Welcome dear students! Today we are going to learn about Reaching the Age of Adolescence from Class 8 Science. In the previous chapter, you have learnt how animals reproduce. It is only after growing up to a certain age that human beings and many other animals can reproduce. Why can humans reproduce only after a certain age? In this chapter, we will learn about the changes that take place in the human body after which a person becomes capable of reproduction. We shall also discuss the role that hormones play in bringing about changes that make a child grow into an adult.
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Let us begin with section seven point one, Adolescence and Puberty. Imagine Boojho celebrating his twelfth birthday. After his friends left, Boojho and Paheli began chatting with their parents. Paheli studies in an all girls school. She started laughing and remarked that many of Boojho school friends, whom she met after a year, had suddenly shot up in height. Some of them were looking very funny with a hairy line above their lips. Her mother explained that the boys had grown up. Growth begins from the day one is born. But upon crossing the age of ten or eleven, there is a sudden spurt in growth which becomes noticeable. The changes taking place in the body are part of growing up. They indicate that you are no longer a child but are on the way to becoming an adult. The period of life, when the body undergoes changes, leading to reproductive maturity, is called adolescence. Adolescence begins around the age of eleven and lasts up to eighteen or nineteen years of age. Since this period covers the teens, which are thirteen to eighteen or nineteen years of age, adolescents are also called teenagers. In girls, adolescence may begin a year or two earlier than in boys. Also, the period of adolescence varies from person to person. The human body undergoes several changes during adolescence. These changes mark the onset of puberty. The most important change which marks puberty is that boys and girls become capable of reproduction. Puberty ends when an adolescent reaches reproductive maturity.
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Now let us move on to section seven point two, Changes at Puberty. 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. Let us look at Activity seven point one. The chart gives the average rate of growth in height of boys and girls with age. The figures give the percentage of the height a person has reached at the age given. For example, by the age of eight, boys have reached seventy two percent and girls seventy seven percent of their full height. At nine years, boys are at seventy five percent and girls at eighty one percent. At ten years, boys are at seventy eight percent and girls at eighty four percent. At eleven years, boys have reached eighty one percent, while a girl has reached eighty eight percent of her full height. At twelve years, boys are at eighty four percent and girls at ninety one percent. At thirteen years, boys are at eighty eight percent and girls at ninety five percent. At fourteen years, boys are at ninety two percent and girls at ninety eight percent. At fifteen years, boys are at ninety five percent and girls at ninety nine percent. At sixteen years, boys are at ninety eight percent and girls at ninety nine point five percent. At seventeen years, boys are at ninety nine percent and girls at one hundred percent. At eighteen years, both boys and girls reach one hundred percent of their full height. These figures are only representative and there may be individual variations.
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Here is a worked example from the textbook. A boy is nine years old and one hundred twenty centimeters tall. At the end of the growth period he is likely to be one hundred twenty multiplied by one hundred divided by seventy five centimeters, which equals one hundred sixty centimeters tall. You can use this table for your friends and work out how tall they are likely to be. Now for Activity seven point two, use the data given in Activity seven point one to draw a graph. Take age on the X axis and per cent growth in height on the Y axis. Highlight the point representing your age on the graph. Find out the percentage of height you have already reached. Calculate the height you might eventually reach. Tally your graph with the one given in Figure seven point one. In this graph, the X axis shows age in years from eight to eighteen, and the Y axis shows percentage from sixty five to one hundred. Two lines are plotted. One line with square markers represents boys, and another line with triangle markers represents girls. Initially, the girls line is higher, showing they grow faster, but by about eighteen years of age, both lines meet at one hundred percent, meaning both reach their maximum height. The rate of growth in height varies in different individuals. Some may grow suddenly at puberty and then slow down, while others may grow gradually. All parts of the body do not grow at the same rate. Sometimes the arms and legs or hands and feet of adolescents look oversized and out of proportion with the body. But soon the other parts catch up and result in a proportionate body. Height depends on the genes inherited from parents. It is very important to eat the right kind of food during these growing years to help bones, muscles and other parts get adequate nourishment.
