NCERT Science Notes - Class 8
Chapter 7 - Reaching the Age of Adolescence

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 8 | Science | Chapter 7 - Reaching the Age of Adolescence. The chapter explores the significant physical and emotional changes that occur during adolescence. This period, typically between ages 11 and 19, marks the transition from childhood to adulthood. Key changes include a sudden increase in height, development of secondary sexual characteristics, and the onset of puberty, which makes individuals capable of reproduction. Hormones play a crucial role in these transformations, affecting both boys and girls differently. The chapter also emphasizes the importance of maintaining a healthy lifestyle and understanding the changes to navigate this critical phase of life effectively.

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NCERT Science Notes - Class 8
Chapter 7 - Reaching the Age of Adolescence

Introduction to Human Reproduction and Growth

1. Reproduction in Animals:
   • Previous chapter discussed how animals reproduce.
2. Age of Reproduction:
   • Humans and many animals can reproduce only after reaching a certain age.
3. Purpose of the Chapter:
   • This chapter focuses on the changes that occur in the human body that enable reproduction.
4. Role of Hormones:
   • Discussion of how hormones contribute to changes that help a child grow into an adult.

7.1 - Adolescence and Puberty

1. Boojho's Birthday:
   • Celebrating his 12th birthday; notices friends have grown taller and have a hairy line above their lips.
2. Growth Explanation:
   • Growth begins at birth; noticeable spurt occurs around age 10 or 11.
3. Adolescence:
   • Period of life with changes leading to reproductive maturity, called adolescence.
   • Begins around age 11 and lasts until 18 or 19.
   • Adolescents are often referred to as teenagers (ages 13-19).
   • Girls may start adolescence 1-2 years earlier than boys.
4. Variability:
   • The period of adolescence varies from person to person.
5. Changes During Adolescence:
   • Several changes occur during this period, marking the onset of puberty.
   • The most significant change is the capability for reproduction.
   • Puberty ends when an adolescent reaches reproductive maturity.
6. Realization:
   • Boojho and Paheli recognize height increase and facial hair as signs of adolescence.

7.2 - Changes at Puberty

• Most noticeable change during puberty is the sudden increase in height.
• Long bones (arms and legs) elongate, making a person taller.

• Growth Chart: Shows average height growth rates for boys and girls with age.
• Table of Growth:

  Age (Years)	% of Full Height (Boys)	% of Full Height (Girls)
  8	72%	77%
  9	75%	81%
  10	78%	84%
  11	81%	88%
  12	84%	91%
  13	88%	95%
  14	92%	98%
  15	95%	99%
  16	98%	99.5%
  17	99%	100%
  18	100%	100%

• Explanation: By age 11, boys reach 81% of their full height, while girls reach 88%. Individual variations exist.

Body Growth in Adolescents

• Uneven Growth: Different body parts grow at different rates during adolescence, causing temporary disproportions (e.g., larger arms or feet compared to the rest of the body).
• Body Proportion: Over time, other body parts catch up, and the body becomes proportionate.
• Genetic Influence: Height is influenced by the genes inherited from parents, making it similar to family members.
• Nutrition Importance: Eating the right food during growth years is crucial to support the development of bones, muscles, and other body parts.

Change in Body Shape

Voice Change

• Boys: At puberty, the voice box (larynx) grows larger, creating a deeper voice. The protruding part in the throat is called the Adam's apple.
• Girls: The larynx remains small and less visible, leading to a higher-pitched voice.
• Temporary Hoarseness: In boys, as the voice box grows, muscles may go out of control, causing a hoarse voice for a few days or weeks before normalizing.

Increased Activity of Sweat and Sebaceous Glands

• During puberty, the secretion of sweat and sebaceous (oil) glands increases.
• This often leads to acne and pimples on the face due to the higher activity of these glands.

Development of Sex Organs

• At puberty, male and female sex organs develop fully.
• In boys, the testes and penis grow and the testes begin producing sperms.
• In girls, the ovaries enlarge, eggs mature, and the ovaries start releasing mature eggs.

