Two Hormones Rewrite the Whole Body in About Four Years
Learn what adolescence and puberty mean and when each begins, trace every physical change back to testosterone or oestrogen, understand the emotional changes and what actually helps, and see why diet and hygiene matter more at this age.
Why do so many changes happen at once, and all in a few years?
Because they all have the same cause — a sudden rise in one hormone.
The pituitary gland begins instructing the gonads to secrete, and from that point a single hormone circulating in the blood reaches every part of the body at once. In boys it is testosterone; in girls it is oestrogen. Height, voice, hair, skin, body shape and the reproductive organs are all responding to the same message.
So the changes are not a list of unrelated events. They are one instruction being carried out in many places. This page covers the second part of the ICSE Class 8 Biology chapter on the endocrine system: adolescence, the changes it brings, and how to look after a body that is rebuilding itself.
The pituitary gland begins instructing the gonads to secrete, and from that point a single hormone circulating in the blood reaches every part of the body at once. In boys it is testosterone; in girls it is oestrogen. Height, voice, hair, skin, body shape and the reproductive organs are all responding to the same message.
So the changes are not a list of unrelated events. They are one instruction being carried out in many places. This page covers the second part of the ICSE Class 8 Biology chapter on the endocrine system: adolescence, the changes it brings, and how to look after a body that is rebuilding itself.
What is the difference between adolescence and puberty?
Puberty is the biological event; adolescence is the whole period of transition that contains it.
Puberty is the stage at which the reproductive organs mature and the body becomes capable of reproduction. It is marked by definite physical changes, and it is brought about by hormones.
Adolescence is the period of transition from childhood to adulthood, covering not only the physical changes of puberty but also the emotional, mental and social changes that go with growing up. A person in this stage is called an adolescent, and the years of rapid growth within it are often called the teens.
Approximate ages, and they are approximations rather than rules:
- Adolescence runs roughly from 11 to 19 years of age.
- Puberty in girls usually begins at about 10 to 12 years.
- Puberty in boys usually begins at about 12 to 14 years.
So girls generally begin puberty earlier than boys, by about a couple of years. This is why, in a Class 7 or Class 8 classroom, many girls are visibly taller than the boys of the same age — and why the boys catch up and often overtake them a few years later. Nothing is wrong in either case; the timetables simply differ.
Why the ages vary so much between individuals. The timing depends on inherited factors, nutrition and general health, so two perfectly healthy people of the same age can be at quite different stages. Starting earlier or later than a friend is normal and is not a sign of any problem.
That is worth stating plainly, because anxiety about being different is itself one of the emotional features of this period, and it is usually based on comparing two normal variations.
The end of adolescence. Growth in height stops when the long bones stop lengthening, which happens once the growth hormone of the previous part can no longer act on them. After that a person may gain weight or muscle but will not get taller — which is why adult height is fixed by the end of adolescence.
Puberty is the stage at which the reproductive organs mature and the body becomes capable of reproduction. It is marked by definite physical changes, and it is brought about by hormones.
Adolescence is the period of transition from childhood to adulthood, covering not only the physical changes of puberty but also the emotional, mental and social changes that go with growing up. A person in this stage is called an adolescent, and the years of rapid growth within it are often called the teens.
Approximate ages, and they are approximations rather than rules:
- Adolescence runs roughly from 11 to 19 years of age.
- Puberty in girls usually begins at about 10 to 12 years.
- Puberty in boys usually begins at about 12 to 14 years.
So girls generally begin puberty earlier than boys, by about a couple of years. This is why, in a Class 7 or Class 8 classroom, many girls are visibly taller than the boys of the same age — and why the boys catch up and often overtake them a few years later. Nothing is wrong in either case; the timetables simply differ.
Why the ages vary so much between individuals. The timing depends on inherited factors, nutrition and general health, so two perfectly healthy people of the same age can be at quite different stages. Starting earlier or later than a friend is normal and is not a sign of any problem.