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Next, we will learn about change in body shape. Have you noticed that boys in your class have broader shoulders and wider chests than boys in junior classes? This is because they have entered the age of puberty when shoulders generally broaden as a result of growth. In girls, the region below the waist becomes wider. In boys, the muscles of the body grow more prominently than in the girls. Thus, changes occurring in adolescent boys and girls are different. Now consider voice change. Did you notice that sometimes the voice of some of the boys in your class cracks? At puberty, the voice box or the larynx begins to grow. Boys develop larger voice boxes. The growing voice box in boys can be seen as a protruding part of the throat called Adam apple. In Figure seven point two, we see a side view of a grown up boy neck. A clear bump is visible on the front of the throat, which is labeled as the Adam 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. This state may remain for a few days or weeks after which the voice becomes normal.
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Let us discuss the increased activity of sweat and sebaceous glands. During puberty the secretion of sweat glands and sebaceous glands, which are oil glands, increases. Many young people get acne and pimples on the face at this time because of the increased activity of these glands in the skin. A few glands such as sweat glands, oil glands and salivary glands release their secretions through ducts. Endocrine glands release hormones directly into the bloodstream. So, they are also termed ductless glands. Now, 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.
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Now, let us look at Reaching Mental, Intellectual and Emotional Maturity. Adolescence is also a period of change in a person way of thinking. Adolescents are more independent than before and are also self conscious. Intellectual development takes place and they tend to spend considerable time thinking. In fact, it is often the time in one life when the brain has the greatest capacity for learning. Sometimes, however, an adolescent may feel insecure while trying to adjust to the changes in the body and mind. But as adolescent learners, you should know that there is no reason to feel insecure. These changes are a natural part of growing up.
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Moving to section seven point three, Secondary Sexual Characters. You have learnt that testes and ovaries are the reproductive organs. They produce the gametes, that is, sperms and ova. In girls, breasts begin to develop at puberty and boys begin to grow facial hair, that is, moustaches and beard. As these features help to distinguish the male from the female they are called secondary sexual characters. 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. Both Boojho and Paheli wish to know what initiates changes at puberty. The changes which occur at adolescence are controlled by hormones. Hormones are chemical substances. These are secretions from endocrine glands, or endocrine system. The male hormone or testosterone begins to be released by the testes at the onset of puberty. This causes changes in boys, for example, the growth of facial hair. Once puberty is reached 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. The production of these hormones is under the control of another hormone secreted from an endocrine gland called pituitary gland.
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Now section seven point four, Role of Hormones in Initiating Reproductive Function. 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. The testes and ovaries secrete sex hormones. These hormones are responsible for the male and female secondary sexual characters. Further, the sex hormones are under the control of hormones from the pituitary gland. Figure seven point three shows a flowchart. At the top, it says hormones from pituitary stimulate testes and ovaries to release testosterone in male and estrogen in female. An arrow points down to released in the blood stream and reach parts of the body, labeled target site. Another arrow points down to stimulate changes in the body at onset of puberty. The pituitary secretes many hormones, one of which makes ova mature in the ovaries and sperms form in the testes. Puberty marks the beginning of the reproductive period when one becomes capable of reproduction.
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Let us study section seven point five, 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, which is ten to twelve years of age, and generally lasts till the age of approximately forty five to fifty 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 twenty eight to thirty 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. Menstruation occurs once in about twenty eight to thirty days. The first menstrual flow begins at puberty and is termed menarche. At forty five to fifty years of age, the menstrual cycle stops. Stoppage of menstruation is termed menopause. Initially, menstrual cycle may be irregular. It takes some time to become regular. Menstrual cycle is controlled by hormones. The cycle includes the maturation of the egg, its release, thickening of uterine wall and its breakdown if pregnancy does not occur. In case the egg is fertilised it begins to divide and then gets embedded in the uterus for further development.