Reaching Mental, Intellectual, and Emotional Maturity

• Adolescence brings changes in thinking and emotional growth.
• Adolescents become more independent and self-conscious.
• Intellectual development occurs, with a heightened capacity for learning.
• Adolescents may feel insecure while adjusting to changes but should understand that these are a natural part of growing up.

7.3 - Secondary Sexual Characters

• Reproductive Organs:
  • Testes in males produce sperms.
  • Ovaries in females produce ova (eggs).
• Development of Secondary Sexual Characters:
  • Girls: Development of breasts begins at puberty.
  • Boys: Growth of facial hair (moustaches, beard) and chest hair.
  • Both Boys and Girls: Hair growth under arms and in the pubic region.
• Role of Hormones:
  • Changes during adolescence are controlled by hormones, chemical substances produced by the endocrine glands.
  • Testosterone: Male hormone released by the testes, responsible for changes in boys (e.g., facial hair).
  • Estrogen: Female hormone released by the ovaries, responsible for changes in girls (e.g., breast development and mammary gland formation).
  • The production of both testosterone and estrogen is regulated by the pituitary gland.

7.4 - Role of Hormones in Initiating Reproductive Function

• Endocrine glands release hormones into the bloodstream, which reach specific body parts called target sites.
• Testes and ovaries secrete sex hormones responsible for male and female secondary sexual characteristics.
• These sex hormones are controlled by hormones from the pituitary gland.
• The pituitary gland secretes hormones that help in the maturation of ova in the ovaries and the formation of sperms in the testes.

Paheli and Boojho have understood that puberty marks the beginning of the reproductive period. However, they are curious if the reproductive phase continues for life or if it ends at some point. To address this:

• The reproductive life does not continue indefinitely.
• In females, the reproductive phase lasts until menopause, which typically occurs around the age of 45 to 50 years.
• In males, reproductive life extends longer but declines with age, though there is no clear cessation as in females.

7.5 - Reproductive Phase of Life in Humans

Key Points

• Adolescents become capable of reproduction when their testes (in males) and ovaries (in females) start producing gametes.
• In females, the reproductive phase begins at puberty (around 10-12 years) and lasts until 45-50 years.
• With puberty, one ovum matures and is released by an ovary once every 28 to 30 days.
• The uterus wall thickens to receive the fertilized egg, resulting in pregnancy.
• If fertilization does not occur, the ovum and uterine lining are shed, causing menstruation.
• Menstruation happens every 28 to 30 days.
• The first menstruation is called menarche, and the cycle stops at menopause (45 to 50 years).
• The menstrual cycle includes egg maturation, release, uterine wall thickening, and breakdown if pregnancy does not happen.
• The menstrual cycle is regulated by hormones.

7.6 - How is the Sex of the Baby Determined?

Boy or Girl?

• The sex of a baby is determined by chromosomes inside the fertilised egg or zygote.
• Human cells have 23 pairs of chromosomes, two of which are sex chromosomes (X and Y).
• Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
• The egg (from the mother) always has an X chromosome.
• Sperms (from the father) are of two types:
  • Some carry an X chromosome.
  • Others carry a Y chromosome.

• If a sperm with an X chromosome fertilises the egg, the zygote will have two X chromosomes (XX), resulting in a female child.
• If a sperm with a Y chromosome fertilises the egg, the zygote will have an X and a Y chromosome (XY), resulting in a male child.

• The father’s sperm determines whether the baby will be a boy or a girl.
• It is wrong to blame the mother for the sex of the baby, as she contributes only an X chromosome.

7.7 - Hormones Other than Sex Hormones

• The pituitary gland stimulates the testes and ovaries to produce hormones and is an endocrine gland attached to the brain.
• Other endocrine glands include the thyroid, pancreas, and adrenals.

