CBSE Class 10 Science HOTs How Do Organisms Reproduce Set 06

This Class 10 Science page gives 24 higher order thinking questions (HOTS) from the chapter How Do Organisms Reproduce, each with a step-by-step answer, a simple-words summary and a teacher's note. The questions test reasoning on DNA copying and variation, vegetative propagation, bread mould, regeneration, the placenta, the testes, chromosome number and the link between pollinators and sexual reproduction.

Higher Order Thinking Questions

Question 1. A piece of potato tuber with an eye produces a new plant while another piece without an eye does not. Give reason.
Answer:
1. A potato is an underground stem (a stem tuber). Its "eyes" are buds.
2. A bud holds the tissue that sprouts into a new shoot and roots, so a piece with an eye grows into a new plant by vegetative propagation.
3. A piece without an eye has no bud, so nothing can sprout and it does not form a new plant.
In simple words: The eye is a bud; no bud means no new plant.

Teacher's Note:
1. Name the part first: the eye is a bud on a stem.
2. Link the answer to vegetative propagation.

Question 2. How are general growth and sexual maturation different from each other?
Answer:
1. General growth means increase in height, weight and size and change in body shape. During this phase the reproductive organs develop only slowly.
2. Sexual maturation is the set of specific changes at puberty that prepare the body for reproduction, such as cracking of voice and new hair patterns in boys, breast development and menstruation in girls.
3. While the body is still growing, its resources go mainly into growth, so the reproductive tissue matures later and the rate of general growth slows down at puberty.
In simple words: Growth makes the body bigger; sexual maturation makes it ready to reproduce.

Teacher's Note:
1. Give one example of each type of change.
2. Mention that growth slows at puberty.

Question 3. Why is it considered better to grow a rose from cutting rather than from seeds?
Answer:
1. A plant grown from a cutting is genetically identical to the parent, so the desirable qualities of the rose (colour, fragrance, size) are preserved.
2. It reaches maturity and produces flowers in a much shorter time than a plant grown from seed.
3. The method is easy, quick and economical for multiplying ornamental plants, and it works even when seeds are not formed.
In simple words: A cutting gives a quick, exact copy of the parent plant.

Teacher's Note:
1. Use the words "identical to the parent" and "faster".
2. Seeds would give variations, which a gardener may not want.

Question 4. Why does bread mould grow profusely on a moist slice of bread rather than on a dry slice?
Answer:
1. The hyphae of bread mould (Rhizopus) secrete enzymes that digest the nutrients of the bread.
2. For this work and for absorbing the digested food, the hyphae need moisture.
3. A moist slice supplies both moisture and nutrients, so the spores germinate and the mould grows fast. A dry slice supplies nutrients but no moisture, so the hyphae fail to grow.
In simple words: Spores need moisture to germinate and grow.

Teacher's Note:
1. Name the two needs: moisture and food.
2. Add that the spores are already present in the air.

Question 5. Why do mouse parents produce mouse-like offspring? Give reasons.
Answer:
1. During reproduction the DNA of the parent is copied and passed on to the offspring through the reproductive cells.
2. DNA carries the information for making proteins, and proteins decide the body design.
3. Because the copy of mouse DNA is passed on, the offspring has the same basic body design, so it is a mouse and not some other animal.
In simple words: The copied DNA carries the mouse body design to the young one.

Teacher's Note:
1. Connect DNA, proteins and body design in one chain.
2. Remember the copies are similar, not exactly identical.

Question 6. Why is it an advantage for the testes to be situated in the scrotal sac outside the main body cavity? Can you think of one disadvantage?
Answer:
1. Advantage: sperm formation needs a temperature 2–3 °C lower than the normal body temperature. The scrotal sac outside the abdominal cavity provides this lower temperature.
2. Disadvantage: being outside the body cavity, the testes are less protected and are more prone to injury.
In simple words: The scrotum keeps the testes cooler, which sperm need, but leaves them exposed to injury.

Teacher's Note:
1. Quote the 2–3 °C lower temperature.
2. Always give one advantage and one disadvantage when asked.

