How do the Organisms Reproduce Notes for Class 10 Science
IMPORTANT TERMS & CONCEPTS
1. Reproduction: It is the process of producing new individuals of the same species by existing organisms of a species i.e., parents.
2. Significance of Reproduction:
I. It allows perpetuation of species.
II. It increases the population of a species.
III. It plays an important role in evolution by transmitting favourable variations from one generation to another generation.
3. Body Design of Organisms: Organisms look similar because their body designs are similar.
- Reproduction at its most basic level involves making copies of the blueprints of body design.
- DNA in the cell nucleus is the information source for making proteins and different proteins lead to different designs.
- A basic event in reproduction is the creation of a DNA copy.
- DNA copying is accompanied by a cell division giving rise to two cells.
- DNA copying always involves some variation; hence DNA copies generated are similar but not identical.
- This tendency of variation during reproduction leads to evolution.
4. Importance of Variation in Organisms:
- Organisms fill well-defined places or niches in the ecosystem with the help of reproduction.
- Organisms having similar body designs use the same niche or place.
- If a niche suitable for a population of organism is drastically changed, the population may be wiped out completely.
- But if some variations are there in few individuals of these populations, there could be chances for survival.
- For example, if there is a population of bacteria living in temperate water and if the water temperature increases due to global warming, most of the bacteria will die. But, the variants resistant to heat will survive and grow further.
- Variation is thus important for survival of species.
5. Types of Reproduction: There are two main methods by which organisms give rise to new individuals —Asexual reproduction and Sexual reproduction.
I. Asexual reproduction: It is the process of producing new organism from a single parent without the involvement of sex cells or gametes.
II. Sexual reproduction: It is the process of producing new organism from two parents by making use of their sex cells or gametes.
6. Characteristics of Asexual Reproduction:
I. Only one individual of an organism is involved.
II. Cell divisions are either amitotic or mitotic.
III. The new individuals produced are genetically identical to their parents.
IV. Asexual reproduction presents a rapid mode of multiplication.
7. Advantages and disadvantages of Asexual Reproduction:
I. Asexual reproduction is simple and fast.
I. Evolutionary change is not possible as no variation is produced. A species consequently cannot adapt to changes in its environment.
II. Asexual reproduction produces identical organism’s generation after generation. In case of any defect in the parent organism, the offspring also inherits it.
8. Modes of Reproduction used by single organisms: Asexual reproduction takes place as follows:
(i) Fission (ii) Fragmentation (iii) Regeneration (iv) Budding (v) Vegetative propagation (vi) Spore formation
9. Fission: This is the simplest method of asexual reproduction in unicellular forms of life such as Amoeba, Paramoecium and other protozoa.
- Fission leads to the creation of new individuals.
- In the process of fission, the parent organism splits or divides to form two or more new organisms.
- Fission is of two types: Binary fission and multiple fission
I. Binary Fission: It is the division of one cell into two similar or identical cells.
- In this method, the nucleus first divides a mitotically into two, followed by the division of the cytoplasm.
- The cell finally splits into two daughter cells.
- Binary fission can be seen in bacteria, yeast and Euglena.
- Among animals, Amoeba and Paramoecium reproduce through binary fission.
II. Multiple Fission: In multiple fission, many individuals are formed from a single individual.
- The nucleus of the cell divides repeatedly, producing many nuclei.
- Each nucleus is surrounded by a small amount of cytoplasm and many daughter cells are produced within the cyst.
- The cyst breaks up under favourable conditions and small offspring are liberated.
- In plants, multiple fission is seen in many algae and in animals; a common example of multiple fission is that of the malarial parasite (Plasmodium).
10. Fragmentation: Multicellular organisms such simple body organisation such as filamentous algae-spirogyra breaks up into two or more small pieces or fragments upon maturation. These fragments grow into new individuals.
Among animals, flatworms show fragmentation of the body which develops into new individuals.
11. Regeneration: It is the ability of a fully differentiated organism to give rise to new individual organisms from its body parts.
- Small cut or broken parts of the organisms’ body grow or regenerate into separate individuals.
