Adolescence is the period of transition from childhood to adulthood. In childhood the sex organs remain normal. In adolescence, there is development of primary and secondary sex characteristics and the child becomes an adult

The fusion of Male and Female gametes (sperm, ovum) is called fertilization. In human, fertilization occurs in fallopian tubes of women's ovary.

The changes are
1. Change in body size
2. Change in body ProPortion
3. Development of primary sex characteristics
4. Development of secondary sex characteristics

Adolescence is the period of reproductive maturity which lies usually between the ages of 11 to 19 Years.

To keep the body healthy, every adolescent needs to have a healthy diet, exercise and hygiene.
Healthy diet:
Adolescence is a stage of rapid growth and development. Hence a diet with proper calories and other nutrients is needed for proper growth and physical activity. A balanced diet is required which includes proteins, carbohydrates, fats and vitamins in requisite proportions. Apart from this, the body needs calcium, phosphorus, iodine and iron because there is an increase in skeletal mass and blood volume during adolescence.
Physical exercise:
Walking, playing outdoor games and doing exercise keep the body fit and healthy. It leads to mental peace.
Hygiene: 
i) Have a bathrobe or twice a day, paying special attention to underarms, groins and genitals.
ii) Change the underwear daily. It should be made of cotton and washed and cleaned everyday.
iii) For adolescents, the increased activity of sweat glands sometimes enhances body odor. If cleanliness is not maintained there are chances of having fungal, bacterial and other infections.

Personal hygiene habits:
(i) Shower or bath daily.
(ii) Always wash your hands before and after meals.
(iii) Keep fingernails clean and dipped.
(iv) Wash your teeth and mouth before and after each meal.
(v) Avoid touching your face, nose or mouth while preparing food.
(vi) Avoid coughing or sneezing around food and close your mouth by using hand kerchief while you cough in public places.
(vii) If you are to taste the food, use a clean spoon.
(viii) Change your clothes regularly and wash them cleanly especially undergarments.
(ix) Don't defecate in open field. Use clean toilets for defecation.
(x) If you are not well, avoid self-medication consult a doctor.

(i) Adolescence is a stage of rapid growth and development
(ii) A diet with proper calories and other nutrients is needed for proper growth and physical activity
(iii) Balanced diet includes proteins, carbohydrates, fats and vitamins in requisite proportions.
(iv) Our Indian Meal of roti/rice, dal, milk, fruits and vegetables forms a balanced food.
(v) The nutritional deficiencies during this period not only retard the physical growth, but also impair the intellectual development and may also delay sexual maturation.
Minerals
(i)  Adolescents need minerals in their diet.
(ii) Since there is an increase in skeletal mass and blood volume during adolescence, the body needs calcium, phosphorus and iron.
(i) Calcium:-
Calcium intake needs to be increased to prevent osteoporosis in later life.
It is present in milk and
(ii) Iodine:-
It helps to prevent thyroid gland related diseases
(iii) Iron:-
(i) Iron builds blood and iron-rich food such as green leaf vegetables, |aggery, meat, dates, fish, chicken, citrus, Indian goose berry and whole pulse are good for adolescents.
(ii) Lack of iron in the diet results in anemia
(iii) To make up for the loss, adolescents need to have a diet rich in iron.
(iv) In boys, iron deficiency occurs due to muscle spurt whereas in girls it occurs due to menstruation in addition to the muscular growth.

(i) When a girl reaches puberty at the age of about 10, the sex hormones released into her blood cause some of the ova in her ovaries to become mature.
(ii) Usually one mature ovum is released from one of the ovaries into the oviduct once in every 28 days. This is called ovulation
(iii) Before ovulation the inner wall of uterus becomes thick and spongy, and full of tiny blood vessels. It prepares itself to receive the fertilized ovum.
(iv) If the ovum does not get fertilized then the thick and soft inner lining of uterus is no longer needed and hence it breaks. So the thick and soft inner lining of uterus along with the blood vessels and the dead ovum comes out of the vagina in the forrn of a bleeding called menstruation.
(v) Menstruation usually occurs 14 days after ovulation and usually lasts for about 3 to 5 days.
(vi) After menstruation is over, the inner lining of the uterus starts building up again so that it may become ready to receive the next ovum.
(vii) If the ovum does not get fertilized even now then menstruation takes place again. This cycle of menstruation is repeated again and again in women after every 28 days. The menstrual cycle is controlled by hormones.

(i) The primary hormones that regulate reproduction are the steroids such as androgens, estrogens and progesterone which have masculinizing, feminizing and gestational effects respectively.
(ii) These hormones are secreted from the gonads which are regulated by the anterior pituitary.
(iii) In male and female, reproductive behaviour and reproduction are mainly under the control of LH and FSH.
(iv) LH stimulates the testes to produce androgens, the male sex hormone
(v) Sperms are then actively produced.
(vi) In man sperm production starts at sexual puberty a4d may continue throughout his life.
Follicle stimulating Hormone (FSH)
(i) FSH in the female influences the development of the Graafian follicle and secretion of estrogens.
(ii) In the male it is necessary for the development of seminiferous tubules, and for spermatogenesis
Luteinizing Hormone (LH)
(i) In the female, it is the hormone necessary for ovulation, and the secretion of the luteal hormone Progesterone, and for the final maturation of the Graafian follicle
(ii) In the male it stimulates the interstitial (Leydig) cells of testes and the secretion of testosterone, and is referred to as the Interstitial cell stimulating Hormone (ICSH).
Prolactin (PRL) or Lactogenic Hormone:-
The main function of this{ormone is milk secretion during lactation.
Oxytocin Hormone:
Oxytocin causes expulsion of milk from the breast and it is also involved in the contraction of some muscles of uterus during child birth

During adolescence, there will be
(i) Changes in body size
(ii) Changes in body proportion
(iii) Development of primary sex characteristics
(iv) Development of secondary sex characteristics
Secondary sex characteristics are the physical feature which distinguishes male from female.