CBSE Class 12 Biology Revision Notes Chapter 1

Introduction

Biology is a crucial subject to master for students planning to pursue a career in science and medical-oriented streams. Class 11 and 12 Biology lays a solid foundation for the preparation of the same. Students are suggested to prepare a study plan and comprehensive notes for Biology because of the extensive CBSC syllabus. Students should get into the practice of creating the chapter notes as they are studying the subject from the NCERT textbook along with other reference books. The CBSC revision notes should include all the important things in line diagrams and pointwise writing. Extramarks’ academic team has created class 12 Biology chapter 1 notes by referring to the NCERT textbook and various other books.

Students may register on Extramarks to access class 12 Biology chapter 1 notes, CBSC class 12  syllabus. In addition to the syllabus, they may also refer to CBSC sample papers, CBSC revision notes, CBSC important questions, CBSC previous year question papers and CBSC extra questions. 

Key Topics Covered In Class 12 Biology Chapter 1 Notes

The span from birth to the natural death of an organism is called its life span. A Butterfly has a lifespan of 1 to 2 weeks, whereas a Parrot has 140 years. During their life span, organisms reproduce several times. Reproduction is essential because it allows the continuity of the species, generation after generation. In the absence of reproduction, there will be no existence of life on earth. In this chapter, students will learn about reproduction in organisms throughout their lifespan. The key topics covered under Class 12 Biology Chapter 1 Notes – Reproduction in Organism include

Reproduction

It is a biological process of producing new young ones (offspring) similar to parents. The young ones grow, mature, and in turn, produce new young ones. Hence, there is a cycle of birth, growth and death. 

Reproducing offspring enables the continuity of the species, generation after generation. Reproduction also facilitates evolution by genetic variation through the intermingling of species and inheritance from the process of reproduction ( in the case of sexual reproduction).

There are two kinds of reproduction. 

1. Asexual Reproduction:

In this mode of reproduction,  progeny is produced by a single parent with or without gamete formation. The offspring are not only identical but are also exact copies of their parent (genetically and morphologically identical) because there is no intermingling of gametes. The term ‘Clone’ is used to describe such similar individuals.

2. Sexual Reproduction:

If two parents of opposite sex reproduce together and the process involves the fusion of male and female gametes to form a zygote, it is called sexual reproduction.

Asexual reproduction and its types

Under this section of class 12 Biology chapter 1 notes, we learn about asexual reproduction. Asexual reproduction is mostly seen in unicellular organisms, as well as in plants and animals with relatively simpler organisations. It is also seen in multicellular organisms. Cell division itself is a mode of reproduction in Protists and Monerans. Parent cell divides by simple mitotic division into two to give rise to new individuals.

The commonly seen modes of asexual reproduction in the case of animals are as follows:

• Fission:  where a cell divides into two halves, and each rapidly grows into an adult is called Binary fission(e.g., Amoeba, Paramecium). Under unfavourable conditions, the Amoeba withdraws its pseudopodia and secretes a three-layered hard covering or cyst around itself. This phenomenon is termed encystation. The encysted Amoeba divides by multiple fission and produces many minute amoebae; when favourable conditions return, the cyst wall bursts out, and the spores are liberated to grow up into many amoebae. This phenomenon is known as sporulation. 
• Budding: In this process, a cell divides unequally, and small buds grow out and remain attached to the parent cell, which eventually gets separated and matures into new organisms (cells), e.g.Yeast cells. In the case of Hydra, the growth of the region of the body occurs called budding that separates out from the main body. In the case of sprouting corals, the bud does not fall off and reproduces as part of a new colony.
• Fragmentation: In this mode of asexual reproduction, if the body breaks into apparent pieces (fragments), each fragment grows and forms an adult that is capable of producing new young ones. A few animal species, including starfish, Flatworms, etc., reproduce using fragmentation.

