Class 8 NCERT Solutions Science Chapter 7 - Reaching the Age of Adolescence

Hormones are chemical substances which are secreted by endocrine glands. They are responsible for changes taking place in the body.

Concept insight: Important from Exam Point of View.

Adolescence is the time period between the beginning of puberty and adulthood. It begins around the age of 11 and lasts till 18 or 19 years of age. The period of adolescence may vary from person to person.During this time two types of changes takes place in the body:

i. Person attains reproductive maturity.
ii. Boys and girls develop secondary sexual characters.

Concept insight: The period of life, when the body undergoes changes, leading to reproductive maturity, is called adolescence.

Menstruation is the process of the shedding of the uterine lining on a regular monthly basis. It begins at puberty. Every month, the uterus prepares itself to receive a fertilized egg. Therefore, the inner lining of the uterus becomes thick and is supplied with blood to nourish the embryo. If the egg is not fertilized, then the lining of the uterus breaks down and gets released in the form of blood through the vagina. This lasts for about two to five days. This cycle occurs every month and is known as the menstrual cycle.

Concept insight: Menstruation is the reproductive cycle of the female body and is controlled by hormones.

Changes at puberty:
i. Sudden increase in height and weight.
ii. Broadening of shoulders and widening of chest in boys. In girls, the region below waist becomes wider.
iii. In boys, under the influence of hormones, the larynx becomes prominent it protrudes out and this is known as Adam's apple. The vocal cords become longer and thicker. These changes cause the voice to become hoarse.
iv. Appearance of hair in areas such as underarms, face, hands, and legs.
v. Appearance of acne as a result of excessive secretion of oil from skin.
vi.Testis grows and starts producing sperms in males, whereas in females, the ovary enlarges and starts producing matured eggs.

Concept insight: The most important change which marks puberty is that boys and girls become capable of reproduction.

Concept insight: List the glands of human endocrine system and the hormones secreted by them.

Sex hormones are chemical substances produced by sex organs. For example, testosterone is the male sex hormone produced by the testis, and oestrogen is the female sex hormone produced by the ovary. These hormones are secreted by sex organs and affect the sexual features of an organism. Hence, they are known as sex hormones.
Functions of sex hormones:
Testosterone: This hormone brings about secondary sexual characters in boys such as the growth of a beard, the voice becoming hoarse, development of reproductive organs, etc.
Oestrogen: This hormone is responsible for the development of secondary sexual characters in females such as the enlargement of breasts, development of female reproductive organs, etc.

Concept insight: List the hormones secreted by sex organs of males and females and their functions.

(a) (ii) Adolescents should be careful about what they eat, because proper diet is needed for the rapid growth taking place in their body.

Concept insight: Adolescence is a stage of rapid growth and development.

(b) (i) Reproductive age in women starts when their menstruation starts.

Concept insight: Menstruation marks the beginning of reproductive phase in females.

(c) (ii) The right meal for adolescents consists of chapatti, dal, vegetables.

Concept insight: Adolescents should consume a balanced diet.

(a) Adam's apple: In human males, the larynx grows larger during puberty and can be seen as a protruding part of the throat. This protrusion is known as the Adam's apple. In boys, under the influence of sex hormones, the larynx becomes prominent, As a result, the vocal cords become longer and thicker, causing the voice to become hoarse.

Concept insight: At puberty, boys develop a larger voice box called Adam's apple.

(b) Secondary sexual characters are those features that help in distinguishing the male and the female body from each other. They are physical or behavioral characteristics that appear in humans at the time of puberty.

Secondary sexual characters in boys:

(i) Appearance of moustache and beard.

(ii) Appearance of chest hair.

(iii) Growth of hair in genital area and other parts of the skin.

Secondary sexual characters in girls:

(i) Increase in breast size and darkening of the skin of nipples present at the tip of the breasts.

(ii) Growth of hair in genital area and other body parts.

Concept insight: Secondary sexual characters are developed during puberty.

(c) Sex determination in an unbom baby:

The sex of a baby is determined by the type of male gamete that fuses with the female gamete.

All human beings have 23 pairs of chromosomes in their nuclei. Out of these 23 pairs, the last pair is known as the sex chromosome.

The human males have 23 pairs of chromosomes including XY sex chromosomes, Therefore, the male gamete has 22 chromosomes and either an X or Y sex chromosome.

Male gametes can be of two types: 22+X or 22+Y

Females have 23 pairs of chromosomes including XX sex chromosomes. Therefore, their gametes can only have 22 chromosomes and one X sex chromosome,

Type of female gametes: 22+X

Thus, as the mother provides only X chromosome, the sex of the baby is determined by the type of male gamete (X or Y) that fuses with the X chromosome of the female.

Concept insight: Sex of the child is determined by the sex chromosome of the father that fertilizes the egg.

Concept insight: Terms used are important from Exam Point of View.

Concept insight: There is a sudden increase in height in boys and girls during puberty.