• Produces the hormone thyroxine.
• Lack of thyroxine can cause goitre, as seen in Boojho and Paheli’s friend Kaka with a bulging throat.

• Produces the hormone insulin, which regulates blood sugar levels.
• Insufficient insulin production can cause diabetes, as explained by their aunt regarding their uncle's condition.

• Maintain salt balance in the blood and secrete the hormone adrenalin, which helps the body deal with stress, anger, or worry.

• Controls other glands by sending signals for hormone release.
• Also secretes growth hormone, essential for normal growth.

Bhoojho asks - Are there hormones in other animals also? Have they any role to play in reproduction?

• Presence of Hormones: Yes, hormones are present in other animals as well. All animals, including mammals, birds, reptiles, amphibians, and even insects, produce hormones that regulate various physiological processes.
• Role in Reproduction:
  • Hormones play a crucial role in reproductive functions in animals, similar to humans. They regulate the development of reproductive organs, the maturation of gametes (sperm and eggs), and the timing of mating or breeding seasons.
  • Sex Hormones: Like in humans, animals produce sex hormones (e.g., testosterone in males and estrogen in females) that influence secondary sexual characteristics and reproductive behaviors.
  • Reproductive Cycles: Hormones help control reproductive cycles, such as estrous cycles in females, which determine the periods when they are fertile.
  • Pregnancy and Lactation: In female animals, hormones regulate pregnancy, the development of the fetus, and lactation for feeding offspring after birth.

7.8 - Role of Hormones in Completing 

the Life History of Insects and Frogs

• Metamorphosis:
  • The transformation from larva to adult in frogs and insects is termed metamorphosis.
  • In frogs, this process is controlled by the hormone thyroxine, which is produced by the thyroid gland.
  • Iodine Requirement: The production of thyroxine requires sufficient iodine in the water. Insufficient iodine levels can hinder tadpoles from developing into adult frogs.
• Insects:
  • In insects, metamorphosis is regulated by insect hormones, which orchestrate the various stages of development.
    
    

• Goitre and Iodine Deficiency:
  • Yes, if people do not consume enough iodine in their diet, they can develop goitre.
  • Goitre is caused by a lack of thyroxine due to insufficient iodine, leading to an enlarged thyroid gland.
  • Ensuring adequate iodine intake is essential for thyroid function and overall health.

Activity 7.3 : Importance of Consuming Iodised Salt

What is Iodised Salt?

• Iodised salt is table salt mixed with a small amount of potassium iodide, a source of iodine.
• It is specifically designed to prevent iodine deficiency in the diet.
  
  

1. Prevention of Iodine Deficiency Disorders:
   • Iodine is essential for the production of thyroid hormones, which regulate metabolism and growth.
   • Deficiency can lead to conditions such as goitre, intellectual disabilities, and developmental delays.
2. Supports Healthy Growth and Development:
   • Adequate iodine intake is crucial for physical and cognitive development in children.
   • Pregnant women require sufficient iodine for fetal brain development.
3. Regulation of Thyroid Function:
   • Iodine is vital for synthesizing thyroid hormones, which control metabolism and energy levels.
   • Ensures proper functioning of the thyroid gland, reducing the risk of hypothyroidism.
4. Reduced Risk of Thyroid Disorders:
   • Regular consumption of iodised salt can help prevent thyroid diseases, including hypothyroidism and hyperthyroidism.
5. Simple and Effective Public Health Strategy:
   • Iodising salt is a cost-effective way to deliver iodine to the population.
   • Widespread use of iodised salt has been shown to significantly reduce iodine deficiency in many regions.
6. Global Health Initiatives:
   • Organizations like the World Health Organization (WHO) promote the consumption of iodised salt to combat iodine deficiency globally.
   • Many countries have implemented legislation to ensure that all salt for human consumption is iodised.
     
     

7.9 - Reproductive Health

Definition of Health:

• Health is defined as the physical and mental well-being of an individual.

