8. HOW DO ORGANISMS REPRODUCE Reproduction: The process of producing new individual of the same species is called reproduction. Type of Reproduction Living organisms reproduce mainly by two methods: (i) Asexual reproduction (ii) Sexual reproduction In Asexual reproduction the offspring arises from a single individual parent. Asexual type of reproduction takes place in unicellular organisms, some plants and certain multi-cellular animals like sponges and hydra. Sexual reproduction essentially involves two individuals of different sexes -a male sex and a female sex. Ex: Human beings Asexual Reproduction Asexual reproduction occurs in various ways (1) Fission-In binary fission, the nuclear division results in the constriction in the cell membrane, which gradually increases inwards and divides the cytoplasm in two daughter organisms when several individuals arise, is called multiple fission. Example-Plasmodium, bacteria, protozoa, Leshmania (causes kala-azar), Plasmodium(malarial parasite) (2) Regeneration- In regeneration, an organism is able to replace its lost body parts. The mature organism breaks into two or more pieces grow into new individuals. The process is called Regeneration. This process is carried out by the specialized cells which proliferates and form large number of cells. Ex Hydra, Planaria and sponges. If hydra gets cut into pieces 1/6 mm of piece then it is capable of regenerating into an entire individual. (3) Budding- In this a bulging on the body of organism appears as a result of repeated mitotic division in the cells. This lateral bulge is called bud. The bud slowly enlarges and develops into a new organism on the body of the parent. Then it separates from the parent body and grows into a new organism. This complete process is known as budding. Ex hydra, yeast cells. (4) Spore formation- In spore formation, the common structure called sporangium (or soprangia) develops from the fungal hypha. The nucleus divides several times within the sporangium and each nucleus, with a bit of cytoplasm, develops into a spore. Spore are covered by thick walls and will come out under suitable conditions. Example- Rhizopus(thread like activity seen in bread mould), Mucor, Penicillium, (5) Vegetative Propagation-. In vegetative propagation, a new plant develops from the vegetative parts of plant, such as root, stem or leaf. Main advantage of vegetative propagation is that it promotes the growth and maturity of fruit or flower in plants. Plants and organism is genetically same as that of parent. Ex some higher plants like Buds on the root of guava, buds on the leaves of Bryophyllum develop into new plants, banana, orange rose and jasmine. (a) Cutting-. In cutting, a part of plant when placed partly under moist soil develops into a new plant, similar to the parent plant. A cutting may be a piece of stem, root, leaf, or a bulb scale. Example Grape, Phalsa and Rose etc. (b) (c) Layering:- In this a part of the stem of a plant is pulled and buried in soil, while it is attached and supported by the parent plant. The layered plant grows roots.ex sugar cane, rose, grapes Grafting:- In Grafting, two parts of different plants are joined together in such a way that they will unite and grow as one plant. The portion of a plant that is grafted on to the other plant is called scion, and the plant in which grafting is performed is called the stock. The stock and the scion are tied together in such a way that the cambium of scion and stock come in contact with each other. Page 1
Example, on citrus root stock, varieties like sweet orange, lemon, lime and grape fruit can be grafted. (6) Parthenogenesis- A process of development of an organism from an unfertilized egg. Sexual Reproduction Sexual Reproduction in Plants The reproductive part of higher plants is the flower. The various parts of flower are-sepals, petals, stamens and carpels. Sepals are usually green and carpels are coloured and showy. In a flower, stamens and carpels constitute the reproductive parts. Each stamen consists of a stalk called filament, and a flattened top called the anther. The anthers produce the pollen-grains. The pollen grain produces two male gametes. Carpels have a swollen ovary at the base, an elongated middle style and a terminal stigma. The ovary contains ovules. Each ovule has an egg-the female gamete. Pollination:- The transfer of pollen grains from anther to stigma is called pollination. Pollen grains are shed from anther and reach the stigma of either the same, or another flower. Many agents, such as wind, water, insects and other agencies, transfer pollen grains. Pollination is of two types (i) Self pollination: - The transfer of pollen from the anther of a flower to the stigma of the same flower, or another flower of the same plant, is known as self-pollination. (ii) Cross-Pollination: - The transfer of pollen from the anther of one flower to the stigma of another flower of a different plant of the same species is cross-pollination. Structure of Pollen Grain Pollen grain is a microscopic structure. It contains two male gametes at maturity. Ovule is a small structure contained in the ovary. Each ovule is attached to the placenta by a stalk called the funicle. The funicle provides nourishment to the ovule. Ovule is surrounded by two protective coats called integuments, bearing a small opening at