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Now, section seven point six, How is the Sex of the Baby Determined? Inside the fertilised egg or zygote is the instruction for determining the sex of the baby. This instruction is present in the thread like structures, called chromosomes in the fertilised egg. Chromosomes are present inside the nucleus of every cell. All human beings have twenty three 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, while a male has one X and one Y chromosome. The gametes, which are egg and sperm, have only one set of chromosomes. The unfertilised egg always has one X chromosome. But sperms are of two kinds. One kind has an X chromosome, and the other kind has a Y chromosome. Figure seven point four illustrates this. It shows a female parent producing eggs with X chromosomes, and a male parent producing two types of sperms, one with X and one with Y. When an X sperm fertilises the egg, the zygote has XX chromosomes and develops into a female child. If a Y sperm fertilises the egg, the zygote has XY chromosomes and develops into a male child. Now you know that the sex chromosomes of the father determine the sex of an unborn baby. The belief that the mother is responsible for the sex of her baby is completely wrong and to blame her for this is totally unjustified.
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Let us look at section seven point seven, Hormones other than Sex Hormones. The hormones secreted by the pituitary stimulate testes and ovaries to produce their hormones. The pituitary gland is an endocrine gland attached to the brain. Apart from the pituitary, the testes and the ovaries, there are other endocrine glands in the body such as thyroid, pancreas and adrenals. Figure seven point five shows the position of endocrine glands in the human body. It labels the pituitary gland in the brain, the thyroid gland in the neck, the pancreas behind the stomach, the adrenal glands on top of the kidneys, and the ovaries and testes in the pelvic region. A boy named Kaka had a very big and bulging throat. He was suffering from goitre, a disease of the thyroid gland. Kaka thyroid gland was not producing the hormone thyroxine. Another person was suffering from diabetes because his pancreas was not producing the hormone insulin in sufficient quantities. Adrenal glands secrete hormones which maintain the correct salt balance in the blood. Adrenals also produce the hormone adrenalin. It helps the body to adjust to stress when one is very angry, embarrassed or worried. Thyroid and adrenals secrete their hormones when they receive orders from the pituitary through its hormones. Pituitary also secretes growth hormone which is necessary for the normal growth of a person.
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Section seven point eight covers the Role of Hormones in Completing the Life History of Insects and Frogs. The tadpole passes through certain stages to become a frog. This 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. For Activity seven point three, collect information from magazines or from doctors and prepare a note on the importance of consuming iodised salt. You can also look for this information on the internet.
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Now we reach section seven point nine, Reproductive Health. The physical and mental well being of an individual is regarded as an individual health. To keep the body healthy, every human being, at any age, needs to have a balanced diet. The person must also observe personal hygiene and undertake adequate physical exercise. During adolescence, however, these become even more essential as the body is growing. For nutritional needs of adolescents, adolescence is a stage of rapid growth and development. Hence the diet for an adolescent has to be carefully planned. A balanced diet means that the meals include proteins, carbohydrates, fats and vitamins in requisite proportions. Our Indian meal of roti or rice, dal which is pulses, and vegetables is a balanced meal. Milk is a balanced food in itself. Fruits also provide nourishment. For infants, mother milk provides all the nourishment that they need. Iron builds blood and iron rich food such as leafy vegetables, jaggery, meat, citrus, Indian gooseberry or amla are good for adolescents. Check items for lunch and dinner in your meal. Is the meal balanced and nutritious? Does it include cereals which give energy and milk, meat, nuts and pulses which provide proteins for growth? Also, does it include fats and sugar that give energy? What about fruits and vegetables which are protective foods? Chips and packed or tinned snacks, though very tasty should never replace regular meals as they do not have adequate nutritional value.
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For Activity seven point four, make a group with your friends. Write down the items of food in your breakfast, lunch and dinner you had on the previous day. Identify the items responsible for proper growth. Also identify the junk food that you consumed the previous day. For Activity seven point five, get ideas from the pictures given in Figure seven point six. The picture shows five nutritious food groups: meat, vegetables, milk and eggs, fruits, and grains. Prepare charts or posters and paste them in the class so that you are aware of the diet for adolescents. You may use your creative ideas and present it like an advertisement. You may even organise a competition on this topic. Regarding 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. If cleanliness is not maintained there are chances of catching bacterial infection. Girls should take special care of cleanliness during the time of menstrual flow. They should keep track of their menstrual cycle and be prepared for the onset of menstruation. Use sanitary napkin or clean homemade pads. Change pads after every four to five hours as per the requirement.