• It is crucial for every individual, regardless of age, to maintain a healthy body through:
  • Balanced Diet: Essential for providing the necessary nutrients for growth and development, especially during adolescence.
  • Personal Hygiene: Important for preventing infections and promoting overall health.
  • Physical Exercise: Necessary for maintaining fitness and a healthy weight, and it helps in physical development during growing years.

Nutritional Needs of Adolescents

Importance of Nutrition During Adolescence:

• Adolescence is a stage marked by rapid growth and development, necessitating a carefully planned diet.

• A balanced diet includes:
  • Proteins: For growth and development.
  • Carbohydrates: For energy.
  • Fats: For energy and essential fatty acids.
  • Vitamins and Minerals: For overall health and well-being.
• An example of a balanced Indian meal includes roti/rice, dal (pulses), and vegetables.
• Milk is a complete food, providing essential nutrients.
• Fruits are also important for nourishment.

• Iron: Essential for building blood. Iron-rich foods include:
  • Leafy vegetables
  • Jaggery
  • Meat
  • Citrus fruits (e.g., oranges)
  • Indian gooseberry (amla)

• Evaluate your lunch and dinner for balance and nutrition:
  • Cereals: Provide energy.
  • Proteins: Ensure inclusion of milk, meat, nuts, and pulses.
  • Fats and Sugars: Present for energy but should be consumed in moderation.
  • Fruits and Vegetables: Act as protective foods.

• While chips and packed/tinned snacks may be tasty, they lack adequate nutritional value and should not replace regular meals.

Personal Hygiene

• Daily Bathing: Everyone should bathe at least once daily, especially teenagers, due to increased sweat gland activity that can cause body odor.
• Body Cleaning: Clean all parts of the body regularly to prevent bacterial infections.
• Menstrual Hygiene:
• Girls should pay special attention to cleanliness during their menstrual flow.
• Keep track of the menstrual cycle and be prepared for menstruation.
• Use sanitary napkins or clean homemade pads.
• Change pads every 4–5 hours as needed to maintain hygiene.

• Importance of Exercise: Engage in walking, exercising, and playing outdoor games to keep the body fit and healthy.
• Fresh Air: Physical activities in fresh air contribute to overall well-being.

• Understanding Myths: Adolescents should recognize and discard myths and taboos related to bodily changes. Here are some common misconceptions:
  1. Pregnancy from Glancing: The belief that a girl can become pregnant just by looking at boys during menstruation.
  2. Mother's Responsibility for Baby's Sex: The idea that the mother solely determines the sex of the child.
  3. Restrictions During Menstruation: The notion that a girl should not work in the kitchen while menstruating.
• Critical Thinking: Armed with knowledge, challenge and refute these myths and any others encountered.

Say “NO” to Drugs

• Adolescence: A time of significant physical and mental activity; confusion and insecurity are normal.
• Rejecting Drugs: Always say "No" to drugs unless prescribed by a doctor.
• Addictiveness: Drugs can create a cycle of dependency; once used, there's a strong desire to use them again.
• Long-term Harm: Drug use leads to long-lasting damage to health and happiness.
• HIV/AIDS Awareness:
• AIDS is caused by the HIV virus.
• HIV can be transmitted through:
• Sharing syringes used for injecting drugs.
• Breastfeeding by an infected mother.
• Sexual contact with an infected person.

• Legal Marriage Age: In India, the legal age for marriage is 18 for girls and 21 for boys.
• Consequences of Early Motherhood:
  • Teenage mothers often lack the mental and physical readiness for motherhood.
  • Early marriage and motherhood can lead to health issues for both mother and child.
  • Employment opportunities for young women may be limited.
  • Responsibilities of motherhood can cause mental stress and agony for unprepared young mothers.

NCERT Science Notes - Class 8 | Science | Chapter 7 - Reaching the Age of Adolescence

NCERT Science Notes - Class 8 | Science | Chapter 7 - Reaching the Age of Adolescence