia
By Leaves (Leaf Buds)	Bryophyllum, Kalanchoe

Artificial Vegetative Propagation

It is a man-made method of plant reproduction.
New plants grow from vegetative parts: stem, root, or leaf.
Special techniques are used so the new plant is similar to the parent plant.

1. Cutting

A piece of the stem or leaf is cut from the parent plant.
The cut piece is planted in soil or water.
It develops roots and becomes a new plant.
Examples: Rose, Hibiscus

2. Grafting

A branch (called scion) of one plant is joined to another plant (called rootstock).
They grow together and act as a single plant.
Useful to combine good fruit quality with strong roots.
Examples: Mango, Apple

3. Layering

A branch is bent down and covered with moist soil while still attached to the parent plant.
Roots form at the buried part.
When roots are strong, the branch is cut and becomes a new plant.
Example: Jasmine

4. Tissue Culture

A few cells or small tissues are taken from a plant and placed in a nutrient medium in the lab.
Under controlled conditions, these cells grow and form many new plants.
Used to produce a large number of plants quickly and to get disease-free plants.

Quick Summary

Cutting: Cut stem/leaf → plant → roots → new plant.
Grafting: Join branch of one plant to another → both grow together.
Layering: Bend branch to ground → cover with soil → roots form → cut → new plant.
Tissue culture: Plant cells in lab → many plants fast; disease-free.

Advantages of Vegetative Propagation

New plants are genetically identical to the parent (clones).
Plants made this way usually survive well (high success rate).
Good qualities of the parent plant (colour, taste, fragrance, resistance) are kept.
Helps produce disease-free plants using tissue culture.
Plants that do not make seeds or take long to grow from seeds can be easily multiplied.

Disadvantages of Vegetative Propagation

Offspring have no genetic variation (all look the same).
If the parent has a disease, it can pass to all new plants.
Not all plants can be propagated this way—only some species.

6. Spore Formation

A type of asexual reproduction in which new individuals are formed from tiny spores produced inside special reproductive structures called sporangia.

The thread-like structures that developed on the bread above are the hyphae of the bread mould (Rhizopus). They are not reproductive parts. On the other hand, the tiny blob called sporangia on-a-stick structures are involved in reproduction.

Sporangia are special reproductive structures found in fungi and some algae. They contain spores, which are tiny, single-celled reproductive units. When the sporangium bursts, the spores are released into the air. Under suitable conditions, each spore germinates to form a new organism.

Example: The black spots you see on old bread (like Rhizopus or bread mould) are groups of spores!

Q. Why is spore formation considered better than other methods of reproduction?

Tough Protection: Spores have a hard shell that keeps them safe from things like being too hot, too dry, or having no food.
Built to Last: They can wait out bad times (unfavorable conditions) and only start growing (germinate) when the conditions become good again.
Travel Light: Spores are very tiny and light, so the wind or water can easily carry them far away. This helps them spread out (distribution).
Fast and Many: Just one spore-producing structure (called a sporangium) can make a huge number of spores, which means they can reproduce and multiply very quickly.

Sexual Reproduction

It's a way to make new individuals that needs two parents—a male and a female.
Both sexes are necessary to produce offspring.
Fusion of male and female Gametes (egg + sperm) to form single cell (zygote) is called fertilisation.
Examples: Bull + Cow gives Calf; Hen + Cock gives Chick.

Advantages of Sexual Reproduction

Creates Differences (Variation): Offspring are not identical to the parents, so the population gets a lot of different traits.
Better Survival (Adaptability): These differences help organisms adjust to changes in their environment.
Evolution: It encourages evolution (species gradually changing over time) through natural selection.
Increases Species Safety (Survival): The differences in offspring increase the chances that the species will survive even when conditions are very bad (adverse conditions).

Sexual Reproduction in Flowering Plants

Parts of a Flower and Their Functions

Part of Flower	Function
Sepals	Green leaf-like parts that protect the flower when it is a bud.
Petals	Bright and colourful to attract insects and other pollinators.
Anther	Makes pollen grains which contain the male gametes (plant sperms).
Filament	A stalk that holds the anther in place; together they form the stamen (male part).
Stigma	Sticky tip of the pistil that catches and holds pollen during pollination.
Style	A tube-like part that connects the stigma to the ovary; the pollen tube grows through it.
Ovary	Swollen base of the pistil containing ovules; after fertilisation it becomes the fruit.
Ovule	Inside the ovary; contains the female gamete (egg). After fertilisation it develops into a seed.
Pistil	The female reproductive part of the flower and is present at the center of flower.
Stamen	The male reproductive part of the flower.

