Chapter 7 — How do Organisms Reproduce?

1. Why do organisms reproduce?

Reproduction is the process through which organisms produce new individuals of their own kind.
It is not essential for the survival of an individual, unlike nutrition, respiration and excretion.
It is essential for the continuity of species.
Reproduction involves copying the organism’s DNA, which carries hereditary information.
DNA copying is not perfectly accurate, so variations can arise.
Variations may help some organisms survive when environmental conditions change.

2. DNA copying and variation

Basic sequence:

DNA copying → formation of cellular apparatus → separation → new cells

DNA contains information needed for making proteins.
Changes in DNA can change proteins and, consequently, body characteristics.
DNA copying is generally accurate but not completely error-free.
Small differences produced during copying are called variations.
Variation may be harmful, neutral or useful.
Useful variations can increase the chances of survival of a species, especially when the environment changes.

Key idea: Reproduction maintains similarities within a species, while variations provide the raw material for evolutionary change.

3. Modes of reproduction

There are two broad modes:

Asexual reproduction	Sexual reproduction
Involves a single individual	Involves two individuals
New generation arises from one parent	DNA from two individuals is combined
Usually produces closely similar offspring	Produces greater variation
Common in simpler organisms	Common in more complex organisms

Asexual reproduction

A. Fission

A single-celled organism divides to form new individuals.

Binary fission: one cell divides into two.
- Examples: Amoeba, many bacteria, protozoa
Multiple fission: one cell produces many daughter cells simultaneously.
- Example: Plasmodium
Leishmania: divides in a definite orientation related to its body structure.
Yeast: reproduces by forming a bud, which grows and eventually separates.

Remember:
Binary = two
Multiple = many

B. Fragmentation

Seen in simple multicellular organisms such as Spirogyra.
The mature organism breaks into smaller pieces.
Each fragment can grow into a new individual.
This works because of the relatively simple organisation of the organism.

C. Regeneration

Some organisms can form complete individuals from body pieces.
Examples: Hydra and Planaria.
Specialised cells divide and produce many cells.
These cells differentiate into different tissues and structures.
Regeneration should not be confused with normal reproduction; an organism does not ordinarily reproduce simply by being cut into pieces.

D. Budding

A small outgrowth called a bud develops on the parent.
Repeated cell division enlarges the bud.
The mature bud separates and becomes an independent organism.
Example: Hydra.

E. Vegetative propagation

New plants can develop from roots, stems or leaves rather than seeds.

Examples:

Sugarcane
Rose
Grapes
Banana
Orange
Jasmine

Advantages:

Plants may produce flowers and fruits earlier than seed-grown plants.
Useful for plants that have lost the ability to produce seeds.
New plants retain characteristics closely similar to the parent.
Methods include layering and grafting.

Tissue culture:
Plant tissue/cells are grown in an artificial medium → rapid cell division produces a callus → hormones promote growth and differentiation → plantlets develop and are transferred to soil.

It can produce many plants from one parent under disease-free conditions.

F. Spore formation

In Rhizopus (bread mould), reproductive structures called sporangia contain spores.
Spores have thick protective walls.
They can survive until suitable conditions, especially moisture, allow them to germinate.
The thread-like hyphae are not the reproductive structures; sporangia are involved in reproduction.

One-line map of asexual reproduction

Fission → Fragmentation → Regeneration → Budding → Vegetative propagation → Spore formation

All these involve one parent.

4. Sexual reproduction

Why is sexual reproduction important?

Asexual reproduction mainly passes on variations already present in one lineage. Sexual reproduction combines variations from two individuals, creating new combinations.

This increases variation within populations and can improve the ability of species to survive changing conditions.

Role of meiosis

A major problem is that combining DNA from two individuals could otherwise double the DNA amount in every generation.

Meiosis solves this by producing specialised germ-cells containing:

Half the chromosome number
Half the DNA amount compared with ordinary body cells.

Fusion of male and female germ-cells restores the normal chromosome number in the new individual.

5. Sexual reproduction in flowering plants

The reproductive structures are found in the flower.

Main reproductive parts

Stamen = male reproductive part

Produces pollen grains.

Pistil = female reproductive part

Stigma – terminal part; may be sticky.
Style – elongated middle portion.
Ovary – swollen basal part containing ovules.
Each ovule contains an egg cell.

Unisexual vs bisexual flowers

Unisexual: contains either stamens or pistil.
- Examples: papaya, watermelon
Bisexual: contains both stamens and pistil.
- Examples: Hibiscus, mustard

Pollination

Transfer of pollen from anther to stigma.

Self-pollination: pollen moves to the stigma of the same flower.
Cross-pollination: pollen moves from one flower to another.
Agents include wind, water and animals.