- For example, simple organisms like Hydra and Planaria can be cut into any number of pieces and each piece grows into a complete organism.
- Regeneration is carried out by specialized cells which proliferate and make large number of cells thus undergoing changes to become various cell types
12. Budding: In budding a small part of the body of the parent grows out as a ‘bud’ which then detaches and becomes a new organism.
- A bud develops as an outgrowth due to repeated cell divisions
- When matured bud gets detached from parent body
- A new independent individual is formed.
13. Vegetative propagation:
In vegetative propagation, new plants are obtained from the pars of old plants like stems, roots and leaves, without the help of any reproductive organ.
There are two ways of vegetative propagation:
(a) Natural Vegetative Propagation and (b) Artificial Vegetative Propagation
14. Natural Vegetative Propagation: Various structures that take part in this type of reproduction are roots, stem and leaves.
I. Natural Vegetative Propagation by Roots: In some plants like Dahlia, sweet potato, etc. the adventitious toots become thick, swollen and tuberous due to storage of food.
II. Natural Vegetative Propagation by Stems: Some plants reproduce by means of stems. They may be aerial like runners, suckers or underground like ginger (rhizome), potato (tuber), onion (bulb).
III. Natural Vegetative Propagation by Leaves: The fleshy leaves of Bryophyllum bear adventitious buds in the notches along the leaf margin.
- When the leaves fall on the soil, the buds develop into small plants under favourable conditions.
- These plantlets on being detached develop into independent plant.
15. Artificial Vegetative Propagation: Some plant growers have developed artificial methods of vegetative propagation like cutting, layering and grafting which are used in agriculture and horticulture.
I. Cutting: In this type of propagation any part of the plant either root, stem or leaf is cut and buried partly in the moist soil.
- Many plants like sugarcane, raised plant, Chrysanthemum, grapes are propagated by means of cutting.
II. Layering: The adventitious roots are produced in the branch of the stem before the plant is detached from the parent plant. The branch of stem is called a layer. This layer is then planted on soil. From this layer new shoot and root produces as a result new plant grows. This process is utilised in the propagation of plant and the phenomenon is called layering. Layering is used in the propagation of plants like Lemon, Guava, Hibiscus, Bougainvillea, Jasmine, Raspberry, Strawberry and many ornamental plants.
III. Grafting: In this method of reproduction, two plants of closely related varieties are joined together so that they live as one plant.
- The portion of a plant that is rafted on the other plant is called scion, and the plant on which grafting is performed is called the stock.
- This method is applied to improve variety of fruits like mango, apple, peas, citrus and guava.
16. Advantages of Vegetative Propagation:
I. Vegetative propagation is a cheaper, easier and more rapid method of propagation in plants than growing plants from their seeds.
II. The traits or characters of the parent plant are preserved by vegetative propagation.
III. Better quality of the plants can be maintained by this method.
IV. It results in propagation of those plants which do not produce viable seeds or produce seeds with prolonged period of dormancy.
V. The plants generated from vegetative means require less time to grow and have the advantage of being more uniform and generically similar to the parent stock.
17. Disadvantages of Vegetative Propagation:
I. Vegetative propagation induces over-crowding.
II. There is no genetic variation, so there is less adaptability to the environment.
III. The disease of the parent plant gets transferred to the offspring.
IV. The plants lose vigour.
V. New characters can neither be introduced nor undesirable characters be eliminated.
18. Tissue Culture: It is the production or propagation of new plants from isolated plant cells or small pieces of plant tissue in a synthetic medium of culture solution. Tissue culture for producing new plants is done as follows:
- Plants are grown by removing tissues or separating cells from the growing tip of the plant and put in an artificial medium.
- The plant tissue divides to form small group of cells or callus.
- The callus is transferred to another medium containing hormones for growth and differentiation that forms plantlets.
- The plantlets produced are transplanted into pots or soil where they can grow to form mature plants.
- This technique is also known as micro propagation in vitro because it takes place outside the body of the parent plant in a test-tube using an artificial environment.
- Micropropagation technique is being used for the production of ornamental plants like Orchids, Dahlia and Carnation.