Asexual reproductive structure

Kingdom Monera, Protista,  Fungi and simple plants such as algae reproduce through special asexual reproductive structures. Tiny, single-celled, thin-walled spores that are extremely resistant to environmental extremes. The spores form new individuals. 

For e.g.,

• Psedopodio spore( with fine pseudopodia) – Amoeba
• Zoospores( motile with flagella) – Chlamydomonas
• Conidia(non-motile) – Penicillium
• Sporangiospores(non-motile) – Rhizopus
• Gemmules( internal buds) -Sponges 

Vegetative Propagation in Plants: The class 12 Biology chapter 1 notes give a detailed explanation of vegetative propagation. In plants, the term vegetative reproduction is used, although it is a type of asexual reproduction. The growth of a new plant from an organ of the mother plant that takes place under suitable environmental conditions is called vegetative propagation. The units of vegetative propagation in plants such as sucker, runner, rhizome, bulb, tuber and offset all have the capability to give rise to the new plant. These structures are called vegetative propagules. These altered organs can develop from the stem, leaf, root, or even the flower. In some filamentous algae, if the body breaks into distinct pieces (fragments), each fragment grows into an adult, e.g.Spirogyra.

Artificial method of vegetative propagation: In this method, a small part of a plant organ (stem, root, leaf) is used to obtain a new plant. A new plant originates invariably from the nodes present in the stem or modified stem. When these nodes come in contact with water or damp soil, they give rise to roots and new plantlets e.g.rhizome of ginger, Banana and sugarcane. Similarly, in Bryophyllum, adventitious buds arise from the notches present on the margin of leaves. Farmers and gardeners use these abilities of plants for artificial propagation of plants by cutting, grafting and layering plant parts.

Looking more deeper into class 12 Biology chapter 1 notes on Vegetative propagation; it is a type of asexual reproduction seen in plants. In this, new plants arise without the involvement of sexual structure. It involves the vegetative part of plants to obtain new plants such as the rhizome, sucker, tuber, bulb, offset etc. Many plants reproduce naturally by vegetative propagation, and many are grown artificially by vegetative propagation, and the offspring produced are genetically identical. E.g., Buds- eyes of Potato and bulb – Onion.

WATER HYACINTH (Terror of Bengal): The class 12 Biology chapter 1 notes give a detailed explanation of the Terror of Bengal.

• This is one of the most invasive weeds due to the phenomenal rate of vegetative propagation.
• Grows wherever there is standing water.
• Uses all the oxygen from water which leads to the death of fish
• It was introduced in India because of its pretty flowers and the shape of its leaves.
• It propagates with the help of offsets.

In an organism with simple organisation, like fungi and algae, asexual reproduction is most common, but if they are in adverse conditions, they shift to a sexual mode of reproduction. Higher plants exhibit asexual (vegetative) as well as sexual modes of reproduction, but in the case of animals, only the sexual mode of reproduction is present most of them.

Sexual Reproduction And Its Phases.

In sexual reproduction, individuals of opposite sex form male and female gametes and a “zygote” are formed by the fusion of these gametes, which develops to form a new individual. Offspring produced are not identical to each other or to the parents. All sexually reproducing animals, whether they are animals, plants or fungi, share a similar pattern of reproduction.

Phases in Sexual Reproduction

Under the class 12 Biology chapter 1 notes, the phases of sexual reproduction are explained in detail and include

• Juvenile phase- The period from birth to sexual maturity is called the juvenile phase. In the case of plants, it is known as the vegetative phase. When the juvenile/ vegetative phase comes to an end, it marks the beginning of the reproductive phase.
• Reproductive phase- In higher plants, the beginning of the reproductive stage is flowering. Few plants show flowering in a particular season, and some others show in all seasons. Few plants like Bamboo species flower only once in a lifetime(after 50 to 100 years), and Strobilanthus kunthiana (neelakuranji) flowers only once in 12 years.

