Reproduction in Plants and Animals
Biological process that results in the generation of new young ones or offspring from an organism is called reproduction. The young ones produced attain similar characteristics to the individual giving birth to its offspring. Thus, reproduction is a part of the cycle where birth, growth and death take place. It enables for the continuity of the species, one generation after another. As all organisms have limited lifespan and are bound to die and reproduction ensures the survival of species in all generations. All organisms undergo reproduction be it microorganisms, plants or animals.
This article is about Reproduction in organisms class 12 Unit VI in NCERT books which also covers sexual reproduction in flowering plants, human reproduction and reproductive health in addition to this. Not only in higher classes, chapter- Reproduction in animals class 8 and also Reproduction class 10 chapter also makes students well aware about the topic. So, this write-up is helpful for students from class 8 onwards.
Types of Reproduction in Organisms
Different methods of reproduction in plants and animals are known, here, we can find majorly two divisions:
Asexual Reproduction
Sexual Reproduction
Asexual Reproduction in Organisms
Commonly seen in Protists, Monerans and Fungi.
For example: Amoeba reproduces by binary fission where the parent cell divides into two halves and each half further develops into a daughter cell. Likewise, bacteria also reproduces by binary fission. Yeast reproduces through budding and other members of Fungi kingdom such as Penicillium reproduces through conidia.
Further, organisms in Anamalia kingdom like Hydra reproduce through reproductive structures called bud and sponge produces internal buds called gemmule that help in asexual reproduction.
Apart from these, Asexual Reproduction is seen in simple plants as well as higher plants such as algae and potato. Algae (Chlamydomonas) produce zoospores that mature later as plants. During unfavourable conditions algae undergoes sexual reproduction.
Vegetative propagation is the asexual reproduction method in plants. It is done by the generation of vegetative propagules in higher plants. For example, specialized structures in potatoes, the stem of the plant, develop buds known as ‘eyes’ that later germinate as new plants. In ginger, rhizomes are the specialized modified stems having buds and nodes undergo vegetative propagation. A bryophyllum leaf has notches on the leaf margins called adventitious buds that later fall off to generate new plants. Runners, offset, bulb and suckers are other examples of vegetative propagules in plants. Farmers and gardeners make full use of this process for commercial production of strawberry farms, potato cultivation and ginger fields. However, excess growth of water hyacinth chokes the Bay of Bengal and kills millions of fish. It is their problematic side.
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Sexual Reproduction in Organisms
Organisms that engage in sexual reproduction vary in their external and internal structures; the pattern of reproduction remains similar.
Juvenile phase, reproductive phase and senescence phase are the three phases of life. Juvenile phase is characterized by the increase in height in all human beings and animals; in plants, emergence of new leaves can be seen in the juvenile phase or vegetative phase that varies in different plants. Juvenile period lasts for 13-16 years in humans and mango trees remain in this phase for around 3-5 years.
The second is the reproductive phase, flowers start appearing in plants and in Mango, a perennial plant, it is difficult to separate the phases as it keeps shedding the leaves throughout the year which is a sign of senescence. Bamboos flower once in 100 years and neelakurinji flowers once in 12 years. Plants develop specialized structures during the reproductive phase, similarly in humans, the reproductive phase starts with puberty and leads to development of sexual characteristics such as facial hair in males and breast in females. There are cyclic changes happening in ovaries and ducts as well as hormones namely menstrual cycle in primates and oestrous or estrous cycles in non-primates. The mammals exhibit the cyclic changes and are seasonal breeders, eg- sheep. Human beings are continuous breeders as they can breed all the time during their reproductive phase.
Senescence phase is characterised by senility, non-existence of menstrual cycle and clowning of metabolism, ultimately leading to death. The phase is controlled by hormones and also environmental factors. In most organisms, the reproductive phase plays an important role as it ensures continuity of life.
Events of Sexual Reproduction
The sequential events of sexual reproductive phase are divided into three, namely:
Pre-Fertilization Events: Gametogenesis and gamete transfer are the two main pre-fertilization events. These are explained in detail in NCERT books of class 12 Biology books.