In flowering plants, new plants are produced by sexual reproduction. The male gamete present in the pollen grain and the female gamete present in the ovule fuse together.
First, pollination takes place in which pollen is transferred from the anther to the stigma.
After this, a pollen tube grows and carries the male gamete to the ovule inside the ovary.
Here fertilisation occurs and a zygote is formed.
The ovule changes into a seed and the ovary develops into a fruit, which protects the seed and helps in seed dispersal.

Pollination:

The process of transfer of pollen grains from the anther (male part) to the stigma (female part) of a flower. Transfer of pollen grains can be done by abiotic components (wind, water) and biotic components (birds, insects, bats, and snails).

Types of pollination: Self pollination and cross pollination.

Self-Pollination	Cross-Pollination
Pollen moves from the male part (anther) to the female part (stigma) on the same flower.	Pollen moves from the anther of one flower to the stigma of another flower (of the same species).
Happens in flowers that have both parts (bisexual) (e.g., Hibiscus).	Can happen in both single-sex (unisexual) or bisexual flowers (e.g., Papaya).
No outside help (agents) is needed.	Requires agents like wind, water, insects, or birds to move the pollen.
Offspring show less variation (they are genetically very similar to the parent).	Offspring show more variation (due to mixing genes from two different plants).

Fertilization: Making the Seed

Fertilization is when the male cell joins the female cell to start a new plant.

What it is: It is the fusion (joining) of the male cell (from the pollen) with the female egg cell inside the ovule.
The Steps:
1. The pollen lands on the sticky part of the flower (stigma).
2. The pollen grows a small tube called a pollen tube.
3. This tube grows down through the flower parts and reaches the ovary.
4. The male cell travels down the tube and fuses with the egg cell inside the ovule.
5. This fusion creates a zygote, which will grow into the new plant embryo.

Post-Fertilization: Making Fruits and Seeds

Post-fertilization is all the changes that happen in a flower after fertilization to form seeds and fruits.

Key Changes:

The Zygote develops into an Embryo.
The Ovule becomes the Seed (with a protective coat).
The Ovary develops into the Fruit.
The flower parts like petals, sepals, stamens, style, and stigma usually dry up and fall off.

Pollination vs. Fertilization
Pollination (Movement)	Fertilization (Joining)
It's the transfer of pollen from the anther to the stigma of a flower.	It's the fusion (joining) of the male cell (from pollen) with the female egg cell inside the ovule.
It is a physical process (pollen movement).	It is a biological process (cells joining).
Happens before fertilization.	Happens after pollination.
No seed or zygote is formed.	Forms a zygote, which leads to the seed and fruit.

SEED-GERMINATION

Seed is the final product of Sexual reproduction.

Germination:It is the process by which a seed develops into a new plant (seedling) under suitable conditions.

PARTS OF A SEED:

Seed Coat: The outer protective covering of the seed.
It protects the seed and embryo from injury, drying, and infection.
Cotyledons: Also called seed leaves.
It contains stored food that nourishes the embryo during germination.
Embryo (Baby Plant): The future plant present inside the seed.

Embryo is made up of two main parts:
- Radicle → develops into the root system.
- Plumule → develops into the shoot system.

Reproduction in Human Beings

Puberty

Puberty is the period when a child’s body grows and becomes sexually mature. During puberty, the reproductive organs develop fully and the body becomes capable of reproduction. It marks the beginning of adolescence with many physical and hormonal changes in boys and girls.

Age of occurrence:

In girls: around 10–12 years
In boys: around 12–14 years

Changes in Girls

Breasts start growing
Skin around nipples becomes darker
Menstruation (periods) begins

Changes in Boys

Facial hair growth (mustache and beard)
Voice becomes deeper
Penis grows and erections can occur

Changes Common in Both Boys and Girls

Hair grows in armpits and genital area
Thin hair appears on arms, legs and face
Skin becomes oily → pimples may appear
Height increases and body shape changes
Become more aware of their own body and emotions

Male vs. Female Germ-Cells (Gametes)

Feature	Male Germ-Cell (Sperm)	Female Germ-Cell (Egg/Ovum)
Production Site	Produced in testes.	Produced in ovaries.
Size & Movement	Very small and motile (can move).	Large and non-motile (cannot move).
Quantity	Millions are produced continuously after puberty.	Usually one ovum is released in a month after puberty.
Shape	Motile with a tail (flagellum) for movement.	Round in shape, no tail.

Male Reproductive System

Functions:

The male reproductive system helps in forming and transporting male gametes (sperms) needed for fertilisation in humans. Here are the main functions:

Testes make sperms which are the male gametes.
Testes also produce testosterone, the male sex hormone that controls puberty changes in boys.
Tubes and glands provide a pathway and fluid to help sperms move easily.
Penis helps in releasing sperms into the female body during sexual reproduction.