one end. This opening is called micropyle. Within the ovule, there is a structure called embryo sac. The embryo sac contains an egg or female gamete towards the micropyle and two polar nuclei ( which fuse to form the secondary nucleus) in the centre. The embryo sac, in addition to an egg and a secondary nucleus, also contains two synergids and there antipodal cells. The embryo sac is surrounded by a nutritive tissue called the nucellus. Process of Fertilization In plants, pollination is followed by fertilization. Once the pollen grains are deposited on the stigma, they form tubes called pollen tubes. One pollen tube grows through the style and reaches the ovary where ovules are located. The pollen tube normally enters the ovule through a small opening called micropyle. Inside the ovule, the pollen tube releases two male gametes into the embroyosac. The embryo sac of the ovule contains the egg. One male gamete fuses with the egg. This fusion of male and female gamete is called syngamy and its product is the zygote. The other male gamete fuses with the two polar nuclei and this process is called triple fusion. Where three nuclei are involved in the fusion process, one male gamete and two polar nuclei. Inside each embryo sac, two fusions, syngamy and triple fusion, take place. This mechanism of two fusions occurring in an embryo sac is called double fertilization. After fertilization ovary develops into the fruit, ovules develop into the seeds. Page 2
Significance of Double fertilization The endosperm produced as a result fertilization supplies food material to the developing embryo, due to which embryo is differentiated property leading to the formation of healthy seeds. Human Reproductive system Human reproduction is highly evolved. The structures associated with reproduction are different in male and female, thereby forming a specialized male reproductive and female reproductive system. In the life of human beings reproductive system becomes functional at a definite age called puberty. Male attain puberty at the age of 13 to 14 years, while females attain it at the age of 10 to 12 years. The male gonad produces sperm and female gonad produces ovum at the age of puberty. The testis and ovary, besides producing gametes, also secrete hormones. The hormones play a very important role in the process of reproduction. They (i) Regulate the process of gametogenesis (formation of sperm and ovum) (ii) Maintain the structure and function of accessory sex organs (iii) Develop secondary sex characters like facial, axial and pubic hair, pitch of the voice and development of mammary glands. Male Reproductive system: - The male reproductive organ is a pair of testis (testes). The testis lies in small sac-like muscular structure outside the abdominal cavity called scrotum. Scrotum provides an optimal temperature for formation of sperms. This temperature is 1 3 C lower than the temperature of the body. The function of testis is to produce sperms and the male hormone (testosterone). Testes form sperms throughout the reproductive life (puberty stage onwards) of man. From each of the testes arises a long tube called Vas deferens. It unites with a duct coming from the urinary bladder to form a common tube called urethra. The urethra is enclosed within a thick muscular organ, called penis, and opens to the outside through a male genital pore. There is only one opening for the urine and sperms. Female Reproductive system Female reproductive system is more complex than that of male. The complexity in structure and function ensures the union of sperm and ovum inside the body of the female. In human females, paired ovaries are located in the abdominal cavity near the kidney. The ovaries perform dual function of (i) production of female gamete (ovum) and (ii) secretion of female hormones (estrogen and progesterone). Each ovary is composed of ovarian follicles. At puberty, these follicles undergo maturation to produce ova. Near the posterior end of ovary, a funnel like structure lead into a long convoluted tube called fallopian tube. The fallopian tube from both sides open into a muscular structure, the uterus. Uterus opens into the vagina. Vagina is tubular structure. It is also called birth canal as through this canal the young one is born after completion of development inside uterus. In females, urinary opening (urethra) and vaginal openings are separate. Sexual Cycle in Females In human female, the ovaries start producing ovum (female gamete) at called puberty (10-12 years). The ovarian follicles grow into mature follicle. One mature follicle develops to surround one ovum. The maturing ovum is then released from the respective ovary by the process called ovulation. At the same time, cycle of events takes place in other parts of the female reproductive organs like uterus and vagina. As the ovarian follicle matures, the inner wall of uterus thickens to get prepared for receiving the developing zygote in case fertilization occurs. Page 3
