Chapter 3: How do Organisms Reproduce?

Solution

Amoeba reproduces asexually by binary fission. The parent cell divides into two equal daughter cells after DNA replication. The division can occur in any plane since Amoeba has no fixed shape. Each daughter cell grows into a mature Amoeba. Budding is seen in Hydra/yeast, fragmentation in Spirogyra, and spore formation in Rhizopus.

Solution

Hydra reproduces asexually by budding. A small outgrowth (bud) develops on the body of the parent. The bud grows, develops tentacles and other structures, and eventually detaches from the parent to become a new independent organism. Amoeba uses binary fission, Planaria uses regeneration, and Spirogyra uses fragmentation.

Solution

Bryophyllum has buds on its leaf margins that can grow into new plantlets when they fall on moist soil. This is vegetative propagation — a form of asexual reproduction where new plants grow from vegetative parts (leaves, stems, roots) of the parent plant without involving seeds or gametes. The offspring are genetically identical to the parent.

Solution

The stamen is the male reproductive part of a flower. It consists of a filament (stalk) and an anther (which produces pollen grains containing male gametes). The pistil (carpel) is the female reproductive part. Sepals protect the bud, and petals attract pollinators.

Solution

In cross-pollination, pollen is transferred from the anther of one plant to the stigma of a different plant of the same species. This means the male and female gametes come from genetically different individuals. When these gametes fuse during fertilisation, the offspring inherits a new combination of genes from two different parents, leading to greater genetic variation. In self-pollination, both gametes come from the same plant, so there is very little new genetic variation.

Solution

After fertilisation, the ovule develops into a seed. The seed contains the embryo (future plant), stored food (endosperm), and a protective seed coat (testa). The ovary (which contains the ovules) develops into the fruit. So: ovule → seed, ovary → fruit. A pollen grain is the male structure, and the embryo is the developing plant inside the seed.

Solution

Fertilisation (fusion of sperm and egg) typically occurs in the fallopian tube (oviduct). The egg released from the ovary during ovulation travels into the fallopian tube. If sperm is present, fertilisation occurs here. The resulting zygote then travels down to the uterus, where it implants in the uterine lining and develops further.

Solution

Menstruation occurs when the released egg is not fertilised. Each month, the uterine lining (endometrium) thickens with blood vessels in preparation for implantation of a fertilised egg. If fertilisation does not occur, the levels of hormones (oestrogen and progesterone) drop, and the thickened uterine lining breaks down and is shed along with blood and the unfertilised egg through the vagina. This lasts about 3–5 days and the cycle repeats approximately every 28 days.

Solution

Condoms are the only common contraceptive method that also provides protection against sexually transmitted infections (STIs) such as HIV/AIDS, gonorrhoea, and syphilis. They act as a physical barrier, preventing the exchange of body fluids between partners. Oral pills prevent ovulation but do not block infections. Copper-T prevents implantation but not infections. Tubectomy is a surgical method that has no effect on STI transmission.

Solution

The testes are located outside the body cavity in the scrotum because sperm production (spermatogenesis) requires a temperature 2–3°C lower than normal body temperature (37°C). The scrotum provides this cooler environment. If the testes are retained inside the body (a condition called cryptorchidism), sperm production is significantly impaired, potentially leading to infertility.

Solution

Tissue culture (micropropagation) involves growing plant cells or tissues in a sterile nutrient medium to produce new plants. Its advantages include: (1) Rapid production of large numbers of plants from a small amount of tissue. (2) All plants are genetically identical (clones) — useful for preserving desirable traits. (3) Plants can be made disease-free by using meristem (growing tip) tissue, which is usually free of viruses. (4) Useful for plants that are difficult to grow from seeds.

Solution

Rhizopus (bread mould) reproduces asexually by spore formation. It produces tiny spores inside a sac-like structure called a sporangium (the black dots you see on mouldy bread). When the sporangium bursts, hundreds of spores are released into the air. Under favourable conditions (moisture, warmth), each spore germinates into a new individual. Amoeba uses binary fission, Hydra uses budding, and Planaria uses regeneration.

Solution

In asexual (binary fission): Minimal to none (clones, barring mutations)

Sexual reproduction creates variation through meiosis and fusion of gametes. Asexual reproduction produces clones.

Solution

In sexual (cross-pollination): High genetic variation due to combination of gametes from two parents

Sexual reproduction creates variation through meiosis and fusion of gametes. Asexual reproduction produces clones.

Solution

In vegetative propagation: No variation (genetically identical to parent)

Sexual reproduction creates variation through meiosis and fusion of gametes. Asexual reproduction produces clones.

