Class X 10 ONE Mark Biology Questions: Do you know the Answers?


Q1.      Name the green dot like structures in some cells observed by a student when a leaf peel was viewed under a microscope.  What is this green colour due to?

Q2.      State the basic difference between the process of respiration and photosynthesis.

Q3.      Name the intermediate and the end products of glucose breakdown in aerobic respiration.

Q4.      Why are bacteria and fungi called decomposers?

Q5.      Mention any one point of difference between Pepsin and Trypsin.


Q6.      What will happen to a plant if its xylem is removed?

Q7.      Name two tissues that provide control and coordination in multicellular animals.

Q8.      How is the spinal cord protected in the human body?

Q9.      Name any two types of tropism.

Q10.    Name one plant growth hormone which retards growth during extremely dry season.

Q11.    Name the information source for making proteins in the cells.


Q12.    List two functions performed by the testis in human beings.

Q13.    Give reason for the statement- Since the ovary releases one egg every month, the uterus also prepares itself every month by making its lining thick and spongy.

Q14.    Name the method by which Plasmodium reproduces. Is this method sexual or asexual?

Q15.    Give an example of a flower which contains both stamens and carpels.


Q16.    Why is variation beneficial for the species, but not necessarily for the individual?

X_Biology_1 Marks_Answers

Class X Science: Syllabus for 2018 CBSE Board Examination


Chemical Reactions: Chemical equation, Balanced chemical equation, implication of a balanced chemical equation, types of chemical reactions: Combination, decomposition, displacement, double displacement, precipitation, neutralization, oxidation and reduction.

Acids, Bases and Salts : Their definitions in terms of furnishing of H+ and OH- ions, General properties, examples and uses, concept of pH scale (Definition relating to logarithm not required), importance of pH in everyday life; preparation and uses of Sodium Hydroxide, Bleaching powder, Baking soda, Washing soda and Plaster of Paris.

Metals and Non-Metals: Properties of metals and non-metals; Reactivity series; Formation and properties of ionic compounds; Basic metallurgical processes; Corrosion and its prevention.

Carbon Compounds: Covalent bonding in carbon compounds. Versatile nature of carbon. Homologous series. Nomenclature of carbon compounds containing functional groups (Halogens, Alcohol, Ketones, Aldehydes, Alkanes and Alkynes), difference between saturated hydrocarbons and unsaturated hydrocarbons. Chemical properties of carbon compounds (combustion, oxidation, addition and substitution reaction). Ethanol and Ethanoic acid (only properties and uses), soaps and detergents.

Periodic Classification of Elements: Need for classification, Early attempts at classification of elements (Dobereiner’s Triads, Newland’s Law of Octaves, Mendeleev’s Periodic Table), Modern periodic table, gradation in properties, valency, atomic number, metallic and non-metallic properties.


Natural Phenomena —12 Marks

Light: Reflection of light by curved surfaces; Images formed by spherical mirrors, centre of curvature, principal axis, principal focus, focal length, mirror formula (Derivation not required), magnification.

Refraction; Laws of refraction, refractive index.

Refraction of light by spherical lens; Image formed by spherical lenses; Lens formula (Derivation not required); Magnification. Power of a lens. Functioning of a lens in human eye, defects of vision and their corrections, applications of spherical mirrors and lenses. Refraction of light through a prism, dispersion of light, scattering of light, applications in daily life.

Effects of Current —-13 Marks

Electric current, potential difference and electric current. Ohm’s law; Resistance, resistivity, Factors on which the resistance of a conductor depends. Series combination of resistors, parallel combination of resistors and its applications in daily life. Heating effect of electric current and its applications in daily life. Electric power, Interrelation between P, V, I and R.

Magnetic Effects of Current : Magnetic field, field lines, field due to a current carrying conductor, field due to current carrying coil or solenoid; Force on current carrying conductor, Fleming’s Left Hand Rule, Electric Motor, Electromagnetic induction. Induced potential difference, Induced current. Fleming’s Right Hand Rule, Electric Generator, Direct Current. Alternating Current


Life Processes: ‘Living Being’. Basic concept of nutrition, respiration, transport and excretion in plants and animals.