Fertilisation

Pollen → pollen tube → through style → ovary → male germ-cell reaches egg → fusion → zygote.

Pollination ≠ fertilisation

Pollination = transfer of pollen
Fertilisation = fusion of male and female germ-cells

After fertilisation

Zygote → embryo

Then:

Ovule → seed
Ovary → fruit
Seed contains the embryo.
Under suitable conditions, the embryo develops into a seedling; this is germination.

6. Reproduction in human beings

Humans reproduce sexually.

Puberty

Puberty is the period during adolescence when reproductive tissues mature and sexual maturation occurs.

Changes occur gradually and not at exactly the same age in everyone.

Examples include:

Hair growth in new body regions
Changes in skin
Changes in body proportions
Development of reproductive features
Menstruation begins in girls
Voice changes and facial hair growth occur in boys

These changes are associated with sexual maturation.

7. Male reproductive system

Major parts and functions

Testes: produce sperm and secrete testosterone.
Scrotum: holds the testes outside the abdominal cavity; sperm formation requires a lower temperature than normal body temperature.
Vas deferens: transports sperm.
Seminal vesicles and prostate gland: add secretions that help sperm transport and provide nutrition.
Urethra: common passage for sperm and urine.
Penis: involved in delivering sperm.

Sperm

A sperm is a very small cell containing mainly genetic material and a tail for movement.

8. Female reproductive system

Major parts

Ovaries: produce eggs and some hormones.
Fallopian tubes/oviducts: carry the egg towards the uterus; fertilisation may occur here.
Uterus: site where the embryo implants and develops.
Cervix: opening between uterus and vagina.
Vagina: passage through which sperm enter and through which childbirth occurs.

Fertilisation and development

Egg + sperm → zygote → embryo → implantation in uterus → foetus

The placenta helps exchange materials between mother and embryo:

Glucose and oxygen reach the embryo.
Waste materials from the embryo are transferred to the mother’s blood.

Development inside the mother takes approximately nine months.

9. Menstruation

If the egg is not fertilised:

The egg survives for about one day.
The uterus had prepared a thick, blood-rich lining for possible pregnancy.
Without fertilisation, this lining is no longer needed.
It breaks down and passes out through the vagina as blood and mucus.
This monthly cycle is called menstruation.
It generally lasts about 2–8 days.

10. Reproductive health

Sexual maturation does not automatically mean that a person is physically or mentally ready for sexual activity or parenthood. The chapter also highlights the importance of considering health and social pressures when making decisions.

Sexually transmitted diseases (STDs)

The chapter mentions:

Gonorrhoea
Syphilis
Warts
HIV-AIDS

Condom use can help reduce transmission of many sexually transmitted infections.

Contraception

Methods described in the chapter include:

Method	Basic principle
Condoms	Mechanical barrier preventing sperm from reaching egg
Oral pills	Alter hormonal balance to prevent egg release/fertilisation
Copper-T	Intrauterine contraceptive device
Surgical methods	Block sperm or egg transport

Contraceptive methods may have side effects, and surgical procedures can have complications if not performed properly.

The chapter also discusses the social harm of illegal sex-selective abortion and notes that prenatal sex determination is prohibited by law.

High-Yield Differences

Binary fission vs multiple fission

Binary: one organism → two daughter cells.
Multiple: one organism → many daughter cells simultaneously.

Asexual vs sexual reproduction

Asexual: one parent; comparatively less variation.
Sexual: two individuals contribute DNA; greater variation.

Pollination vs fertilisation

Pollination: transfer of pollen from anther to stigma.
Fertilisation: fusion of male and female germ-cells to form a zygote.

Regeneration vs reproduction

Regeneration: formation of body parts/organisms from pieces through specialised cellular processes.
Reproduction: a normal biological process that produces new individuals.

Must-Remember Chains

DNA copying → variation → adaptation/survival of species

Asexual reproduction → one parent → less variation

Sexual reproduction → two individuals → combination of variations → greater variation

Anther → pollen → stigma → pollen tube → ovule → fertilisation → zygote

Zygote → embryo → implantation → foetus

Ovule → seed

Ovary → fruit

No fertilisation → uterine lining breaks down → menstruation

Exam Focus: Learn These First

Importance of DNA copying and variation
All six modes of asexual reproduction with examples
Advantages of vegetative propagation
Why sexual reproduction produces more variation
Role of meiosis
Parts and functions of a flower
Pollination vs fertilisation
Changes during puberty
Male and female reproductive systems
Placenta and embryo nourishment
Menstruation
Major contraceptive methods
How reproduction contributes to stability and survival of species