Reproduction in organisms: the Complete Explanation 2021

Biology is a limited and interesting area of science in which the students find themselves very free and enjoyable. Because this is the actual source that unfolds the mystery of the beginning of the life on the earth and the revaluation that makes us the most advanced human beings (Homo Sapience Sapience). So here I am going to mention some great biological activities that happen in living beings. And I hope it will be a really helpful for the students of 12th standard. So here is the list of top 10 biological activities.

Each and every organism lives for a certain period of time. The period from birth to the natural death of an organism represents its life span. Is not it very interesting to find out that the life span of different organisms can be as short as a few days or as long as a few thousand years? Between these two extremes are the life spans of most other living organisms. You may note that life spans of organisms are not necessarily correlated with their sizes; the sizes of crows and parrots are not very different yet their life spans show a wide difference. Similarly, a mango tree has a much smaller life span as compared to peepal tree. Whatever be the life span, death of every individual organism is certainty. For example no individual is immortal, except single celled organisms. A question might appear here that why do we say that there is no natural death in single celled organisms? Given this reality, have you ever wondered how vast numbers of plant and animal species have existed on earth for several thousands of years? There must be some processes in living organisms that ensure thus continuity. Yes, we are taking about reproduction, something that we for granted. Reproduction is defined as biological process in which an organism gives rise to young ones similar to itself. The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle of birth, growth and death. Reproduction enables the continuity of the species, generation after generation. There is a large diversity in the biological world and each organism has evolved its own mechanism to multiply and produce offspring. The organism’s habitat, its internal physiology and several other factors are collectively responsible for how it reproduces. Based on whether there is participation of one organism or two in the process of reproduction, it is of two types. When offspring is produced by a single parent with or without the involvement of gamete formation, the reproduction is asexual. When two parents participate in reproduction process and also involve fusion of male and female gametes, it is called sexual reproduction.

  • Asexual reproduction

In this method, a single individual is capable of producing offspring. As a result, the offspring that are produced are not only identical to one another but are also exact copies of their parent. The term clone is used to describe such morphologically and genetically similar individuals. Let us see how widespread asexual reproduction is, among different groups of organisms. Asexual reproduction is common among single celled organisms, and in plants and animals with relatively simple organizations. In protists and monerans the organism or the parent cell divides into two to give rise to new individuals. Thus, in these organisms cell division is itself a mode of reproduction. Many single celled organisms reproduce by binary fission, where a cell divides into two halves and each rapidly grows into an adult. In yeast the division is unequal and small buds are produced that remain attached initially to the parent cell which , eventually get separated  and mature into new yeast organisms. Under unfavorable condition the Amoeba withdraws its pseudopodia and secretes a three layered hard covering or cyst around itself. This phenomenon is called as encystation. When favorable condition return, the encysted amoeba divides by multiple fission and produces many minute amoeba or pseudopodiospores; the cyst wall burst out and the spores are liberated in the surrounding medium to grow up into many amoeba. This phenomenon is known as sporulation. Members of the kingdom fungi and simple plants such as algae reproduce through special through special asexual reproductive structures. The most common of these structures are zoospores that usually are the microscopic motile structures. Other common reproductive structures are conidia, buds and gemmules.

While in animals and other simple organisms the term asexual is used unambiguously, in plants, the term vegetative reproduction is frequently used in the plants, the units of vegetative propagation such as runner, rhizome, sucker, tuber, offset, bulb are all capable of giving rise to new offspring. These structures are called vegetative propagules. Obviously, since the formation of these structures does not involve two parents, the process involved is asexual. In some organisms if the body breaks into distinct pieces and each fragment grows into an adult capable of producing offspring. This also a mode of asexual reproduction called fragmentation.

You must have heard about the scourge of the water bodies. This is nothing but the aquatic plant ‘water hyacinth’ which is one of the most invasive weeds found growing wherever there is standing water. It drains oxygen from the water , which leads to the death of fishes. You may find it interesting to knoew that this plant was introduced in India Because of its beautiful flowers and shape of leaves. Since it can propagate vegetatively at a phenomenal rate and spread all over the water body in a short period of time. it becomes very difficult to get rid of them.

Are you aware that how plants like potato, sugarcane, banana, ginger and dahlia are cultivated? Have you seen small plants emerging from the buds of the potato tuber, from the rhizomes of banana and ginger? When you carefully try to determine the site of origin of the new arise from the nodes  present in the modified stems of these plants. When the nodes come in contact with damp soil or water, they produce roots and new plants. Similarly, adventitious buds arise from the notches present at margins of leaves of bryophyllum. This ability is fully exploited by the gardeners and farmers for commercial propagation of such plants. It is interesting to note that asexual reproduction is the common method of reproduction in organism that has a relatively simple organization. Like algae and fungi and that they shift to sexual method of reproduction just before the onset of adverse conditions. Asexual as well as sexual modes of reproduction are exhibited by the higher plants. On the other hand, only sexual mode of reproduction is present in most of the animals.

  • Sexual reproduction

Sexual reproduction involves formation of the male and female gametes, either by the same individual or by different individuals of the opposite sex. These gametes fuse to form the zygote which develops to form the new organism. It is an elaborate, complex and slow process as compared to asexual reproduction. Because of the fusion of male and female gametes, sexual reproduction results in offspring that aren’t identical to parents or among-st themselves. A study of diverse organisms-plants, animals or fungi  show that though they differ so greatly In external morphology, internal structure and physiology, when it comes to sexual mode of reproduction surprisingly, they share a similar pattern. Let us first discuss what features are common these diverse organisms.  All organism have reach a certain stage of growth and maturity in their life, before they can reproduce sexually. The period of growth is called the juvenile phase. It is known as vegetative phase in plants. The phase is of variable durations in different organisms.  The end of juvenile phase which marks the beginning of the reproductive phase can be seen very easily in the   higher plants when they come to flower.

