HOW DO ORGANISMS REPRODUCE
Reproduction is an integral feature of all living beings. The process by which a living organism produces a copy like its own is called reproduction.
Importance of Reproduction:
Reproduction is important for each species, because this is the only way for a living being to continue its lineage. Apart from being important for a particular individual, reproduction is also important for the whole ecosystem. Reproduction helps in maintaining a proper balance among various biotic constituents of the ecosystem. Moreover, reproduction also facilitates evolution because variations come through reproduction; over several generations.
Types of Reproduction:
There are two main types, viz. asexual and sexual reproduction.
Asexual Reproduction: When a single parent is involved and no gamete formation takes place; the method is called asexual reproduction. No meiosis and syngamy occours during asexual reproduction.
Types of asexual reproduction
Binary Fission: Binary fission occurs in unicellular animals and is somewhat similar to mitosis. The mother cell divides into two daughter cells; and each daughter cell begins its life like a new individual. The parent generation ceases to exist, after binary fission. Amoeba is a very good example of the organism which reproduces by binary fission.
Multiple Fission: in this type The nucleus divides into several nuclei and each daughter nucleus is surrounded by a membrane. At maturity the parent cell bursts releasing the daughter nuclei. The daughter nuclei; in turn; grow into new individuals. (E.g)Plasmodium and entamoeba
Budding: It occurs in yeast and hydra. A small bud grows at any end of the yeast cell. Nucleus gets elongated and a part of it protrudes into the bud. The nucleus then divides into two nuclei. One of the nuclei goes into the bud. The bud grows to certain extent and gets detached from the mother cell.
Fragmentation: Reproduction by fragmentation is seen in filamentous algae, e.g. spirogyra. The filament of spirogyra divides into many pieces and each piece develops into a new individual.
Regeneration: Some simple organisms can regenerate a new individual from a part which has been accidentally cut off. Planaria shows good example of reproduction by regeneration. If a planaria is cut into several pieces; each piece develops into a new individual.
Spore Formation: Most of the fungi, bryophytes and pteridophytes reproduce by this method. Spores are produced in special spore-bearing organs; called sporangium. When spores mature; the sporangium bursts open to release them.
Vegetative Propagation: Vegetative propagation is a special case, as it happens in higher plants; which otherwise have the capability to reproduce sexually. When a vegetative part of a flowering plant reproduces a new plant, it is called vegetative propagation. Some examples of vegetative propagation are given below.
Tuber of Potato: The potato tuber is a modified stem. Many notches can be seen on the surface of potato. These are called ‘eyes’ of potato. Each ‘eye’ of a potato can give rise to a new potato plant. Farmers utilize this capability of potatoes to grow potato more quickly; which is not possible by using the seeds of potato.
Rhizomes of Ginger and Turmeric: Rhizomes are examples of modified underground stems for food storage. These contain nodes, internodes and scaly leaves. When buried under the soil, the rhizomes produce new plants.
Leaf of Bryophyllum: Leaves of bryophyllum have notches on the margin. If a leaf is put under the soil, small saplings grow from the notches on the leaf margin.
Artificial Vegetative Propagation: Man has used artificial vegetative propagation to grow many plants. This has enabled farmers and horticulturists to grow many plants in shorter duration and has helped them to earn more profit. Artificial vegetative propagation has also helped in developing many new varieties of plants. Stem cutting, layering and grafting are the preferred means of artificial vegetative propagation.
Advantages of Asexual Reproduction:
The organism does not have to depend on another organism for carrying out reproduction; because a single parent is needed.
It takes less time than sexual reproduction and hence more number of offspring can be produced in shorter time.
The offspring are exact clones of their parent.
Desirable characteristics can be easily incorporated into plants with artificial vegetative propagation.
Disadvantages of Asexual Reproduction:
As a single parent is involved, so there is negligible chance of variation.
In most of the cases in simple organisms, the parent generation ceases to exist after asexual reproduction.
Asexual reproduction cannot give rise to biodiversity which is important for a healthy ecosystem.