Question 7. What is a clone? Why do offspring formed by asexual reproduction exhibit remarkable similarity?
Answer:
1. A clone is an offspring that is a genetically identical copy of its parent.
2. In asexual reproduction only one parent is involved and no gametes fuse. The offspring arises by cell division, which copies the parent's DNA exactly, so the offspring is nearly identical to the parent and to one another.
In simple words: Clones are exact genetic copies made from a single parent.

Teacher's Note:
1. Define clone in one line.
2. Reason: single parent, no gametes, copied DNA.

Question 8. What is the effect of DNA copying which is not perfectly accurate in the reproduction process?
Answer:
1. No biochemical reaction is absolutely reliable, so DNA copies are similar but not always identical.
2. The small differences produce variations in the offspring.
3. Some variations help individuals survive when the environment changes, so they are useful for the survival of the species and form the basis of evolution.
In simple words: Small copying errors create variations, and variations help a species survive.

Teacher's Note:
1. Use the word variation.
2. Link it to survival and evolution.

Question 9. Colonies of yeast fail to multiply in water, but multiply in sugar solution. Give one reason for this.
Answer:
1. Yeast needs food to obtain the energy for growth and reproduction.
2. Water gives no nutrition, so the yeast cells do not multiply in it.
3. Sugar solution supplies nutrition, so the yeast cells get energy, grow and form buds (budding), and the colony multiplies.
In simple words: Yeast multiplies only when it has food, and sugar is its food.

Teacher's Note:
1. Say clearly that sugar is a source of nutrition and energy.
2. Name the process: budding.

Question 10. In a bisexual flower inspite of the young stamens being removed artificially, the flower produces fruit. Provide a suitable explanation for the above situation.
Answer:
1. A bisexual flower has both stamens and pistil. Removing the stamens removes only the male part.
2. The pistil (carpel) remains intact.
3. Pollen grains can arrive from another flower by wind, insects or other agents (cross-pollination), so fertilisation takes place. The ovule becomes a seed and the ovary ripens into a fruit.
In simple words: The female part is still there, so pollen from another flower can fertilise it.

Teacher's Note:
1. State what remains: the pistil.
2. Say that cross-pollination supplies the pollen.

Question 11. In an asexually reproducing species, if a trait X exists in 5% of a population and trait Y exists in 70% of the same population, which of the two traits is likely to have arisen earlier? Give reason.
Answer:
1. In asexual reproduction the DNA copies are almost identical, so a new trait appears only through a rare copying error (variation) and is then inherited by the offspring.
2. A trait found in a larger part of the population must have been passed on through many generations.
3. So trait Y (70%) is likely to have arisen earlier, because it has had more time to spread than trait X (5%).
In simple words: The trait present in more individuals has been passed on for longer.

Teacher's Note:
1. Do not confuse this with sexual reproduction, where traits mix.
2. Give the reason in terms of time and generations.

Question 12. Megha cut a celery plant into two pieces. She placed the lower part of the cut celery in a jar of water. The pictures show what Megha observed after a week: new leaves and stems had grown from the cut celery part. What can Megha conclude from her activity?
Answer:
1. New leaves and stems grew from the cut celery piece, which means a whole celery plant can grow from a piece of the plant.
2. This is vegetative propagation: new plants grow from a vegetative part such as the stem.
3. It is a fast and economical method, and the new leaves and stems are genetically similar to the parent plant.
In simple words: A piece of celery can grow into a new plant, which is vegetative propagation.

Teacher's Note:
1. Name the process from the observation.
2. Mention that offspring are like the parent.

Question 13. Akshit had two dogs. The male dog has white ears and the female dog has brown ears. One of the puppies born to these parents has both brown and white ears. He observed that the puppy (offspring) is similar to the parents but not identical. What is the likely reason for this?
Answer:
1. Dogs reproduce sexually, so two parents are involved.
2. Each parent passes on only half of its genetic information (DNA) through its gamete, and the two sets combine in the zygote.
3. So the puppy has genes from both parents and is similar to them, but the new combination of genes (plus small changes in DNA copying) makes it different, with both ear colours.
In simple words: Genes from both parents mix in a new way, so the puppy is similar but not identical.