That is worth stating plainly, because anxiety about being different is itself one of the emotional features of this period, and it is usually based on comparing two normal variations.
The end of adolescence. Growth in height stops when the long bones stop lengthening, which happens once the growth hormone of the previous part can no longer act on them. After that a person may gain weight or muscle but will not get taller — which is why adult height is fixed by the end of adolescence.
Which physical changes does each hormone cause?
Testosterone from the testes drives the changes in boys; oestrogen from the ovaries drives them in girls. Both are secreted on instructions from the pituitary.
Changes in both boys and girls:
- A sudden increase in height — the growth spurt, the fastest growth since infancy
- Sweat and oil glands become more active, so more perspiration, body odour, and often pimples or acne on the face
- Growth of hair in the armpits and the pubic region
- The reproductive organs mature and begin to function
- Increased appetite, and more muscle and bone being built
Changes in boys, from testosterone:
- The voice deepens, as the larynx (voice box) enlarges — the visible lump it produces in the throat is the Adam's apple
- Facial hair appears, on the upper lip and then the chin
- Shoulders and chest broaden, and muscles develop
- Hair may grow on the chest and arms
- The testes begin producing sperm
Changes in girls, from oestrogen:
- Breasts develop
- The hips broaden, as the pelvis widens
- The voice becomes higher and thinner than a boy's
- The ovaries begin releasing ova, so ovulation starts
- Menstruation begins — the first menstruation is called menarche
How the growth spurt differs between them. Girls begin theirs earlier and finish earlier; boys begin later, grow for longer and usually end up taller. Both spurts last a few years and then stop.
The point that ties this to the previous part. These are called secondary sexual characters — features that distinguish males from females but are not themselves the reproductive organs. A deep voice does not help reproduction directly; it is a side effect of testosterone acting on the larynx. This is exactly how a hormone behaves: released into the blood, it reaches every organ and produces an effect in each one built to respond.
A misconception to clear. Pimples are caused by the oil glands becoming more active, not by eating oily food or by poor character. They are a normal hormonal effect, they improve as the hormone levels settle, and squeezing them makes scarring more likely rather than less.
Changes in both boys and girls:
- A sudden increase in height — the growth spurt, the fastest growth since infancy
- Sweat and oil glands become more active, so more perspiration, body odour, and often pimples or acne on the face
- Growth of hair in the armpits and the pubic region
- The reproductive organs mature and begin to function
- Increased appetite, and more muscle and bone being built
Changes in boys, from testosterone:
- The voice deepens, as the larynx (voice box) enlarges — the visible lump it produces in the throat is the Adam's apple
- Facial hair appears, on the upper lip and then the chin
- Shoulders and chest broaden, and muscles develop
- Hair may grow on the chest and arms
- The testes begin producing sperm
Changes in girls, from oestrogen:
- Breasts develop
- The hips broaden, as the pelvis widens
- The voice becomes higher and thinner than a boy's
- The ovaries begin releasing ova, so ovulation starts
- Menstruation begins — the first menstruation is called menarche
How the growth spurt differs between them. Girls begin theirs earlier and finish earlier; boys begin later, grow for longer and usually end up taller. Both spurts last a few years and then stop.
The point that ties this to the previous part. These are called secondary sexual characters — features that distinguish males from females but are not themselves the reproductive organs. A deep voice does not help reproduction directly; it is a side effect of testosterone acting on the larynx. This is exactly how a hormone behaves: released into the blood, it reaches every organ and produces an effect in each one built to respond.
A misconception to clear. Pimples are caused by the oil glands becoming more active, not by eating oily food or by poor character. They are a normal hormonal effect, they improve as the hormone levels settle, and squeezing them makes scarring more likely rather than less.
What emotional changes happen, and what actually helps?
The same hormonal changes affect the brain, so the mental and emotional shifts of adolescence are as normal and as biological as the physical ones.
