NCERT Solutions for Class 10 Science Chapter 7 How do Organisms Reproduce

DNA found in chromosomes in the nucleus of the cell has the information to create proteins which lead to body design of an organisms. If the organisms are to make exact copies of themselves, the DNA should replicate to make an exact copy of itself. DNA replicate in the cell with the help of various enzymes and this is accompanied by division of the basic unit of every organism, i.e., the cell.

The sexually act is very intimate connection of bodies and thus many diseases can be sexually transmitted. These include bacterial infections like gonorrhoea and syphilis and viral infection like warts and HIV-AIDS (Human Immunodeficiency Virus-Acquired Immuno Deficiency Syndrome). HIV-AIDS causes damages to the immune system of human body.

Variations are useful for the survival of species in changed environmental situations. For example, if a population of reproducing organisms were suited to a particular niche (well defined place of abode) and if the niche is drastically changed, the population could be wiped out. However, if some variations were to be present in a few individuals in these populations, there would be some chance for them to survive. Thus, if there is a population of bacteria living in temperate waters and if water temperature increases by global warming, most of bacteria would die. But few a variants resistant to heat would survive and grow further. Variation is thus useful for the survival of species over time.

The methods used for regulation of child birth are:
(i) Barrier method: In this method physical devices like condoms, cervical cap and diaphragm are used to prevent sperms to reach up to the ovum.
(ii) Chemical method: In this method specific drugs are used by females. These drugs may be (a) oral pills or (b) vaginal pills. Oral pills mainly contains hormones and are called oral contraceptives. They disturb hormonal balance so that eggs are not released and fertilised.
(iii) Intrauterine contraceptive devices (IUCDs): ICUD like copper-T is placed safely in the uterus by skill doctor. ICUDs prevents implantation of the fertilized ovum inside the uterus.
(iv) Surgical method: this method is applicable to both male and female. In males, a small portion of vas deferens and the fallopian tube in female is surgically removed or ligated (tied). It is known as vasectomy in males which prevents release of sperms from the testes. In females, it is called tubectomy.

Multi cellular organisms cannot simply divide cell by cell because:
(i) Many multi cellular organism are not simply a random collection of cells. Specialised cell are organised as tissues and tissues are organised into organs which are placed at a definite position in the body.
(ii) Each organ performs a specific function/functions.
In such a carefully organised situation, cell by cell division (for reproduction) would be impracticable.

The uterus develops thick and spongy lining to receive fertilised egg. When the egg is not fertilised, uterine lining slowly break and comes out as blood and mucous. This cycle takes place roughly every month and is known as menstrual cycle (menstruation).

Fragmentation: The mode of reproduction of simple multicellular organisms in which body simply breaks up into smaller pieces upon maturity and each piece or fragment grows into a new individual is called fragmentation. The relatively simple multicellular organisms which are simply a random collection of cells are involved in fragmentation type of reproduction. Example: Spirogyra.

Role of seminal vesicle and Prostate gland.
Along the path of the vas deferens, these glands add their secretions so that the sperms are now in a fluid which makes their transport easier and this fluid also provides nutrition to sperms.

The transfer of pollen grains form anther of stamen to stigma is called 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 called cross pollination. In this case, pollen grains of the same flower do not pollinate its stigma. In cross pollination, the transfer of pollen grains may take place through some agency like wind, insect, bird, water, mammals etc.
Transfer of pollen from the anther of a flower to the stigma of the same flower or a flower on the same plant is called self pollination.

DNA found in chromosomes in the nucleus of the cell has the information to create proteins which lead to body design of an organisms. If the organisms are to make exact copies of themselves, the DNA should replicate to make an exact copy of itself. DNA replicate in the cell with the help of various enzymes and this is accompanied by division of the basic unit of every organism, i.e., the cell.

Variations are useful for the survival of species in changed environmental situations. For example, if a population of reproducing organisms were suited to a particular niche (well defined place of abode) and if the niche is drastically changed, the population could be wiped out. However, if some variations were to be present in a few individuals in these populations, there would be some chance for them to survive. Thus, if there is a population of bacteria living in temperate waters and if water temperature increases by global warming, most of bacteria would die. But few a variants resistant to heat would survive and grow further. Variation is thus useful for the survival of species over time.

Bryophyllum Advantages: (i) Many buds are produced in the notches along the margin of a single leaf. Each leaf-bud of Bryophyllum produces a plant. (ii) The plants from the buds are exactly similar to the parent plant. (iii) A large number of young plants are produced by a single leaf of a plant that helps in survival of the species.

Budding: A bud, as in Hydra, develops as an outgrowth due to replicated cell division at a specific site. These buds when mature detach from the parent body and become new individual. Spore Formation: In spore formation, as in Rhizopus, a specific part called Sporangia that produce spores. The spores are covered by thick walls that protect them until a spore gets favourable conditions to grow into a new (Rhizopus) plant.

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.

The testes are the male reproductive organs that are located outside the abdominal cavity within a pouch called scrotum. Functions of testes: Produce sperms Produce a hormone called testosterone, which brings about secondary sexual characters in boys.

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.

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. 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.

Contraceptive methods are mainly adopted because of the following reasons: (i) To prevent unwanted pregnancies. (ii) To control population rise or birth rate. (iii) To prevent the transfer of sexually transmitted diseases.