NCERT 8th Class (CBSE) Science: Reproduction

Question: Why is reproduction important? Answer: Reproduction is important for producing new individuals of the same kind. Question: How is sexual reproduction different from asexual reproduction? Answer: Sexual reproduction requires two parents, one male and one female, to reproduce. Whereas in Asexual reproduction only one parent is required to reproduce. Question: What is metamorphosis? Answer: It is transformation of the larva into an adult through drastic changes is called metamorphosis. Question: What is menstruation? Answer: If the ovum is not fertilized by a sperm, it is expelled from the uterus along with some uterine muscles and blood. This is known as menstruation. Question: What is cell division? Answer: After fertilization, the zygote travels down the oviduct and divides over and over again, to form a ball of cells. This is known as cell division. Question: List the various stages in the life cycle of a mosquito. Answer: The stages are: When the eggs hatch, a worm like Larva comes out of each egg. In about two weeks, it changes into an encased form called pupa which has the shape of a comma (,). The pupa changes into mosquito inside the case, and in about a week the mosquito wriggles out of the case. Question: Differentiate between external and internal fertilization, giving two examples of each. Answer: External fertilization: Sea animals, such as starfish and jellyfish, gather in large groups in water. They lay their eggs and sperms together on the surface of the water. The sperm fuse with the eggs on the water to form zygotes. The zygotes develop into new individuals in the water. This type of fertilization is known as external fertilization as it occurs outside the animal's body. During spring and the rainy season, frogs and toads move to ponds and slow-moving streams. The male and female clasp each other in water. The female lays hundreds of eggs. The eggs are delicate and do not have a shell. A layer of jelly-like substance holds them together and provides protection to them. The male ejects sperms directly on the cluster of the floating eggs. The sperms swim in water with the help of their long tail, and fertilize the eggs. Internal fertilization: In birds, reptiles and mammals, including humans, fertilization occurs inside the female's body. This is known as internal fertilization. In earthworms also, fertilization is internal. An earthworm, though a hermaphrodite, does not fertilize itself. Two earthworms come in contact with each other and exchange sperms. The eggs are fertilized internally with the sperms received from the partner. Question: How does fertilization occur in frogs? Answer: A frog passes through three stages during its development from an egg to an adult frog: Egg → Tadpole (larva) → adult frog The tadpole that hatches out of the egg looks very different from an adult frog. It undergoes changes to finally become a frog. Question: What happen in the female body if the ovum is not fertilized by a sperm. Answer: If the ovum is not fertilized by a sperm, it is expelled from the uterus along with some uterine muscles and blood. This is known as menstruation. Question: How does Hydra reproduce? Answer: Yeast and Hydra reproduce by budding. In Hydra, a bud appears on the body wall which grows into full Hydra in a day or two and separates from the parent. Question: Explain the difference between viviparous and oviparous animals. Answer: Viviparous animals The animals that give birth to young ones are called viviparous animals. Example: Except platypus all mammals. Embryo is developed inside the mother's womb. Oviparous animals The animals that lay eggs are called oviparous animals. Examples: Fishes, birds. Embryo is developed inside a hard shell outside the mother's body. Example: Hen's egg. Question: Explain what happens after a hen lays a fertilized egg. Answer: After fertilization, the zygote divides repeatedly and travel down the oviduct as in humans. However, as it travels down, many protective layers are formed around it. The hard shell of a hen's egg is one such layer. After the hard shell is formed around the embryo, the hen lays the egg. The hen then sits on the egg to keep it warm. Development of the chick takes place inside the shell. It takes about 3 weeks for the embryo to develop into a chick. After its development is complete, the chick comes out by bursting open the egg shell. Question: 1. Give details of two different ways in which a zygote develops in case of animals where fertilization is internal. Answer: Internal fertilization in earthworm: An earthworm , though a hermaphrodite, does not fertilize itself. Two earthworms come in contact with each other and exchange sperms. The eggs are fertilized internally with the sperms received from the partners. Thus, a zygote is developed. Internal fertilization in human: In human fertilization takes place on meeting of male and female gamete inside body. Fertilization is internal, and the development of the zygote into a fully formed individual also occurs inside the body of the female. Question: What is puberty? What is it caused by? What changes occur in boys and girls during puberty? Answer: The onset of sexual maturity occurs in boys at the age of 11-16 years, and in girls at the age