Control and Co-Ordination in Animals and Plants: Tropic movements in plants; Introduction of plant hormones; Control and co-ordination in animals; Nervous system; Voluntary, involuntary and reflex action; Chemical co-ordination: animal hormones.

Reproduction: Reproduction in animals and plants (asexual and sexual) reproductive health-need and methods of family planning. Safe sex vs HIV / AIDS. Childbearing and women’s health.

Heredity and Evolution: Heredity; Mendel’s contribution – Laws for inheritance of traits: Sex determination: brief introduction; Basic concepts of evolution.

Natural Resources – 07 Marks

Sources of Energy: Different forms of energy, conventional and non-conventional sources of energy: Fossil fuels, solar energy; biogas; wind, water and tidal energy; Nuclear energy. Renewable versus non-renewable sources of Energy.

Our Environment : Eco-system, Environmental problems, Ozone depletion, waste production and their solutions. Biodegradable and non-biodegradable substances.

Management of Natural Resources : Conservation and judicious use of natural resources. Forest and wild life; Coal and Petroleum conservation. Examples of people’s participation for conservation of natural resources. Big dams: advantages and limitations; alternatives, if any. Water harvesting. Sustainability of natural resources.

Marking Scheme for Class X Science Paper for 2018

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Class X Biology: Sexual Reproduction in HUMANS


1. The reproductive organs in human beings ie. Testis in males and ovary in females become functional only on Puberty. During this period of adolescence, the rate of general body growth begins to slow down and reproductive tissues begin to mature.

2. Puberty: It is the age at which sex hormones or games begin to be produced and boy or girl becomes sexually mature. At puberty, physical changes occur in the body.

(i) Changes common to Boys and Girls:-
 Appearance of hair in armpits, genital areas, legs, arms, and face
 The appearance of pimples due to the occurrence of oily skin

(ii) Changes difference in Boys and Girls:
 Girls – Start of menstruation, darkening of nipple skin and enlargement of breasts
 Boys- appearance of facial hair, change of voice, enlargement and occasional erection of Penis

Male Reproductive System

3. The male reproductive system consists of portions which produce the germ-cells and other portions that deliver the germ-cells to the site of fertilisation.

4. The male reproductive system consists of the following organs:

(i) Testes (singular testis) are the oval-shaped primary reproductive organ in man. A pair of testis lies in a small sac-like muscular structure outside the abdominal cavity called scrotum. The function of the testis is to produce sperm and male sex hormone testosterone. In addition to regulating the formation of sperms, testosterone brings about changes in appearance seen in boys at the time of puberty. The scrotum provides the optimal temperature for formation of sperms.

Male Reproductive Organ3


(ii) Epididymis is a coiled tube-like structure firmly attached to the testis and serves as the storehouse of sperms. Inside the epididymis, sperms become mature and develop motility.

(iii) Vas Deferens – The sperms are carried by a long tube called vas deferens or sperm duct into an organ called seminal vesicles, where the sperms get nourishment and are stored.

(iv) Urethra – It is a common duct for passage of both urine and spermatic fluid. Urethra carries the sperms to an organ called Penis and opens to the outside through a male genital pore.

(v) Penis – It forms the external male genital organ. It is a copulatory organ with thick muscular walls.

(vi) Accessory Glands – Seminal vesicles are a pair of thin-walled muscular elongated sac which secretes fluid which makes transport of sperms easier and this fluid also provides nutrition.

(vii) Prostate Glands – they produce fluid which is released in the urethra along with secretion of seminal vesicle. The secretion of accessory glands together with sperms is called semen.

(viii) Sperms are tiny bodies that consist of mainly genetic material and a long tail which help them to move towards the female germ cell.

Female Reproductive System

5. Female Reproductive system – The female germ cells or eggs are made in ovaries and are responsible for the production of hormones. The female reproductive system consists of –

(i) Ovaries – They are a pair of small and oval-shaped organs, located in the abdominal cavity near the kidney. Ovaries are female reproductive organs which perform the dual function of production of female gamete or ovum and the secretion of female sex hormones, estrogen, and progesterone.