Among animals, birds lay eggs seasonally, the female placental mammals exhibit cyclical changes in the activities of hormones, ovaries and accessory glands during the reproductive phase of sexual reproduction. In non-primate mammals like sheep, cows, rats, deers, dogs, tigers, etc., such cyclical changes during reproduction are called the oestrus cycle, whereas, in primates (monkeys, apes, and humans), it is called the menstrual cycle.

Many mammals exhibit such cycles only during favourable seasons, hence are called seasonal breeders. Other mammals who actively reproduce throughout their reproductive phase are called continuous breeders. 

• Senescent phase- This phase is the end of the reproductive phase, and old age ultimately leads to death.

Transitions between the three phases happen in both plants and animals due to hormones. Interaction between hormones and certain environmental factors regulates the reproductive processes.

Events In Sexual Reproduction: 

All sexually reproducing organisms show events and processes that have remarkable fundamental similarities. Its three different stages are; Prefertilization, Fertilisation, and Post-fertilisation.

Pre-fertilization: Under class 12 Biology chapter 1 notes, students will study pre-fertilisation. It includes gametogenesis and gamete transfer.

Gametogenesis

The procedure of development of female and male gametes. Gametes are haploid cells (having a single set of chromosomes) that may be similar or dissimilar in structure. In algae, both gametes are similar in a structure called homogametic(isogametes). In a higher organism that reproduces sexually, two morphologically different gametes are formed, called heterogametes, Female gametes are called ovum or egg, and male gametes are called antherozoid or sperm.

Are meiosis and gametogenesis always interlinked? The class 12 Biology chapter 1 notes provide answers to this. Gametogenesis is the process of gamete formation; during gametogenesis, a diploid organism(2n) needs to reduce its chromosome number to half (n) so that both the parents can contribute equally( n+n=2n). Meiosis is the kind of cell division that reduces the number of chromosomes by half that occurs during gametogenesis. Hence, meiosis and gametogenesis are said to be interlinked.

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

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

Students will also learn about Cell division in the course of gamete formation in class 12 Biology chapter 1 notes. 

Cell division in the course of gamete formation: In all heterogametic species, gametes are of two types, namely, male and female. Gametes are always haploid though the parent plant body may be either diploid or haploid. A haploid (n)parent produces gametes by mitotic division. 

Many organisms belonging to monera, algae, fungi, and bryophytes have haploid plant bodies, but in organisms belonging to pteridophytes, gymnosperms, angiosperms and most animals, including human beings, the parental body is diploid. In diploid(2n) organisms, specialised cells called meiocytes (gamete mother cells) undergo meiosis and form haploid gamete. One set of chromosomes gets incorporated into each gamete at the end of meiosis. 

For example, 

• Ovary – Diploid (2n)
• Anther – Diploid (2n)
• Pollen – Haploid (n)
• Egg – Haploid (n)
• Male gamete – Haploid (n)
• Zygote – Diploid (2n)

Gamete Transfer

Students can learn about Gamate Transfer by referring to class 12 Biology chapter 1 notes. In a majority of organisms, the male gamete is motile, which exhibits movement and the female gamete is stationary. In a few algae and fungi, both types of gametes are motile. In lower plants like algae, bryophytes and pteridophytes, water is the medium through which male and female gametes move. The number of male gametes is much more than female gametes as most of the male gametes fail to reach the female gametes.

In angiosperms and gymnosperms, pollen grains are the carriers of male gametes, and the ovule has the egg. Pollen grains must be transmitted to the stigma( receptive part of the female reproductive organ) to facilitate fertilisation. The transfer of pollen from the anther to the stigma is called pollination.

Pollination may be self (pollen transfer from anther to the stigma of the same flower) or cross (pollen transfer from anther to the stigma of a different flower). Pollen grains germinate and give rise to a tube-like structure on the stigma, called pollen tubes, which carry the male gametes to the ovule and discharge male gametes near the egg. Coming together of both the gametes is essential for fertilisation.

The Extramarks class 12 Biology chapter 1 notes provide more information to enable students to understand the concepts better. 

