Class-10 Science Chapter-8 notes

 Chapter- 8 
How do Organisms Reproduce?

→ Reproduction is the process by which living organisms produce new individuals similar to themselves. It ensures continuity of life on earth.

→ Nucleus of the cell contains DNA (Deoxyribose Nucleic Acid) which is the heredity material.

→ DNA replicates and forms new cells causing variation. So, these new cells will be similar but may not be identical to original cell.

→ Variations are useful for the survival of the individual and species over time as well as basis for evolution.

Types of Reproduction

Asexual Reproduction

→ A single individual give rise to new individual.

→ Gametes are not formed.

→ New individual is identical to parent.

→ It is extremely useful as a means of rapid multiplication.

→ Adopted by lower organisms.

Sexual Reproduction

→ Two individuals i.e., one male and one female are needed to give rise to new individual.

→ Gametes are formed.

→ New individual is genetically similar but not identical to parents.

→ It is useful to generate more variations in species.

→ Adopted by higher organisms.

Modes of Asexual Reproduction

Fission

→ The parent cell divides into daughter cells.

• Binary fission: 2 cells are formed. Example: amoeba.

• Multiple fission: Many cells are formed. Example: Plasmodium.

Fragmentation

→ The organism breaks-up into smaller pieces upon maturation, each piece develops into new individual. Example: Spirogyra.

Regeneration

→ If organism is somehow cut or broken into many pieces, each piece grows into a complete organism. Example: Planaria, Hydra.

Budding

→ A bud is formed which develops into tiny individual. It detaches from parent body upon maturation and develops into new individual. Example: Hydra

Vegetative Propagation

→ In many plants, new plants develops from vegetative parts such as:

• By roots: Example: dahlias, sweet potato.

• By stem: Example: potato, ginger.

• By leaves: Example: bryophyllum (leaf notches bear buds which develop into plants).

Artificial methods in Vegetative Propagation

(i) Grafting: Example: Mango

(ii) Cutting: Example: Rose

(iii) Layering: Example: Jasmine

(iv) Tissue culture: New plants are grown by using growing tip of a plant. 

→ These growing cells are kept in a culture medium leads to the formation of callus. Callus is then transferred to hormone medium which causes growth and differentiation.

Example: ornamental plants, orchid.

• Benefits of tissue culture

→ We can grow plants like banana, rose, jasmine etc. that have lost the capacity to produce seeds.

→ New plants are genetically similar to parents.

→ Helps in growing seedless fruits.

(v) Spore Formation: Spores are small bulb like structures which are covered by thick walls. Under favourable conditions, they germinate and produce new organism.

Example: Rhizopus

Sexual Reproduction

→ When reproduction takes place as a result of the fusion of male and female gametes is called sexual reproduction.

→ Fusion of gametes is called fertilization which results in variation.

Sexual Reproduction in Plants

→ Flowers are the reproductive organs of plants.

→ A typical flower consists of four main whorls namely sepals, petals, stamen and pistil.

Types of Flowers

• Bisexual flower: Both male and female reproductive parts are present.

Example: Hibiscus, mustard.

• Unisexual flower: Either male or female reproductive part is present.

Example: Papaya, watermelon.

Structure of Flower

Process of Seed Formation

→ Pollen grains, produced in the anther, are transferred to the stigma of same flower (self pollination) or stigma of another flower (cross pollination) through agents like air, water or animals.

→ Pollen grains germinate and form pollen tubes which pass through style to reach upto the ovules present in ovary.

→ The fusion of male and female gametes is called fertilization. Zygote is produced inside the ovary.

→ Zygote divides to form embryo. Ovule develops thick coat and changes into seed gradually.

→ Ovary changes into fruit and other parts of flower fall off.

→ The seed germinates to form a plant under suitable conditions such as air, moisture etc.

Reproduction in Human Beings

→ Humans use sexual mode of reproduction.