In human beings, the embryo develops inside the uterus. If fertilization does not take place the thickened inner wall of the uterus breaks down along with its blood vessels, and moves out of the vagina in the form of bleeding, called menstrual flow (menstruation). It lasts for about 4 7 days. The cycle of events taking place in the ovaries and uterus every twenty-eight days the menstrual flow, is called menstrual cycle (sexual cycle in human female). In a normal healthy woman, ovulation takes place in the mid of the menstrual cycle around 14 th day. Menstruation occurs every 28 to 30 days. The commencement of menstruation at puberty is called menarche, and marks the beginning of reproductive life of woman. The cycle of events in ovary and uterus stops around the age of 50 years in a human female. It is marked by the absence of menstrual flow. The stoppage of menstrual flow and other events at this age is termed menopause. The period between menarche and menopause (12 to 50 years) is the normal reproductive life in human female. The function of ovaries and uterus change during pregnancy. Both ovulation and menstruation stop till birth of the offspring. Hormones released from hypothalamus and pituitary and the female hormones control the functions of the ovary and other reproductive organs. Fertilization In human beings, fertilization in internal. The male gamete (sperm) is introduced inside the female genital tract by the process called copulation (matting). The sperms are highly active and mobile. Millions of sperms are released in the vagina, and they move up through cervix and uterus. Finally, in the fallopian tube only one sperm fertilizes the ovum. After the union of the sperm and the ovum (fertilization), zygote is formed, if copulation has taken place during ovulatory period (middle of the menstrual cycle). The embryonic development of the zygote starts immediately in the fallopian tube and pregnancy begins. The embryo moves down to reach the uterus, and gets attached to its thickened inner wall. The close attachment of the embryo with the uterus is called implantation. After implantation a special tissue develops between uterine wall and the embryo (foetus) called placenta. Through the placenta, the developmental needs of the foetus, such as nutritional, respiratory and excretory are met from the maternal body. The development of foetus inside the uterus till birth is called gestation. On completion of gestation, the birth of the fully developed foetus is termed as parturition. The average duration of human pregnancy is about 280 days or 40 weeks and it is calculated from the first day of the last menstrual cycle. The newborn child, after 40 weeks of gestation, weighs about 3.5 kg. Differences between Asexual and Sexual Reproduction Asexual Reproduction Only one parent is involved Gametes are not formed No fertilization It involves only mitosis Daughter organisms are genetically identical to the parent. Multiplication occurs rapidly. Since there is no variation, so it does not contribute to evolution of the species. Sexual Reproduction Two parents are involved Gametes are always formed. Fertilization takes place. It involves both meiosis and mitosis. Daughter organisms are genetically different from the parent. Multiplication is not so rapid as in asexual reproduction. Since there are variations, so it contributes to evolution of the species. Page 4
Population control The techniques have been developed to prevent and manage pregnancy. These methods are broadly categorized as Barrier Chemical Surgical methods. Use of specific drugs by females. A small portion of vasdeferens in There are two types of such male, and the fallopian tube in drugs, oral pills and vaginal pills. female, is surgically removed or Oral pills are mainly hormonal tied. It is called vasectomy in preparations, and are called oral males and tubectomy in females.. contraceptives (OCs). The use of Intrauterine Contraceptive Devices (IUCDs) is also very effective and popular. A practicing doctor or a skilled nurse places a copper-t safely inside the uterus Physical devices such as condom, diaphragm and cervical caps They prevent the entry of sperm in the female genital tract during copulation Form the family planning point of view; it is always advisable to adopt any of the fertility regulation methods Reproductive Health and Sexually Transmitted Diseases Infectious diseases are spread from an infected person to a healthy person by sexual contact. Such diseases are called sexually transmitted diseases (STDs), Health includes our physical, social and mental well being. Reproductive health forms a crucial part of the general health. Bacteria, viruses, protozoans and fungi cause many infectious diseases. STDs diseases are spread by means of air, water and food. Common STDs are gonorrhea, syphilis and trichomoniasis. In most cases, the symptoms of these diseases are burning sensation at urination and urethral discharge. These diseases are curable. Acquired Immuno Deficiency Syndrome (AIDS) is also sexually transmitted diseases (STDs) that cause death. It has no definite cure till now. The causative agent for this disease is Human Immunodeficiency Virus (HIV) World Health Organization (WHO) has prepared guidelines and considered reproductive health as fundamental human right. The National Population Policy and National Health Policy of India have covered these aspects under their programme. The family planning programme of the government has taken enough steps to create awareness and provide relevant information about reproductive health. Non Government Organizations (NGO) is also sharing this responsibility of national importance to a great extent. Page 5