Class 8 Science Notes Chapter 10 (Reaching the Age of Adolescence) – Science Book

Dear readers, here are the detailed notes for Chapter 10: Reaching the Age of Adolescence, NCERT Class 8 Science, tailored for government exam preparation.

Chapter 10: Reaching the Age of Adolescence

1. Introduction: Adolescence and Puberty

• Adolescence: The period of life when the body undergoes changes, leading to reproductive maturity. It generally begins around the age of 11 and lasts up to 18 or 19 years. Adolescents are also called 'teenagers' (covering ages 13 to 19).
• Puberty: The specific period during adolescence when the body undergoes changes that make an individual capable of reproduction. It marks the onset of adolescence. The end of puberty marks the completion of adolescent reproductive maturity.

2. Changes at Puberty

• a) Increase in Height:
  • Sudden increase in height is a conspicuous change.
  • Growth rate varies among individuals.
  • Initially, girls may grow faster than boys, but both reach their maximum height around 18 years of age.
  • Formula to calculate approximate full height (cm): (Present Height (cm) / % of full height at this age) * 100. (Note: The percentage table is usually provided in the textbook; memorizing the table itself is less crucial than understanding the concept).
• b) Change in Body Shape:
  • Boys: Shoulders broaden, chest becomes wider, muscles grow more prominently.
  • Girls: Region below the waist becomes wider, hips broaden.
• c) Change in Voice:
  • Boys: Voice box (larynx) begins to grow, becoming larger and protruding (Adam's apple). This causes the voice to become deep and hoarse. Sometimes the voice 'cracks' initially.
  • Girls: Larynx grows slightly, voice generally remains high-pitched.
• d) Increased Activity of Sweat and Sebaceous Glands:
  • Secretion of sweat glands (sudoriferous glands) and oil glands (sebaceous glands) increases.
  • Increased sebaceous gland activity can lead to acne and pimples on the face.
• e) Development of Sex Organs:
  • Males: Testes and penis develop completely. Testes start producing sperm.
  • Females: Ovaries enlarge, and eggs begin to mature. Ovaries start releasing mature eggs (ova).
• f) Reaching Mental, Intellectual and Emotional Maturity:
  • Adolescence involves changes in thinking patterns. Individuals become more independent, self-conscious.
  • Intellectual development takes place; learning capacity increases.
  • Emotional fluctuations are common due to hormonal changes and adapting to physical/social changes.

3. Secondary Sexual Characters

• These are features that help distinguish males from females but are not directly involved in reproduction.
• Boys:
  • Growth of facial hair (moustache, beard).
  • Hair growth on the chest, underarms, and pubic region.
  • Deepening of voice.
  • Broadening of shoulders.
• Girls:
  • Development of breasts (mammary glands).
  • Hair growth underarms and pubic region.
  • Widening of hips.
  • Onset of menstruation.
• These changes are controlled by hormones.

4. Role of Hormones in Initiating Reproductive Function

• Hormones: Chemical substances secreted by endocrine glands (ductless glands) directly into the bloodstream. They travel through the blood to target sites/organs and stimulate specific changes.
• Endocrine System: The system of endocrine glands responsible for hormone production.
• Pituitary Gland: An endocrine gland located at the base of the brain. It secretes hormones that regulate many other endocrine glands, including the testes and ovaries. Often called the "Master Gland".
• Sex Hormones:
  • Testosterone: Male sex hormone produced by the testes. Responsible for male secondary sexual characters (facial hair, deep voice, etc.) and sperm production.
  • Estrogen: Female sex hormone produced by the ovaries. Responsible for female secondary sexual characters (breast development, etc.), maturation of eggs, and regulation of the menstrual cycle.
• Hormonal Control Mechanism:
  1. Pituitary gland secretes hormones.
  2. These pituitary hormones stimulate the testes (in males) or ovaries (in females).
  3. Testes produce testosterone; Ovaries produce estrogen.
  4. Testosterone/Estrogen are released into the bloodstream and reach target sites throughout the body.
  5. These hormones trigger the changes associated with puberty (secondary sexual characters, maturation of sex organs).
• The production of these hormones is under the control of hormones from the pituitary gland.

5. Reproductive Phase of Life in Humans

• Onset: Begins at puberty with the maturation of sex organs and the production of gametes (sperm in males, ova in females).
• Duration:
  • Males: The reproductive phase generally lasts much longer than in females, potentially throughout life after puberty.
  • Females: Begins at puberty (around 10-12 years) and generally lasts until the age of 45-50 years.
• Menstruation (in Females):
  • The cycle of producing and releasing one mature ovum (egg) and the thickening/shedding of the uterine lining.
  • Ovulation: Release of a mature egg from the ovary (approx. once every 28-30 days).
  • Uterine Preparation: The uterus wall thickens to receive a potentially fertilized egg.
  • Menstruation: If fertilization does not occur, the released egg and the thickened uterine lining, along with blood vessels, are shed. This causes bleeding, which is called menstruation. It occurs approximately once every 28-30 days.
  • Menarche: The first menstrual flow, occurs at puberty.
  • Menopause: The stoppage of menstruation, typically occurs between 45-50 years of age. Marks the end of the reproductive phase in females.
  • Menstrual Cycle: The entire cycle from one menstruation to the next, regulated by hormones (Estrogen and Progesterone, influenced by pituitary hormones).