→ Sexual maturation: The period of life when production of germ cells i.e. ova (female) and sperm (male) start in the body. This period of sexual maturation is called puberty.

Changes at Puberty

• Common in male and female

→ Thick hair growth in armpits and genital area.

→ Skin becomes oily, may result in pimples.

• In girls

→ Breast size begin to increase.

→ Girls begin to menstruate.

• In boys

→ Thick hair growth on face.

→ Voice begin to crack.

These changes signals that sexual maturity is taking place.

Male Reproductive System

(i) Testes

→ A pair of testes are located inside scrotum which is present outside the abdominal cavity. 

→ Scrotum has a relatively lower temperature needed for the production of sperms.

→ Male germ cell i.e. sperms are formed here.

→ Testes release male sex hormone (testosterone).

Function of testes:

→ Regulate production of sperms.

→ Bring changes at puberty.

(ii) Vas deferens

→ It passes sperms from testes upto urethera.

(iii) Urethera

→ It is a common passage for both sperms and urine. Its outer covering is called penis.

(iv) Associated glands 

→ Seminal vesicles and prostate gland add their secretion to the sperms. This fluid provide nourishment to sperms and make their transport easy.

→ Sperm along with secretion of glands form semen.

Female Reproductive System

(i) Ovary

→ A pair of ovary is located in both sides of abdomen.

→ Female germ cells i.e. eggs are produced here.

→ At the time of birth of a girl, thousands of immature eggs are present in the ovary.

→ At the onset of puberty, some of these eggs start maturing.

→ One egg is produced every month by one of the ovaries.

(ii) Oviduct or Fallopian tube

→ Receives the egg produced by the ovary and transfer it to the uterus.

→ Fertilisation i.e. fusion of gametes takes place here.

(iii) Uterus

→ It is a bag-like structure where development of the baby takes place.

→ Uterus opens into vagina through cervix.

Fertilisation of egg

• When egg is fertilised

→ The fertilized egg called zygote is planted in uterus and develops into an embryo.

→ The embryo gets nutrition from the mother’s blood with the help of a special tissue called placenta. It provides a large surface area for the exchange of glucose, oxygen and waste material.

→ The time period from fertilization upto the birth of the baby is called gestation period. It is about 9 months.

• When egg is not fertilised

→ The uterus prepares itself every month to receive fertilized egg.

→ The lining of the uterus becomes thick and spongy, required to support the embryo.

→ When fertilisation had not taken place, this lining is not needed any longer.

→ This lining breaks and comes out through vagina as blood and mucus.

→ This cycle takes around 28 days every month and called menstruation.

Reproductive Health

→ Reproductive health means a total well-being in all aspects of reproduction i.e. physical, emotional, social and behavioural.

• Sexually Transmitted Diseases (STDs)

→ Many diseases can be sexually transmitted such as:

(i) Bacterial : Gonorrhoea and syphilis

(ii) Viral : Warts and HIV-AIDS

→ Use of condom prevents these infections to some extent.

→ Contraception: It is the avoidance of pregnancy, can be achieved by preventing the fertilisation of ova.

• Methods of contraception

(i) Physical barrier

→ To prevent union of egg and sperm.

→ Use of condoms, cervical caps and diaphragm.

(ii) Chemical methods

→ Use of oral pills

→ These change hormonal balance of body so that eggs are not released.

→ May have side effects.

(iii) Intrauterine contraceptive device (IUCD)

→ Copper-T or loop is placed in uterus to prevent pregnancy.

(iv) Surgical methods

→ In males the vas deferens is blocked to prevent sperm transfer called vasectomy.

→ In females, the fallopian tube is blocked to prevent egg transfer called tubectomy.

Female Foeticide

→ The practice of killing a female child inside the womb is called female foeticide.

→For a healthy society, a balanced sex ratio is needed that can be achieved by educating people to avoid malpractices like female foeticide and prenatal sex determination.

→ Prenatal sex determination is a legal offence in our country so as to maintain a balanced sex ratio.