Teacher's Note:
1. Mention sexual reproduction and mixing of genes.
2. Use the words similar but not identical.

Question 14. Anjali puts the pieces of potato having eyes in soil in one pot. In another pot, she puts the seeds of chillies. Everyday she watched the plants and expected the plants in both the pots to come out at the same time. But the pot having pieces of potatoes grew faster than the pot with chilli seeds. Why does this happen?
Answer:
1. A potato is an underground stem, and its eyes are buds. Each piece already has stored food and a bud, so it sprouts quickly.
2. This is vegetative propagation, which is a cheaper, faster and easier way to grow plants.
3. Chilli seeds must first germinate and the seedling must grow, so the plant appears later.
In simple words: A potato piece grows from a ready bud with stored food, while a seed has to germinate first.

Teacher's Note:
1. Name the method: vegetative propagation.
2. Compare the starting point of the two pots.

Question 15. While going for a nature trail, Sanjay saw a leaf having small plants growing in its margin. He plucked the leaf and showed it to all the students of his class. They were surprised to see this but Sanjay knew the secret behind it. What could have been explained by Sanjay to his own class?
Answer:
1. The leaf was a leaf of Bryophyllum.
2. Buds are produced in the notches along the leaf margin. When the leaf falls on moist soil, these buds develop into new plants.
3. This is a method of vegetative propagation, which is a type of asexual reproduction.
In simple words: Bryophyllum leaves carry buds in their notches that grow into new plants.

Teacher's Note:
1. Name the plant and the part where the buds form.
2. State that it is asexual reproduction.

Question 16. Plants produced through vegetative propagation are genetically identical to their parents. What could be the biggest disadvantage of vegetative propagation?
Answer:
1. All the plants are genetically identical, so there is almost no genetic variation in the population.
2. Both the parent plant and the progeny are susceptible to the same pathogen, so a disease can wipe out the whole population.
3. No new varieties are produced, so the plants adapt less well to a changing environment.
In simple words: No variation means one disease or one change can harm all the plants.

Teacher's Note:
1. Say "no variation" and give its consequence.
2. This is the opposite of the advantage of sexual reproduction.

Question 17. (A) Reproduction is not necessary to maintain the life of an individual organism, unlike essential life processes like nutrition, respiration or excretion. Then why do living organisms reproduce? (B) What could be the two most likely reasons for unicellular organisms to reproduce only through asexual reproduction?
Answer:
(A) Organisms reproduce to increase the population of their species, for the perpetuation (continuity) of the species and to bring variations that lead to evolution.
(B) Two likely reasons: (1) they can produce a large number of offspring in a short time by simple cell division; (2) the offspring are suited to the same environment and the characters of the parent are preserved.
In simple words: Reproduction keeps the species going; unicellular organisms use simple cell division because it is fast.

Teacher's Note:
1. Write at least two points for each part.
2. Keep (A) and (B) separate.

Question 18. A child observed green slimy patches on the ground near a pond. He even saw entangled mats which were floating on the surface of water. He put a part of it in a polythene bag and wanted to know what it is and how it reproduces. Answer his query.
Answer:
1. The green slimy patches and the floating entangled mats are Spirogyra, a filamentous green alga.
2. Under favourable conditions it reproduces by fragmentation: the filament breaks into small pieces when it is mature, and each piece grows into a new filament.
3. Only one parent is involved and no gametes are formed, so this is asexual reproduction.
In simple words: The mats are Spirogyra, which multiplies by breaking into pieces.

Teacher's Note:
1. Name the organism and the method.
2. Add that it is asexual.

Question 19. Planaria can regenerate lost body parts due to the presence of specialised cells called neoblasts. These specialised cells multiply and make a large mass of cells from which different cells undergo changes to become different types of cells and tissues. How do the characteristics of a Planaria formed by regeneration compare with the characteristics of the original Planaria? Justify your answer.
Answer:
1. The characteristics will be the same as those of the original Planaria.
2. Regeneration does not involve gametes or the mixing of genes. The new body is built by repeated cell division of the cells of the original body, so the new cells carry the same DNA.
In simple words: Regeneration copies the same cells, so the new Planaria is like the old one.