6. How is the Sex of the Baby Determined?

• Sex is determined by sex chromosomes.
• Human cells have 23 pairs of chromosomes in the nucleus. One pair is the sex chromosomes (X and Y).
• Females: Have two X chromosomes (XX). All ova (eggs) carry only an X chromosome.
• Males: Have one X and one Y chromosome (XY). Sperm can carry either an X or a Y chromosome (50% chance for each).
• Determination:
  • If a sperm carrying an X chromosome fertilizes the egg (X), the zygote will be XX (Female).
  • If a sperm carrying a Y chromosome fertilizes the egg (X), the zygote will be XY (Male).
• Conclusion: The sex chromosome contributed by the male (sperm) determines the sex of the child. It is scientifically incorrect and unfair to blame the mother for the sex of the baby.

7. Hormones Other Than Sex Hormones

• The endocrine system produces various hormones regulating different bodily functions. Examples mentioned:
  • Thyroxine: Produced by the Thyroid gland. Regulates metabolism. Deficiency can cause Goitre.
  • Insulin: Produced by the Pancreas. Regulates blood sugar levels. Deficiency causes Diabetes.
  • Adrenaline: Produced by the Adrenal glands. Helps the body cope with stress/emergency situations ("fight or flight" hormone).

8. Role of Hormones in Completing Life History of Insects and Frogs

• Metamorphosis: The process of transformation from larva to adult through drastic changes.
• Insects: Insect metamorphosis (e.g., silkworm, butterfly) is controlled by insect hormones.
• Frogs: Metamorphosis from tadpole to adult frog is controlled by Thyroxine (produced by the thyroid gland).
  • Thyroxine production requires iodine in the water.
  • If the water lacks sufficient iodine, tadpoles cannot metamorphose into frogs.

9. Reproductive Health

• Health is crucial during adolescence due to rapid growth and development.
• a) Nutritional Needs:
  • Adolescents need a balanced diet (includes proteins, carbohydrates, fats, vitamins, minerals in adequate proportions).
  • Roti, rice, dal, vegetables, milk, fruits are important.
  • Iron: Builds blood; important especially for girls (due to blood loss during menstruation). Iron-rich foods: leafy vegetables, jaggery, meat, citrus, amla.
  • Calcium: For bone growth. Found in milk and milk products.
  • Avoid junk food/packaged snacks as they lack sufficient nutritional value.
• b) Personal Hygiene:
  • Essential to prevent infections.
  • Daily bathing is necessary due to increased sweat gland activity.
  • Keep all body parts clean.
  • Maintain cleanliness during menstruation (use sanitary napkins or tampons, change regularly).
• c) Physical Exercise:
  • Walking, playing outdoor games, regular exercise keeps the body fit and healthy.
• d) Say NO to Drugs:
  • Adolescence is a vulnerable age. Peer pressure can lead to experimenting with drugs.
  • Drugs are addictive and harmful to health and happiness. Avoid them completely.
  • HIV/AIDS: Caused by a virus (HIV), can spread through sexual contact with an infected person or sharing infected syringes. No cure, prevention is key.

10. Myths and Taboos

• Many societal myths and taboos exist regarding menstruation and adolescent changes. It's important to rely on scientific facts and disregard unfounded beliefs.

Key Terms for Quick Revision:

• Adolescence: Period of transition from childhood to adulthood (approx. 11-19 years).
• Puberty: Process of sexual maturation during adolescence.
• Hormones: Chemical messengers produced by endocrine glands.
• Endocrine Glands: Ductless glands secreting hormones (e.g., Pituitary, Thyroid, Adrenal, Pancreas, Testes, Ovaries).
• Testosterone: Male sex hormone.
• Estrogen: Female sex hormone.
• Secondary Sexual Characters: Physical traits distinguishing sexes, not directly involved in reproduction.
• Adam's Apple: Protruding part of the larynx in boys.
• Menstruation: Monthly shedding of uterine lining and blood in females.
• Menarche: First menstruation.
• Menopause: Stoppage of menstruation.
• Gametes: Reproductive cells (Sperm and Ova).
• Zygote: Fertilized egg.
• Sex Chromosomes: X and Y chromosomes determining sex.
• Balanced Diet: Diet containing all necessary nutrients in right proportions.
• Metamorphosis: Transformation from larva to adult (e.g., tadpole to frog).
• Thyroxine: Hormone from the thyroid gland, crucial for frog metamorphosis.

This chapter highlights a crucial developmental stage. Understanding the biological, hormonal, and health aspects is vital. Focus on the definitions, specific changes, roles of hormones, sex determination mechanism, and reproductive health aspects for exam preparation.