Fertilization events: It is an important event where the fusion of gametes occurs called syngamy that results in zygote formation. Also, external fertilization and internal fertilization are the two terms which should be well understood.
Post-Fertilization Events: It includes the formation of diploid zygote and the development of embryo from the zygote called embryogenesis during which cell division and cell differentiation occurs.
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FAQs on Reproduction in Organism
1. What is reproduction and what is its primary importance for organisms?
Reproduction is a fundamental biological process by which new individual organisms, or 'offspring,' are produced from their 'parents.' Its primary importance is to ensure the continuity of a species, generation after generation. Without reproduction, a species would fail to perpetuate itself and eventually become extinct.
2. What is the main difference between asexual and sexual reproduction?
The main difference lies in the involvement of gametes. Asexual reproduction involves a single parent and produces offspring that are genetically identical (clones), without the fusion of gametes. In contrast, sexual reproduction typically involves two parents who produce male and female gametes; the fusion of these gametes results in offspring with a unique combination of genes from both parents.
3. What are some common types of asexual reproduction found in organisms?
Several types of asexual reproduction exist, each suited to different organisms. Common examples include:
- Binary Fission: A parent cell divides into two daughter cells, common in bacteria and Amoeba.
- Budding: A new individual develops from an outgrowth or bud on the parent, as seen in Hydra and yeast.
- Fragmentation: The parent's body breaks into distinct pieces, each of which can grow into a new organism, like in Planaria and Spirogyra.
- Vegetative Propagation: New plants arise from vegetative parts like roots, stems, or leaves. Examples include runners in grass and tubers in potatoes.
4. What are the key events that take place during sexual reproduction?
Sexual reproduction is a complex process that can be grouped into three distinct stages:
- Pre-fertilisation Events: This includes all processes before the fusion of gametes, namely gametogenesis (formation of gametes) and gamete transfer.
- Fertilisation (Syngamy): This is the crucial event where the male and female gametes fuse to form a diploid zygote.
- Post-fertilisation Events: This stage involves the events that occur after zygote formation, primarily embryogenesis, which is the development of the embryo from the zygote.
5. How do oviparous and viviparous animals differ in their reproductive strategy?
The key difference is where the zygote develops. Oviparous animals, like reptiles and birds, lay eggs. The development of the embryo occurs outside the female's body, and the egg's calcareous shell provides protection. In contrast, viviparous animals, like most mammals, give birth to live young. The zygote develops into an embryo inside the female parent's body, receiving nourishment and protection directly.
6. Why is the zygote considered a vital link in the life cycle of sexually reproducing organisms?
The zygote is considered a vital link because it is the first cell of the new generation, carrying the complete diploid set of chromosomes from both parents. It connects one generation to the next, ensuring the transfer of genetic information. Every sexually reproducing organism, including humans, begins its life as a single cell—the zygote.
7. From an evolutionary perspective, why is sexual reproduction considered more advantageous than asexual reproduction?
Sexual reproduction is considered more advantageous because it leads to genetic variation among offspring. The fusion of gametes from two different parents creates new combinations of genes, which is the raw material for natural selection. This variation increases the species' ability to adapt to changing environments, survive diseases, and evolve over time, whereas the genetically identical offspring from asexual reproduction are all equally vulnerable.
8. How does embryogenesis differ in oviparous and viviparous animals after fertilisation?
After fertilisation, embryogenesis in oviparous animals occurs externally within an egg, which contains a finite amount of yolk for nourishment. The embryo is vulnerable to environmental threats and predators. In viviparous animals, embryogenesis occurs internally within the mother's uterus. The embryo receives continuous nourishment and protection, leading to a much higher chance of survival for the offspring post-birth.
9. What is the difference between monoecious and dioecious conditions in flowering plants?
This terminology describes how male and female reproductive structures are distributed on plants. Monoecious plants (or bisexual condition) have both male (staminate) and female (pistillate) flowers on the same individual plant. Examples include maize and coconut. Dioecious plants have male and female flowers on separate individual plants—one plant is male, and another is female. Examples include papaya and date palm.