What commonly happens:
- Mood swings — feeling cheerful and then low without an obvious cause, and being more easily irritated
- Self-consciousness about appearance, height, weight, skin and voice
- A strong desire for independence and privacy, and a tendency to question rules and authority
- Increased interest in the opposite sex
- Strong influence of friends, and peer pressure to do what the group is doing
- Anxiety about studies, examinations and the future
- Becoming more aware of others' feelings, and more concerned with fairness and identity
Practical ways to manage stress:
- Talk to someone — a parent, an older sibling, a teacher or another trusted adult. Most of what feels uniquely wrong turns out to be ordinary once said aloud.
- Get regular physical activity — a sport, cycling, or simply walking. Exercise is one of the most reliable ways to lift mood.
- Get enough sleep. Adolescents need more sleep than adults, and most of the irritability blamed on hormones is worsened by getting too little.
- Manage time — break a large task into small parts with short breaks, rather than facing the whole of it at once.
- Keep a hobby — music, drawing, reading, gardening — something with no marks attached.
- Avoid comparing yourself with others. Since everyone's timetable differs, the comparison is not measuring what it seems to.
- Resist peer pressure and say no clearly to tobacco, alcohol and drugs. These are especially harmful during a period of rapid growth, and addiction forms more readily at this age.
- Try deep breathing, yoga or meditation for a few minutes a day.
Why knowing the cause helps in itself. A mood swing understood as a hormonal effect is easier to sit out than one taken as evidence that something is wrong with you. This is genuinely the most useful thing in the chapter — the changes are temporary, they are universal, and they settle as the hormone levels stabilise.
And one thing worth saying to an adult. Asking for help is not a weakness. Persistent low mood, being unable to sleep, or feeling unable to cope are reasons to speak to a parent or a doctor, exactly as a persistent physical symptom would be.
What commonly happens:
- Mood swings — feeling cheerful and then low without an obvious cause, and being more easily irritated
- Self-consciousness about appearance, height, weight, skin and voice
- A strong desire for independence and privacy, and a tendency to question rules and authority
- Increased interest in the opposite sex
- Strong influence of friends, and peer pressure to do what the group is doing
- Anxiety about studies, examinations and the future
- Becoming more aware of others' feelings, and more concerned with fairness and identity
Practical ways to manage stress:
- Talk to someone — a parent, an older sibling, a teacher or another trusted adult. Most of what feels uniquely wrong turns out to be ordinary once said aloud.
- Get regular physical activity — a sport, cycling, or simply walking. Exercise is one of the most reliable ways to lift mood.
- Get enough sleep. Adolescents need more sleep than adults, and most of the irritability blamed on hormones is worsened by getting too little.
- Manage time — break a large task into small parts with short breaks, rather than facing the whole of it at once.
- Keep a hobby — music, drawing, reading, gardening — something with no marks attached.
- Avoid comparing yourself with others. Since everyone's timetable differs, the comparison is not measuring what it seems to.
- Resist peer pressure and say no clearly to tobacco, alcohol and drugs. These are especially harmful during a period of rapid growth, and addiction forms more readily at this age.
- Try deep breathing, yoga or meditation for a few minutes a day.
Why knowing the cause helps in itself. A mood swing understood as a hormonal effect is easier to sit out than one taken as evidence that something is wrong with you. This is genuinely the most useful thing in the chapter — the changes are temporary, they are universal, and they settle as the hormone levels stabilise.
And one thing worth saying to an adult. Asking for help is not a weakness. Persistent low mood, being unable to sleep, or feeling unable to cope are reasons to speak to a parent or a doctor, exactly as a persistent physical symptom would be.
Why do hygiene and diet matter more during adolescence?
Because the body is both growing fastest and producing more sweat and oil than at any other time.
Personal hygiene, and the reason for each habit:
- Bathe daily, and more often in hot weather. The sweat glands are more active, and bacteria acting on sweat cause body odour.