Bisexual Flowers: The flowers having both female and male reproductive parts are considered bisexual flowers. The androecium is the male reproductive structure( anther), while the female reproductive structure is the gynoecium( pistil). For instance, Solanum Lycopersicum (Tomato), Hibiscus (Gudhal), Brassica (Mustard), Helianthus (Sunflower), and Narcissus (Daffodil) are some examples of bisexual flowers. 

Unisexual flowers: Flowers that bear stamens( male reproductive structure) are called staminate flowers, while the flowers that bear pistil( female reproductive structure) are known as pistillate flowers. Cucurbit plants bear unisexual flowers, i.e., separate male and female flowers but are on the same plant.

The staminate flowers of cucurbit have bright yellow petals. It obviously has stamens which function as the male reproductive structures of a flower. The female reproductive structure is present on pistillate flowers, and the mode of pollination is cross-pollination. If a female flower is fertilised, a small fruit may be visible. Another plant that bears unisexual flowers is Papaya.

Fertilization:

The class 12 Biology chapter 1 notes details on fertilisation. The fusion of male and female gametes to form a diploid zygote is called syngamy or fertilisation. When a female gamete develops into a new organism without fertilisation, it is called Parthenogenesis.

External fertilisation is a mode of reproduction where syngamy( fusion of male and female gamete) occurs in the external medium(water), i.e., outside the body of the organisms. External fertilisation is observed in most algae, fishes and amphibians such as frogs. However, there is a certain disadvantage of external fertilisation:

• The possibility of survival of the gametes is low.
• Not all the eggs are fertilised and are wasted 
• The predators usually eat the eggs.
•  The offspring are extremely vulnerable to predators.
• A lack of adequate parental care results in a low survival rate. 

Difference between External Fertilisation and Internal Fertilisation.

Vegetative reproduction is considered to be a type of asexual reproduction as it does not involve the fusion of male and female gametes. In this, a new plant grows from some vegetative part of the plant, such as root stem or leaves and is genetically identical to the parent plant.

Post-fertilisation: As explained in class 12 Biology chapter 1 notes, the post-fertilisation process includes

• The formation of the diploid zygote is universal in all sexually reproducing organisms.

• The 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, begins life as a single cell–the zygote.

• In organisms with external fertilisation, a zygote is formed in the external medium, whereas in those exhibiting internal fertilisation, the zygote is formed inside the organism’s body.

• In fungi and algae, the zygote is thick-walled and resistant to desiccation and damage. It germinates after a period of rest.

• In organisms with a haplontic life cycle, the zygote divides by meiosis to form haploid spores that grow into haploid individuals.

For example, the post-fertilisation changes in a flower include:

1. After the process of fertilisation, the sepals, petals and stamens of the flower fall off, but the pistil remains attached to the plant.
2. The zygote develops into an embryo through the process of embryogenesis.
3. The ovule turns into the seed.
4. The ovary grows into the fruit, which develops a thick wall called the pericarp, which is protective in function.

Embryogenesis

As per class 12 Biology chapter 1 notes, embryogenesis is the process of development of an embryo from the zygote. The zygote undergoes cell division (mitosis) and cell differentiation. Cell divisions increase the number of cells;  cell differentiation means groups of cells go through certain modifications to create specialised tissues and organs to form an organism.

Following are the main differences between gametogenesis and embryogenesis as mentioned in class 12 Biology chapter 1 notes.

Oviparous: The development of the zygote occurs outside the body of the female parent; they lay fertilised or unfertilised eggs. After a certain period of incubation, young ones hatch out. e.g., Reptiles and Birds.

Viviparous: The development of the zygote takes place inside the female parent’s body. The offspring are delivered out of the body of the female after a certain time. e.g., Human beings and the majority of mammals.