SEXUAL REPRODUCTION
Sexual reproduction involves two parents and gamete formation. Gametes are special cells which are formed after meiosis. There are two types of gametes, viz. male and female gametes. The number of chromosomes is haploid in the gametes. When gametes fuse during fertilization, the number of chromosomes becomes diploid. This is important for maintaining the unique identity of a particular species which reproduces by sexual method.
In sexual reproduction, the offspring gets sets of genes from two different individuals. This leads to subtle variation through subsequent generations. These variations accumulate over thousand of generations and finally may give rise to a new species
DNA Replication:
DNA replication is the process by which DNA makes a copy of itself. DNA replication happens during the S – phase (synthesis phase) of the cell cycle. This is important because the daughter cells would need additional copies of the DNA. The process of DNA replication is a foolproof process, yet some alterations do take place
Sexual Reproduction in Flowering Plants:
Flower is a modified shoot which bears special organs and plays the role of the reproductive system in plant.
Structure of a typical Flower:
A typical flower is composed of four distinct whorls, viz. calyx, corolla, androecium and gynoecium.
Calyx: The outermost whorl of the flower is called calyx. It is composed of green leaf-like structures; called sepals.
Corolla: The second whorl of the flower is called corolla. It is composed of colourful leaf-like structures; called petals. Petals are colourful so that insects and birds can be attracted; to assist the flower in pollination.
Androecium: This is the third whorl in the flower. It is composed of stamens. Stamen is made of a slender stalk and anthers on top. Anthers produce the pollen-grains. Pollen grains are the male gametes.
Gynoecium: This whorl is at the centre of the flower. It is composed of a swollen base; called ovary. A slender style stands upright on the ovary. It has a flat top; called stigma. Ovules are inside the ovary. Ovules are the female gametes.
Pollination: The transfer of pollen grains from anther to the stigma is called pollination. If the pollen grains from the same flower or the same plant are transferred to the stigma; it is called self pollination. If pollen grains from a different plant are transferred to the stigma; it is called cross-pollination.Many agents help plants in cross pollination, e.g. insects, animals, air, water, etc. Insects are the main pollinators for the plant kingdom.
FERTILISATION:-
The fusion of male and female gametes is called fertilization. The product of fertilization is called zygote. Zygote undergoes several rounds of mitosis and develops into an embryo. Subsequently, the embryo develops into a new individual.
Fertilization in flowering plant:
After landing at the stigma, pollen grains absorb moisture and germinate. A pollen grain develops a pollen tube; which penetrates through the tissue of the style and reaches the ovule. Pollen nuclei are transferred through the pollen tube. After fertilization, zygote is formed; which finally develops into the embryo. The ovary turns into the fruit. The ovule turns into seed. Once the seed becomes mature, fruit dries up so that dispersal of seeds can take place.
Structure of Seed: A seed contains an embryo, some reserve food and is enclosed by a protective covering; called seed coat. The reserve food is stored in the cotyledons. The embryo has two pointed parts. The upper part is called plumule which gives rise to the shoot system. The lower part is called radicle which gives rise to the root system. Cotyledons supply food when the embryo needs it during germination. Seed germination is the process by which the embryo in the seed kick-starts a new life.
REPRODUCTION IN HUMAN BEINGS
Male Reproductive System:-The male reproductive system in human beings is composed of following parts:
Testis: There is a pair of testes; which lie in a skin pouch; called scrotum. Scrotum is suspended outside the body; below the abdominal cavity. This helps in maintaining the temperature of testes below the body temperature. This is necessary for optimum sperm production. Testis primarily serves the function of sperm production. Sperms are the male gametes. Apart from that, testis also produces testosterone. Testosterone is also called the male hormone, as it is responsible for developing certain secondary sexual characters in boys.
Vas Deferens: Vas deferens is the tube which carries sperms to the seminal vesicle.
Seminal Vesicle: This is the place where sperms are stored. Secretions from the seminal vesicle and prostate gland add up to make the semen.
Penis: It is a muscular organ which serves the genitor-urinary functions. The urethra works as the common passage for urine as well as for sperms.
Female Reproductive System:
The female reproductive system in human beings is composed of following parts:
Uterus: This is pear-shaped hollow muscular organ. Uterus is the place where the embryo gets implanted and develops into a newborn baby. The wall of the uterus provides safety and nutrition to the growing foetus.