19. Spore Formation: When a slice of bread is kept in moist dark place for a few days, spores of Rhizopus present in air settle on the bread to form new fungus plants of Rhizopus.
- The Rhizopus consists of fine thread-like projections called hyphae. It has a knob like structure which is involved in reproduction called sporangia, containing spores, which develop into new Rhizopus.
20. Sexual Reproduction: It is a type of reproduction in which the two sexes, namely, male and female are involved.
- The male sexual unit is known unit is known as male gamete or sperm while female sexual unit is termed as female gamete or ova.
- Thus, the two major processes, i.e., formation of gametes and fusion of gametes constitute sexual reproduction.
21. Significance of Sexual Mode of Reproduction:
I. Sexual reproduction promotes diversity of characters in the offsprings.
II. It results in new combinations of genes brought together in the gamete and this reshuffling increases genetic variation.
III. It plays a prominent role in the origin of new species.
IV. The sexual mode of reproduction incorporates process of combining DNA from two different individuals during reproduction.
Angiosperms: It is a phylum comprising of the flowering plants. The gametes are produced within the flowers and the ovules are enclosed in a carpel.
23. Angiosperms: It is a phylum comprising of the flowering plants. The gametes are produced within the flowers and the ovules are enclosed in a carpel.
24. Sexual Reproduction in Flowering Plants: The sexual reproduction in plants takes place in the following steps:
- The reproductive parts of angiosperms are located in the flower.
- Stamens and carpels are the reproductive parts of a flower which contain the germ cells.
- The male organ of a flower called ‘stamen’ makes the male gametes, which are present in pollen grains of the plant.
- The female organ of a flower called ‘carpel’ or ‘pistil’ makes the female gametes, which are present in ovules of the plant.
- The male gametes present in pollen grains fertilise the female gametes present in ovules.
- The fertilized ovules grow and become seeds.
- The seeds produce new plants.
25. Parts of a Flower: The flowers are usually bisexual, i.e., male and female reproductive parts are present in the same plant. The flower is attached to the plant by a stalk or pedicel.
- The main parts of a flower are – sepals, petals, stamens and carpels, (pistil).
- Sepals are usually green, leaf-like parts in the outermost circle of a flower. The function of sepals is to protect the flower in its initial stages when it is in the form of a bud.
- Petals are the colourful parts of a flower whose base lies inside the sepals. The function of petals is to attract insects for pollination and to protect the reproductive organs, which are at the centre of the flower.
- Stamen and carpel constitute the reproductive parts of a flower.
- Stamen is the male reproductive organ of the plant. A flower usually has a number of stamens in it.
- Each stamen is made up of two parts – a filament, the stalk of stamen and an anther, the swollen top of stamen.
Another is the upper bilobed part and each lobe contains two pollen sacs within numerous pollen grains are produced that are yellowing in colour.
26. Unisexual: it is the plant whose flowers contain either stamens or carpels but not both. Example: Papaya, Watermelon.
27. Bisexual: It is the plant whose flowers contain both stamens and carpels. Example: Hibiscus, Mustard.
It is the transfer of pollen grains from the anther of a stamen to the stigma of a carpel. The pollen grains are transferred by many agents, such as insects (bees and butterflies), birds, man wind and water.
Pollination is of two types – self-pollination and cross-pollination.
29. Self-Pollination: It is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or another flower of the same plant. It is seen in pea and China rose plant.
I. Self-pollination in bisexual flowers ensures continuity of the race.
II. It helps to preserve the parental characters, as the gametes from the same flower are involved.
III. It is not necessary for flowers to produce nectar or scent or be colourful.
I. New varieties cannot be obtained by self-pollination.
II. The genetic defects of the breed cannot be removed.
III. Repeated self-pollination leads to loss of vigour and vitality of the species.
30. Cross-Pollination: It is the transfer of pollen grains from the anther of one flower to the stigma of another flower of a different plant of the same species. It is common in majority of flowering plants. Cross-pollination occurs with the help of two groups of agents – biotic such as insects, birds and man; and abiotic such as wind and water.