Furthermore, in class 12 Biology chapter 1 notes, students learn that Oviparous animals lay eggs(fertilised or unfertilised), and the foetus gets mature inside the eggs. Eggs laid are at a bigger risk of being destroyed by other animals (predators) or environmental factors. In the case of viviparous organisms, the foetus develops inside the female, and because of proper embryonic care and protection, the chances of survival of offspring are greater in viviparous organisms.

In flowering plants, the zygote develops inside the ovule. After fertilisation, the sepals, petals and stamens fall off. The zygote develops into the embryo (progenitor and the next generation)  ovules develop into the seed. The ovary develops into the fruit within a thick-walled protective covering called pericarp. After dispersal, seeds germinate in the presence of favourable conditions to give rise to new plants.

Despite its complex nature, higher organisms undergo sexual reproduction because the organisms produced by the fusion of two different gametes are genetically different. Due to these differences, they are well adapted to changing and challenging environmental conditions and usually have higher survival rates.

Difference between Sexual and Asexual reproduction.

Studying deeper into class 12 Biology chapter 1 notes, students must understand the difference between sexual and asexual reproduction. Learning about the difference can help students understand the two modes of reproduction better. 

Chances of survival: Sexual reproduction better promotes genetic diversity through a new combination of genes during gametogenesis( meiosis ), and progeny inherit a mixture of genes from both male and female parents, so they are different from each other and their parents. So they can adapt to new or changing environments, which gives them a survival advantage. e.g., the disease is less likely to infect all the individuals in a population. However, asexual reproduction is faster and consumes less energy and time. So sometimes, offspring produced by asexual reproduction can survive better.

The difference in offspring: The progeny originated by asexual reproduction involves a single parent and is genetically identical(clone) to the parent, whereas the progeny originated by sexual reproduction are formed when male and female gametes ( with a new combination of genes) fuse together, and offspring are genetically and morphologically unique.

Other dissimilarities between sexual and asexual reproduction as per class 12 Biology chapter 1 notes are mentioned below:

The Extramarks class 12 Biology chapter 1 notes differences between a zoospore and a zygote are mentioned below:

Class 12 Biology Chapter 1 Notes: Exercises & Answer Solutions

Extramarks provides a Biology syllabus where class 12 students can get proper knowledge of every chapter with ease. Referring to the syllabus helps students to be prepared with the necessary study materials. Subjects like Biology will become so easy to study once students access the syllabus, refer to the notes and concentrate on important questions. Students may register on Extramarks to access the Class 12 Biology chapter 1 notes any time they need them on Extramarks.

Class 12 Biology chapter 1 notes Reproduction in Organism is provided here with a simple explanation. The notes are based on the CBSE syllabus and can benefit the students immensely. All questions and solutions are from the NCERT books of class 12 chapter 1. In addition to Class 12 Biology chapter 1 notes, students may access study materials by below. 

1. CBSE revision notes
2. CBSE sample papers
3. CBSE previous year question papers
4. CBSE extra questions

The Class 12 Biology chapter 1 notes and study materials provided by Extramarks are prepared by experts and are 100% accurate.

NCERT Exemplar for Class 12 Biology Chapter 1 Notes

Extramarks platform provides NCERT class 12 Biology chapter 1 Exemplar, which has a wide range of CBSE extra questions for students to increase their knowledge. With the help of these notes, students would have all the study material related to chapter 1 Biology in one place. 

Solving CBSE sample papers will also help the students to understand how much and which content should be given more time to study. Study material such as class 12 Biology chapter 1 notes enable quick and thorough revision for subjects like Biology. Students can score really well in class 12 CBSE board exams by getting access to academic materials from Extramarks.

Key Features of Class 12 Biology Chapter 1 Notes

Key features of Class 12 Biology Chapter 1 Notes are as under.

• It follows the CBSE Syllabus.
• It is prepared by the experts at Extramarks.
• It provides a broad understanding of all the concepts.
• It is focused on preparing short and pointwise notes for quick revision.
• It provides authentic knowledge in an easy language.
• It enhances time management skills.