Teacher's Note:
1. Contrast it with sexual reproduction, where genes mix.
2. Link the answer to the absence of gametes.

Case Based Higher Order Questions

Question 20. The developing human embryo gets nutrition from the mother through a special tissue called placenta. (A) Mention two structural designs of the placenta that help the embryo to get nutrition. (B) Is the placental tissue designed for one way transport? Justify your answer. (C) The placenta serves as the nutritive, respiratory and excretory organs of the foetus. Justify the above statement.
Answer:
(A) (1) The placenta has villi on the embryo's side. (2) On the mother's side it has blood spaces that surround the villi, giving a large surface area for exchange.
(B) No. Nutrients and oxygen pass from the mother to the embryo, and the waste made by the embryo is passed back into the mother's blood through the same tissue, so transport is in both directions.
(C) The foetus gets glucose and other nutrients from the mother (nutritive), takes in oxygen and gives out carbon dioxide (respiratory), and passes out its wastes into the mother's blood (excretory).
In simple words: The placenta feeds the baby, gives it oxygen and takes its wastes away.

Teacher's Note:
1. Mention villi and blood spaces for part (A).
2. Part (B) answer is two-way transport.

Question 21. Given below are certain situations. Analyse and describe its possible impact on a person: (A) Testes of a male boy are not able to descend into scrotum during his embryonic development. (B) Vas deferens of a man is plugged. (C) Prostate and seminal vesicles are not functional. (D) Egg is not fertilised in a human female. (E) Placenta does not attach to the uterus optimally.
Answer:
(A) Sperm formation will be badly affected, because sperms need a temperature lower than the body temperature.
(B) Sperms cannot be carried from the testes to the urethra, so they are not transferred and fertilisation cannot take place.
(C) The sperms will not get the fluid medium and nourishment that the secretions provide, so their transfer is not easy.
(D) The egg lives for about one day. The thick lining of the uterus then breaks down and comes out with blood and mucus as menstruation.
(E) The embryo will not get enough nutrition and oxygen and waste removal will be poor, which can seriously affect its growth.
In simple words: Each organ has a job; if it fails, sperm formation, sperm transfer, or the baby's supply is affected.

Teacher's Note:
1. Answer the five parts one by one.
2. For each part, name the organ and its normal function first.

Question 22. Suppose there is no meiosis and the gametes are formed by mitotic divisions. What kind of situation do you expect under such circumstances?
Answer:
1. Normally two haploid gametes fuse to form a diploid zygote, because gametes are formed by meiosis, which halves the chromosome number.
2. If gametes were formed by mitosis they would be diploid (2n), not haploid.
3. Two diploid gametes would fuse to give a 4n zygote, and the next generation would give 8n, and so on.
4. The chromosome number and the amount of DNA would double in every generation, so the cell machinery could not work properly and the species could not continue.
In simple words: Without meiosis the DNA amount would keep doubling in each generation.

Teacher's Note:
1. Write the 2n, 4n, 8n sequence.
2. Meiosis is what keeps the chromosome number constant.

Question 23. During the process of sexual reproduction, DNA from two different individuals combine. If each new generation is to be the combination of DNA copies from two pre-existing individuals, then each new generation will end up having twice the amount of DNA that the previous generation had. (A) DNA does not double in the new generation. How can this happen? (B) (i) How will you differentiate between the two germ cells? (ii) Why is DNA copying an essential part of the process of reproduction?
Answer:
(A) The reproductive cells are formed by a special division (meiosis) in which the number of chromosomes and the amount of DNA become half. When the two gametes fuse, the full number is restored, so the DNA does not double.
(B)(i) The male gamete is small and motile; the female gamete is larger, non-motile and contains stored food.
(B)(ii) DNA copying passes the body design and characters from parent to offspring, and its small inaccuracies give the variations that help the species survive.
In simple words: Gametes carry half the DNA, so fusion brings it back to the normal amount.

Teacher's Note:
1. Mention meiosis and halving in part (A).
2. For part (B)(i) compare size and motility.

Teacher's Note:
1. Read each column of the table before answering.
2. Link sexual reproduction with variation and survival.