Reproduction: How Life Continues Class 9 Science Notes

Every living organism has a definite life span. It is born, grows, matures, reproduces and eventually dies. Reproduction is the biological process by which living organisms produce new individuals of their own kind.

Reproduction ensures the continuity of a species. For example, a mango tree may die after completing its life span, but its seeds can grow into new mango plants. Similarly, cows give birth to calves and humans give birth to children.

Living organisms reproduce mainly by two methods:

• Asexual reproduction: A single parent produces offspring that are generally genetically identical to the parent.
• Sexual reproduction: Male and female gametes take part in the formation of a new individual. The offspring receive a combination of genetic material, resulting in variations.

Asexual reproduction involves only one parent. It is found in many unicellular organisms, such as bacteria, Amoeba and yeast, as well as in simple multicellular organisms such as Hydra and sponges. It also occurs in many plants.

In this method, offspring are generally genetically identical to the parent because they are produced through cell division without the fusion of gametes.

Some important methods of asexual reproduction are:

• Vegetative propagation in plants
• Budding in yeast and Hydra
• Spore formation in fungi

Some plants can produce new individuals from their roots, stems or leaves instead of producing seeds. This method is called vegetative propagation.

The vegetative parts of a plant contain growing regions that can develop into new shoots and roots. Since only one parent plant is involved, the new plants are generally genetically identical to it.

Examples of natural vegetative propagation

• Potato: New shoots develop from the buds or eyes on the stem tuber.
• Ginger: New plants develop from the underground stem called a rhizome.
• Money plant: Stem cuttings can grow into new plants.
• Sugarcane: Stem cuttings containing nodes can develop into new plants.
• Bryophyllum: Small plantlets develop along the margins of its leaves.

Artificial methods of vegetative propagation

Gardeners, farmers and horticulturists use several techniques to multiply plants with desirable characteristics.

• Cutting: A piece of stem is cut from a healthy plant and placed in suitable soil. Under favourable conditions, it develops roots and shoots. Sugarcane and money plant can be propagated through stem cuttings.
• Grafting: A stem piece from one plant is joined to the rooted plant of another. The two parts grow together. This method is used to combine desirable qualities of different varieties.
• Layering: A flexible branch is bent towards the ground and a portion is covered with soil. Roots develop from the covered part. It can then be separated from the parent plant.
• Tissue culture: New plantlets are produced from small pieces of plant tissue under controlled laboratory conditions. It allows a large number of healthy plants to be produced.

Importance in agriculture

Vegetative propagation helps farmers multiply plants quickly and maintain desirable characteristics. It is useful for cultivating selected varieties on a large scale. Tissue culture is used, for example, to produce healthy banana plantlets.

Budding is a method of asexual reproduction in which a small outgrowth, called a bud, develops on the parent organism. The bud grows and may eventually separate to live independently.

Budding in yeast

Yeast is a unicellular fungus. During budding, a small outgrowth appears on the parent cell. The cell divides, and the bud receives a copy of the genetic material. The bud grows and may separate from the parent cell.

Budding in Hydra

Hydra is a simple multicellular animal. Repeated cell division at a particular region of its body produces a small bud. The bud grows, develops structures similar to the parent and eventually separates to live independently. Several buds may sometimes be seen on one Hydra.

Fungi such as Rhizopus reproduce asexually through spore formation. Spores are tiny reproductive structures that can develop into new individuals under favourable conditions.

In Rhizopus, thread-like structures called hyphae form the body of the fungus. Some hyphae grow upright and bear a sac-like structure called a sporangium at the tip. Numerous spores develop inside the sporangium.

When the sporangium matures, it releases spores. These spores are light and can be carried by air. When they land on a suitable surface with adequate moisture and nutrients, they germinate and grow into new fungal individuals.

Fungi are also important decomposers. They break down dead organic matter and help recycle nutrients. Some fungi are used in the production of antibiotics, such as penicillin.

In the asexual reproduction examples discussed above, the central process of cell division is generally mitosis. Mitosis produces daughter cells with the same number of chromosomes as the parent cell.

When the genetic material is copied and distributed equally, the resulting offspring are generally genetically identical to the parent. Such genetically identical individuals are called clones.

Sexual reproduction involves the formation of a new individual through the contribution of genetic material from two parents. The male and female reproductive cells are called gametes.

The male and female gametes fuse during fertilisation. This fusion forms a zygote, which develops into a new individual.

Sexual reproduction creates new combinations of genetic information. As a result, offspring may differ from their parents and from one another. These differences are called variations.

Why is meiosis necessary?

If gametes carried the full chromosome number, the number of chromosomes would double after fertilisation in every generation. Meiosis solves this problem by reducing the chromosome number to half during gamete formation.

• Cells with two sets of chromosomes are called diploid.
• Gametes with one set of chromosomes are called haploid.
• During fertilisation, the haploid male and female gametes combine and restore the diploid chromosome number in the zygote.