Fallopian Tubes: One fallopian tube comes out from each side at the top of the uterus. The fallopian tubes end in finger-like structures; called flimbriae. Fertilization happens in the fallopian tube.
Ovary: There are two ovaries; one near each fallopian tube. Ovary produces the eggs or the female gametes. All the eggs are produces by the ovary when the female child is still in the womb. One egg matures in each ovulation cycle and is released from the ovary. The egg is caught by the flimbriae and transferred to the fallopian tube.
Vagina: The cervix (mouth of the uterus) opens into the vagina. Vagina is a muscular tube-like organs; which serves as the passage for the sperms and also as the canal during the child birth.
PUBERTY
Human beings are complex animals and hence there is a distinct phase in their life cycle which marks the onset and attainment of sexual maturity. This period is called puberty. It usually starts at around 10 – 11 years of age in girls and at around 12 – 13 years of age in boys. It usually ends at around 18th year of age in girls and at around 19th year of age in boys. Since the years during puberty end in ‘teens’; hence this phase is also called teenage.
Changes in Boys during Puberty: The boys suddenly grow in height dramatically. Voice becomes deep and the Adam’s apple becomes prominent. Shoulders become broad and body becomes muscular. Facial hairs begin to grow. Hairs also grow under the armpit and in the pubic region.
Changes in Girls during Puberty: The voice becomes thin. Shoulders and hip become rounded. Breasts get enlarged. Hairs grow under the armpit and in the pubic region.
Sexual Dimorphism: The physical dissimilarities in the male and female of a species which give them different appearances is called sexual dimorphism.
Secondary Sexual Characters: Features which highlight sexual dimorphism are called secondary sexual characters.
Menstruation: Menstruation is a trait which is unique to humans and some primates. During each ovulation cycle, the uterus prepares itself in anticipation of a possible pregnancy. The uterine wall develops an additional lining. When the egg is not fertilized, it gets disintegrated and so does the additional lining in the uterine wall. The fragments of disintegrated tissues are shed; along with blood. This is observed in the form of bleeding through the vagina which can last from 3 to 7 days. The whole sequence of events during an ovulation cycle is called menstrual cycle. The bleeding which occurs for few days is called menstruation. The first menstrual flow is called menarche and the last menstrual flow (which happens in the late 40s) is called menopause.
TEXTUAL QUESTIONS
1. What is the importance of DNA copying in reproduction?
Answer-DNA is the genetic material present in the cells of all organisms. The genetic information from generation to generation is carried by DNA .It is therefore possible for the organism to produce organism of its own type due to DNA copying only.For the inheritance of traits of the parent,DNA copying is a must.DNA copying also brings about variation ,which forms the basis for the origin of new species.
2. Why is variation beneficial to the species but not necessarily for the individual?
Answer-Variations are beneficial to the species than individual because sometimes for a species, the environmental conditions change so drastically that their survival becomes difficult. For example, if the temperature of water increases suddenly, then most of the bacteria living in that water would die. Only few variants that are resistant to heat would be able to survive. However, if these variants were not there, then the entire species of bacteria would have been destroyed. Thus, these variants help in the survival of the species. However, all variations are not necessarily beneficial for the individual organisms.
1. How does binary fission differ from multiple fission?
Answer-In binary fission a single cell divides into two equal halves.Amoeba and Bacteria divide by binary fission.
In multiple fission, a single cell divides into many daughter cells simultaneously. Plasmodium divide by multiple fission.
2. How will an organism be benefited if it reproduces through spores?
Answer-Advantages of spore formation:
→ Large numbers of spores are produced in one sporangium.
→ Spores are distributed easily by air to far-off places to avoid competition at one place.
1. What is the importance of DNA copying in reproduction?
Answer-DNA is the genetic material present in the cells of all organisms. The genetic information from generation to generation is carried by DNA .It is therefore possible for the organism to produce organism of its own type due to DNA copying only.For the inheritance of traits of the parent,DNA copying is a must.DNA copying also brings about variation ,which forms the basis for the origin of new species.
