Class 12 Biology Chapter 1 Reproduction in Organisms

Reproduction In Organisms: Access comprehensive notes and solutions for Class 12 Biology Chapter 1 on Reproduction in Organisms. Enhance your understanding of the topic with detailed explanations, diagrams, and key concepts to excel in your studies.

Class 12 Biology Chapter 1 Reproduction in Organisms

Life Span: The period which begins from birth and ends with the natural death of an organism is known as its life span.

Reproduction

Reproduction is a biological process wherein younger ones produced are identical to their parents. This phenomenon is significant in the continuity of the species, generation after generations. Typically, reproduction is observed in all living organisms from single-celled entities such as amoeba to multicellular entities of the most advanced forms, such as human beings. Reproduction is carried out in two modes, depending upon the participation of one or both parents.

Types of Reproduction:

Based on whether there is one or two organisms taking part in the process of reproduction.

• Asexual Reproduction
• Sexual Reproduction

Asexual Reproduction: When the offspring is produced by single parents with or without the involvement of gamete formation, the reproduction is called asexual reproduction.

Sexual Reproduction: When two parents (opposite sex) participates in reproduction process and also involves the fusion of male and female gametes, it is called sexual reproduction.

Mode of Asexual Reproduction:

• Usually followed by organisms with relatively simpler organizations.
• Offsprings produced by single parent.
• With/ without involvement of gamete formation.
• Clones: Offsprings produced are genetically and morphologically similar to each other and to the parent, i.e. they are clones.
• In Protista and Monera, the parent cells divides into two to give rise to new individuals. Thus, in these organisms cell division is the mode of reproduction itself.
• Binary fission in this method of asexual reproduction, a cell divides into two halves and rapidly grows into an adult. Ex. amoeba, paramecium.
• Budding small buds are produced that remain attached initially with parents and get separated on maturation. Ex. Yeast.

• Fungi and simple plants like algae reproduce through special reproductive structures like zoospores (motile structure), conidia (penicillium), buds (hydra) and gemmules (sponges).
• In plants, vegetative reproduction occurs by vegetative propagules like runner, rhizome, sucker, tuber, offset and bulb.
• Asexual reproduction is the most common method of reproduction in organisms having simpler body like in algae and fungi but during unfavourable condition they shift to sexual reproduction.

Vegetative Propagation:

It is a type of process in which new plants are obtained without the production of sexual structures i.e., seeds or spores. It involves the propagation of plants through different types of vegetative parts such as the rhizome, sucker, tuber, bulb, etc. In this, a fusion of the male and the female gamete does not take place and requires only one parent. This is grouped into natural and artificial.

Vegetative Propagules:

• Runner- oxalis
• Sucker- Mint and Chrysanthemum
• Tuber- potato
• Offset- water hyacinth, pistia
• Bulb- onion, garlic
• Rhizome- ginger
• Bulbil- agave
• Leaf buds- Bryophyllum

Artificial Methods: In this type of method, only a small part of the plant organ is utilized for obtaining a new complete plant. Amongst them, the most common methods which are used are cutting, layering, and grafting.

Cutting: In cutting, a small piece of root is cut and when planted in moist soil, it will lead to the artificial inducement and development of adventitious roots. For example, in lemon.

In Rose, sugarcane, hibiscus, and chrysanthemum plants are developed by cuttings that involve stem pieces with the presence of nodes. The small cuttings are planted in moist soil to develop new plants. Underground parts of the stem leading to the development of adventitious roots, whereas buds develop and sprout on the aerial parts of stems. The new plants are in a common language known as cutting. Later, these cuttings are transplanted in different prepared places.

Layering: This method is used for growing rose, lemon, grape, hibiscus, and jasmine. The lower branches of these plants are bent a little bit and covered with soil in such a way that the tip of the branch protrudes from the ground and the middle part of the plant is inside the soil. It will then develop adventurous roots from this buried area of the stem of the plant, at that time this branch is cut off and separated from the mother plant, whereby a new plant is obtained.

Grafting: Grafting is carried out on plants that are having difficulty in forming roots or that generally have a weak root system. This method involves joining two plants of the same or different species, this is achieved by connecting the tissues of the two plants directly to The. When brought into contact, the meristematic tissue of both plants divides and multiplies, and finally the cells of each plant fuse. 

The rooted plant is called the stem plant. The plant that is grafted onto it is called the sprout. A plant is selected as the “scion” that has superior and desirable properties. The stock is generally strong, robust, and resilient, mango, apple, pear, citrus, guava, lychee, and many other fruit plants are obtained and kept in this way. The graft can be of several types, namely, bud graft, lateral graft, and tongue graft, wedge graft, and crown graft, depending on the methods of joining the two parts.

Significance of Vegetative Reproduction:

• Vegetative reproduction is an ideal method of reproduction in plants in which we want to preserve parental characteristics.
• It is best for plants that are less efficient sexually, small seeds, long seed dormancy, poor seed viability, etc. They can also be easily multiplied by this method.
• Vegetative propagation is useful for obtaining disease-free plants.
• By using grafting, the desired characters can be brought together from two varieties.

Water Hyacinth (Terror of Bengal)

• One of the most invasive weeds.
• Grows wherever there is standing water.
• Drains oxygen from water- leads to death of fishes.
• Introduced in India because of its pretty flowers & shape of leaves.
• Vegetative propagation occurs at a phenomenal rate.

