Sexual reproduction in animals, and adolescence
In animals, an egg and a sperm fuse to form a zygote. Humans become able to reproduce at puberty, during adolescence, when the body and mind change.
Sexual reproduction in animals
| Term | Meaning | Examples |
|---|---|---|
| Fertilisation | An egg (from the female) and a sperm (from the male) fuse to form a zygote, which grows into an embryo and then a full individual | All animals that reproduce sexually |
| Oviparous | Lay eggs; the young develop partly or fully inside the egg | Fish, frogs, reptiles, birds |
| Viviparous | The young develop inside the mother's womb and are born alive | Cats, dogs, cows, humans (mammals) |
| Hermaphrodite | Both male and female sex organs in one individual | Earthworm, tapeworm |
| Separate sexes | Males have testes; females have ovaries | Most animals, including humans |
Changes at puberty
| Kind of change | What happens |
|---|---|
| Physical (body) | A growth spurt in height and weight. The sex organs mature. Hair grows in the armpits and the pubic area. Breasts develop in girls; facial hair grows and the voice deepens in boys. |
| Physiological (body working) | Sweat and oil glands become more active, so pimples and body odour can appear. Appetite increases. Growth hormone and sex hormones rise: testosterone in boys, oestrogen and progesterone in girls. Girls begin to menstruate. |
| Psychological (mind) | Ability to think in abstract ways; mood swings; feeling self-conscious. Self-image, identity and friendships matter a lot. Interest in others grows, and peer pressure can cause stress. |
Growing up healthy
The timing and pace of these changes differ from person to person, so friends of the same age may look different. Each person is unique, and every pattern is normal. Bathe daily and wash all parts of the body, including the genitals. To reduce pimples, wash your face with soap and water, drink plenty of water, eat fruits and vegetables, and do not pick them. A boy may release semen in his sleep (a wet dream); this is normal. Healthy relationships rest on equality, respect and care. Decisions about sex need correct information, an understanding of the results, and a sense of responsibility. Adolescents may be able to reproduce, but they are not yet ready for parenthood. Your NIOS lesson gives the legal minimum age of marriage in India as 18 for women and 21 for men; laws can change, so check the current rule. Pregnancy in the teenage years is risky for both the young mother and her baby.
Menstruation and menstrual hygiene
- In girls, the reproductive years begin at puberty and last till about 45 to 50 years of age.
- From puberty, one ovum (egg) matures and is released by one of the two ovaries about once every 28 to 30 days.
- Each month the lining of the uterus thickens to receive a fertilised egg. If there is no fertilisation, the lining and its blood vessels are shed as bleeding. This is menstruation, or a period.
- The first period is the menarche. The stopping of periods, usually between 45 and 55 years, is the menopause.
- Cycles of 21 to 35 days are common, and a period usually lasts about 4 days. Early cycles may be irregular or painful; see a doctor if the problem continues.
- Menstruation is a normal body process, not an illness. A girl can do her usual activities, and there is no scientific reason to keep her away from the kitchen or other places.
- Use a sanitary pad or a clean cloth and change it every 6 to 8 hours. Wash a reusable cloth with mild detergent and dry it in the sun.
Worked example: estimating adult height from a growth chart
A girl aged 11 is 140 cm tall. Growth charts show that at 11, girls have reached about 88% of their full height. Estimate her adult height.
- Adult height ≈ present height ÷ (% of full height reached) × 100
- = 140 cm ÷ 88 × 100
- = 1.591 cm × 100 = 159.1 cm
About 159 cm. This is only an estimate, because each person grows at their own pace.
Teenagers need more food because they grow fast, so dal, milk, eggs, green vegetables and fruit matter in these years. For questions about body changes, a parent, a teacher, a health worker or a doctor is the right person to ask.
2 marks: “Sort these animals into oviparous and viviparous, and say what each word means: frog, cat, sparrow, cow.” Oviparous (lay eggs; the young develop inside the egg): frog, sparrow. Viviparous (the young develop in the mother's womb and are born alive): cat, cow.
3 marks: “List the physical, physiological and psychological changes at puberty, one or two of each.” Physical: growth spurt; hair in the armpits and pubic area; breasts in girls; facial hair and a deeper voice in boys. Physiological: more active sweat and oil glands; more appetite; sex hormones rise; periods begin in girls. Psychological: mood swings; self-consciousness; friendships matter a lot.
2 marks: “What is menstruation? Explain why a pregnant woman has no periods.” The monthly shedding of the thickened uterus lining and its blood vessels when the egg is not fertilised. In pregnancy the egg has been fertilised and the growing embryo is attached to the thick lining, so the lining is not shed.
Ovi means egg and vivi means alive: oviparous animals lay eggs; viviparous animals give birth to live young.