Female Reproductive Organ2

(ii) Fallopian tube or Oviduct – It is a pair of long convoluted tubes that carry ova or eggs from the ovary to the uterus. The fallopian tube has a funnel-shaped opening near the ovary. These tubes from both the sides open into an elastic bag-like structure, t heh uterus.

(iii) Uterus or Womb – The two oviducts unite into an elastic bag-like structure known as the uterus. The uterus opens into the vagina through the cervix. It is a hollow, pear-shaped organ within which the embryo develops.

(iv) Vagina – It is a tubular structure also called “birth canal”. It receives sperms from male and also serves as the passage through the fully developed foetus is born. The uterus opens into the vagina through the cervix.

6. Sexual Reproduction in Human Beings

(i) The sperms enter through the vaginal passage during sexual intercourse.

(ii) Sperms travel through cervix into the uterus and reach the oviduct where they may encounter the egg.

(iii) Only one sperm fertilises the ovum/egg to form a zygote. This is known as fertilization.

(iv) Fertilization occurs only during the ovulatory period. The fertilised egg, the zygote, gets implanted in the lining of the uterus and starts dividing. The lining of the uterus thickens every month and is richly supplied with blood to nourish the growing embryo.

(v) The embryo gets nutrition from the mother’s blood with the help of a special tissue called placenta, where the exchange of nutrients, oxygen and waste products take place. This is a disc which is embedded in the uterine wall. It contains villi on the embryo’s side of the tissue. This provides a large surface area for glucose and oxygen to pass from the mother to the embryo.

(vi) The development of the child inside the mother’s body takes approximately nine months. The child is born due rhythmic contractions of the muscles in the uterus.

7. Menstruation

(i) Ovary releases one egg every month, the uterus also prepares itself every month to receive a fertilised egg. The uterus stores food in the form of blood to nourish the embryo and its lining becomes thick and spongy.

(ii) If the egg is not fertilised, it lives for about one day. As the food is no more required, the lining slowly breaks and comes out through the vagina as blood and mucous.

(iii) This cycle takes place roughly every month and is known as menstruation. It usually lasts for about two to eight days.

8. Reproductive Health deals with those aspects of general health which helps a person to lead a normal and reproductive life.

(i) Sexually transmitted diseases are those diseases which spread by sexual contact from an infected person to a healthy person. These include bacterial infections such as gonorrhoea and syphilis, and viral infections such as warts and HIV-AIDS. A condom for covering the penis during sex can prevent transmission these infections.

(ii) Gonorrhoea – It is caused by bacterium Neisseria gonorrhoea. It is characterised by inflammation of urinogenital tract and the patient feels burning sensation during urination. This bacteria infects the ureter in men and the cervix in women.

(iii) Syphilis – It is caused by Treponema pallidium bacteria. It is characterised by lesions in the mucous membrane of urinogenital tract and ulcers on genitalia.

(iv) AIDS (Acquired Immune Deficiency syndrome)– It is caused by HIV virus which suppresses the body’s immune mechanism and thereby making it susceptible to any disease. It can be transmitted by Sexual Contact, Blood transfusion, infected needles used for injections, from mother to child during pregnancy.

10. Methods to avoid Pregnancy – There are various ways are :

(i) Mechanical Methods – In this type of contraception, a physical barrier is placed to prevent the entry of sperms into the uterus. It includes condoms used by males and diaphragm cap used by females. Diaphragm cap is also fixed over the cervix but is not a permanent arrangement These devices can also act as effective guard against sexually transmitted diseases.

(ii) Contraceptive Devices – These are also called intrauterine device (IUD). Here a loop or the copper-T is inserted by a doctor into the uterus to prevent the implantation of the fertilized egg.

(iii) Chemical /Hormonal Methods – It involves the intake oral pills which contain hormones which stop the ovaries from releasing ovum/egg into the fallopian tube. These are called oral contraceptive pills which changes the hormonal balance of the body so that egg is not released and therefore fertilisation cannot occur.