How does meiosis create variation?

During meiosis, the chromosomes of each pair separate, so each gamete receives one chromosome from each pair. The combinations can differ from one gamete to another. This random distribution produces different combinations of genetic information.

Variations are important because some individuals may be better able to survive when environmental conditions change. Over many generations, variation contributes to evolution.

In flowering plants, the flower is the reproductive organ. A flower contains structures that take part in the formation of male and female gametes and the production of seeds.

Structure of a flower

A complete flower generally has four main parts:

• Sepals: The outermost green parts that protect the flower during the bud stage.
• Petals: Often coloured and sometimes fragrant. They help attract pollinators.
• Stamen: The male reproductive part. It consists of a filament and an anther. The anther produces pollen grains containing male gametes.
• Pistil: The female reproductive part. It consists of the stigma, style and ovary. The ovary contains ovules, and each ovule contains an egg cell.

Pollination

Pollination is the transfer of pollen grains from the anther to the stigma of a flower. It is an essential step in sexual reproduction in flowering plants.

Types of pollination

Self-pollination: Pollen is transferred to the stigma of the same flower or another flower on the same plant.

Cross-pollination: Pollen is transferred from a flower on one plant to a flower on another plant of the same species.

Agents of pollination

Pollination may take place with the help of external agents called pollinators. These include wind, water, insects and birds.

• Wind: Grasses such as maize and wheat produce large quantities of light pollen. Their stigmas may be long and feathery to trap pollen.
• Water: In aquatic plants such as Vallisneria and Hydrilla, water currents help carry pollen.
• Insects: Bees and butterflies visit flowers such as sunflower, hibiscus and marigold. Bright colours, fragrance and nectar attract them.
• Birds: Some flowers are pollinated by birds such as sunbirds.

Pollination and reproductive success

Different plants use different strategies to improve the chances of pollen reaching a suitable stigma. Wind-pollinated plants may release very large quantities of pollen because much of it does not reach a flower. Insect-pollinated plants often use attractive flowers and sticky pollen to improve transfer.

The textbook gives an example comparing approximate pollen production and seed formation. Wind-pollinated grasses may release about 5,00,000–10,00,000 pollen grains per flower and produce an estimated 50–200 seeds. Insect-pollinated plants may release about 20,000–40,000 pollen grains and produce an estimated 800–1,000 seeds.

After a compatible pollen grain reaches the stigma, it germinates and forms a pollen tube. The tube grows through the style towards the ovary. The male gamete travels through the pollen tube and reaches the ovule.

The male gamete fuses with the egg cell inside the ovule. This fusion is called fertilisation. It forms a fertilised egg called the zygote.

What happens after fertilisation?

• The fertilised egg develops into an embryo.
• The ovule develops into a seed.
• The ovary enlarges and develops into a fruit.
• When suitable conditions are available, the seed germinates and grows into a new plant.

In sexual reproduction, male and female gametes must meet and fuse. Animals show different reproductive strategies depending on their habitat and the way their embryos develop.

External fertilisation

In external fertilisation, the fusion of male and female gametes takes place outside the female's body. It is common in many aquatic animals, including most fish and frogs.

The female releases eggs into the water, and the male releases sperm over them. A large number of eggs may be produced because many are lost due to water currents or are eaten by other organisms.

Internal fertilisation

In internal fertilisation, the fusion of male and female gametes takes place inside the female's body. It occurs in reptiles, birds and mammals.

The fertilised egg or developing embryo receives greater protection inside the female reproductive system. The chances of survival of the young are generally higher than in external fertilisation.

Different ways of development

In some animals, the young hatch as larvae. A larva is an intermediate stage that may look very different from the adult and often feeds and grows before changing into the adult form.

For example, a butterfly passes through egg, larva, pupa and adult stages. Frogs also undergo changes as they develop from tadpoles into adult frogs.

In birds and reptiles, the embryo develops inside an egg containing yolk that provides nourishment. In mammals, the embryo develops inside the female body.

In human beings, reproductive maturity develops during adolescence. The reproductive organs mature and begin producing gametes: sperm in males and eggs in females.

When a sperm fuses with an egg inside the female reproductive system, a zygote is formed. It develops into an embryo and later into a foetus in the uterus.

The male reproductive system

The male reproductive system produces sperm and helps transfer them into the female reproductive system.

• Testes: A pair of organs that produce sperm and the hormone testosterone.
• Scrotum: A pouch of skin that holds the testes outside the main body cavity. It keeps them slightly cooler than normal body temperature, which is important for sperm formation.
• Vas deferens: A tube through which sperm travel from the testes towards the urethra.
• Seminal vesicles and prostate gland: These glands add fluids that nourish sperm and help them remain active and move.
• Urethra: A passage through which urine and semen leave the body at different times.
• Penis: An external reproductive organ that transfers semen into the female reproductive tract.

