Class 10 Science Notes Chapter 7 — How do Organisms Reproduce?#
Why reproduce#
- Reproduction is not needed to keep one individual alive, but it keeps the species going.
- The basic event is DNA copying, along with making extra cell apparatus. DNA holds the information for making proteins, and so for the body design.
- Copying is never perfect, so it produces small variations. Variations help some individuals survive when the environment changes (for example, heat-resistant bacteria when water gets warmer). Variation is useful for the survival of the species, not necessarily for the individual.
Asexual reproduction#
A new organism comes from a single parent. The offspring are almost identical to the parent.
| Mode | How | Examples |
|---|---|---|
| Binary fission | Cell divides into two equal halves | Amoeba (any plane), Leishmania (definite plane, because of its whip-like structure) |
| Multiple fission | Cell divides into many daughter cells at once | Plasmodium (malaria parasite) |
| Fragmentation | Body breaks into pieces; each piece grows into a new organism | Spirogyra |
| Regeneration | A body part grows into a whole organism, using specialised cells | Hydra, Planaria |
| Budding | A bud grows out of the parent and later detaches | Hydra, yeast |
| Spore formation | Spores in sporangia are released and grow into new organisms; thick walls protect them | Rhizopus (bread mould) |
| Vegetative propagation | New plants grow from roots, stems or leaves | Buds on the leaf margin of Bryophyllum; layering and grafting in sugarcane, rose, grape |
- Advantages of vegetative propagation: plants that do not make viable seeds (banana, orange, rose, jasmine) can be grown; plants bear flowers and fruit earlier; all plants are genetically like the parent.
- Tissue culture: cells from the growing tip of a plant are grown in a nutrient medium to form a mass of cells (callus), which is moved to a hormone medium to form plantlets. Used for ornamental plants.
- Regeneration is not the same as reproduction: most organisms cannot be cut into pieces to make new ones.
Sexual reproduction#
- Two parents each give a germ cell (gamete). Germ cells have half the DNA of body cells, so the zygote has the normal amount.
- It creates more variation, because it combines DNA from two individuals.
- Usually one germ cell is large with food stored (female, egg) and the other small and motile (male, sperm).
In flowering plants#
- The flower is the reproductive part. Stamen is the male part (anther makes pollen grains); pistil is the female part (stigma, style, ovary with ovules).
- A flower is unisexual if it has only stamens or only a pistil (papaya, watermelon), and bisexual if it has both (hibiscus, mustard).
- Pollination — transfer of pollen from the anther to the stigma. Self-pollination is within the same flower; cross-pollination is to another flower, by wind, water or animals.
- Fertilisation: a pollen tube grows down the style to the ovule. The male germ cell fuses with the female germ cell (egg) in the ovule to form the zygote.
- After fertilisation, the zygote forms the embryo, the ovule becomes the seed (with a seed coat), and the ovary becomes the fruit. The petals, sepals, stamens, style and stigma may fall off.
- Germination: the seed has a future shoot (plumule), a future root (radicle) and stored food (cotyledons).
In human beings#
- Puberty — the period when the reproductive tissues mature, as general body growth slows. Common changes: thick hair in the armpits and genital area, oily skin, pimples. Girls: breasts develop, menstruation begins. Boys: facial hair, voice cracks.
| Male system | Function |
|---|---|
| Testes (in the scrotum, outside the body, because sperm need a lower temperature) | Make sperms and testosterone |
| Vas deferens | Carries sperms; joins the tube from the bladder |
| Urethra | Common passage for sperms and urine |
| Seminal vesicles, prostate gland | Add fluid that helps sperms move and nourishes them |
| Female system | Function |
|---|---|
| Ovaries | Make eggs (one egg a month from one ovary) and hormones |
| Oviduct (fallopian tube) | Carries the egg to the uterus; fertilisation happens here |
| Uterus | Where the embryo grows; opens into the vagina through the cervix |
| Vagina | Receives sperms |
- The zygote divides to form an embryo, which is implanted in the thick lining of the uterus.
- Placenta — a disc in the uterine wall with villi on the embryo’s side. It passes glucose and oxygen from the mother to the embryo and removes the embryo’s wastes.
- Pregnancy lasts about nine months. The baby is born by rhythmic contractions of the uterus muscles.
- Menstruation: if the egg is not fertilised (it lives about one day), the thick uterine lining breaks down and leaves as blood and mucus. This happens roughly every month and lasts 2–8 days.
Reproductive health#
- Sexually transmitted diseases (STDs): bacterial — gonorrhoea, syphilis; viral — warts, HIV-AIDS. A condom helps prevent their spread.
| Contraceptive method | How it works |
|---|---|
| Mechanical barrier (condoms) | Stops sperm reaching the egg; also protects against STDs |
| Hormonal (oral pills) | Changes hormone balance so eggs are not released; may have side effects |
| Devices in the uterus (loop, copper-T) | Prevent pregnancy; may cause side effects. Do not protect against STDs |
| Surgical (blocking the vas deferens or fallopian tube) | Permanent; surgery can cause infection if not done properly |
- Female foeticide — illegal sex-selective abortion of female foetuses has caused the child sex ratio to fall. Prenatal sex determination is banned by law.
Common mistakes#
- Saying fertilisation in humans happens in the uterus. It happens in the oviduct (fallopian tube).
- Mixing up pollination (pollen to stigma) and fertilisation (fusion of male and female germ cells).
- Saying the ovule becomes the fruit. The ovule becomes the seed; the ovary becomes the fruit.
- Saying a copper-T protects against STDs. It does not.