Solution

In sexual: Significant variation (each offspring genetically unique)

Sexual reproduction creates variation through meiosis and fusion of gametes. Asexual reproduction produces clones.

Solution

In budding: Minimal to none (clones of parent)

Sexual reproduction creates variation through meiosis and fusion of gametes. Asexual reproduction produces clones.

Solution

Regeneration is the ability of an organism to regrow a lost or damaged body part, and if cut into pieces, each piece may grow into a complete organism (as in Planaria). However, it is not a normal mode of reproduction — organisms do not deliberately cut themselves to produce offspring. It is a response to injury. True reproduction is a planned biological process for producing the next generation. Also, regeneration requires specialised cells and is not possible in all organisms — you cannot regenerate a whole dog from a piece of its body.

Solution

The placenta is a disc-like structure embedded in the uterine wall that serves as the interface between the mother and the developing foetus. It provides oxygen and nutrients from the mother's blood to the foetus and removes carbon dioxide and waste products from the foetus to the mother. Importantly, the mother's blood and foetus's blood do NOT mix directly — exchange occurs across the thin placental membrane by diffusion. The placenta also produces hormones (like progesterone and oestrogen) to maintain pregnancy.

Solution

In asexual reproduction, only one parent is involved. The offspring is produced by mitotic division (DNA is replicated and the cell divides), creating copies of the parent's DNA. There is no fusion of gametes from two different parents and no genetic recombination (which occurs in meiosis during sexual reproduction). Therefore, the offspring are genetically identical to the parent — essentially clones. Minor variations may still arise due to occasional DNA copying errors (mutations), but these are rare.

Solution

Sexual reproduction involves two parents, each contributing half the genetic material through gametes (sperm and egg). During meiosis (gamete formation), crossing over and independent assortment create new combinations of genes. When gametes from two different parents fuse during fertilisation, the offspring receives a unique combination of genes from both parents. This is why siblings (except identical twins) look different from each other and from their parents.

Solution

The fruit serves two important functions: (1) It protects the developing seeds from physical damage, desiccation, and pathogens. (2) It aids in seed dispersal. Fleshy fruits attract animals that eat them and disperse seeds through their droppings (e.g., mango, berry). Some fruits have hooks (Xanthium) for attachment to animals, wings (maple) for wind dispersal, or burst open to scatter seeds (balsam). Dispersal prevents competition between parent and offspring for resources.

Solution

Pollination is the transfer of pollen from the anther to the stigma — it is a step in sexual reproduction in flowering plants, not a method of asexual reproduction. Binary fission (Amoeba), budding (Hydra, yeast), and spore formation (Rhizopus) are all modes of asexual reproduction where only one parent is involved and no gamete fusion occurs.

Solution

Vasectomy is a permanent surgical contraceptive method for males in which the vas deferens (the tubes that carry sperm from the testes) are cut, tied, or sealed. This prevents sperm from being released in the semen during ejaculation, effectively preventing fertilisation. The corresponding procedure in females is tubectomy, where the fallopian tubes are cut and tied.

Solution

The Assertion is true — DNA is the blueprint for proteins that determine body design. Copying DNA during reproduction ensures that the basic body design features are passed to the next generation. However, the Reason is false — DNA copying is NOT 100% accurate. Small errors (mutations) occur during replication, creating variations in the offspring. These variations are actually beneficial for evolution, as they provide raw material for natural selection.

Solution

Both are true and the Reason correctly explains the Assertion. The uterine wall is thick and muscular with a rich blood supply because the developing embryo implants in the uterine lining and receives all its nourishment, oxygen, and waste removal through the placenta (which forms from uterine and embryonic tissues). The thick muscular wall also helps during childbirth, contracting to push the baby out. The rich blood supply ensures adequate nutrients reach the growing foetus throughout the 9-month pregnancy.

Solution

Grafting is the most reliable method. By taking a branch (scion) from the prize-winning tree and joining it to a hardy rootstock, the gardener ensures that the fruit-producing part is genetically identical to the prize-winning tree. Seeds (option A) would produce offspring with genetic variation due to sexual reproduction and may not retain the prize-winning qualities. Cross-pollination (option C) would introduce new genetic combinations. Mango trees do not produce spores (option D).

Solution

Removing the anther (emasculation) eliminates the source of self-pollen but does not affect the female parts (stigma, style, ovary). When pollen from another plant of the same species is placed on the intact stigma, cross-pollination occurs. The pollen tube will grow, fertilisation will happen, and seeds will form normally. This is actually a technique used in plant breeding to ensure controlled cross-pollination and prevent self-pollination — the anther is removed before maturity, and desired pollen is manually applied to the stigma.