- Wash the face with a mild soap twice a day, since the oil glands are more active. Do not squeeze pimples — it spreads infection and causes scarring.
- Wear clean, washed underclothes every day, and prefer cotton, which absorbs sweat and lets air through.
- Brush teeth twice a day, trim nails short, and wash hands before eating.
- Girls need extra care during menstruation — change the sanitary pad or cloth regularly, wash the hands before and after, keep the area clean and dry, and dispose of used material properly wrapped. A clean cloth must be washed with soap and dried in sunlight before reuse.
- Boys should keep the genital area clean and dry.
A balanced diet, and what each part is for:
- Extra energy for the growth spurt — from cereals such as rice, wheat, millets and bread. An adolescent genuinely needs more food than a younger child.
- Protein to build the new tissue — pulses and dals, milk and curd, eggs, fish, meat, soya, groundnuts.
- Calcium and vitamin D for bones growing rapidly — milk, curd, paneer, ragi, green leafy vegetables, and sunlight for vitamin D.
- Iron for the blood — green leafy vegetables, jaggery, dates, pulses, liver. Girls need more iron than boys after menarche, since blood is lost each month, and iron deficiency causing anaemia is common at this age.
- Vitamins and minerals from fruits and vegetables, in variety rather than in quantity.
- Plenty of water, particularly with more sweating.
What to limit: deep-fried snacks, aerated drinks, sweets and packaged junk food. The objection is not that they are enjoyable but that they are high in energy and low in nutrients — they fill the appetite without supplying the protein, calcium and iron the growing body actually needs.
And do not skip breakfast. A body that has gone the whole night without food and is growing rapidly needs the meal most, and skipping it makes concentration and mood worse for the rest of the morning.
Why deficiencies show up now more than before. A child's requirement is modest and is easily met. An adolescent's requirement rises sharply, so a diet that was adequate at nine may become inadequate at thirteen without anything changing. That is the real reason this topic sits in the chapter — not because adolescents eat badly, but because the same diet is no longer enough.
Personal hygiene, and the reason for each habit:
- Bathe daily, and more often in hot weather. The sweat glands are more active, and bacteria acting on sweat cause body odour.
- Wash the face with a mild soap twice a day, since the oil glands are more active. Do not squeeze pimples — it spreads infection and causes scarring.
- Wear clean, washed underclothes every day, and prefer cotton, which absorbs sweat and lets air through.
- Brush teeth twice a day, trim nails short, and wash hands before eating.
- Girls need extra care during menstruation — change the sanitary pad or cloth regularly, wash the hands before and after, keep the area clean and dry, and dispose of used material properly wrapped. A clean cloth must be washed with soap and dried in sunlight before reuse.
- Boys should keep the genital area clean and dry.
A balanced diet, and what each part is for:
- Extra energy for the growth spurt — from cereals such as rice, wheat, millets and bread. An adolescent genuinely needs more food than a younger child.
- Protein to build the new tissue — pulses and dals, milk and curd, eggs, fish, meat, soya, groundnuts.
- Calcium and vitamin D for bones growing rapidly — milk, curd, paneer, ragi, green leafy vegetables, and sunlight for vitamin D.
- Iron for the blood — green leafy vegetables, jaggery, dates, pulses, liver. Girls need more iron than boys after menarche, since blood is lost each month, and iron deficiency causing anaemia is common at this age.
- Vitamins and minerals from fruits and vegetables, in variety rather than in quantity.
- Plenty of water, particularly with more sweating.
What to limit: deep-fried snacks, aerated drinks, sweets and packaged junk food. The objection is not that they are enjoyable but that they are high in energy and low in nutrients — they fill the appetite without supplying the protein, calcium and iron the growing body actually needs.
And do not skip breakfast. A body that has gone the whole night without food and is growing rapidly needs the meal most, and skipping it makes concentration and mood worse for the rest of the morning.
