CBSE Class 10 Science
Ch-8 How do Organisms Reproduce?
How do Organisms Reproduce? – CBSE Notes for Class 10 Science
1. Reproduction is the process by which a living organism is able to produce new individuals of its own kind. Unlike other life processes such as nutrition, respiration, etc., it is not essential to, maintain the life of an individual organism. But it is important for the existence and continuity of the species.
2. Reproduction involves the creation of DNA copy and additional cellular apparatus by the cell involved in the process.
3. The process of DNA copying leads to variations. This inbuilt tendency for variations during reproduction is the basis for evolution.
4. Living organisms’ reproduce mainly through :
(i) Asexual reproduction
(ii) Sexual reproduction
5. ASEXUAL REPRODUCTION
(a) Single ceiled organisms reproduce through following ways:
(ii) Budding (also by multicellular organisms)
(iii) Spore formation (also by multicellular organisms)
(b) Asexual reproduction by multicellular organisms:
(i) Fragmentation and Regeneration
6. Fission: In unicellular organisms when cell becomes fully mature, it splits into two or more parts. It is called the fission. In organisms such as Amoeba, splitting can take place in any plane. But in organisms like Leishmania, having whip like structure at one end of the cell, binary fission occurs in a definite orientation in relation to these structures.
7. Regeneration : It is the ability to give rise to new organism. When the individual is cut or broken up into many pieces. It can be seen in Hydra and Planaria and is known as regeneration.
Regeneration is carried out by specialised cells. These cells proliferate and rqgkeJarge numbers of cells. From this mass of cells, different cells undergo changes to become various cell types and tissues. These changes take place in an organised sequence referred to as ” development. However, regeneration is not the same as reproduction, since most organisms would not normally depend on being cut up to be able to reproduce.
8. Budding: Organisms such as Hydra use regenerative cells for reproduction in the process of budding. In Hydra, a bud develops as an outgrowth due to repeated cell division at one specific site. These buds develop into tiny individuals and when fully mature, detach from the parent body and become new independent individuals.
9. Spore Formation (Sporulation): Some bacteria and lower organisms make spores. During spore formation, knob like structure called sporangium develops from the fungal hypha. Sporangia contain spores that eventually develop into new individual. The spores are covered by thick walls that protect them until they come in contact with moist surface or substratum and can begin to grow.
10. Fragmentation : It can be seen in Spirogyra. During this process filament of spirogyra simply breaks up into smaller pieces upon maturation. These pieces or fragments grow into new individuals. This process occurs under favourable conditions of moisture, temperature, light and nutrient availability.
11. Vegetative propagation: It is the simplest method of reproduction in some higher plants in which new plant is produced from any vegetative part of the plant such as root, stem, leaf, etc.
Advantages of vegetative propagation : Vegetative propagation is useful for plants those have lost the capacity to produce seeds, e.g. banana, rose, jasmine. Moreover, all plants produced are genetically similar to the parent plant.
Natural Vegetative Propagation: In some plants like guava, sweet potato, dahlia, roots sprout and grow into new plants during favourable conditions. In some other,stems grow horizontally and develop root below and leaves above the ground. Many other common examples of vegetatively propagating plants are onion, banana, garlic, ginger, turmeric, bryophyllum and water hyacinth.
12. Vegetative propagation in Bryophyllum: Bryophyllum reproduces by the vegetative propagation method. During this method, buds produced in the notches along the leaf margin of bryophyllum fall on the soil and develop into new plants.
13. SEXUAL REPRODUCTION :
Sexual reproduction involves two individuals for producing a new individual. Sexual reproduction begins with fertilization, which is defined as the union of two different gametes. The motile germ-cell fptrUeh or sperm) is called the male gamete and germ-cell containing stored food (egg or ovum) is called the female gamete. The process of fusion of two gametes is called fertilization. After fertilization, a zygote is formed , which develops into a new organism.
14. Sexual reproduction in Plants : The flowering plants or angiosperms bear special reproductive parts located in the flower. Various parts of flower are; sepals, petals, stamens and carpels.