of 10-15 years. This is known as puberty. In boys, puberty is marked by the voice becoming deeper, growth of hair on the face and body, development of muscles and production of sperms by the testes. In girls, breasts become bigger and hips get more rounded. These changes are caused by hormones that are different in girls and boys. Question: Discuss how a zygote develops into an individual in humans. Answer: After fertilization, the zygote travels down the oviduct and divides over and over again, to form a ball of cells. This is known as cell division. The cells then begin to form different tissues and organs of the body. This developing structure is called the embryo. Embryo gets embedded in the wall of the uterus for nutrition and protection. During the development, the different groups of cells change their size and shape, and turn into the different body parts such as head, hands, legs, etc. This is called cell differentiation or cell maturation. The stage at which all the body parts of the embryo could be recognized is called, the foetus. The foetus keeps developing further. When all body parts assume the right size and forms, the development is complete and the individual is born. Question: Explain the life cycle of a frog, with the help of a labelled diagram. Answer: A frog has different stages of development between hatching of the eggs and formation of the individual. A frog passes through three stages during its development from an egg to an adult frog: Egg → Tadpole (larva) → adult frog The tadpole that hatches out of the egg looks very different from an adult frog. It undergoes changes to finally become a frog. The life cycle of a frog diagram: Question: Can the process of a child changing into an adult be called metamorphosis? Answer: No, the process of a child changing into an adult cannot be called metamorphosis because the basic body structure does not change. Question: Frogs and fishes lay hundreds of eggs at a time. However, their population is very little as compared to the number of eggs they lay. Why? Answer: Most of the eggs, tadpoles and young frogs get eaten by predators. That is why their population is much smaller than the number of eggs they lay. Question: Why do you think it is necessary for frogs and fishes to lay so many eggs? Answer: Frogs and fishes lay hundreds of eggs for the survival of their species. This is because their fertilization is external and a large number of the eggs that hatch in the open get eaten by predators. Question: What do you think would have happened if humans also gave birth to a large number of babies at the same time? Answer: Since mortality of infants is very low as compared to animals. Therefore if they gave birth to a large number of babies at the same time, their population would have become very high and unsustainable. Question: Can a woman with a blocked fallopian tube give birth to a child? Why? Answer: No, a blocked fallopian tube prevents the ova to fertilize with sperm cell as the male gametes from the father cannot come in contact with female gametes of mother. Question: Under what circumstances can twins are born in humans? Answer: When two ova can be produced by ovaries and both get fertilized, it can result in twins. Question: Why is it that dogs always produce several puppies whereas human beings usually produce only one child at a time? Answer: Dogs produce more than one egg at a time. Hence, more puppies are born to them at the same time. Whereas humans usually produce one egg at a time, hence produce only one child at a time. Question: Describe the the process of fertilization in human beings. Answer: A single egg comes out of the ovary every month, and reaches the fallopian tube. Sperms reach the fallopian tube where a sperm fertilizes the egg. This results in formation of zygote. Question: Give two differences between a zygote and a foetus. Answer: The difference is as follows: Question: Define asexual reproduction. Describe two methods of asexual reproduction in animals. Answer: When a single parent is involved in reproduction and gametes are not formed, it is called asexual reproduction. Two methods of asexual reproduction are as follows: Budding: This method is seen in multicellular animals which are highly simple in structure. A small bud develops on the body. The bud develops and starts resembling its mother. After that, the bud gets detached from the mother’s body to begin life as a new individual. Examples: Hydra and sponges. Binary Fission: This method is seen in unicellular animals, e.g. amoeba. The cell of amoeba divided into two daughter cells. Each daughter cell becomes a new individual. Question: Differentiate between internal fertilization and external fertilization. Answer: The difference is as follows:

Class Notes NCERT Solutions for CBSE Students

NCERT 8th Class (CBSE) Science: Reproduction

Question: What do you think would have happened if humans also gave birth to a large number of babies at the same time?

Question: Can a woman with a blocked fallopian tube give birth to a child? Why?

Question: Under what circumstances can twins are born in humans?

Question: Why is it that dogs always produce several puppies whereas human beings usually produce only one child at a time?

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