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Reaching the Age of Adolescence

CBSE Class 8 Science • Chapter 15 • Detailed Master Notes

Chapter Overview

Growing up is a natural process. Humans become capable of reproduction after reaching puberty. In this chapter, we study the bodily changes organized by hormones that prepare humans for their reproductive phase.

15.1 Adolescence and Puberty

Growth begins from the day one is born. However, crossing the age of 10 or 11, there is a sudden spurt in growth which becomes noticeable.

Adolescence: The period of life, when the body undergoes changes, leading to reproductive maturity, is called adolescence. It begins around the age of 11 and lasts up to 18 or 19 years of age. Since this period covers the 'teens' (13 to 19 years of age), adolescents are also called teenagers.

The human body undergoes several changes during adolescence. These changes precisely mark the onset of puberty. Puberty ends when an adolescent reaches reproductive maturity.

15.2 Changes at Puberty

Increase in Height: The most conspicuous change is the sudden increase in height. The long bones (bones of the arms and legs) elongate.
Change in Body Shape: In boys, the shoulders broaden, and muscles grow prominently. In girls, the region below the waist becomes wider.
Voice Change: The voice box or larynx begins to grow. In boys, it protrudes as the Adam's apple, causing the voice to become deep or hoarse. Girls' voices are usually high-pitched.
Activity of Sweat and Sebaceous Glands: The secretion of sweat glands and sebaceous glands (oil glands) increases, often causing acne or pimples on the face.
Development of Sex Organs: Male sex organs develop completely. The testes begin to produce sperms. In girls, ovaries enlarge, and eggs begin to mature.
Mental and Emotional Maturity: Adolescents become more independent and self-conscious. This period involves significant brain learning capacity.

15.3 Secondary Sexual Characters

Features that help to distinguish the male from the female are called secondary sexual characters.

In Girls: Breasts begin to develop. Hair grows under the arms and in the pubic region.
In Boys: Facial hair (moustaches and beards) appears. Hair grows on the chest.

Role of Hormones

The changes at puberty are controlled by hormones. Hormones are chemical substances secreted by endocrine glands (ductless glands). They pour their secretions directly into the bloodstream.

Testosterone: The male hormone secreted by the testes. It causes the development of facial hair and other male traits.
Estrogen: The female hormone secreted by the ovaries. It causes the breasts to develop.

The production of these sex hormones is controlled by a master gland called the Pituitary Gland, located in the brain.

AI Image Prompt: A medical illustration of the human body showing the location of major endocrine glands: the Pituitary gland in the brain, Thyroid gland in the neck, Adrenal glands on top of the kidneys, Pancreas, and the reproductive organs. Clean and educational.

15.4 Reproductive Phase of Life in Humans

The reproductive phase begins at puberty (10-12 years) and lasts until roughly 45-50 years in females. In males, the capacity for maturation and production of gametes lasts much longer.

Menstruation: In females, one ovum matures and is released by one of the ovaries once in about 28 to 30 days. The wall of the uterus becomes thick to receive the egg. If fertilization does not occur, the egg and the thickened uterine lining shed off, causing bleeding. This is called menstruation.

Menarche: The first menstrual flow begins at puberty.
Menopause: At 45 to 50 years of age, the menstrual cycle stops completely.

15.5 Sex Determination of the Baby

Inside the fertilized egg or zygote is the instruction for determining the sex of the baby. This instruction is present in the thread-like structures called chromosomes.

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 (XX).
A male has one X and one Y chromosome (XY).

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. If an X-sperm fertilizes the egg, the zygote becomes female. If a Y-sperm fertilizes the egg, the zygote becomes male.

Topic Practice: Hormones

Q1. Why are endocrine glands called 'ductless glands'?

Ans: Endocrine glands do not have any tubes or ducts. They release their hormone secretions directly into the blood stream to reach a specific target site in the body.

15.6 Hormones Other Than Sex Hormones

Gland	Hormone	Function / Deficiency Disease
Thyroid	Thyroxine	Deficiency causes Goitre (swelling of the neck).
Pancreas	Insulin	Deficiency causes Diabetes, failing to control blood sugar levels.
Adrenal	Adrenalin	Helps the body adjust to severe stress when highly angry, embarrassed, or worried.

15.7 Nutritional Needs during Adolescence

Adolescence is a stage of rapid growth and development. It is vital to consume a balanced diet including proteins, carbohydrates, fats, vitamins, and minerals. Junk food is heavily discouraged as it lacks adequate nutritional value. A diet rich in iron builds blood, so leafy vegetables and jaggery are highly recommended.