I. Cross-pollination results in healthier offsprings.
II. Seeds produced by cross-pollination have much better germinating capacity.
III. More abundant and viable seeds are produced.
IV. Variations arte introduced by cross-pollination.
I. Plants have to depend on external agencies for pollination.
II. The pollen grains have to be produced in large quantity to ensure pollination.
31. Gametes: the cells involved in sexual reproduction are called gametes. The male gamete in animals is called ‘sperm’ or spermatozoan and the female gamete in animals are called ‘ovum’ of egg.
32. Zygote: The cell which is formed by the fusion of a male gamete and a female gamete is called zygote, i.e., it is a ‘fertilised ovum’ or ‘fertilised egg’.
33. Embryo: It is the stage of development between the zygote or fertilised egg and the newly formed offspring.
34. Fertilisation: It is defined as the fusion of a male gamete (sperm) with a female gamete (an ovum or egg) to form a zygote during sexual reproduction.
35. Fertilisation in Plants: Pollination is followed by fertilization in plants.
- After the pollen lands on a suitable stigma, it has to reach the female germ cells in the ovary.
- The pollen tube grows out of the pollen grain through the style to reach the ovary.
- After fertilization, the zygote divides several times to form an embryo within the ovule.
- The ovule then develops a tough coat and gets converted into a seed.
- The seed contains the future embryo which develops into seedling.
- The ovary develops and ripens to form a fruit.
- The process of double fertilization occurs inside each embryo sac, in which two fusions, syngamy and triple fusion take place.
- When one male gamete fuses with the egg contained in the embryo sac of the ovule, this fusion of male and female gametes 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.
Note: After fertilization ovary changes into Fruit and ovule changes into seed while sepal petal stamen carpel degenerate.
Germination: It is the initial stages in the growth of a seed to develop into al seeding under appropriate conditions.
36. Reproduction in Human Beings:
- The reproductive organs of human beings, i.e., testis in male and ovary in female become functional only after attaining sexual maturity.
- In males, sexual maturity is attained at the age of 13-14 years, while in females at the age of 10-12 years.
This is known as the age of puberty.
- The testes and ovary produce viable gametes and also secrete hormones like testosterone (male hormone from testes) and estrogen and progesterone (female hormones from ovary).
37. Puberty: It is the age at which the sex hormones or gametes begin to be produced and the boy or girl becomes sexually mature
38. Sexual Maturity in Human beings: Various changes take place in human body at the time of sexual maturity or puberty.
I. Changes Common to both Boys and Girls.
- Thick hair growth in armpits and genital area between the thighs.
- Thinner hair on legs, arms, and face.
- Oily skin and appearance of pimples.
II. Changes Different to both Boys and Girls.
In Girls In Boys
Breast size begins to increase. Thick facial hair growth.
Darkening of nipple skin. Voice begins to crack.
Start of menstruation cycle. Penis occasionally begins to become enlarged and erect.
39. Male Reproductive System: It consists of portions that produce the germ-cells and other portions that deliver the germ-cells to the site of fertilization. The human male reproductive system consists of the following organs:
- Testes (singular testis) are the oval-shaped primary reproductive organs in man. A pair of lies in a small saclike muscular structure outside the abdominal cavity called scroutm. The function of testis is to produce sperm and male sex hormone called testosterone. The scrotum provides the optimal temperature for formation of sperms.
- Epididymis is a coiled tube-like structure firmly attached to the testis and serves as the storehouse of sperms. Inside the epididymis, sperms become mature and develop motility.
- Vas deferens: The sperms are carried by a long tube called vas deferens of sperm duct into organs called seminal vesicles, where the sperms get nourished and stored.
- Urethra is a common duct for the passage of both urine and spermatic fluid. Urethra carries the sperms to an organ called penis which opens to the outside through a male genital pore.
- Penis forms the external male genital organ. It is a copulatory organ with thick muscular walls.
- Accessory Glands: Seminal vesicles are a pair of thin-walled muscular elongated sac which secretes fluid for nourishment of sperms.
- Prostate Glands also produce fluid which is released in the urethra along with secretion of seminal vesicle. The secretion of accessory glands together with sperms is called semen.