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Physical exercise is vital. 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. For Activity seven point six, collect data on the number of children in your class who exercise regularly and who do not exercise regularly. Did you notice any difference in their fitness and health? Prepare a report on the benefits of regular exercise. Now, let us discuss myths, taboos, do and do nots. There are many wrong notions which you should now be able to discard as informed adolescents. For example, there are myths and taboos regarding bodily changes that adolescents experience. Some of these are given below and you can now argue why these are myths and not facts. First, a girl becomes pregnant if she looks at boys during menstruation. Second, the mother is responsible for the sex of her child. Third, a girl should not be allowed to work in the kitchen during menstruation. You may come across many other myths and taboos. Discard them. Say no to drugs. Adolescence is a period of much activity in the body and mind which is a normal part of growing up. So do not feel confused or insecure. If anybody suggests that you will get relief if you take some drugs, just say no unless prescribed by the doctor. Drugs are addictive. If you take them once, you feel like taking them again and again. They harm the body in the long run. They ruin health and happiness. You must have heard about AIDS which is caused by a dangerous virus, HIV. This virus can pass on to a normal person from an infected person by sharing the syringes used for injecting drugs. It can also be transmitted to an infant from the infected mother through her milk. The virus can also be transmitted through sexual contact with a person infected with HIV.
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Let us talk about adolescent pregnancy. In our country, the legal age for marriage is eighteen years for girls and twenty one years for boys. This is because teenage mothers are not prepared mentally or physically for motherhood. Early marriage and motherhood cause health problems in the mother and the child. It also curtails employment opportunities for the young woman and may cause mental agony as she is not ready for responsibilities of motherhood. Now, let us review what we have learnt. Humans become capable of reproduction after puberty sets in. Children between the ages of eleven and nineteen years are called adolescents. The onset of puberty brings about growth of the reproductive organs. Hair grow at various places on the body. Breasts develop in girls and facial hair, that is moustache and beard, appear in boys. Voice of boys becomes hoarse as voice box enlarges during adolescence. Children gain height during adolescence. The onset of puberty and maturity of reproductive parts are controlled by hormones. Hormones are secretions of endocrine glands which pour them directly into the blood stream. Pituitary gland secretes hormones which include growth hormone and hormones that make other glands such as the testes, ovaries, thyroids and adrenals, secrete hormones. Pancreas secretes insulin, thyroid produces thyroxine and adrenals produce adrenalin. Testosterone is the male hormone and estrogen, the female hormone. The uterine wall in females prepares itself to receive the developing fertilised egg. In case there is no fertilisation, the thickened lining of the uterine wall breaks down and goes out of the body along with blood. This is called menstruation. Sex of the unborn child depends on whether the zygote has XX or XY chromosomes. It is important to eat balanced food and maintain personal hygiene during adolescence.
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Now we will solve the exercises from your textbook. Exercise one asks, what is the term used for chemical secretions of endocrine glands responsible for changes taking place in the body? The answer is hormones. Exercise two asks, define adolescence. The definition is, the period of life, when the body undergoes changes, leading to reproductive maturity, is called adolescence. Exercise three asks, what is menstruation? Explain. Menstruation is the monthly shedding of the thickened lining of the uterus along with blood vessels and the unfertilised egg. It occurs once in about twenty eight to thirty days in females of reproductive age. If fertilisation does not occur, this breakdown and bleeding happens. The first menstrual flow is called menarche, and its stoppage around age forty five to fifty is called menopause. Exercise four asks, list changes in the body that take place at puberty. The changes are sudden increase in height, change in body shape with broader shoulders in boys and wider hips in girls, voice change with deepening in boys and development of Adam apple, increased activity of sweat and sebaceous glands leading to acne, complete development of sex organs like testes and ovaries, growth of facial, chest, underarm and pubic hair, and mental, intellectual and emotional maturity.