(iv) Surgical/Sterilisation method – Sterilisation is a surgical procedure that involves cutting of the tubes that conduct the gametes. In males, it is called vasectomy in which each vas deferens is cut/blocked. In females, it is called tubectomy in which the oviducts are cut/blocked. They are as good as permanent methods of prevention of pregnancy.

11. Sex Ratio – It is the ratio of the number of females to the number of males in a population. The female-male sex ratio must be maintained for healthy society. Due to preference for males in society and female foeticides the number of females per 1000 males is decreasing rapidly. Therefore, prenatal sex determination has been prohibited by Law.

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Class X Biology: Sexual Reproduction in Flowering Plants


1. Sexual Mode of Reproduction – In this process, the exchange of genetic materials takes place by a process called conjugation.

2. sexual reproduction involves the production of germ cells or gametes and their subsequent fusion to produce a new individual.

3. In sexual reproduction, a male gamete fuses with a female gamete to form a new cell called zygote.

4. This zygote then grows and develops into a new organism in due course of time.

5. One germ-cell is large and contains the food-stores while the other is smaller and likely to be motile. Conventionally, the motile germ cell is called the male gamete and the germ-cell containing the stored food is called the female gamete.


1. The plants in which the sex organs are carried within the flowers and the seeds are enclosed in a fruit are called angiosperms (flowering plants).

2. The flowering plants are reproduced by sexual reproduction method.

3. The function of a flower is to make male and female gametes and to ensure that fertilisation will take place to make new seeds for the reproduction of the plant.

4. The different reproductive parts of a flower are – sepals, petals, stamens, and carpels. Stamens and carpels are the reproductive parts of a flower which contain the germ-cells.

5. The flower may be unisexual (papaya, watermelon) when it contains either stamens or carpels or bisexual (Hibiscus, mustard) when it contains both stamens and carpels.

6. Parts of a Flower: The main parts of a flower are:

(i) Receptacle: The base of a flower to which all the parts of a flower are attached is called receptacle.

Parts of a Flowering Plant2

(ii) Sepals (group name calyx): Sepals are green outermost leaf-like floral organs which protect the flower in the bud stage.

(iii) Petals (group name corolla): The colourful parts of a flower are called petals. The petals lie inside the sepals. Petals attract insects to flowers for pollination. They also provide protection to centrally placed reproductive organs.

(iv) Stamen: Stamen is the male reproductive part and it produces pollen grains that are yellowish in colour. The stalk of stamen is called filament and the swollen top of stamen is called anther. The anther of stamen makes the pollen grains and stores them.

(v) Carpel: Carpel is present in the centre of a flower and is the female reproductive part. It is made up of three parts. The swollen bottom part is ovary, middle elongated part is the style and the terminal part which may be sticky is the stigma. The ovary contains ovules and each ovule has an egg cell.

7. Pollination: The transfer of pollen grains from the anther of a stamen to the stigma of a carpel is called pollination.


(i) If the transfer of pollen occurs in the same flower or another flower on the same plant, it is called self-pollination.

(ii) If the pollen is transferred from the anther of a flower on one plant to the stigma of a flower on another similar plant, it is called cross-pollination.

(iii) Pollination is done by insects, birds, winds, and water.

(iv) After the pollen lands on a suitable stigma, a tube grows out of the pollen grain and travels through the style to reach the ovary leading to fertilisation.

8. Fertilization: The fusion of the male gamete with the female gamete is called fertilisation. Fertilization occurs when the male gamete present in pollen grain joins with the female gametes present in ovule.

(i) A male gamete moves down the pollen tube. The pollen tube enters the ovule in the ovary. The tip of pollen tube bursts open and male gamete comes out of pollen tube.

(ii) In ovary, the male gamete of pollen combines with the nucleus of female gamete or egg present in ovule to form a fertilized egg called zygote.

(iii) After fertilization, the zygote divides several times to form an embryo within the ovule. The ovule develops a tough coat and is gradually converted into a seed. The seed contains the future plant or embryo which develops into a seedling under appropriate conditions. This process is known as germination.

(iv) The ovary grows rapidly and ripens to form a fruit. The other parts of flower like sepals, petals, stamen, stigma and style dry up and fall off.