Why deficiencies show up now more than before. A child's requirement is modest and is easily met. An adolescent's requirement rises sharply, so a diet that was adequate at nine may become inadequate at thirteen without anything changing. That is the real reason this topic sits in the chapter — not because adolescents eat badly, but because the same diet is no longer enough.
Exam tip
Exam tip: attribute each change to the right hormone
Every physical change should be traced to its hormone: testosterone from the testes in boys, oestrogen from the ovaries in girls, both on instructions from the pituitary.
Sort the changes into three groups when answering — those in both, those in boys only, those in girls only. Listing them all together loses the structure the marks are given for.
Call them secondary sexual characters, and define the term as features that distinguish the sexes without being the reproductive organs themselves.
Keep puberty and adolescence distinct: puberty is the biological maturing of the reproductive organs; adolescence is the whole transition including emotional and social changes.
Give the ages as approximations: adolescence about 11 to 19 years, puberty in girls about 10 to 12, in boys about 12 to 14 — and say that girls generally begin earlier.
Name the larynx and the Adam's apple for the deepening voice, and menarche for the first menstruation.
For diet, pair each nutrient with its purpose — protein for tissue, calcium for bones, iron for blood, with girls needing more after menarche.
And for stress, give practical measures — talking to a trusted adult, exercise, sleep, time management, saying no to peer pressure — not general advice to stay positive.
Sort the changes into three groups when answering — those in both, those in boys only, those in girls only. Listing them all together loses the structure the marks are given for.
Call them secondary sexual characters, and define the term as features that distinguish the sexes without being the reproductive organs themselves.
Keep puberty and adolescence distinct: puberty is the biological maturing of the reproductive organs; adolescence is the whole transition including emotional and social changes.
Give the ages as approximations: adolescence about 11 to 19 years, puberty in girls about 10 to 12, in boys about 12 to 14 — and say that girls generally begin earlier.
Name the larynx and the Adam's apple for the deepening voice, and menarche for the first menstruation.
For diet, pair each nutrient with its purpose — protein for tissue, calcium for bones, iron for blood, with girls needing more after menarche.
And for stress, give practical measures — talking to a trusted adult, exercise, sleep, time management, saying no to peer pressure — not general advice to stay positive.
Did you know
Why does a boy's voice crack instead of simply getting deeper?
A deepening voice does not arrive smoothly. For a while it jumps unpredictably, dropping mid-sentence and then squeaking back up.
The cause is a mismatch in timing. Testosterone makes the larynx grow larger and the vocal cords longer and thicker, and longer, thicker cords vibrate more slowly — which means a lower frequency and therefore a lower pitch, exactly as a longer string on a sitar sounds lower.
But the muscles that tension those cords are controlled by fine adjustments the brain learned years earlier on a smaller instrument. While the instrument is changing size week by week, the old settings no longer produce the expected note, and the pitch overshoots in both directions until the control is relearned.
So the cracking is not a fault in the voice box. It is a temporary calibration problem, and it ends once the larynx stops growing and the adjustments catch up — which is also why it passes without any treatment at all.
The cause is a mismatch in timing. Testosterone makes the larynx grow larger and the vocal cords longer and thicker, and longer, thicker cords vibrate more slowly — which means a lower frequency and therefore a lower pitch, exactly as a longer string on a sitar sounds lower.
But the muscles that tension those cords are controlled by fine adjustments the brain learned years earlier on a smaller instrument. While the instrument is changing size week by week, the old settings no longer produce the expected note, and the pitch overshoots in both directions until the control is relearned.
So the cracking is not a fault in the voice box. It is a temporary calibration problem, and it ends once the larynx stops growing and the adjustments catch up — which is also why it passes without any treatment at all.
Key takeaways
Adolescence and puberty: quick revision
- Puberty is the biological stage at which the reproductive organs mature. Adolescence is the whole transition from childhood to adulthood, including emotional and social changes.