Observe a few trees in your area. Do they flower during the same month year after year? Why do think the availability of fruits like mango, apple, jackfruit is seasonal? Are their some plants that flower throughout the year and some others that show seasonal flowering?

Plants, the annual and the biennial types. A few plants exhibit unsual flowering phenomenon some of them such as bamboo flower only once in their lifetime, generally after 50-100 years, produce large number of fruits and die. Another plant Stobilanthus Kunthiana  flowers once in 12 years. As many of us would know that this plant flower during September-October 2006. Its mass flowering transformed large tracks of hilly areas in Kerala, Karnataka and Tamil Nadu into blue stretches and attracted a large number of tourists. In animals, the juvenile phase is followed by morphological and physiological changes prior to active reproductive behavior. The reproductive phase is also variable duration in different organisms.

Among animals fro example birds, do they lay eggs all through year? Or is it seasonal phenomenon? What about other animals like frogs and lizards? You will obviously notice that birds living in nature by laying eggs only seasonally. However birds in captivity can be made to lay eggs throughout the year. In this case laying of eggs is not related to reproduction but it is a commercial exploitation for human welfare. The female of placental mammals exhibit cyclical change in the activities of ovaries and accessory ducts as well as hormones during the reproductive phase. In non-primate mammals like cows, rats, deers, dogs, tiger etc. such cyclical changes during reproduction are called oestrus cycle where as in primates it is called menstrual cycle. Many mammals especially those living in natural, wild conditions exhibit such cycles only during favorable seasons in their reproductive phase and are therefore called seasonal breeders. Many other mammals are reproductively active throughout their reproductive phase and hence are called continuous breeders.

That we all grow old is something that we recognize. But what is meant by growing old? The end of reproductive phase can be considered as one of the parameters of senescence or old age. There are concomitant changes in the body during this last phase of life span. Old age ultimately leads to death. In both plants and animals hormones are responsible from the transition between three phases. Interaction between hormones and certain environmental factors regulate reproductive processes and the associated behavioral expression of organisms.

EVENTS IN SEXUAL REPRODUCTION: after attainment of maturity, all sexually reproducing organisms’ exhibit events and processes that have remarkable fundamental similarity, even though the structures associated with sexual reproduction are indeed very different. The events of sexual reproduction though elaborate and complex follows a regular sequence sexual reproduction is characterized by the fusion of the male and female gametes, the formation of zygote and embryogenesis. For convenience these sequential events may be grouped into three distinct stages namely, the pre fertilization, fertilization and the post fertilization events.


The primary sex organs- the testis in the males and the ovaries in the females produces gametes fro example sperms and ovum, respectively by the process called gametogensis. In testis the immature germ cells produce sperms by spermatogenesis that begins the puberty. The spermatogonia present in the inside wall of the seminiferous tubules multiply by mitotic division and increase in numbers. Each spermatogonium is diploid and contain 46 chromosomes. Some of the spermatogonia is called as primary spermatocytes periodically undergo meiosis. A primary spermatocyte completes the first mitotic division leading to formation of two equal, haploid spermatids. The spermatids are transformed into spermatozoa by the process called spermiogenesis. After spermiogenesis sperm heads become embedded in the sertoli cells, and are finally released from the seminiferous tubules by the process called spermiation.

Spermatogenesis starts at the age of puberty due to significant increase in the secretion of gonadotropin releasing hormone. This if you recall, is a hypothalamic hormone. The increased level of GnRH hen acts as the anterior pituitary gland and stimulates secretion of two gonadotropins – luteinizing hormone and follicle stimulating hormone. LH acts at the leydig cells and stimulates synthesis and secretion of androgens. Androgens, in turn, stimulate the process of spermatogenesis. FSH acts on the sertoli cells and stimulates secretion of some factors which help in the process of spermiogenesis.

Let us examine the structure of a sperm. It is microscopic structure composed of a head, neck and middle piece and a tail. A plasma membrane envelope the whole body of the sperm.  The head of the sperm contains an elongated haploid nucleus, the anterior portion of which is covered by a cap-like structure, acrosome. The acrosome is filled with enzymes the helps in the fertilization of the ovum. The middle piece possesses numerous mitochondria, which produces the energy for the movement of tail that facilitate sperm motility, at least 60 per cent sperms must have normal shape and size and at least and 40 percent of them must show vigorous motility . Sperms released from the seminiferous tubules, are transported by the accessory ducts.  Secretions of epididymis, vas deference, seminal vesicle and prostate are essential for maturation and motility of the sperms. The seminal plasma along with the sperms constitutes the semen. The function of male sex accessory ducts and glands are mentained by the testicular hormones.

The process of formation of a mature female gamete is called oogenesis which is markedly different rom spermatogenesis. Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells are formed within each fetal ovary; no more oogonia are formed and added after birth. These cells start division and enter into prophase 1 of the meiotic division and get temporarily arrested at the stage, called primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells and is called the primary follicle. A large number of these follicles degenerate during the phase from the birth to puberty. Therefore at puberty only 60000 to 80000 primary follicles are left in each ovary. The primary follicles get sororunded by more layers of granulose cells and a new theca and are called as secondary follicles.

The secondary follicle soon transforms into a tertiary follicle which is chrcterised by a fluid filled cavity called antrum. The theca layer is organized into an inner theca interna and an outer theca externa . itisiportant to draw your attention that is at this stage that the primary oocyte within the tritary follicle grows in size and completes its first meiotic division. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body.

Leave a Comment