Chapter 10 Reaching the Age of Adolescence Class 8 Notes | EduRev

Science - Short Notes for Class 8th

Created by: Sushil Solanki

Class 8 : Chapter 10 Reaching the Age of Adolescence Class 8 Notes | EduRev

 Page 1


Chapter 10 - Reaching the Age of Adolescence 
 
Adolescence – Age group (11 – 19 years of age) during which body undergoes several changes to make itself capable of 
reproduction. 
Puberty – The age at which an individual matures sexually. (It can vary from 13-19 depending upon person to person) 
Changes that occur with puberty: 
1. Sudden increase in height - Height of an individual depends on the genes inherited from his/her parents but 
there is a sudden increase in height of most individuals during puberty. 
2. Change in body shape – Shoulders broaden in boys while the area below the waist broadens in girls. 
3. Change in voice – the voice box of larynx enlarges in boys and is visible near the throat as Adam’s apple. The 
voice of boys become heavy and deep while girls have a sharp voice. 
4. Sexual Organs - the male sexual organs such as the penis and testes and in females’ ovaries enlarges and start 
releasing mature eggs. 
5. Adolescents become more independent, self-conscious and intellectual. 
 
Secondary Sexual Characters – Features apart from changes in sexual organs such as growth of body hair that occur 
during puberty are known as secondary sexual characters. 
Examples: 
1. Boys develop chest hair 
2. Hair develops on thighs, in pubic region, under the arms in both boys and girls. 
Causes – Hormones control the changes that occur during puberty. 
Hormones – Proteins produced by endocrine glands that effect various body functions and features. Hormones are 
secreted at one site (gland) but are transported to other site, called the target site through blood stream to produce its 
effect. 
 E.g., Testosterone is a hormone produced by testes, which causes changes in boys during puberty while estrogen is 
produced by ovaries that cause various changes in girls during puberty. 
Pituitary gland, also called the master gland, which is present in the brain, controls the functioning of all the other 
glands. 
 
Glands – the organs that produces/secretes hormones and digestive juices. 
1. Endocrine glands – Glands whose secretions travel to the target site through blood stream. E.g. Glands that 
secretes hormones such as thyroid gland, adrenal gland, pituitary gland etc. Pituitary gland produces growth 
hormone and adrenal gland produces adrenaline when we are afraid or angry. Insulin is produced by pancreas 
and breaks down sugar in the blood. Deficiency of insulin causes diabetes. Thyroid gland produces thyroxine that 
controls the iodine level of the body. It also helps in development of frog from tadpole (metamorphosis). 
 
2. Exocrine glands – Glands whose secretions travel to the target site through tubular structures called ducts. E.g. 
Glands of digestive juices such as pancreas, salivary gland etc. 
 
Page 2


Chapter 10 - Reaching the Age of Adolescence 
 
Adolescence – Age group (11 – 19 years of age) during which body undergoes several changes to make itself capable of 
reproduction. 
Puberty – The age at which an individual matures sexually. (It can vary from 13-19 depending upon person to person) 
Changes that occur with puberty: 
1. Sudden increase in height - Height of an individual depends on the genes inherited from his/her parents but 
there is a sudden increase in height of most individuals during puberty. 
2. Change in body shape – Shoulders broaden in boys while the area below the waist broadens in girls. 
3. Change in voice – the voice box of larynx enlarges in boys and is visible near the throat as Adam’s apple. The 
voice of boys become heavy and deep while girls have a sharp voice. 
4. Sexual Organs - the male sexual organs such as the penis and testes and in females’ ovaries enlarges and start 
releasing mature eggs. 
5. Adolescents become more independent, self-conscious and intellectual. 
 
Secondary Sexual Characters – Features apart from changes in sexual organs such as growth of body hair that occur 
during puberty are known as secondary sexual characters. 
Examples: 
1. Boys develop chest hair 
2. Hair develops on thighs, in pubic region, under the arms in both boys and girls. 
Causes – Hormones control the changes that occur during puberty. 
Hormones – Proteins produced by endocrine glands that effect various body functions and features. Hormones are 
secreted at one site (gland) but are transported to other site, called the target site through blood stream to produce its 
effect. 
 E.g., Testosterone is a hormone produced by testes, which causes changes in boys during puberty while estrogen is 
produced by ovaries that cause various changes in girls during puberty. 
Pituitary gland, also called the master gland, which is present in the brain, controls the functioning of all the other 
glands. 
 