2. Why is variation beneficial to the species but not necessarily for the individual?
Answer-Variations are beneficial to the species than individual because sometimes for a species, the environmental conditions change so drastically that their survival becomes difficult. For example, if the temperature of water increases suddenly, then most of the bacteria living in that water would die. Only few variants that are resistant to heat would be able to survive. However, if these variants were not there, then the entire species of bacteria would have been destroyed. Thus, these variants help in the survival of the species. However, all variations are not necessarily beneficial for the individual organisms.
1. How does binary fission differ from multiple fission?
Answer-In binary fission a single cell divides into two equal halves.Amoeba and Bacteria divide by binary fission.
In multiple fission, a single cell divides into many daughter cells simultaneously. Plasmodium divide by multiple fission.
2. How will an organism be benefited if it reproduces through spores?
Answer-Advantages of spore formation:
→ Large numbers of spores are produced in one sporangium.
→ Spores are distributed easily by air to far-off places to avoid competition at one place.
→ Spores are covered by thick walls to prevent dehydration under unfavourable conditions.
3. Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?
Answer-Higher complex organisms cannot give rise to new individuals through regeneration because complex organisms have organ-system level of organization. All the organ systems of their body work together as an interconnected unit. They can regenerate their lost body parts such as skin, muscles, blood, etc. However, they cannot give rise to new individuals through regeneration.
4. Why is vegetative propagation practised for growing some types of plants?
Answer-Vegetative propagation is practiced for growing some types of plants because of following advantages:
Answer-Higher complex organisms cannot give rise to new individuals through regeneration because complex organisms have organ-system level of organization. All the organ systems of their body work together as an interconnected unit. They can regenerate their lost body parts such as skin, muscles, blood, etc. However, they cannot give rise to new individuals through regeneration.
4. Why is vegetative propagation practised for growing some types of plants?
Answer-Vegetative propagation is practiced for growing some types of plants because of following advantages:
→ It is used to grow a plant in which viable seeds are not formed or very few seeds are produced such as Orange, Banana, Pineapple.
→ It helps to introduce plants in new areas where the seed germination fails to produce mature plant due to change in environmental factors and the soil.→ It is more rapid, easier and cheaper method.
→ By this method a good quality of a race or variety can be preserved.
→ By this method a good quality of a race or variety can be preserved.
5. Why is DNA copying an essential part of the process of reproduction?
Answer-DNA copying is an essential part of reproduction as it passes genetic information from parents to offspring. It determines the body design of an individual. The reproducing cells produce a copy of their DNA through some chemical reactions and result in two copies of DNA. The copying of DNA always takes place along with the creation of additional cellular structure. This process is then followed by division of a cell to form two cells.
1. How is the process of pollination different from fertilization?
Answer-Pollination is the process of transfer of pollens from anther to stigma. It occurs with the help of certain pollinators such as air, water, birds, or some insects.
Fertilization, is the fusion of the male and female gametes. It occurs inside the ovule and leads to the formation of zygote.
2. What is the role of the seminal vesicles and the prostate gland?
Answer-The secretions from seminal vesicles and prostate glands lubricate the sperms and provide a fluid medium for easy transport of sperms. Their secretion also provides nutrient in the form of fructose, calcium, and some enzymes.
3. What are the changes seen in girls at the time of puberty?
Answer-The changes seen in girls at the time of puberty are:
→ Increase in breast size and darkening of skin of the nipples present at the tips of the breasts.
→ Appearance of hair in the genital area and on other areas of skin like underarms, face, hands, and legs.
→ Increase in the size of uterus and ovary.
→ Beginning of menstrual cycle.
→ More secretion of oil from the skin, which results in the appearance of pimples.
4. How does the embryo get nourishment inside the mother's body?
Answer-After fertilization the lining of uterus thickens and is richly supplied with blood to nourish the growing embryo. The embryo gets nutrition from the mother’s blood with he help of a special tissue called placenta. It is embedded in the uterine wall. Placenta contains Villi on the embryo’s side of the tissue and blood spaces on mother’s side surrounding the villi. This provides a large surface from mother to the embryo and waste products from embryo to mother.