The female reproductive system

The female reproductive system produces eggs and provides the site where fertilisation and the development of the embryo can occur.

• Ovaries: A pair of organs that produce eggs and release female reproductive hormones.
• Oviducts or fallopian tubes: Tubes that connect the ovaries to the uterus. Fertilisation usually takes place in an oviduct.
• Uterus: A muscular, bag-like organ in which the embryo implants and the foetus develops.
• Cervix: The narrow lower passage of the uterus that opens into the vagina.
• Vagina: A muscular passage connecting the cervix to the outside of the body.

The process of formation of reproductive cells is called gametogenesis. In human beings, it takes place in the testes and ovaries.

Gametes are formed by meiosis. The chromosome number is reduced to half so that fertilisation restores the normal chromosome number.

From puberty onwards, usually one mature egg is released from an ovary during a menstrual cycle. This process is called ovulation.

During sexual intercourse, sperm may enter the female reproductive tract and travel towards the oviduct. If a sperm successfully fuses with an egg, a zygote is formed.

The zygote undergoes repeated mitotic divisions as it travels towards the uterus. It eventually attaches to the thickened inner lining of the uterus. This attachment is called implantation and marks the beginning of pregnancy.

If an egg is not fertilised, it remains viable for about a day and then degenerates. The thickened inner lining of the uterus is no longer needed and is shed along with some blood through the vagina. This process is called menstruation or a period.

Menstruation usually lasts about 3–7 days. The cycle of ovulation, preparation of the uterus and menstruation repeats typically every 21–35 days, often around 28 days. The cycle usually begins during puberty and continues until menopause.

Main stages of a typical 28-day cycle

• Days 1–5: Menstruation occurs as the uterine lining is shed.
• Days 6–14: The uterine lining gradually rebuilds and an egg matures in an ovary.
• Around day 14: Ovulation may occur, releasing a mature egg from the ovary.
• Days 15–28: The uterine lining becomes thicker and richer in blood vessels. If fertilisation does not occur, the lining begins to break down and the cycle repeats.

Menstruation is a normal biological process. Maintaining menstrual hygiene includes changing menstrual products regularly, washing hands before and after changing them, cleaning the external genital area with water and disposing of used products responsibly.

Human pregnancy usually lasts about nine months and is divided into three stages called trimesters. During this period, the embryo and later the foetus develop inside the uterus.

First trimester

During the first two months, the developing fertilised egg is called an embryo. Major organs begin to form. From about the ninth week, the developing baby is called a foetus.

Second trimester

The foetus grows bigger and stronger. The mother can usually begin to feel its movements.

Third trimester

The baby grows rapidly and prepares for life outside the womb. The uterus protects and nourishes the developing baby throughout pregnancy.

During childbirth, contractions of the uterine muscles help push the baby through the birth canal. If a normal vaginal birth is not possible or safe, medical or surgical procedures may be needed.

Care during pregnancy

• Eat a balanced diet containing proteins, vitamins and minerals.
• Attend regular medical check-ups.
• Follow medical advice about rest and suitable physical activity.
• Avoid smoking, alcohol and medicines taken without medical advice.
• Give attention to emotional well-being and seek support when needed.

Care after birth

Newborn babies need warmth, gentle handling, timely vaccination and appropriate nutrition. Breast milk provides nutrition and helps protect infants from many diseases. The mother's health and well-being also require care, rest and support.

During adolescence, the body undergoes changes that lead to reproductive maturity. The production of sperm in boys and the development of menstrual cycles in girls are examples of these changes.

However, physical maturity does not mean that a person is automatically ready for all adult responsibilities. Emotional maturity develops over time and includes understanding feelings, communicating clearly and making thoughtful decisions.

Sexually transmitted infections

Some infections can spread through sexual contact. These are called sexually transmitted infections (STIs). The chapter mentions gonorrhoea, herpes, syphilis, genital warts and HIV. HIV can eventually lead to AIDS.

Condoms help reduce the risk of transmission of many STIs and also help prevent pregnancy. Anyone concerned about possible infection should seek advice from a qualified healthcare professional.

Methods of preventing unwanted pregnancy

• Barrier methods: Condoms and vaginal barriers help prevent sperm from reaching the egg. Condoms also reduce the risk of many STIs.
• Hormonal methods: Oral contraceptive pills use hormones to prevent pregnancy, for example by changing the release of eggs. They do not protect against STIs.
• Intra-Uterine Devices (IUDs): Devices such as Copper-T are placed inside the uterus to help prevent pregnancy.
• Surgical methods: Procedures can block the vas deferens in males or the fallopian tubes in females, preventing sperm and egg from meeting.

Responsible reproductive health

Reproductive health involves informed and responsible decisions, access to reliable information and appropriate medical care. Prenatal sex determination is strictly prohibited by law in India. Discrimination based on a child's sex can harm society and contribute to an imbalance in the sex ratio.