CBSE 2019 Science (Biology) Class question paper and solutions key

This article provides solutions to all the questions of Biology of CBSE Board's X Class question paper of 2019 Main Set-I, Set-2, Set-3 question papers. This will help the students to learn the technique of writing the answers in a proper and correct way by using exact keywords.

Science 2019 Main Exam Main Set-I/Set-2/Set-3 Questions and Solutions


List two components of an ecosystem. (Set-1) (1 Mark)
Ans. The two components of an ecosystem are Biotic (living) and Abiotic (non-living).

Q. Write various steps of the terrestrial food chain of four trophic levels. (Set-2) (1 Mark)
Ans. Plant (Producers)-------->Goat (Herbivore)----------->Fox(Carnivore)------->Lion(Top carnivore)

Q.Generally, we do not clean natural ponds or lake but an aquarium needs a periodic cleaning. Why? (Set-3) (1 Mark)

Ans. Aquarium forms an incomplete ecosystem so that they do not have decomposers to decompose the waste. But a natural ecosystem will have micro-organisms to decompose the waste and clean on its own. Thus the aquarium needs periodic cleaning.


How do auxins promote the growth of a tendril around a support? (OR) What is nerve impulse? State the direction followed by nerve impulse while traveling in the body of an organism.

(Set-1) (2 Marks)
Ans. Auxin is a plant hormone that is synthesized at the tip of the shoot. When a tendril comes in contact with the support, it stimulates faster growth of cells on the opposite side. Thus the tendril curls around the support.


Ans. A nerve impulse is a wave of irritability that sweeps over a neuron or The electrical disturbance that sweeps over a neuron is known as an impulse. A nerve impulse will always travel from dendrites to cyton end of the neuron to the axon end.

Q. What is insulin? Why are some patients of diabetes treated by giving an injection of insulin? (OR) How is the movement of leaves of the sensitive plant different from the movement of a shoot towards light?
(Set-2) (2 Marks)

Ans. Insulin is a hormone secreted by the endocrine gland pancreas. The insulin hormone controls the blood sugar level in our body. In diabetes patients, insulin hormone is not properly secreted by the pancreas so the blood sugar levels increase. So, to control blood sugar level insulin injection will be given to the diabetes patients.


Ans. Movement of a leaf in the sensitive plant is a nastic movement while the movement of a shoot towards light is tropic.

Movement of a leaf in the sensitive plant is due to stimulus touch whereas the movement of a shoot towards light is due to the stimulus light.

Movement of a leaf in the sensitive plant is not dependent on the direction of stimulus whereas the movement of a shoot towards light is dependent on the direction of the stimulus.

Q.List in tabular form two differences between a reflex action and walking. (OR) List in a tabular form two differences between pepsin and trypsin. (Set-3) (2 Marks)

Ans. A reflex action is an involuntary action mediated by the spinal cord whereas walking is a voluntary action controlled by the brain.

A reflex action is initiated by some stimulus (touch, pain, pressure, etc.) while walking is initiated by a willing thought of the brain.

A reflex action involves muscles and glands whereas walking only involves muscles.

Ans. Pepsin is a digestive enzyme secreted by gastric glands of the stomach in gastric juice while trypsin is a digestive enzyme secreted by the pancreas in pancreatic juice.

Pepsin enzyme works in acidic medium whereas trypsin enzyme works only in alkaline medium.


Q.List four functions of the human heart. Why is double circulation necessary in the human body? (Set-1) (3 Marks)

  1. The heart pumps oxygenated blood to reach every part of the body to supply oxygen.

  2. Heart pumps deoxygenated blood to lungs for its oxygenation.

  3. Heart pumps blood with digested nutrients to reach each every cell and tissue of the body.

  4. Heart helps to transport metabolic wastes along with the circulation of blood to kidneys for their elimination.

Double circulation is necessary for the human being to separate oxygenated and de-oxygenated blood. This makes their circulatory system more efficient and helps in maintaining constant body temperature.

Q. (a) Name the part of the human brain which controls (i) Voluntary actions and (ii) Involuntary actions.
Ans. (i) Cerebrum or forebrain and (ii) Medulla oblongata. (Set-2)

Q. Write the function of the Peripheral nervous system. Name the components of this system stating their origin. (Set-2)
Ans. The primary role of the Peripheral nervous system is to connect the Central Nervous System to the organs, limbs, and skin.

The peripheral nervous system (PNS) has two components- the somatic nervous system and the autonomic nervous system. The PNS consists of all of the nerves that lie outside the brain and spinal cord.

Q. State the short term and long term aims of exploiting resources.

Ans. Advantages of exploiting resources with short term aim:-
  • It provides immediate benefit to a few people.

  • Progress in science and technology for development in a country.

  • Urbanization and Industrialisation of an area can be hastened.

Three advantages of using a long term perspective:-
  • Resources will be made available for sustainable development and for future generations.

  • Provides a valuable contribution to socio-economic development.

  • The quality of the environment will be conserved.

Question 4

Q. Explain how glucose is broken down in the absence or shortage of oxygen. (Set-1)

Ans. In the absence of oxygen, the glucose molecule gets broken down into pyruvate molecules in the cytoplasm of cells of the lower group of organisms like yeast. Then these pyruvate molecules get reorganized into form ethyl alcohol.

Glucose (Glycolysis in Cytoplasm) ------------> Pyruvate (in the absence of O2) ---------->2 C2H5OH + 2 CO2 + 2 ATP

In the muscle cells in the absence of oxygen, the glucose molecule gets broken down into pyruvate molecules in the cytoplasm of muscle cells. Later in the absence of oxygen get reorganized into lactic acid and 2 ATP.