- Sperms are tiny bodies that consist of mainly genetic material and a long tail which help them to move towards the female germ cell.
40. Female Reproductive System: The female germ-cells or eggs are made in the ovaries and are responsible for the production of some hormones. The human female reproductive system consists of the following organs:
- Ovaries are a pair of small and oval-shaped organs, located in the abdominal cavity near the kidney. Ovaries are the female primary reproductive organs which perform dual functions of production of female gamete or ovum and the secretion of female sex hormones, estrogen and progesterone.
- FallopianTube or Oviduct is a pair of long convoluted tubes that carry oval or eggs from the ovary to the uterus. The fallopian tube has a funnelshaped opening near the ovary. These tubes from both the sides opening near the ovary. These tubes from both the sides open into an elastic bag-like structure, the uterus.
- Uterus or womb is a hollow, pear-shaped organ within which the embryo develops. Its upper portion is broader, while its lower portion is narrower, called cervix.
- Vagina: The uterus opens into the vagina through the cervix. Vagina is a tubular structure called “birth canal”. It receives sperms from the male and also serves as the passage through which the fully developed foetus is born.
41. Sexual Reproduction in Human Beings:
- The male gamete (sperm) is introduced inside the female genital tract (vagina) by the process of copulation or mating. Fertilization occurs in the fallopian tube.
- Sperms are highly active and mobile which move up through cervix into the uterus and then pass into the fallopian tubes.
- In the fallopian tube only one sperm fertilises the ovum to form a zygote. This is called fertilization.
- Fertilization occurs only if copulation takes place during the ovulatory period.
- The embryonic development of the zygote starts immediately in the fallopian tube and pregnancy starts while menstruation stops.
- The embryo moves down to reach the uterus. The embedding of embryo in the thick inner lining of the uterus is called implantation.
- Then, a special tissue develops between the uterine wall and the embryo (foetus) called placenta where the exchange of nutrients, oxygen and waste products take place.
- The time period from the development of foetus inside the uterus till birth is called gestation.
- The act of giving birth of the fully developed foetus at the end of gestation period is termed as parturition.
- The development of the child inside the mother’s body takes approximately nine months.
- The breakdown and removal of the inner, thick and soft lining of the uterus along with its blood vessels in the form of vaginal bleeding is called menstrual flow or menstruation.
- The cycle of events taking place in the ovaries and uterus every twenty eight days or roughly one month and marked by the menstrual flow is called menstrual cycle or sexual cycle in human female. It lasts for about two to eight days.
43. Reproductive health is all those aspects of general health which help a person to lead a normal, safe and satisfying reproductive life.
44. Sexually Transmitted Diseases (STDs)
These are the diseases which are spread by sexual contact from an infected person to a healthy person. They are caused by various micro-organisms that live in warm and moist environments of the vagina, urethra, anus and mouth. Some of the common sexually transmitted diseases are:
I. Gonorrhea: It is caused by bacterium Neisseria gonorrhea. It is characterised by inflammation of urinogenital tract and the patient feels burning sensation during urination. This bacteria infects the ureter in men and the cervix in women.
II. Syphilis: It is caused by bacterium Treponema palladium. It is characterised by lesions in the mucous membrane of urinogenital tract and ulcers on genitalia.
III. Trichomoniasis: It is caused by protozoan Trichomonas vaginalis. It is characterised by some vaginal discharge at the urinogenital tract of the female.
IV. AIDS (Acquired Immune Deficiency Syndrome): it is caused by a virus called HIV (Human Immunodeficiency Virus) which suppresses the body’s immune mechanism and thereby making it susceptible to any disease.
Mode of transmission of AIDS is as follows:
- By having sexual contact with an infected person.
- By the transfusion of blood from an infected person.
- Through infected needles used for injection.
- Through the placenta from the mother to child during pregnancy.
45. Methods to Avoid Pregnancy:
A number of techniques have been developed to prevent and control pregnancy.
I. Mechanical Barrier Methods: In these methods, physical devices such as condoms, diaphragm and cervical caps are used. These devices prevent the entry of sperm in the female genital tract during copulation, thus acting as a barrier between them.
