How do Organisms Reproduce? — CBSE Class 10 Science Important Questions

Answer: (d) Rose – root. Rose is propagated by stem cuttings, not roots. Bryophyllum reproduces through buds on leaf margins, and potato and money plant reproduce through stems, so those matches are correct.

Answer: (c) Hydra. Hydra (and yeast) reproduce by budding. Amoeba uses binary fission, Leishmania undergoes fission along a definite plane, and Plasmodium reproduces by multiple fission.

Answer: (a) seed and fruit. The fertilized ovule matures into the seed (enclosing the embryo), while the ovary wall grows into the fruit that protects the seeds.

Answer: (d) Diabetes. Diabetes is a metabolic disorder, not transmitted sexually. Syphilis and gonorrhoea are bacterial STDs and HIV-AIDS is a viral STD.

Answer: (a) Both A and R are true and R is the correct explanation of A. Because asexual reproduction uses one parent with no gamete fusion, the DNA is copied without recombination, so the offspring (clones) are genetically identical to the parent.

Regeneration is the ability of a fully differentiated organism to grow a complete new individual from its body fragments if it is cut or broken. Example: Planaria (also Hydra). It differs from reproduction because regeneration is carried out by specialised cells that proliferate and re-organise; it is not the normal mode by which such organisms multiply, whereas true reproduction is the deliberate creation of new individuals to continue the species.

In sexual reproduction two germ cells fuse to form the zygote. If each germ cell carried the full set, the zygote would have double the number of chromosomes (and double the DNA), and the amount would keep doubling in every generation. The cell's design, which is controlled by its DNA, would then break down.

Because each germ cell has half the number of chromosomes, their fusion restores the normal number in the zygote, so the DNA content of the species stays constant from generation to generation.

(a) Budding in Hydra: 1. Due to repeated cell division at a specific site, a small outgrowth called a bud develops on the body of Hydra.
2. The bud grows and develops into a tiny new individual, complete with a mouth and tentacles, while still attached to the parent.
3. When mature, the bud detaches from the parent body and lives as an independent organism.

(b) Spore formation in Rhizopus:

1. Rhizopus (bread mould) grows as thread-like structures called hyphae.
2. Some hyphae bear tiny blob-on-a-stick structures called sporangia, which contain spores.
3. Each spore can develop into a new Rhizopus individual. Spores are covered by thick walls that protect them until they come in contact with a moist surface, where they begin to grow.

Labelled L.S. of a bisexual flower (also showing a pollen grain germinating on the stigma):

Pollination: the transfer of pollen grains from the anther of a flower to the stigma of a carpel. It is self-pollination when pollen reaches the stigma of the same flower, and cross-pollination when carried (by wind, water or insects) to a different flower.

Fertilization: after pollination, the pollen grain germinates on the stigma and grows a pollen tube down the style into the ovary; the male gamete travels through this tube and fuses with the egg (female gamete) inside the ovule to form a zygote. The zygote develops into an embryo, the ovule becomes the seed and the ovary becomes the fruit.

Stability of populations: reproduction continuously produces new individuals to replace those that die, keeping the population of a species stable over generations.

Role of DNA copying: reproduction essentially copies the DNA blueprint of the parent, so offspring have the same basic body design and belong to the same species — this maintains the species' identity and stability.

Role of variation: DNA copying is never perfectly accurate, and sexual reproduction adds recombination, producing small variations. If the environment changes drastically (heat, cold, disease), individuals with a suitable variation can survive and reproduce, so the species is not wiped out. Thus copying ensures continuity while variation ensures long-term survival, together stabilising the species.

(a) Formation of sperms requires a lower temperature than the normal body temperature, so the testes lie outside the abdominal cavity in the scrotum.

(b) Testosterone. It regulates the formation of sperms and brings about the changes in appearance seen in boys at puberty.

(c) The seminal vesicles and the prostate gland. Their secretions make the transport of sperms easier and also provide nutrition to the sperms.

Main parts and functions:

1. Ovaries (a pair): produce the female gametes (eggs/ova) and secrete female hormones (oestrogen and progesterone). At puberty, one egg is usually released each month (ovulation).
2. Oviducts / Fallopian tubes: carry the egg from the ovary towards the uterus; fertilization normally occurs here when a sperm meets the egg.
3. Uterus (womb): a muscular, bag-like organ where the fertilized egg is implanted and the embryo develops into a baby. Its lining thickens to receive the embryo.
4. Cervix: the narrow opening of the uterus that leads into the vagina.
5. Vagina: receives sperm during mating and serves as the birth canal.

Fate of the egg:
(i) If fertilized: the zygote divides to form an embryo that gets implanted in the uterine lining; the placenta develops to supply the embryo with nutrients and oxygen and remove wastes, and pregnancy continues.
(ii) If not fertilized: the egg lives about a day and the thickened uterine lining, no longer needed, breaks down and is shed with blood through the vagina — this is menstruation, lasting 2–8 days and repeating about every 28 days.

(a) Placenta: a special disc-shaped tissue embedded in the wall of the uterus that connects the developing embryo to the mother.

Structure: on the embryo's side it bears finger-like projections called villi; on the mother's side there are blood spaces (sinuses) surrounding the villi. This provides a large surface area for exchange.

Functions:

• Supplies glucose, nutrients and oxygen from the mother's blood to the embryo.
• Removes carbon dioxide and nitrogenous wastes from the embryo into the mother's blood.
  (The two bloodstreams do not mix; exchange occurs by diffusion across the villi.)

(b) Changes at puberty:

• Common to both boys and girls: thick hair grows in the armpits and the genital area; the skin often becomes oily and may develop pimples. (Thinner hair also appears on the legs, arms and face.)
• Specific to girls: the breasts grow larger and menstruation begins.
• Specific to boys: thick facial hair (moustache and beard) grows and the voice begins to crack.

(i) Condom is a barrier method. Its added benefit is that it also prevents the spread of sexually transmitted diseases (STDs) such as HIV-AIDS, syphilis and gonorrhoea.

(ii) Males – vasectomy (the vas deferens is blocked/cut so sperm are not delivered); females – tubectomy (the fallopian tubes are blocked/cut so the egg cannot reach the uterus).

(iii) Oral pills contain hormones that change the hormonal balance of the body so that eggs are not released (ovulation is stopped), and hence fertilization cannot occur.

(iv) Prenatal sex determination often leads to selective abortion of female foetuses, which has caused a falling child sex ratio. To stop this social evil and protect the female child, determining the sex of the unborn baby is legally banned.