Glands – the organs that produces/secretes hormones and digestive juices. 
1. Endocrine glands – Glands whose secretions travel to the target site through blood stream. E.g. Glands that 
secretes hormones such as thyroid gland, adrenal gland, pituitary gland etc. Pituitary gland produces growth 
hormone and adrenal gland produces adrenaline when we are afraid or angry. Insulin is produced by pancreas 
and breaks down sugar in the blood. Deficiency of insulin causes diabetes. Thyroid gland produces thyroxine that 
controls the iodine level of the body. It also helps in development of frog from tadpole (metamorphosis). 
 
2. Exocrine glands – Glands whose secretions travel to the target site through tubular structures called ducts. E.g. 
Glands of digestive juices such as pancreas, salivary gland etc. 
 
Menstruation Cycle in Humans 
One eggs matures in one of the ovary every month and is released. The walls of the uterus thickens with blood and 
mucus for the zygote just in case fertilization occurs. If the fertilization occurs the zygote is formed and implanted in the 
walls of the uterus and develops to form a baby. 
However, if there is no fertilization, the egg remains unfertilized, and the uterine walls does not receive zygote, it starts 
shedding off. At the end of the month the walls, along with blood mucus and eggs are released over a period of time (3-
4 days). This process leading to the release of blood and mucus is known as menstruation or periods. 
Menstruation begins with puberty and stops at an age of about 45 to 50. The stopping of menstruation is known as 
menopause. 
 
Sex Determination (Is Baby a boy or a girl?) 
Chromosomes are thread-like structures present in the cells. There are twenty-three pair of chromosomes in humans 
and one of the pair is sex chromosome.  
Males have one ‘X’ and another ‘Y’ – sex chromosome. 
Females have both sex chromosomes as ‘X’ 
The male and female gametes have 23 chromosomes each. When these gametes fuse during fertilization the zygote gets 
23 pairs of chromosomes i.e. 23*2=46 chromosomes. 
Sex of the child is decided only by the sex chromosome. 
 
 
 
 
 
 
 
                                     Sex Determination in Humans 
Explanation – The mother sex cells or mother gamete cells have 22 + 1(sex) pair or 44 + 2(sex) chromosomes. Males 
have 44 + XY while the females have 44 + XX.  
The gametes receives only one chromosome from each pair randomly and therefore has 22 + 1(sex) chromosomes. 
Since, only the sex chromosomes determine the sex of the child we only consider those chromosomes.  
A sperm may have either an ‘X’ chromosome or a ‘Y’ chromosome while the egg always have an ‘X’ chromosome. If the 
sperm with ‘X’ chromosome fertilizes with the egg the zygote has XX chromosomes and develops into a girl. If the sperm 
with ‘Y’ chromosome fertilizes with the egg the zygote has XY chromosomes and develops into a boy. 
Hence, the male is responsible for the sex of the child and not the mother. 
 
 
Page 3


Chapter 10 - Reaching the Age of Adolescence 
 
Adolescence – Age group (11 – 19 years of age) during which body undergoes several changes to make itself capable of 
reproduction. 
Puberty – The age at which an individual matures sexually. (It can vary from 13-19 depending upon person to person) 
Changes that occur with puberty: 
1. Sudden increase in height - Height of an individual depends on the genes inherited from his/her parents but 
there is a sudden increase in height of most individuals during puberty. 
2. Change in body shape – Shoulders broaden in boys while the area below the waist broadens in girls. 
3. Change in voice – the voice box of larynx enlarges in boys and is visible near the throat as Adam’s apple. The 
voice of boys become heavy and deep while girls have a sharp voice. 
4. Sexual Organs - the male sexual organs such as the penis and testes and in females’ ovaries enlarges and start 
releasing mature eggs. 
5. Adolescents become more independent, self-conscious and intellectual. 
 
