CBSE Notes Class 8 Science Chapter 7 - Reaching the Age of Adolescence 

Adolescence is the period of transition from childhood to adulthood, during which various biological and psychological changes occur. It is the stage of rapid growth and development, which begins around the age of eleven and lasts up to eighteen or nineteen years. This period covers the 'teens' (thirteen to eighteen or nineteen years of age); adolescents are called 'teenagers'.

1.0Puberty 

• The human body undergoes several changes during adolescence, marking puberty's onset. 
• The age at which sex hormones and gametes begin to be produced, and the boy is biologically ready to become a father, and the girl is biologically ready to become a mother, is called puberty.

Changes at Puberty

The following changes start taking place in the body of boys and girls at puberty : 

• Increase in height
• Change in body shape
• Voice change
• Increased activity of sweat and sebaceous gland
• Reaching mental, intellectual and emotional maturity
• Development of sex organs 
• Secondary sexual characters - The features that help distinguish the male from the female are called secondary sexual characteristics. The reproductive organs are the testes and ovaries, which produce the gametes, sperm in males and ova in females. 

Secondary sexual characters in male 

• Widening of shoulders. Deepening of voice. 
• Appearance of beard and mustache. 
• Growth of sex organs (testes and penis). 

Secondary sexual characteristics in females

• Widening of the pelvis and hips. 
• High-pitched voice. Initiation of the menstrual cycle. 
• Growth of mammary glands (breasts). 
• Maturation of secondary sex organs like fallopian tubes and uterus. 

2.0Endocrine system

• Hormones control all the changes which occur at adolescence. 
• Hormones are chemical substances secreted from endocrine glands or the endocrine system. 
• Endocrine glands are ductless glands that secrete their secretions directly into the blood. 

Some important endocrine glands are as follows:

Reproductive phase of life in humans

Adolescents become capable of reproduction when their testes and ovaries begin to produce gametes. 

• The capacity for maturation and production of gametes lasts much longer in males than in females. 
• In females, the reproductive phase of life begins at puberty (10 to 12 years of age) and generally lasts till the age of approximately 45 to 50 years. 
• The ova begin to mature with the onset of puberty. One ovum matures and is released by one of the ovaries once in about 28 to 30 days. 
• During this period, the uterus's wall becomes thick enough to receive the egg if it is fertilized, and the zygote begins to develop. 
• This results in pregnancy. If fertilization does not occur, the released egg, the thickened lining of the uterus, and its blood vessels are shed off. 
• This causes bleeding in women, which is called menstruation. Initially, the menstrual cycle may be irregular. 
• It takes some time to become regular.

3.0Reproductive health 

• Health is a state of complete mental, physical and social well-being.
• Nutritional needs of the adolescents
• Personal hygiene
• Physical exercise
• Say "NO" to drugs and alcohol
• Adolescent pregnancy

AIDS (Acquired immunodeficiency syndrome) 

• AIDS is a disease which is caused by HIV (Human immunodeficiency virus). 
• This virus destroys the body's natural defense mechanism and makes it susceptible to diseases. 

Transmission of AIDS 

• Sharing the needle of the syringe for injecting drugs. 
• Unprotected sex with an infected person. 
• They are transfusing blood into the body of a healthy person from an infected person. 
• Infected mothers can pass HIV through milk to an infant.

How to prevent AIDS 

• Use disposable syringes. 
• Get blood from registered blood banks for transfusion. 
• Have safe and protected sex. 
• HIV-infected mothers should not breastfeed their babies.

4.0Sex determination

• The sex of the baby is determined by the sex chromosomes XX and XY inside the fertilized egg or zygote. 
• All human beings have 23 pairs of chromosomes in the nuclei of their cells. 
• Two chromosomes out of these are the sex chromosomes, named X and Y. 
• A female has two X chromosomes, while a male has one X and one Y chromosome. 
• The gametes (egg and sperm) have only one set of chromosomes. 
• The unfertilized egg always has one X chromosome. 
• But sperms are of two kinds. One kind has an X chromosome, and the other has a Y chromosome.
• When a sperm containing an X chromosome fertilizes the egg, the zygote will have two X chromosomes and develop into a female child. 
• If the sperm contributes a Y chromosome to the egg (ovum) at fertilization, the zygote will develop into a male child. 
• The sex chromosomes of the father determine the sex of an unborn baby.