II. Chemical methods: In these methods, specific drugs are used by females, which are of two types – oral pills and vaginal pills.
III. Surgical Methods: In these methods, a small portion of vas deferens in male and the fallopian tube in female is surgically removed or tied. It is called vasectomy in males and tubectomy in females. In this case, if the vas deferens in male is blocked sperm transfer will be prevented and if the fallopian tube in the female is blocked, the egg will not be able to reach the uterus, thus fertilization will not take place.
46. Sex Ratio: It is the ratio of the number of females to the number of males in a population. The female-male sex ratio must be maintained for a healthy society. The female child sex ratio is declining at an alarming rate in some sections of our society because of reckless female feticide. Therefore, prenatal sex determination has been prohibited by law.
47. Population Size: Organisms increase their population with the help of reproduction. The rates of birth and death in a given population determine its size. The increase in a population occurs when the birth rate is higher than the death rate.
Important Questions How do the Organisms Reproduce Class 10 Science
Very Short Answer type Questions :
Question. Select two plants from the following which are grown by vegetative propagation process:
Banana, Wheat, Mustard, Jasmine, Gram.
Answer: Banana and jasmine are grown by vegetative propagation, because they do not produce seeds.
Question. What is the advantage of reproduction through spores?
How will organism be benefitted if it reproduces through spores?
Answer: Spore with a cell wall can survive even in adverse conditions. Large number of spores are produced in one sporangium. It is easily dispersed through wind as they are large in number and light in weight.
Question. What is the effect of DNA copying which is not perfectly accurate in the reproduction process?
Answer: DNA copying will lead to variation in populations which helps in evolution of the species.
Question. What are the functions performed by testis in human beings?
Answer: The function of testis is to produce sperms and male sex hormone called testosterone.
Question. How does reproduction help in providing stability to the population of species?
Answer: Stability is provided by equalizing the birth and death ratio. Thus, the rate of birth should approximately be equal to the rate of death.
Question. The mode of reproduction depends on which feature of the organism?
Answer: The mode of reproduction depends upon the body design of organism. Those with simple body design may reproduce asexually. Those with complex body design may form gametes and may undergo sexual reproduction.
Question. Name two simple organisms which have ability of regeneration.
Answer: Planaria and Hydra
Question. What happens when egg is not fertilised?
Answer: When egg is not fertilised, blood and mucus comes out through vagina. This process is called menstruation.
Question. Explain how do organisms create an exact copy of themselves.
Answer: The cell uses biochemical reactions to make exact copies of DNA or genetic material.
Short Answer Type Questions :
Question. (a) State in brief the functions of following female reproductive system
(i) Ovary, (ii) Fallopian tube, (iii) Uterus
(b) State in brief the functions of following male reproductive system
(i) Scrotum, (ii) Testes, (iii) Vas deferens
Answer: (a) (i) Ovary: It produces ova and female sex hormones.
(ii) Fallopian tube: It is the site of fertilisation and transfer of female gamete from the ovary.
(iii) Uterus: Implantation of zygote and it keeps foetus till complete development.
(b) (i) Scrotum: It protects testes.
(ii) Testes: It produces sperms.
(iii) Vas deferens: It delivers sperms from testes to urethra.
Question. Why must pollination occur before fertilisation? How is pollination different from fertilisation? What does a pollen contain inside?
Answer: Pollination must occur before fertilisation as it has to reach the male gametes present in pollen grain which will germinate to form a pollen tube and carry the male gametes to the ovum. Pollination is different from fertilisation because here the pollen grain with its male gametes just reaches the stigma, whereas fertilisation of male gamete to ovum results in formation of diploid zygote which is the foundation of new generation.
Question. Mention the changes observed in flower after fertilisation.
Answer: (i) Ovary ripens and developed into fruit.
(ii) Ovules develop into seed.
(iii) Petals and sepals wither and drop.
(iv) Stigma, style and stamens dry up and fall off.
Question. What are the two roles of testosterone?
Answer: (i) Testosterone regulates formation of sperms.