Secondary Sexual Characters – Features apart from changes in sexual organs such as growth of body hair that occur 
during puberty are known as secondary sexual characters. 
Examples: 
1. Boys develop chest hair 
2. Hair develops on thighs, in pubic region, under the arms in both boys and girls. 
Causes – Hormones control the changes that occur during puberty. 
Hormones – Proteins produced by endocrine glands that effect various body functions and features. Hormones are 
secreted at one site (gland) but are transported to other site, called the target site through blood stream to produce its 
effect. 
 E.g., Testosterone is a hormone produced by testes, which causes changes in boys during puberty while estrogen is 
produced by ovaries that cause various changes in girls during puberty. 
Pituitary gland, also called the master gland, which is present in the brain, controls the functioning of all the other 
glands. 
 
Glands – the organs that produces/secretes hormones and digestive juices. 
1. Endocrine glands – Glands whose secretions travel to the target site through blood stream. E.g. Glands that 
secretes hormones such as thyroid gland, adrenal gland, pituitary gland etc. Pituitary gland produces growth 
hormone and adrenal gland produces adrenaline when we are afraid or angry. Insulin is produced by pancreas 
and breaks down sugar in the blood. Deficiency of insulin causes diabetes. Thyroid gland produces thyroxine that 
controls the iodine level of the body. It also helps in development of frog from tadpole (metamorphosis). 
 
2. Exocrine glands – Glands whose secretions travel to the target site through tubular structures called ducts. E.g. 
Glands of digestive juices such as pancreas, salivary gland etc. 
 
Menstruation Cycle in Humans 
One eggs matures in one of the ovary every month and is released. The walls of the uterus thickens with blood and 
mucus for the zygote just in case fertilization occurs. If the fertilization occurs the zygote is formed and implanted in the 
walls of the uterus and develops to form a baby. 
However, if there is no fertilization, the egg remains unfertilized, and the uterine walls does not receive zygote, it starts 
shedding off. At the end of the month the walls, along with blood mucus and eggs are released over a period of time (3-
4 days). This process leading to the release of blood and mucus is known as menstruation or periods. 
Menstruation begins with puberty and stops at an age of about 45 to 50. The stopping of menstruation is known as 
menopause. 
 
Sex Determination (Is Baby a boy or a girl?) 
Chromosomes are thread-like structures present in the cells. There are twenty-three pair of chromosomes in humans 
and one of the pair is sex chromosome.  
Males have one ‘X’ and another ‘Y’ – sex chromosome. 
Females have both sex chromosomes as ‘X’ 
The male and female gametes have 23 chromosomes each. When these gametes fuse during fertilization the zygote gets 
23 pairs of chromosomes i.e. 23*2=46 chromosomes. 
Sex of the child is decided only by the sex chromosome. 
 
 
 
 
 
 
 
                                     Sex Determination in Humans 
Explanation – The mother sex cells or mother gamete cells have 22 + 1(sex) pair or 44 + 2(sex) chromosomes. Males 
have 44 + XY while the females have 44 + XX.  
The gametes receives only one chromosome from each pair randomly and therefore has 22 + 1(sex) chromosomes. 
Since, only the sex chromosomes determine the sex of the child we only consider those chromosomes.  
A sperm may have either an ‘X’ chromosome or a ‘Y’ chromosome while the egg always have an ‘X’ chromosome. If the 
sperm with ‘X’ chromosome fertilizes with the egg the zygote has XX chromosomes and develops into a girl. If the sperm 
with ‘Y’ chromosome fertilizes with the egg the zygote has XY chromosomes and develops into a boy. 
Hence, the male is responsible for the sex of the child and not the mother. 
 
 
Health of Adolescent 
1. Balance diet – A diet that contain right amount of nutrients such as carbohydrates, proteins, vitamins and 
minerals is known as balanced diet. It is important to consume diet rich in proteins (pulses and green leafy 
vegetables) and carbohydrate to meet the energy requirement of the growing adolescents. 
2. To maintain personal hygiene by bathing daily to prevent any bacterial infection. 
3. Physical exercise helps to keep the body fit. 
4. Avoid the use of drug such as smoking, tobacco or alcohol to keep the body healthy and proper development of 
various organs during adolescence.   
5. AIDS is a disease caused by HIV virus and can be transmitted through sexual contact, infected syringes or 
mother’s milk to the infant. 
The legal age to get marriage in our country is 18 years for girls and 21 years for boys because teenage girls are not 
prepared physically and mentally for motherhood.  
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