Organ	Location / Structure	Function
Testes	Present inside the scrotum, outside the abdominal cavity.	Make sperms and release testosterone (male hormone).
Scrotum	Sac-like structure around the testes.	Keeps testes cooler than body temperature for sperm formation.
Vas deferens	Tube connected from testes to urethra.	Carries sperms from testes to urethra.
Seminal vesicle	Located near the bladder, along vas deferens.	Adds f luid to sperms → gives nutrition and helps in movement.
Prostate gland	Located near urethra, connected to vas deferens.	Adds fluid to sperms → makes transport easy.
Urethra	Tube that passes through penis.	Common passage for urine and sperms.
Penis	External male organ.	Transfers sperms into female body.

Female Reproductive System

Organ / Process	Location / Structure	Function
Ovary	Two small organs in the lower abdomen, one on each side of the uterus.	Make female sex cells called ova/eggs and also produce female hormones (estrogen and progesterone).
Ovulation	This is a monthly process happening in the ovary.	Release of a mature egg (ovum) from an ovary.
Fallopian Tubes (Oviducts)	Thin tubes that connect the ovaries to the uterus.	Carry the egg from ovary to uterus. Fertilisation of egg and sperm happens here.
Uterus (Womb)	A strong, hollow organ in the pelvic region.	The foetus develops here after the fertilised egg implants in its soft lining (endometrium).
Cervix	Narrow lower part of the uterus.	Connects uterus to vagina; allows passage of sperms; widens (dilates) during childbirth.
Vagina	A muscular passage from the cervix to the outside of the body.	Receives sperms during intercourse; functions as the birth canal during delivery.

Process of Reproduction in Females:

During copulation, the male releases millions of sperms into the female reproductive system.
Sperms swim towards the egg in the fallopian tube using their tails.
Only one sperm is able to enter and fertilize the female egg.
After fertilization, a zygote (fertilized egg) is formed.
The zygote slowly moves to the uterus.
In the uterus, the zygote implants into the soft inner lining called the endometrium.
The zygote keeps dividing to form an embryo.
The embryo grows and develops into a foetus inside the uterus.

Implantation: It is the process where the fertilized egg (zygote) attaches to the lining of the uterus (endometrium) to develop into an embryo.

Placenta: It is a special tissue connection between the embryo (developing baby) and the wall of the uterus in the mother. It is formed by the embryonic and maternal tissues.

Functions of Placenta:

Provides nourishment to the embryo.
Removes waste materials from the embryo’s blood.
Allows exchange of gases — oxygen and carbon dioxide.
Transports nutrients and oxygen from the mother’s blood to the embryo.
Transfers antibodies and hormones from mother to the foetus.

How Placenta Helps in Nourishment:

The mother’s blood rich in oxygen and nutrients passes into the embryo’s blood through the placenta by diffusion, while waste products from the embryo diffuse back into the mother’s blood for removal.

Menarche: This is a girl's very first period, which happens when she starts puberty, usually between the ages of 9 and 16. It means her body is becoming capable of reproduction.

Menstruation: Also called a "period," this is the monthly process where the uterus sheds its inner lining (blood and tissue) because the egg doesn't fertilise. This natural cycle continues from menarche until menopause, typically every 4–5 weeks.

Menopause: This is when a woman's periods naturally and permanently stop, marking the end of her reproductive years. It usually occurs around 45 to 55 years of age.

Reproductive Health

Reproductive health means being healthy in every way—physically, mentally, and socially—when it comes to having children and sexual well-being.
It involves practicing safe sex. It includes knowing how to prevent diseases.

Sexually Transmitted Diseases (STDs): STDs are sicknesses that are passed from one person to another, mostly through sexual activity.

Examples of STDs:

Bacterial Infections (can often be cured with medicine): Gonorrhea, Syphilis
Viral Infections (cannot usually be cured, but symptoms can be managed): HIV/AIDS, Genital Warts

Contraception: Contraception (or birth control) is simply about preventing pregnancy. It uses methods that either block the sperm from meeting the egg or stop a fertilized egg from starting to grow inside the uterus.

Method	Examples	How it works
Mechanical/ Barrier	Condom	Prevents the sperm and ovum from meeting Prevent STDs.
Hormonal Chemical methods	Pills	Prevent ovulation, alter cervical mucus, or change the uterus lining to prevent implantation, May have side effect Does not prevent STDs.
Intra Uterine Contraceptive Devices (IUD)	Copper-T or Loops	Planted in uterus to prevent implantation Can cause uterine irritation
Surgical	Vasectomy (males), Tubectomy (females)	Blocks vas deferens to prevent sperm transfer in males. Blocking the fallopian tube to prevent egg transfer Surgery is safe in long run but may have risk like infections.

NAMAN GARG

Sound – Class 9 Science Complete Notes