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Exercise five asks, prepare a table having two columns depicting names of endocrine glands and hormones secreted by them. Here is the complete list. Pituitary gland secretes growth hormone and hormones that stimulate other glands. Thyroid gland secretes thyroxine. Pancreas secretes insulin. Adrenal glands secrete adrenalin and hormones for salt balance. Testes secrete testosterone. Ovaries secrete estrogen. Exercise six asks, what are sex hormones? Why are they named so? State their function. Sex hormones are chemical substances secreted by the reproductive endocrine glands, specifically the testes and ovaries. They are named so because they are responsible for the development of secondary sexual characters and reproductive functions. Testosterone in males causes facial hair growth, deepening of voice, and sperm production. Estrogen in females causes breast development, maturation of ovaries, and regulation of the menstrual cycle. Exercise seven is multiple choice. Part a, adolescents should be careful about what they eat, because the correct option is two, proper diet is needed for the rapid growth taking place in their body. Part b, reproductive age in women starts when their correct option is one, menstruation starts. Part c, the right meal for adolescents consists of correct option is two, chapati, dal, vegetables.
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Exercise eight asks for notes. Part a, Adam apple. The Adam apple is a protruding part of the throat in boys, which is the enlarged voice box or larynx that develops during puberty. It causes the voice to deepen. Part b, secondary sexual characters. These are features that develop during puberty and help distinguish males from females. In boys, they include facial hair, chest hair, broader shoulders, and deep voice. In girls, they include breast development and wider hips. Both develop underarm and pubic hair. Part c, sex determination in the unborn baby. The sex is determined by the sex chromosomes in the zygote. Females have XX chromosomes and males have XY. The egg always carries an X chromosome. Sperm can carry either X or Y. If an X sperm fertilises the egg, the zygote is XX and develops into a girl. If a Y sperm fertilises the egg, the zygote is XY and develops into a boy. Therefore, the father determines the sex.
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Exercise nine is a word game. Across clues. Three, protruding voice box in boys. Answer is ADAMSAPPLE. Four, glands without ducts. Answer is ENDOCRINE. Seven, endocrine gland attached to brain. Answer is PITUITARY. Eight, secretion of endocrine glands. Answer is HORMONES. Nine, pancreatic hormone. Answer is INSULIN. Ten, female hormone. Answer is ESTROGEN. Down clues. One, male hormone. Answer is TESTOSTERONE. Two, secretes thyroxine. Answer is THYROID. Three, another term for teenage. Answer is ADOLESCENCE. Five, hormone reaches here through blood stream. Answer is TARGETSITE. Six, voice box. Answer is LARYNX. Seven, term for changes at adolescence. Answer is PUBERTY. Exercise ten provides a table of height data for boys and girls at ages zero, four, eight, twelve, sixteen, and twenty. The heights for boys are fifty three, ninety six, one hundred fourteen, one hundred twenty nine, one hundred fifty, and one hundred seventy three centimeters. The heights for girls are fifty three, ninety two, one hundred ten, one hundred thirty three, one hundred fifty, and one hundred sixty five centimeters. If you draw graphs showing height and age for both on the same paper, you will see that initially both grow at similar rates. Between ages eight and twelve, girls grow faster and are taller than boys at age twelve. By age sixteen, both reach one hundred fifty centimeters. After sixteen, boys continue to grow taller, reaching one hundred seventy three centimeters at age twenty, while girls reach one hundred sixty five centimeters. The conclusion is that girls experience a growth spurt earlier than boys, but boys eventually surpass girls in height by late adolescence.
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For extended learning, you are encouraged to find out from elder relatives about the legal status of early marriage and write a two minute speech on why it is harmful. Collect newspaper cuttings on HIV and AIDS and write a fifteen to twenty sentence article. Research the sex ratio from the two thousand eleven census, understand concerns about the low ratio, learn what amniocentesis is, and why its use for sex identification is banned in India. Finally, put your ideas together and write a short note on the importance of knowing facts about reproduction. You can visit the websites mentioned in your textbook for more information. I hope this lesson has helped you understand the fascinating changes that occur during adolescence. Remember to eat well, stay active, maintain hygiene, and always rely on scientific facts rather than myths.
Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]