Answer-DNA copying is an essential part of reproduction as it passes genetic information from parents to offspring. It determines the body design of an individual. The reproducing cells produce a copy of their DNA through some chemical reactions and result in two copies of DNA. The copying of DNA always takes place along with the creation of additional cellular structure. This process is then followed by division of a cell to form two cells.
1. How is the process of pollination different from fertilization?
Answer-Pollination is the process of transfer of pollens from anther to stigma. It occurs with the help of certain pollinators such as air, water, birds, or some insects.
Fertilization, is the fusion of the male and female gametes. It occurs inside the ovule and leads to the formation of zygote.
2. What is the role of the seminal vesicles and the prostate gland?
Answer-The secretions from seminal vesicles and prostate glands lubricate the sperms and provide a fluid medium for easy transport of sperms. Their secretion also provides nutrient in the form of fructose, calcium, and some enzymes.
3. What are the changes seen in girls at the time of puberty?
Answer-The changes seen in girls at the time of puberty are:
→ Increase in breast size and darkening of skin of the nipples present at the tips of the breasts.
→ Appearance of hair in the genital area and on other areas of skin like underarms, face, hands, and legs.
→ Increase in the size of uterus and ovary.
→ Beginning of menstrual cycle.
→ More secretion of oil from the skin, which results in the appearance of pimples.
4. How does the embryo get nourishment inside the mother's body?
Answer-After fertilization the lining of uterus thickens and is richly supplied with blood to nourish the growing embryo. The embryo gets nutrition from the mother’s blood with he help of a special tissue called placenta. It is embedded in the uterine wall. Placenta contains Villi on the embryo’s side of the tissue and blood spaces on mother’s side surrounding the villi. This provides a large surface from mother to the embryo and waste products from embryo to mother.
5. If a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases?
Answer-No, because copper-T will not prevent contact body fluids. Thus it will not protect her from sexually transmitted diseases.
Excercise
1. Asexual reproduction takes place through budding in ► (b) yeast.
2. Which of the following is not a part of the female reproductive system in human beings? ► (c) Vas deferens
3. The anther contains ► (d) pollen grains.
Answer-No, because copper-T will not prevent contact body fluids. Thus it will not protect her from sexually transmitted diseases.
Excercise
1. Asexual reproduction takes place through budding in ► (b) yeast.
2. Which of the following is not a part of the female reproductive system in human beings? ► (c) Vas deferens
3. The anther contains ► (d) pollen grains.
4. What are the advantages of sexual reproduction over asexual reproduction?
Answer-Advantages of sexual reproduction:
→ In sexual reproduction, more variations are produced. Thus, it ensures survival of species in a population.
→ The new formed individual has characteristics of both the parents.
→ Variations are more viable in sexual mode than in asexual one. This is because in asexual reproduction, DNA has to function inside the inherited cellular apparatus.
5. What are the functions performed by the testis in human beings?
Answer-Functions of testes:
→ Produce sperms, which contain haploid set of chromosomes of father.
→ Produce a hormone called testosterone, which brings about secondary sexual characters in boys.
6. Why does menstruation occur?
Answer-Menstruation is a process in which blood and mucous flows out every month through the vagina. This process occurs every month because one egg is released from the ovary every month and at the same time, the uterus (womb) prepares itself to receive the fertilized egg. Thus, the inner lining of the uterus gets thickened and is supplied with blood to nourish the embryo. If the egg does not get fertilised, then the lining of the uterus breaks down slowly and gets released in the form of blood and mucous from the vagina.
7. Draw a labelled diagram of the longitudinal section of a flower.
Answer
Answer-Advantages of sexual reproduction:
→ In sexual reproduction, more variations are produced. Thus, it ensures survival of species in a population.
→ The new formed individual has characteristics of both the parents.
→ Variations are more viable in sexual mode than in asexual one. This is because in asexual reproduction, DNA has to function inside the inherited cellular apparatus.
5. What are the functions performed by the testis in human beings?
Answer-Functions of testes:
→ Produce sperms, which contain haploid set of chromosomes of father.
→ Produce a hormone called testosterone, which brings about secondary sexual characters in boys.