9. The male germ-cell produced by pollen grain fuses with the female gamete present in the ovule. This fusion of the germ-cells or fertilisation gives us the zygote which will growing into a new plant.

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Class X Biology: Asexual Reproduction

1. Reproduction is defined as the production of individuals of the same species, that is the next generation of the species. Reproduction is thus essential for the continuation of the species. It ensures that the genetic material of one generation is transmitted to the next.

2. Importance of Variation– reproduction involves making copies of the blueprints of body design. The chromosomes in the nucleus of a cell contain information for the inheritance of features from parents to next generation in the form of DNA (Deoxyribose Nucleic Acid) molecules. Cells use chemical reactions to build copies of their DNA. The process of copying DNA will have some variations each time. The DNA copies generated will be similar but may not be identical to the original. This gives rise to variations. Over time these variations accumulate and give rise to new species or help in adapting to the environment and if not the organisms perish.

3. Types of Reproduction – There are two main methods of reproduction in living-

(i) Asexual/Vegetative reproduction – The production of a new organism from a single parent without the involvement of sex cells (or gametes) is called asexual reproduction.

(ii) Sexual reproduction – The process of production of a new organism from two parents by making use of sex cells.


4. Methods of Asexual Reproduction [ 6 methods]-

(i) Fission – In the process of fission, a unicellular organism split to form two or more new organisms. It is of two types.

(a) Binary fission: In Binary Fission, the parent organism splits to form two new organisms. for example- Amoeba, Paramecium Leishmania (which cause kala-azar), Bacteria etc are reproduced by binary fission.

Binary Fission

(b) Multiple Fission – In multiple fission, the parent organism splits to form many new organisms at the same time. For example Plasmodium. Some organisms during unfavorable condition a cyst or protective wall are formed around the cell of Plasmodium. Inside the cyst, the nucleus of cell splits several time to form many daughter nuclei and then cytoplasm collects around each daughter nuclei and thin membrane forms so many new daughter cells are formed within a cyst. When a favorable condition comes, the cyst breaks open and daughter cells are released each farming a new organism.

Multiple Fission

(ii) Budding – In budding a small part of a body of parent organism grows out as a bud which then detaches and becomes a new organism. For Example Hydra, yeast reproduces by budding. In Hydra, a small outgrowth (bud) is formed on the sites of its body by the repeated mitotic division of its cell. This bud then grows gradually by developing mouth and tentacles and then tiny new hydra detaches itself from parent organism and lives as a separate organism.


(iii) Spore formation – In spare formation, the parent plant produces 100 of microscopic reproductive units called “spores” within the spore case. When the spore case burst, then the spores spread into the air. When these airborne spores land on food or soil, under favorable condition they germinate and produce new plants for example -Most of the fungi such as Rhizopus ( bread mold), mucor, bacteria, non-flowering plants, ferns, and mosses.

Spore formation 3


(iv) Regeneration – The process of getting back off full organism from its body part is called regeneration. For example, simple animals like Hydra and Planaria show regeneration. Planaria (Flatworm) is found in freshwater ponds. If the body of Planaria somehow gets cut into a number of pieces then each body piece can regenerate into complete Planaria.


(v) Fragmentation – The breaking up of body of simple multicellular organism into two or more fragments or maturing, each of which subsequently grows to form a new complete organism is known as fragmentation. Spirogyra is a green filamentous algae. Spirogyra breaks into two or more fragments on maturing and each fragment then grows into new spirogyra.


(vi) Vegetative Propagation – In vegetative propagation new plants are obtained from the parts of old plants (stem, leaves, or root) without the help of any reproductive organ. This can be done in two ways

(a)Natural methods of vegetative propagation: The green grass grows in the field after rain from the dry, old stem of grass plant present in the field by the method of vegetative propagation. Bryophyllum plants can be reproduced by vegetative propagation by using leaves.

a. Plant                     Vegetative propogule
b. Potato                  Potato tuber (stem)
c. Begonia               Leaves
d. Money plant      Stem

(b) Artificial method of vegetative propagation – The process of growing many plants from one plant by man-made method is called vegetative propagation.

