Study Material and Notes of Ch 10 Reaching the Age of Adolescence Class 8th Science

Topics in the chapter

• Adolescence
• Puberty
→ Increase in height
→ Change in body shape
→ Change in the voice pattern
→ Change in activity of sweat and sebaceous glands
→ Change in sex organs
→ Change in intellectual level
• Secondary sexual characteristics in boys
• Secondary sexual characteristics in girls
• Hormones
→ Characteristics of hormones
• Endocrine glands
→ Pituitary gland
→ Thyroid gland
→ Parathyroid gland
→ Pancreas
→ Adrenal gland
• Gonads
• Historical background and discovery of HIV
→ Structure of HIV
→ Transmission of HIV
→ Prevention of HIV
→ Tests for detection of HIV
• Personal health and hygiene in adolescents
• Sex determination in humans


→ The time period when the body undergoes changes to reach reproductive maturity is known as

→ It begins around the age of 11 and lasts till about 18 or 19 years of age.

→ Adolescence in girls can begin one or two years earlier than boys.


→ The various changes that occur in the body during adolescence marks the onset of puberty.

→ Puberty ends when teenagers attain sexual maturity.

→ Changes that take place during puberty

Increase in height

→ It is caused by the growth in long bones of the arms and legs.

→ Girls grow faster than boys initially but both reach their maximum height by the age of 18

Change in body shape

→ Boys develop broader shoulders, wider chests, and prominent muscles.
→ In girls the region below the waist becomes wider.

Change in the voice pattern

→ Voice box or larynx starts growing during puberty.

→ It protrudes in males in the neck region and is called Adam’s apple.

→ Boys develop deep low-pitched voice.

→ Girls develop high-pitched voice.

Change in activity of sweat and sebaceous glands

→ The activity of sweat glands increases during puberty, resulting in production of more sweat.

→ The oily secretions from sebaceous glands increase. The accumulation of oil and bacterial action leads to acne problems in teenagers.

Changes in sex organs

→ Testes and penis develop completely in boys.

→ Testes start producing sperms.

→ Ovaries develop completely and start producing eggs in girls.

Change in intellectual level

→ The learning capacity of brain increases.

→ Intellectual development takes place during adolescence.

→ Development of secondary sexual characteristics.

Secondary sexual characteristics in boys

→ Appearance of moustaches and beard.

→ Appearance of hair on chest.

→ Growth of hair in genital area and other parts.

Secondary sexual characteristics in girls

→ Increase in breast size

→ Growth of hair in the pubic region.


→ Hormones are chemical secretions that bring about various changes in the body.

→ They are produced by endocrine glands.

→ These glands release hormones into blood to reach specific target site.

→ Production of hormones is under the control of hormones produced from pituitary gland.

Characteristics of hormones

→ Hormones act as chemical messengers.

→ They are secreted by living cells/tissues or organs called glands.

→ They are secreted in very small quantities by glands.

→ They act upon specific cells, tissues, or organs called the target sites.

→ They are generally slow in action, but have long lasting effects.

→ They either accelerate or inhibit a reaction.

Endocrine glands

→ Hormones are secreted by endocrine glands such as the pituitary gland, thyroid gland, adrenal gland, pancreas etc.

• Major endocrine glands in humans are

(i) Pituitary
(ii) Hypothalamus
(iii) Pineal
(iv) Thyroid
(v) Parathyroid
(vi) Thymus
(vii) Pancreas
(viii) Adrenal
(ix) Testis in men /ovary in women

→ A feedback mechanism (positive and negative) regulates the action of the hormones.

Pituitary gland

→ It is a pea sized gland situated at the base of the brain. It secretes a growth hormone (GH).

→ It is required for proper body growth.

→ The hyposecretion of growth hormone causes a condition called dwarfism.

→ The hypersecretion causes gigantism in children and acromegaly in adults.

Thyroid gland

→ It is located close to trachea in the neck. It produces a hormone called thyroxine.