6. Why does menstruation occur?
Answer-Menstruation is a process in which blood and mucous flows out every month through the vagina. This process occurs every month because one egg is released from the ovary every month and at the same time, the uterus (womb) prepares itself to receive the fertilized egg. Thus, the inner lining of the uterus gets thickened and is supplied with blood to nourish the embryo. If the egg does not get fertilised, then the lining of the uterus breaks down slowly and gets released in the form of blood and mucous from the vagina.
7. Draw a labelled diagram of the longitudinal section of a flower.
Answer
8. What are the different methods of contraception?
Answer-The contraceptive methods can be broadly divided into the following types:
→ Natural method: It involves avoiding the chances of meeting of sperms and ovum. In this method, the sexual act is avoided from day 10th to 17th of the menstrual cycle because during this period, ovulation is expected and therefore, the chances of fertilization are very high.
→ Natural method: It involves avoiding the chances of meeting of sperms and ovum. In this method, the sexual act is avoided from day 10th to 17th of the menstrual cycle because during this period, ovulation is expected and therefore, the chances of fertilization are very high.
→ Barrier method: In this method, the fertilization of ovum and sperm is prevented with the help of barriers. Barriers are available for both males and females. Condoms are barriers made of thin rubber that are used to cover penis in males and vagina in females.
→ Oral contraceptives: In this method, tablets or drugs are taken orally. These contain small doses of hormones that prevent the release of eggs and thus fertilization cannot occur.
→ Implants and surgical methods: Contraceptive devices such as the loop or Copper-T are placed in uterus to prevent pregnancy. Some surgical methods can also be used to block the gamete transfer. It includes the blocking of vas deferens to prevent the transfer of sperms known as vasectomy. Similarly, fallopian tubes of the female can be blocked so that the egg will not reach the uterus known as tubectomy.
→ Oral contraceptives: In this method, tablets or drugs are taken orally. These contain small doses of hormones that prevent the release of eggs and thus fertilization cannot occur.
→ Implants and surgical methods: Contraceptive devices such as the loop or Copper-T are placed in uterus to prevent pregnancy. Some surgical methods can also be used to block the gamete transfer. It includes the blocking of vas deferens to prevent the transfer of sperms known as vasectomy. Similarly, fallopian tubes of the female can be blocked so that the egg will not reach the uterus known as tubectomy.
9. How are the modes for reproduction different in unicellular and multicellular organisms?
Answer-In unicellular organisms, reproduction occurs by the division of the entire cell. The modes of reproduction in unicellular organisms can be fission, budding, etc. whereas in multicellular organisms, specialised reproductive organs are present. Therefore, they can reproduce by complex reproductive methods such as vegetative propagation, spore formation, etc. In more complex multicellular organisms such as human beings and plants, the mode of reproduction is sexual reproduction.
10. How does reproduction help in providing stability to populations of species?
Answer-Reproduction is the process of producing new individuals of the same species by existing organisms of a species, so, it helps in providing stability to population of species by giving birth to new individuals as the rate of birth must be at par with the rate of death to provide stability to population of a species.
11. What could be the reasons for adopting contraceptive methods?
Answer-Contraceptive methods are mainly adopted because of the following reasons:
→ To prevent unwanted pregnancies.
→ To control population rise or birth rate.
→ To prevent the transfer of sexually transmitted diseases.
Answer-In unicellular organisms, reproduction occurs by the division of the entire cell. The modes of reproduction in unicellular organisms can be fission, budding, etc. whereas in multicellular organisms, specialised reproductive organs are present. Therefore, they can reproduce by complex reproductive methods such as vegetative propagation, spore formation, etc. In more complex multicellular organisms such as human beings and plants, the mode of reproduction is sexual reproduction.
10. How does reproduction help in providing stability to populations of species?
Answer-Reproduction is the process of producing new individuals of the same species by existing organisms of a species, so, it helps in providing stability to population of species by giving birth to new individuals as the rate of birth must be at par with the rate of death to provide stability to population of a species.
11. What could be the reasons for adopting contraceptive methods?
Answer-Contraceptive methods are mainly adopted because of the following reasons:
→ To prevent unwanted pregnancies.
→ To control population rise or birth rate.
→ To prevent the transfer of sexually transmitted diseases.
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