(ii) The appearance of secondary sexual characteristics at puberty like beard, genital hair, change in voice is due to testosterone.
Question. What are the advantages of sexual reproduction over asexual reproduction?
Answer: In asexual reproduction, the offspring is almost identical to the parent because they have the same gene as their parent. Thus, variation is not present.
Sexual reproduction involves fusion of male and female gametes. The offspring exhibits diversity of characters because they receive some genes from the mother and some from the father. The mixing of genes in different combinations; results in genetic variations. This variation leads to the continuous evolution of various species to produce various organisms.
Question. Why does menstruation occur?
Answer: Menstruation occurs in females when the egg produced inside the cervix is not fertilized. Since the egg does not fuse with the male gamete, so the thick and soft lining of uterus having a lot of blood capillaries in it are not required. This unfertilized egg dies within a day and the lining breaks down shedding blood along with other tissues. This comes out of the vagina in the form of bleeding.
Question. What are the different methods of contraception?
Answer: Contraception is the method to avoid pregnancy. Various methods of contraception are as follows:
Physical Barrier Methods: Use of condoms, diaphragms, cervical caps can be used.
These prevent the entry of sperms into the female genital tract by acting as a barrier between them.
Chemical Methods: Oral pills can be used which change the hormonal balance and stop release of egg. Vaginal pills kill the sperms.
Surgical Methods: This includes vasectomy (sperm duct is removed) in males and tubectomy (removal of small portion of fallopian tube) in females.
Question. What could be the reasons for adopting contraceptive methods?
Answer: The reasons for adopting contraceptive methods could be:
1. Protection from sexually transmitted diseases such as HIV-AIDS, gonorrhoea, syphilis, warts etc.
2. Restricting the number of children.
3. Sufficient gap between successive births.
4. Enjoying a good reproductive health.
5. Controlling population.
Question. List three distinguishing features between sexual and asexual types of reproduction in tabular form.
Question. Describe the structure and function of placenta.
Answer: Structure of placenta: It is a special disc like tissue embedded in mother’s uterine wall and connected to the foetus/embryo.
Functions of placenta: It provides a large surface area for glucose and oxygen/nutrients to pass from mother’s body to the developing/developed embryo/foetus and also helps in passing the waste from the foetus/embryo to the mother’s body.
Question. How do sperms reach the female genital tract? Where fertilisation and implantation of the embryo does take place? For how long does the embryo remain attached to uterine walls?
Answer: Sperms travel upwards through uterus to oviduct where they may meet an ovum and hence fertilise it. Fertilisation takes place in oviduct to give rise to single celled zygote. The zygote divides to become multicellular embryo which gets fixed in uterus. The embryo remains attached to uterine walls throughout gestation period which is about 40 weeks in humans.
Question. Draw a labelled diagram of longitudinal section of pistil of flower showing germination of pollen grains on the stigma.
Question. List any four steps involved in sexual reproduction and write its two advantages.
Answer: The four main steps involved in sexual reproduction are:
(i) In the first stage of sexual reproduction, meiosis process occurs and the number of chromosomes reduces from diploid (2n = 46) to haploid (n = 23) for each gamete.
(ii) In the second stage, there is transfer of male gametes into the female body.
(iii) In the third stage, the two gametes will fuse together after fertilisation, a single male gamete will fuse with a female gamete. i.e. fertilisation process takes place.
(iv) After fusion of male and female gametes, they form a zygote, in which the number of chromosomes is restored to diploid (2n = 46).
The two main advantages of sexual reproduction are:
(i) There are more variations, which leads to better adaptability of the offsprings in the environment.
(ii) Promotes the diversity in the characteristics of offspring, because it results by fusion of gametes.
Question. Write one main difference between asexual and sexual mode of reproduction. Which species is likely to have comparatively better chances of survival – the one reproducing asexually or the one reproducing sexually? Give reason to justify your answer.
Answer: Any one of the following differences:
(i) In sexual reproduction two opposite sexes are involved whereas in asexual reproduction only one individual is involved.
(ii) In sexual reproduction male and female gametes formation take place whereas in asexual reproduction no gamete formation occurs.