→ It is required for regulating metabolism in the body.

→ The hyposecretion of thyroxine causes hypothyroidism.

→ This condition causes abnormalities like simple goitre, myxoedema and cretinism.

→ Lack of iodine leads to deficiency of thyroxine, which results in a disease called goitre.

→ The excess secretion of thyroxine causes hyperthyroidism. It results in high metabolism, protrusion of the eye balls, high BP, nervous tension, etc.

Parathyroid Gland

→ There are four parathyroid glands present on back side of thyroid glands that secrete parathyroid hormone or parathormone (PTH).

→ This hormone regulates the level of calcium ions in the bloodstream.

→ Excess of parathyroid hormone removes calcium from bones and makes them soft.


→ It produces two hormones- Insulin and Glucagon.

→ These hormones maintain blood sugar level.

→ Deficiency of insulin results in diabetes.

Adrenal Gland

→ There are two adrenal glands located one on upper part of each kidney.

→ It has two parts- cortex and medulla.

→ Cortex secretes the hormones like cortisol that regulates the rate of metabolism.

→ The medulla secretes a hormone like adrenaline that prepares the body to face various stressful situations.


→ It includes testes in males and ovaries in females.

→ Male sex hormone is testosterone. It is produced by the testes on the onset of puberty.

→ Female sex hormones produced by ovaries are estrogen and progesterone.

→ Deficiency of estrogen causes infertility.

→ Process of Hormonal Action

→ Endocrine glands release their secretions (hormones) into the bloodstream.

→ Hormones, on reaching their target site, bring about necessary changes to maintain proper functioning of the body.

Historical background and Discovery of HIV

→ The first cases of AIDS were recognized in U.S.A in the year 1981.

→ The AIDS virus was first discovered by the team of French scientists lead by Luc Montagnier in1983.

→ In 1984, the American virologist named Robert Charles Gallo gave the first report on the virus causing AIDS.

→ The name HIV was suggested by the International committee on the nomenclature of viruses.

→ In India the firsts AIDS patient was identified in Chennai in the year 1987.

→ AIDS or Acquired Immune deficiency syndrome is a viral disease, caused by the deadly virus (HIV).

Structure of HIV

→ Human Immunodeficiency virus (HIV) is spherical in shape and contains RNA as its genetic material.

→ Externally, the virus is covered by the double layered membrane made up of fatty substances.

→ Inside the fatty membrane a core of proteins is found that surrounds the viral RNA along with the enzyme reverse transcriptase.

Transmission of HIV

→ Sharing of syringes during drug abuse.

→ Unsafe sexual contact.

→ Transfusion of infected blood

→ From infected mother to her infant through milk.

Prevention of HIV

→ Avoid sexual contact with infected persons

→ Ensure use of disposable syringes

→ Screening blood from blood banks

Tests for detection of HIV

→ PCR (Polymerase Chain Reaction)

→ ELISA (Enzyme linked Immuno Sorbent Assay)

→ Western Blot

Personal health and hygiene in adolescents

→ Adolescents should have a balanced diet with right proportions of various nutrients.

→ Adolescents should maintain cleanliness to prevent bacterial infections.

→ They should indulge in some physical exercises to keep their bodies fit.

→ They should avoid the consumption of drugs and alcohol.

Sex determination in humans

• Autosomes: First 22 pairs of chromosomes that do not determine the sex of an individual.

• Sex chromosomes: Last pair of chromosomes, represented as X and Y.

→ Females have two X chromosomes,so can be represented as 44+XX.

→ Males have one X and one Y chromosome, so can be represented as 44+XY.

→ Each gamete receives half of the chromosomes i.e. 22+X or 22+Y.

→ Male gametes have 22 autosomes and either X or Y sex chromosome.

→ Male gametes can be of two types, 22+X or 22+Y.

→ Female gametes can be of only one type, 22+X.

→ Sex of a baby is determined by the type of the male gamete (X or Y) that fuses with the female gamete.
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