- Approximate ages: adolescence about 11 to 19 years; puberty in girls about 10 to 12, in boys about 12 to 14 — so girls generally begin earlier. All of these vary between individuals, and being early or late is normal.
- The pituitary instructs the gonads; testosterone from the testes and oestrogen from the ovaries cause the changes.
- In both: the growth spurt, more active sweat and oil glands (body odour and pimples), armpit and pubic hair, and maturing reproductive organs.
- In boys: voice deepens as the larynx enlarges (the Adam's apple), facial hair, broader shoulders and chest, more muscle, and sperm production begins.
- In girls: breasts develop, hips broaden, the voice becomes higher, ovulation starts, and menstruation begins at menarche.
- These are secondary sexual characters — features distinguishing the sexes without being the reproductive organs.
- Pimples come from more active oil glands, not from oily food, and should not be squeezed.
- Emotional changes: mood swings, self-consciousness, a desire for independence and privacy, interest in the opposite sex, peer pressure, and anxiety about studies.
- Managing stress: talk to a trusted adult, exercise, enough sleep, time management, a hobby, avoiding comparison, refusing tobacco, alcohol and drugs, and breathing exercises or yoga.
- Hygiene: daily bath, wash the face twice daily, clean cotton underclothes daily, brush teeth, trim nails; extra care during menstruation with regular changing, clean hands and proper disposal.
- Diet: extra energy for the growth spurt, protein for tissue, calcium and vitamin D for bones, iron for blood — girls needing more after menarche, since anaemia is common — plus fruits, vegetables and water.
- Limit fried snacks, sweets, aerated drinks and junk food, which are high in energy and low in nutrients, and do not skip breakfast.
- Requirements rise sharply at this age, so a diet adequate earlier may no longer be enough.
Test yourself by sorting the physical changes into the three groups and naming the hormone for each, then listing one purpose for each nutrient — those two answers cover most of what this chapter asks.
- Approximate ages: adolescence about 11 to 19 years; puberty in girls about 10 to 12, in boys about 12 to 14 — so girls generally begin earlier. All of these vary between individuals, and being early or late is normal.
- The pituitary instructs the gonads; testosterone from the testes and oestrogen from the ovaries cause the changes.
- In both: the growth spurt, more active sweat and oil glands (body odour and pimples), armpit and pubic hair, and maturing reproductive organs.
- In boys: voice deepens as the larynx enlarges (the Adam's apple), facial hair, broader shoulders and chest, more muscle, and sperm production begins.
- In girls: breasts develop, hips broaden, the voice becomes higher, ovulation starts, and menstruation begins at menarche.
- These are secondary sexual characters — features distinguishing the sexes without being the reproductive organs.
- Pimples come from more active oil glands, not from oily food, and should not be squeezed.
- Emotional changes: mood swings, self-consciousness, a desire for independence and privacy, interest in the opposite sex, peer pressure, and anxiety about studies.
- Managing stress: talk to a trusted adult, exercise, enough sleep, time management, a hobby, avoiding comparison, refusing tobacco, alcohol and drugs, and breathing exercises or yoga.
- Hygiene: daily bath, wash the face twice daily, clean cotton underclothes daily, brush teeth, trim nails; extra care during menstruation with regular changing, clean hands and proper disposal.
- Diet: extra energy for the growth spurt, protein for tissue, calcium and vitamin D for bones, iron for blood — girls needing more after menarche, since anaemia is common — plus fruits, vegetables and water.
- Limit fried snacks, sweets, aerated drinks and junk food, which are high in energy and low in nutrients, and do not skip breakfast.
- Requirements rise sharply at this age, so a diet adequate earlier may no longer be enough.
Test yourself by sorting the physical changes into the three groups and naming the hormone for each, then listing one purpose for each nutrient — those two answers cover most of what this chapter asks.