Most flowers have both male and female reproductive organs. The flower may be unisexual (papaya, watermelon) when it contains either stamen or carpel or bisexual (Hibiscus, Mustard) when it contains both stamens and carpels. It has male reproductive part cal led stamen and a female reproductive part called carpel. Carpel is made of three parts. The swollen bottom part is the ovary, middle elongated part is the style and the terminal part which may be sticky is the stigma. The ovary contains ovules and each ovule has an egg cell. Each stamen consists of stalk called filament, and a flattened fertile top called anther. The anthers produce the pollen grains. The pollen grains produce male gametes which fuse with (egg cel I) female gamete present in the ovule. This fusion of the germ-cells or fertilization gives zygote which grows into a new plant. Pollination: It is the process of transfer of pollen grains from the anther to the stigma of flower. If this transfer of pollen occurs in the same flower, it is referred to as self-pollination, whereas if the pollen is transferred from one flower to another, it is known as cross-pollination. This transfer is carried out by different agencies like wind, water, insects or animals.
Fertilization : A tube grows out of pollen grain and travels through the style to reach the female germ-cells present in ovule in the ovary. Out of two male gametes present in pollen tube one fuses with egg to form zygote. This fusion is called fertilization. After fertilization, the zygote divides several times to form an embryo within the ovule. The ovule develops a tough coat and gradually turns into a seed. The ovary grows rapidly and ripens to form a fruit. Meanwhile the petals, sepals, stamens, style and stigma may shrivel and fall off.
15. Reproduction in human beings : The reproductive organs of human beings are called gonads. These are testes in male and ovaries in female. The male gonad produces sperms and female gonad produces ova (eggs) at the age of puberty (after attainment of sexual maturity). Various changes occur in girls and boys at this age.
16. Male Reproductive System consists of the following organs:
Testes: A pair of testes are situated in scrotum that lie outside the abdominal cavity and behind the penis. Testes produce sperms and hormone, Testosterone hormone. Testosterone brings about changes in appearance of boys at the time of puberty.
VAS deferens: From each testis, a duct arises which is known as vas deferens which unites with a tube coming from urinary bladder. It brings sperms from testis.
Urethra: Vas deferens tube opens into a common tube called urethra. It runs through a muscular organ called Penis. Penis is male copulatory organ.
Accessory Glands: Glands like prostate and seminal vesicles and Cowper’s gland add their secretions which make transport of sperms easier and this fluid also provides nutrition.
17. Female Reproductive System : It consists of the following organs:
Ovaries: Paired ovaries are located in the abdominal cavity near the kidney. Ovaries produce female gamete (ovum or egg) and secrete female hormones (estrogen and progesterone). One egg is produced every month alternately by one of the ovaries.
Fallopian Tube: The egg is carried from the ovary to womb/uterus through a thin oviduct or fallopian tube.
Uterus: The two oviducts unite into an elastic bag like structure known as the uterus.
Vagina : Uterus opens into vagina. It is female copulatory organ.
18. Sexual Cycle in female: After puberty, only one egg is produced alternately from one ovary after a period of 28 days. Egg in fallopian tube encounter sperms which enter through the vaginal passage during sexual intercourse. This fertilized egg (zygote) gets implanted in the lining of uterus which later forms embryo. Embryo gets nutrition from the mother’s blood with the help of special tissue called placenta.
If the egg is not fertilized, if lives for about one day since the ovary releases one egg every month, the uterus prepares itself every month to receive the fertilized egg. Thus, its lining becomes thick and spongy. If it does not get zygote, the developed lining slowly breaks down and comes out through vagina as blood and mucus. This cycle takes place roughly every month and is known as menstruation. It usually lasts for about 2-5 days.
19. Reproductive Flealth: Reproductive organs need a lot of care and hygiene. Otherwise they are susceptible to many infections or diseases. The diseases which spread through sexual routes are known as sexually transmitted diseases e.g., bacterial infections like syphilis, gonorrhoea and viral infections such as warts and HIV- AIDS. Condom helps to prevent transmission of many of these infections to some extent.
Frequent pregnancy causes many health problems and also adds to an already exploding population. Many ways have been devised to avoid pregnancy. Contraception can be achieved by:
• Mechanical barrier method (use of condoms).
• Chemical methods (use of pills).
• Use of contraceptive devices (copper-T).
• Surgical methods (vasectomy in males and tubectomy in females)