CBSE – 8th Standard Science

CBSE - 8th Standard Science

Reaching The Age Of Adolescence

Adolescence and Puberty

  • Growth begins from the day one is born. Growing up is a natural process. The period of life, when the body undergoes changes, leading to reproductive maturity, is called adolescence.


  • Adolescence begins around the age of 11 and lasts upto 18 or 19 years of age. the period of adolescence varies from person to person. changes mark the onset of puberty.


Changes at Puberty


Increase in Height

  • The most conspicuous change during puberty is the sudden increase in height. At this time the long bones, that is, the bones of the arms and the legs elongate and make a person tall.


Change in Body Shape

  • In boys, the muscles of the body grow more prominently than in the girls. Thus, changes occurring in adolescent boys and girls are different.


Voice Change

  • At puberty, the voice box or the larynx begins to grow. Boys develop larger voice boxes. The growing voice box in boys protruding throat is called Adam’s apple.  In girls the larynx is hardly visible from the outside because of its small size.
  • Generally, girls have a high pitched voice, whereas boys have a deep voice. In adolescent boys, sometimes, the muscles of the growing voice box go out of control and the voice becomes hoarse..


Development of Sex Organs

  • At puberty, male sex organs like the testes and penis develop completely. The testes also begin to produce sperms.
  •  In girls, the ovaries enlarge and eggs begin to mature. Also ovaries start releasing mature eggs.


Reaching Mental, Intellectual and Emotional Maturity

  • Adolescence is also a period of change in a person’s way of thinking. Adolescents are more independent than before and are also self conscious.


Secondary Sexual Characters

  • In girls, breasts begin to develop at puberty called and boys begin to grow facial hair, that is, moustaches and beard. As these features help to distinguish the male from the female


Increased Activity of Sweat and Sebaceous Glands

  • During puberty the secretion of sweat glands and sebaceous glands (oil glands) increases. Many young people get acne pimples on skin.
  • Boys also develop hair on their chest. In both, boys and girls, hair grows under the arms and in the region above the thighs or the pubic region.
  • The changes which occur at adolescence are controlled by hormones. Hormones are chemical substances. These are secretions from endocrine glands.
  • The production of these hormones is under the control of another hormone secreted from an endocrine gland called pituitary gland.
  •  Endocrine glands release hormones into the bloodstream to reach a particular body part called target site. The target site responds to the hormone. There are many endocrine glands or ductless glands in the body


Role of Hormones in Initiating Reproductive Function

  • The male hormone or testosterone begins to be released by the testes at the onset of puberty. This causes changes in boys
  • In girls, ovaries begin to produce the female hormone or estrogen which makes the breasts develop. Milk secreting glands or mammary glands develop inside the breasts.


 Reproductive Phase of Life in Humans

  • Adolescents become capable of reproduction when their testes and ovaries begin to produce gametes. The capacity for maturation and production of gametes lasts for a much longer time in males than in females.
  • In females, the reproductive phase of life begins at puberty (10 to 12 years of age) and generally lasts till the age of approximately 45 to 50 years. The ova begin to mature with the onset of puberty.
  • One ovum matures and is released by one of the ovaries once in about 28 to 30 days. During this period, the wall of the uterus becomes thick so as to receive the egg, in case it is fertilised and begins to develop. This results in pregnancy.
  • If fertilisation does not occur, the released egg, and the thickened lining of the uterus along with its blood vessels are shed off. This causes bleeding in women which is called menstruation. The first menstrual flow is termed menarche. At 45 to 50 years of age, the menstrual cycle stops it is termed menopause.
  • In case the egg is fertilised it begins to divide and then gets embedded in the uterus for further development.


Sex Determination

  • Inside the fertilised egg  is the instruction for determining the sex of the baby. 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


  • When a sperm containing X chromosome fertilises the egg, the zygote would have two X chromosomes and develop into a female child.
  • If the sperm contributes a Y chromosome to the egg (ovum) at fertilisation, the zygote would develop into a male child.


Hormones other than Sex Hormones

  • Pituitary also secretes growth hormone which is necessary for the normal growth of a person
  • Thyroid and adrenals secrete their hormones when they receive orders from the pituitary through its hormones..
  • Thyroid gland  produce the hormone thyroxine.
  • Adrenal glands secrete hormones which maintain the correct salt balance in the blood. insulin in sufficient Adrenals also produce the hormone adrenalin. It helps the body to adjust to stress when one is very angry, embarrassed or worried.


  • The change from larva to adult is called metamorphosis
  • Metamorphosis in insects is controlled by insect hormones. In a frog, it is controlled by thyroxine, the hormone produced by thyroid.
  • Thyroxine production requires the presence of iodine in water. If the water in which the tadpoles are growing does not contain sufficient iodine, the tadpoles cannot become adults.


Reproductive Health

  • The physical and mental well being of an individual is regarded as an individual’s health. During adolescence, balanced diet, personal hygiene and undertake adequate physical exercise become even more essential as the body is growing.


Nutritional Needs of the Adolescents

  • Adolescence is a stage of rapid growth and development. Hence the diet for an adolescent has to be carefully planned.
  • Balanced diet means that the meals include proteins, carbohydrates, fats and vitamins in requisite proportions Milk is a balanced food in itself. Fruits and iron-rich food such as leafy vegetables, jaggery, meat, citrus,are good for adolescents.
  • For infants, mother milk provides all the nourishment that they need.


Personal Hygiene

  • Everyone should have a bath at least once everyday. It is more necessary for teenagers because the increased activity of sweat glands sometimes makes the body smelly. All parts of the body should be washed and cleaned everyday.


Physical exercise

  • Walking and playing in fresh air keeps the body fit and healthy. All young boys and girls should take walks, exercise and play outdoor games.
  • Adolescence is a period of much activity in the body and mind which is a normal part of growing up Drugs are addictive They harm the body in the long run, ruin health and happiness.