Reproduction: Class 10 Biology

Key points

Reproduction is the production of new individuals of the same species. It keeps the species alive from generation to generation. There are two types: asexual reproduction, which needs only one parent, and sexual reproduction, which usually needs two parents and the fusion of gametes.

Asexual reproduction gives offspring that are genetically identical to the parent. Common methods are:

• Binary fission: a cell divides into two equal parts, as in Amoeba and bacteria.
• Budding: a small outgrowth forms on the parent and separates, as in yeast and Hydra.
• Spore formation: spores grow into new individuals, as in Rhizopus and ferns.
• Fragmentation and regeneration: pieces of the parent grow into whole organisms, as in Planaria.
• Vegetative propagation in plants: new plants form from roots, stems or leaves, as in potato tubers, onion bulbs, runners of strawberry and cuttings of rose.

Sexual reproduction involves male and female gametes, which fuse in fertilisation to form a zygote. The offspring show variation because they receive genes from both parents. Gametes are made by meiosis, so they carry half the usual number of chromosomes (haploid), and fertilisation restores the full number (diploid).

A flower is the reproductive organ of a flowering plant. Its parts are the sepals (protect the bud), petals (attract insects), stamens (male part) and carpels (female part). A stamen has an anther, which makes pollen grains, on a filament. A carpel has a stigma that receives pollen, a style, and an ovary that holds ovules.

Pollination is the transfer of pollen from an anther to a stigma. Self pollination happens within the same flower or plant, and cross pollination happens between different plants. Agents of cross pollination include insects, wind, water and birds. Insect pollinated flowers have large coloured petals, scent and nectar. Wind pollinated flowers have small petals and light pollen in large amounts, with feathery stigmas.

After pollination the pollen grain grows a pollen tube down the style to the ovule. The male nucleus fuses with the egg cell in the ovule, which is fertilisation. After fertilisation the ovule becomes a seed and the ovary becomes the fruit.

The male reproductive system of humans consists of:

• Testes: two organs inside the scrotum that make sperms and the hormone testosterone. The scrotum keeps them slightly below body temperature.
• Epididymis: stores sperms while they mature.
• Sperm duct (vas deferens): carries sperms towards the urethra.
• Glands (seminal vesicles and prostate): add fluid that nourishes the sperms. Sperms and this fluid together form semen.
• Urethra and penis: pass semen and also urine out of the body.

The female reproductive system consists of:

• Ovaries: two organs that make egg cells (ova) and the hormones oestrogen and progesterone.
• Oviducts (fallopian tubes): carry the egg to the uterus, and fertilisation normally occurs here.
• Uterus (womb): a muscular organ where the embryo grows.
• Cervix and vagina: the cervix is the neck of the uterus, and the vagina receives sperms and forms the birth canal.

The menstrual cycle lasts about 28 days. An egg is released from an ovary around the middle of the cycle, which is called ovulation. The lining of the uterus thickens in preparation for a possible embryo. If the egg is not fertilised, the lining breaks down and leaves the body through the vagina as menstruation, which lasts about 3 to 5 days. The cycle is controlled by hormones from the pituitary and the ovaries.

If a sperm fertilises the egg, the zygote divides repeatedly and the embryo sinks into the uterus wall, which is implantation. The placenta forms and joins the embryo to the mother. Through it the foetus receives oxygen and nutrients and gets rid of carbon dioxide and urea. Pregnancy lasts about nine months, and birth is by strong contractions of the uterus.

Sexually transmitted diseases (STDs) are spread by sexual contact. Examples are gonorrhoea, syphilis and AIDS, which is caused by the human immunodeficiency virus (HIV). HIV also spreads through infected blood and needles, and from mother to child. These diseases are prevented by avoiding risky contact, by using safe, sterilised needles and by screening blood.

Study tip: Draw and label the male and female reproductive systems and a flower, since these appear often. Remember the order of events: gametes, fertilisation, zygote, embryo, foetus, and write the function next to every labelled part.

Definitions and flashcards

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Short questions with model answers

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Common mistakes

• Confusing pollination with fertilisation. Pollination is transfer of pollen, fertilisation is fusion of gametes.
• Saying that fertilisation takes place in the uterus. It normally takes place in the oviduct.
• Writing that the ovary produces sperms. Sperms are made in the testes.
• Writing that mother and foetus blood mix in the placenta. Only materials are exchanged.
• Thinking that asexual reproduction gives variation. Offspring are identical clones.

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