Sexual Reproduction:

• Involves formation of male and female gamete by two individuals of the opposite sex.
• Offspring produced by fusion of male and female gametes not identical to each other or to the parents.
• All sexually reproducing organisms share a similar pattern of reproduction.
• In sexual reproduction, fusion of male and female gametes results in offspring that are not identical to parents.

Different Phases in Sexual Reproduction:

Juvenile phase: It is the period of growth after birth in an individual organism, and before it meets reproductive maturity.

Reproductive phase: It is the time when a human organism sexually reproduces.

Senescent phase: It is the end of reproductive phase. Old age ultimately leads to death

Events in Sexual Reproduction: Pre-fertilization, Fertilization, Post-fertilization

Pre-fertilization: all the events prior to fusion of gametes are included in it. It includes gametogenesis and gamete transfer.

Gametogenesis: Is the process of formation of male and female gametes. Gametes are haploid cells which may be similar or dissimilar in structure. In algae, When both gametes are similar in structure called homogametes (isogametes). In higher organism that reproduces sexually, two morphologically distinct gametes are formed called heterogametes, male gametes are called antherozoid or sperm and female gametes are called ovum or egg.

Isogametes Heterogametes:

In fungi and plants, homothallic and monoecious terms are used to denote the bisexual condition and heterothallic and dioecious are used for unisexual condition. In flowering plants, the unisexual male flower is staminate, i.e., bearing stamens, while the female is pistillate or bearing pistils.

In animals, species which possess both male and female reproductive organs in same individual are called bisexual or hermaphrodites (earthworm, sponges, tapeworm etc.) and both having either male or female reproductive organs are called unisexual (cockroach, human).

Gametes are always haploid( having half set of chromosome ), although organisms may be haploid and diploid. Diploid organisms form gametes by meiotic division. The organisms belonging to algae, fungi, and bryophytes have haploid plant body and pteridophytes, gymnosperms, angiosperms and most of animals are diploid (having double set of chromosome)

In diploid organisms, gamete mother cell (meiocyte) undergoes meiosis in which one set of chromosome is present in gametes.

Gamete Transfer:

In majority of organisms, male gametes are motile and females gametes are non-motile, except in fungi and algae in which both gametes are motile. In simple plants like algae, fungi, bryophytes and pteridophytes water is the medium through which male and female gametes moves. The number of male gametes are much more than number of female gametes as most of male gametes fail to reach the female gametes.

In higher plants pollen grains are carrier of male gametes and ovule has eggs. Pollen grains must be transferred from anther to stigma to facilitate fertilization.

Pollination: The transfer of pollen grains from anther to stigma is called pollination. Pollination may be self (anther to stigma of same flower) or cross (anther to stigma of different flower).

Pollen grains germinate on stigma to produce pollen tube that delivers the male gametes near the ovule.

Fertilization: The fusion of male and female gamete is called fertilization or syngamy. It results in the formation of diploid zygote.

Parthenogenesis: The process of development of new organisms without fertilization of female gametes is called parthenogenesis. For example honey bee, rotifers, and lizards.

There are two types of fertilization:

1. External Fertilization: Syngamy occurs outside the body of the organism Large numbers of gametes are released in the surrounding medium. Example: Bony fishes and Amphibians.
2. Internal Fertilization: Syngamy occurs inside the body of the organism Numbers of ova produced are less, but large numbers of male gametes are released and they travel towards the ovum. Example: Birds and Mammals.

Post Fertilization Events: events in the sexual reproduction after formation of zygote.

Zygote is the vital link that ensures continuity of species between organisms of one generation and the next. Every sexually reproducing organism, including human beings, begin life as a single cell–the zygote.

In the organisms, having external fertilization, zygote is formed in external medium (water) and those having internal fertilization zygote is formed inside the body of female.

In algae and fungi, zygote develops a thick wall resistant to desiccation and damage. This germinates after a period of rest.

In the organisms having haplontic life cycle, zygote divides to form haploid spores that germinate to form haploid individual.

Embryogenesis: the process of development of embryo from the zygote. During this, zygote undergoes mitotic division and cell differentiation. Cell division increase the number and cell differentiation help information of new group of cells and organs.

There are two types of animals:

Oviparous: Development of zygote takes place outside the body of organisms and lay fertilized of unfertilized eggs. Example: Reptiles and birds.

Viviparous: Development of zygote takes place inside the body of organisms and produces young ones. Example: Human, dog, horse etc.

Pericarp: In flowering plants, zygote is formed inside the ovule. After fertilization, sepals, petals and stamens of flower fall off. The zygote develops into embryo and ovules into seeds. The ovary develops into fruits which develop a thick wall called pericarp, protective in function.

In animals:

By Copulation: e.g., Reptiles, Birds and Mammals.

By External medium: e.g., Fishes and Amphibians.

Sporulation: During unfavourable conditions organisms like Amoeba surrounded by resistant coat (three layered—hard covering) or cyst. This is called encystation. Within cyst a number of spores are formed. On returning favourable conditions, the cyst burst and spores are liberated and gradually grows in adults. This process is known as sporulation (multiple fission).

Fragmentation: It is a type of asexual reproduction where an organism splits into fragments. Their fragments develops into fully grown individual. e.g. spirogyra, fungi and some annelids.

Regeneration: It is a process of renewal, restoration and growth. It can occur at the level of the cell, tissue and organ. It is common in Hydra, Planaria and echinoderms

• In human, liver has power of regeneration, if it is partially damaged.
• During danger a lizard discard a part of its tail which can regenerate later.

Class 12 Biology Chapter 1 Reproduction in Organisms Mind Map