Glucose (Glycolysis in the cytoplasm) ------------> Pyruvate (in the absence of oxygen)-------->Lactic acid + 2 ATP

Q. Explain the following:- (a) Speciation (b) Natural Selection
Mendel, in one of his experiments with pea plants, crossed with a variety of pea plants having round seeds with one having wrinkled seeds. Mendel's observation giving reasons for F1 and F2 progeny of the cross. Also, list any two contrasting characters, other than round seeds of the pea plants that Mendel used in his experiment. (Set-2)

Ans. (a) Speciation:- The origin of new species through a time period from pre-existing species is known as speciation. The four ways by which speciation occurs is
  1. Geograhical isolation
  2. Genetic drift
  3. Natural selection
  4. Mutations

(b) Natural Selection:- Natural selection is the process by which organisms having some special features are at an advantage for better survival in the changing environment.

Example:- The variation that confers an advantage to an individual organism will survive in a population. Suppose there is a population of red beetles in the green bushes and color variation arises during sexual reproduction so that one beetle is now green in color (instead of red). This variation offers an advantage of survival because of the green beetles can mix up with green bushes. So it cannot be eaten up crows and hence their population increase. If however variation had produced blue beetles then the color could not offer any survival advantage because blue beetles in green bushes could be easily spotted by a crow and eaten by it.


F1 generation – all plants with round and wrinkled seeds.
F2 generation – plants with round and wrinkled seeds.
-Tall/dwarf plants
-Yellow / green seeds
-White/purple flowers

Q. List in a tabular form three distinguishing features between Cerebrum and Cerebellum. (Set-3)

  1. Cerebrum is the largest part of the brain whereas cerebellum is the smallest part of the brain.

  2. Cerbrum control voluntary activities of the body while cerebellum maintains equillibrium and balance of the body.

  3. Cerebrum controls the functioning of sense organs while cerebellum controls posture of the body.

Question 5

Q. Why are forests consider "biodiversity hotspots"? Suggest four approaches towards the conservation of forests. (Set-1)

Ans. Forests serve as the reservoir of biodiversity containing different species of plants, animals and all other sorts of living organisms.

Approaches for conservation of forests:-
  1. Reforesting those forest lands where large number of trees cut.

  2. Establishing National parks and sanctuaries.

  3. Protection of forests by taking help from people.

  4. By enforcing laws strictly for the protection of forests.

Q. A village of South India people started cultivating crops all around a lake. They added fertilizers to their fields to enhance the yield. Soon after it was discovered that the water body is filled with green floating plants and the aquatic plants and aquatic animals started dying in large numbers.

Analyze the situation and give reasons for the excessive growth of plants and the death of animals. (Set-2)

Ans. As the farmers are adding fertilizers heavily to their fields, fertilizers get washed off to the lake when rains precipitate in this area. Due to the enrichment of the lake with nutrients, the green plants have grown vigorously. This situation is known as eutrophication. Due to the excessive growth of algae and plants over water, most of the O2 is consumed by them and the aquatic animals do not get O2 and thus they die in large numbers.

Q. What is transpiration? List its two functions. (Set-3)

Ans. The loss of excess water from the plant in the form of water vapor through the aerial parts of the plant is known as transpiration.

Functions of transpiration:-
  1. Transpiration helps to cool the plant body.

  2. Transpiration help to distrubute water and minerals throughout the lant body.

Question 6

Q. What is reproduction? list its two types. How are the modes of reproduction different in unicellular and multicellular organisms?

Ans. Producing its own kind of organisms to maintain its race constant in nature is known as reproduction. Reproduction is of two kinds- Asexual reproduction and Sexual reproduction.

In unicellular organisms, the asexual mode of reproduction occurs while mostly sexual reproduction occurs in multicellular organisms.

A single parent is required in asexual reproduction in unicellular organisms while two parents are required for sexual reproduction in multicellular organisms.

In unicellular organisms no need for fusion of gametes for asexual reproduction while a fusion of gametes occurs in the sexual reproduction of multicellular organisms.

Q. What are Sexually Transmitted diseases (STD)? List two viral and bacterial STDs.
What is contraception? List three reasons for adopting contraceptive methods.

Ans. Diseases that are spread through sexual intercourse through sexual secretion or transfusion blood between infected persons, etc.
Two viral diseases:- Herpes, HIV
Two bacterial diseases:- Syphilis, Gonorrhea.

Contraception is the method of preventing pregnancy by adopting various family planning methods.
Reasons for adopting contraceptive methods:-
  1. By using condoms sexually transmitted diseases can be prevented.

  2. The size of the family can be minimized.

  3. Population growth rate can be kept under control.

Question 7

In the experiment of preparing a temporary mount of a leaf peel to observe stomata, we use two liquids other than water. Name these two liquids and state when and why these liquids are used.

Ans. The two liquids used other than waters are Saffranine and glycerine.
Saffranine is used to color the cells so that we can view the cells of the peel and stomata very clearly. Glycerine liquid is used to prevent the drying away the moisture of the peel.


Q. List four precautions in proper sequence which we observe while preparing a temporary mount of a leaf peel.

Ans. Preparation has taken in preparing a mount of leaf peel:-
  1. The peel taken from the leaf should be first kept in a watch glass containing water to prevent drying away of the peel.

  2. Saffranine stained peel should be washed with water to remove excess stain from the peel.

  3. One or two drops of glycerine should be added to the peel on the slide to prevent its drying.

  4. A coverslip should be placed over the leaf peel on slide gently and carefully so that no air bubbles sets under the covert slip.

Also chedk CBSE 2018 Science (Biology) Main Set-I X Class Question Paper and Solutions Key


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