• Sexually reproducing organisms have better chances of survival.
• This is because more variations are generated.
Question. What is vegetative propagation? State two advantages and two disadvantages of this method.
Answer: Vegetative propagation is a mode of asexual reproduction in which new plants are obtained from vegetative parts of the plants. It does not involve the production of seeds or spores for the propagation of new plants.
Two advantages of vegetative propagation are:
(i) Plants which do not produce seeds are propagated by this method, for example sugarcane, potato, etc.
(ii) Vegetative propagation is a cheaper, easier, rapid method of propagation in plants than growing plants from their seeds. For example, lilies grow very slowly and take 4 to 7 years to develop flowers when grown through their seeds, but flowers are produced only after a year or two when grown vegetatively.
Two disadvantages of vegetative propagation are:
(i) As there is no genetic variation, there is no chance of development of new and better varieties.
(ii) The vegetatively propagated plants are more prone to diseases that are specific to the species. This can result in the destruction of an entire crop.
Question. Define the term pollination. Differentiate between self pollination and cross pollination. What is the significance of pollination?
Answer: Pollination: The process in which pollen is transferred from stamen to the stigma is called pollination.
Significance: After the pollen lands on suitable stigma, it needs to reach the female germ cells in the
ovary where fertilisation takes place, zygot is formed. Zygot divides several times to form embryo with
in ovule which develops a tough coat around it called seed coat and seed is formed. The ovary grows
rapidly and ripens to form fruit.
Question. (a) List in tabular form two differences between binary fission and multiple fission.
(b) What happens when a mature spirogyra filament attains considerable length.
(b) A mature spirogyra filament breaks into smaller fragments and each fragment grows into a new plant. This process of reproduction is called fragmentation.
Question. How does reproduction help in providing stability to the populations of species?
Answer: It is because reproduction is a process by which species increase their population. If the rate of birth is more than death, the size of population will increase and will be more stable.
Question. (a) What is the location of following:
(i) DNA in a cell,
(b) Expand DNA.
Answer: (a) (i) DNA is found in nucleus of cell,
(ii) Genes are located on the chromosomes.
(b) Deoxyribonucleic acid.
Question. When a cell reproduces, what happens to its DNA?
Answer: During the process of reproduction, transmission of DNA from parents to offspring takes place. Before reproduction, DNA is replicated, which means two copies of DNA are produced. When the cell divides into two, these two copies are distributed equally between the two daughter cells so that similar amount and type of DNA is transferred from the parent cell to the daughter cells. It maintains the consistency in the amount and type of DNA in the living organism of a particular species.
Question. Reproduction is one of the most important characteristics of living beings. Give three reasons in support of the statement.
Answer: Reproduction is an energy consuming process which is not essential for the survival of an individual.
But it is the most important characteristic of all living beings, because of the following reasons:
(i) Reproduction helps in increasing the number of members of a population.
(ii) By replacing the dead members with the new ones, it minimises the risk of extinction of a species.
(iii) It brings about variations in species, thus leading to their evolution.
Question. Explain budding in hydra with the help of labelled diagrams only.
Answer: A bud is an outgrowth from the body part of parent due to repeated cell division at a specific site. These buds develop into new individuals, which detach from the parent body on maturation.
In hydra, the cells divide very fast at a specific site as an outgrowth called bud. These bud develops into a small individual, while remaining attached to the parent. When it matures, it detaches from the parent body to exist as an independent organism.
Question. What happens when:
(a) Accidently, Planaria gets cut into many pieces?
(b) Bryophyllum leaf falls on the wet soil?
(c) On maturation sporangia of Rhizopus bursts?
Answer: (a) When Planaria gets cut into many pieces, it will undergo a process known as regeneration due to which each piece will grow into a new planaria organism.
(b) When bryophyllum leaf falls on the wet soil, the buds that are produced in the notches along the leaf will develop into new plants by the process known as vegetative propagation.
(c) When the sporangia of Rhizopus bursts upon maturation, the spores present inside it spread in the open environment. Then, with the help of different agents, they are carried to different places and when they land on a favourable surface, they start growing and produce new organism.