NCERT Solutions For Class 8 Science Chapter 10
Reaching the Age of Adolescence

Adolescence is the period in human life cycle that occurs after childhood and before adulthood, between the age 10 and 19. Adolescence is defined as the period of life when the body undergoes changes, leading to reproductive maturity.

The term used for secretions of endocrine glands responsible for changes taking place in the body is hormones. Hormones are the chemical substances that are directly released into blood steam by the endocrine glands.

(a) (ii) proper diet is needed for the rapid growth taking place in their body.

(b) (i) menstruation starts.

(c) (ii) chapati, dal, vegetables.

The hormones that are produced by sex organs are termed as sex hormones. For example, testes release sex hormone called testosterone and ovary releases sex hormone called estrogen. They are named so because of their affect on the sexual behaviour of an organism.

Functions of sex hormones are:

1. Testosterone: It is responsible for the development of secondary sexual characters in males, such as growth of facial hair and body hair, deepening of voice, broadening of shoulder, growth and development of reproductive organs, production of sperms, etc.
2. Estrogen: It is responsible for the development of secondary sexual characters in females, such as breast enlargement, widening of region below waist, growth and development of reproductive organs, production of ova, etc.

Menstruation is the periodic discharge of thickened lining of uterus along with blood through vagina. It begins during puberty. Every month, female’s body releases an ovum and prepares itself for pregnancy by thickening the uterine walls. If pregnancy does not occur, then the thickened lining of uterus shed off along with the blood vessels. This discharge of blood is called menstruation.

Following conclusions can be drawn from the graphs:

1. At the time of birth, the height of girls and boys remain same.
2. Up to the age of 8 years, the height of boys is more than that of the girls.
3. After the age of 8 years, the height of girls outpaces the height of boys.
4. After the age of 12 years, the height of boys speedily increases and reaches up to the height of girls at 16^th year.
5. After 16 years to 20 years, height of girls and boys increase but boys' height is much more than that of the girls.

(a) Adam’s apple: In boys, during puberty, the voice box grows in size. This growing voice box can be seen as a protruding part of the throat called Adam’s apple. Due to this, boys have a deep voice.

(b) Secondary sexual characters: These are the features that help in distinguishing a male body from a female body. The secondary sexual characters appear at the time of puberty. Characters, like growth of hair in genitals and other body parts such as face, armpit, legs, chest etc., deepening of the voice, broadening of shoulder, and growth of muscles are male secondary sexual characters. On the other hand, characters, like growth of breasts, growth of hair in genitals and other body parts such as armpits, etc., widening of the region below waist are some of the female secondary sexual characters.

(c) Sex determination in the unborn baby: The sex of the unborn baby is determined by the type of male gamete that fuses with the female gamete. Human beings possess 23 pairs of chromosomes in the nucleus of their cells. 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 and 22 + Y.

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

When a male gamete carrying X chromosome fuses with a female gamete, then a female child will be born after the gestation period. When a male gamete carrying Y chromosome fuses with a female gamete, then a male child will be born after the gestation period.

Certain changes that take place in the body of boys at puberty are:

1. Sudden increase in height, during the age 13-14 years, that slows down around 18.
2. Voice changes as the larynx begins to grow. The growing larynx can be seen as Adam’s apple.
3. Change in body shape, like broadening of shoulders and muscular growth, also takes place.
4. Organs, like testicles and penis, starts growing in size and attains their full size by the age of 17-18 years.
5. Growth of pubic hair, facial hair, leg, chest and armpit hair starts in boys during puberty.
6. Sweat glands and sebaceous glands become more active and result into pimples and acne.

Changes that take place in the body of girls at puberty are:

1. Sudden increase in height, during the age of 11 years that slows down around 16.
2. A drastic change in the shape of the body takes place due to the growth of breasts and widening of the region below the waist. 
3. Organs, like ovaries and uterus, start growing in size. Ovaries start releasing mature eggs.
4. Pubic hair growth, as well as armpit and leg hair start growing in girls during puberty.
5. Sweat glands and sebaceous glands become more active and result into pimples and acne.
6. Onset of menstruation also takes place during puberty.