How do Organisms Reproduce?

The One Thing

Reproduction produces new individuals and ensures the continuity of a species. It may occur asexually, producing genetically similar offspring from one parent, or sexually, through the formation and fusion of gametes, producing variation.

Reactions, Processes and Experiments

Key Terms

• Reproduction: The biological process by which organisms produce new individuals of their own kind.
• Asexual reproduction: Reproduction involving a single parent, without the fusion of gametes.
• Binary fission: Asexual reproduction in which one unicellular organism divides into two individuals, as in Amoeba.
• Multiple fission: Asexual reproduction in which one parent cell divides to form many daughter cells, as in Plasmodium.
• Budding: A new individual develops as a small outgrowth, or bud, on the parent, as in Hydra and yeast.
• Fragmentation: The parent body breaks into pieces, and each piece develops into a complete organism, as in Spirogyra.
• Regeneration: The ability of some organisms to regrow lost body parts; in organisms such as Planaria, body fragments may form complete individuals.
• Vegetative propagation: Asexual reproduction in plants using roots, stems, or leaves instead of seeds.
• Spore formation: Production of microscopic spores that can grow into new organisms under suitable conditions, as in Rhizopus.
• Sexual reproduction: Reproduction involving two parents or two types of gametes and the fusion of male and female gametes.
• Variation: Differences among individuals of the same species, which may help populations adapt and evolve.
• Gamete: A reproductive cell; the sperm is the male gamete and the ovum is the female gamete in humans.
• Pollination: The transfer of pollen grains from the anther to the stigma of a flower.
• Fertilisation: The fusion of male and female gametes to form a zygote.
• Zygote: The first cell formed after fertilisation, which develops into an embryo.
• Male reproductive system: The system consisting mainly of testes, scrotum, sperm ducts, accessory glands, urethra, and penis, which produces and transports sperm.
• Testes: Male reproductive organs that produce sperm and the hormone testosterone.
• Female reproductive system: The system consisting mainly of ovaries, oviducts, uterus, cervix, and vagina, which produces ova and supports development of the embryo.
• Ovaries: Female reproductive organs that produce ova and hormones such as oestrogen and progesterone.
• Menstruation: The periodic shedding of the thickened uterine lining when fertilisation does not occur.
• Ovulation: The release of a mature ovum from an ovary, usually once during each menstrual cycle.
• Implantation: The attachment of the embryo to the lining of the uterus.
• Placenta: A temporary organ that allows exchange of nutrients, oxygen, and wastes between the mother and developing embryo without direct mixing of their blood.
• Puberty: The stage when hormonal changes cause reproductive organs to mature and secondary sexual characteristics to appear.
• Contraception: Methods used to prevent pregnancy, including condoms, oral pills, intrauterine devices, surgical methods, and natural methods.
• Sexually transmitted infection: An infection spread mainly through sexual contact; examples include gonorrhoea, syphilis, HIV infection, and AIDS.
• Reproductive health: A state of physical, mental, and social well-being related to the reproductive system and its functions.

Easily Confused

• Asexual reproduction vs sexual reproduction: Asexual reproduction involves one parent and no fusion of gametes, whereas sexual reproduction involves male and female gametes and their fusion.
• Binary fission vs multiple fission: Binary fission produces two individuals from one parent cell, whereas multiple fission produces many daughter cells.
• Fragmentation vs regeneration: Fragmentation begins when the parent body breaks into pieces, whereas regeneration refers to regrowth of lost parts; in Planaria, fragments may also form complete individuals.
• Pollination vs fertilisation: Pollination is the transfer of pollen from anther to stigma, whereas fertilisation is the fusion of male and female gametes.
• Ovulation vs menstruation: Ovulation is the release of a mature ovum from an ovary, whereas menstruation is the shedding of the uterine lining when fertilisation does not occur.
• Fertilisation vs implantation: Fertilisation forms a zygote, normally in the oviduct, whereas implantation is the attachment of the embryo to the uterine lining.
• Contraception vs prevention of sexually transmitted infections: Contraception primarily prevents pregnancy; condoms also reduce the risk of many sexually transmitted infections.
• HIV transmission vs casual contact: HIV can spread through infected blood, contaminated needles, unprotected sexual contact, and transmission from an infected mother to child, but not through ordinary casual contact.
• Testes vs ovaries: Testes produce sperm and testosterone, whereas ovaries produce ova and hormones such as oestrogen and progesterone.
• Zygote vs embryo: The zygote is the first cell formed after fertilisation; it develops into the embryo.

What Gets Asked

• Compare asexual and sexual reproduction: Marks depend on stating that asexual reproduction uses one parent and no gamete fusion, while sexual reproduction involves gametes, fertilisation, and genetic variation.
• Identify examples of asexual reproduction: Questions may require matching Amoeba with binary fission, Plasmodium with multiple fission, Hydra and yeast with budding, Spirogyra with fragmentation, Planaria with regeneration, and Rhizopus with spore formation.
• Explain vegetative propagation: Specific examples may be required: potato by stem tuber, Bryophyllum by leaf buds, and rose or sugarcane by stem cuttings.
• Describe flowering-plant reproduction: The required sequence is pollen production in the anther, ovules inside the ovary, pollination, fertilisation, development of the ovule into a seed, and development of the ovary into a fruit.
• Describe human fertilisation and pregnancy: Questions may test that fertilisation normally occurs in the oviduct, implantation occurs in the uterus, and the placenta exchanges nutrients, oxygen, and wastes without direct mixing of maternal and embryonic blood.
• Explain contraception and infection prevention: Answers should distinguish methods that prevent pregnancy from the additional infection-reducing role of condoms, and should include the stated HIV transmission routes rather than claiming transmission through ordinary casual contact.