I. Cutting: A small part of plant which is removed by making a cut with sharp knife is called cutting. A cut may be a piece of stem, root or leaves. For example-The plant like rose, sugarcane, cactus is grown by cutting. It is necessary that there are some buds on it.


II. Layering: In this method a branch of plant is pulled towards the ground and a part of it is covered with moist soil leaving the tip of branch exposed above the ground. after some time new roots develop and then it cut off from parent plant and grows as new plant. for example- Jasmine, strawberry, hibiscus, and guava etc.


III. Grafting: It is the method in which the cut stem of two different plant one with root and other without root and joined together in such a way that the two stem joint and grows as a single plant. This new plant has the characteristics of both the plants. The cut stem of plant having root is called stock and the cut stem of another plant is called Scion. For example- Apple, peach, apricot, pear etc.


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Class X Science: Management of Natural Resources

1. Environment refers to the natural elements that make up the earth and surround living organisms, making life processes possible.

(i) Soil, water, air, light, and temperature are the physical aspects of environment and are known as its ‘abiotic‘ components.

(ii) All plants and animals collectively make up the biological aspects of environment and are referred to as its ‘biotic‘ components.

2. Natural Resources are materials or substances occurring in nature which can be exploited for economic gain. They can be divided into two types:

(i) Inexhaustible Natural Resources – Resources that are unlimited in nature and are not likely to be exhausted by human activities. Solar radiation, air, water, precipitation (rainfall, snowfall, etc.,) and atomic power are some instances of such resources.

(ii) Exhaustible Natural Resources – Natural resources that are limited in nature and are liable to be degraded in quantity and quality by human activities are exhaustible natural resources. Examples are forests, soil, wild animals, minerals, fossil fuels etc.

3. Environmental conservation has gained enormous importance since the last quarter of the 20th century. The terms environmental pollution, global warming, climate change, and destruction of rainforests, are all issues of global concern that are already affecting our lifestyles and sense of well-being.

4. Natural resources like forests, wildlife, water, coal and petroleum and see what are the issues at stake in deciding how these resources are to be managed for sustainable development.

5. While conservation and utilization of natural resources in a sustainable manner are what responsible citizens and the governments should aim to achieve. To ensure sustainable use of resources in our environment utilize the principle of ‘The Three R’s.’ – Reduce, Reuse, and Recycle.

Reduce: This means that you useless. You save electricity by switching off unnecessary lights and fans. You save water by repairing leaky taps. You do not waste food.

Recycle: This means that you collect plastic, paper, glass and metal items and recycle these materials to make required things instead of synthesizing or extracting fresh plastic, paper, glass or metal. To recycle, we first need to segregate our wastes so that the material that can be recycled is not dumped along with other wastes.

Reuse: In the ‘reuse’ strategy, you simply use things again and again. Instead of throwing away used envelopes, you can reverse it and use it again. The plastic bottles in which you buy various food-items like jam or pickle can be used for storing things in the kitchen.

6. The concept of sustainable development encourages forms of growth that meet current basic human needs while preserving the resources for the needs of future generations. Sustainable development implies a change in all aspects of life. It depends upon the willingness of the people to change their perceptions of the socio-economic and environmental conditions around them, and the readiness of each individual to alter their present use of natural resources.

7. Economic development is linked to environmental conservation.


1. The management of natural resources utilization should be for long-term gains and not for short-term gains.

2. The management should also ensure equitable distribution of resources so that all, and not just a handful of rich and powerful people, benefit from the development of these resources.

3. Sustainable development method should be considered while exploiting natural resources to limit the damage caused to the environment while these resources are either extracted or used. For example, mining causes pollution because of a large amount of slag discarded for every tonne of metal extracted.

4. Here consider management of: –

(i) Forests and Wild Life
(ii) Water
(iii) Coal and Petroleum

5. Conservation – Conservation is defined as the controlled utilization of natural resources for the benefit of all life so that it may yield sustainable benefit to the present generations as well as the future generations.


1. Forests are ‘biodiversity hot spots’. One measure of the biodiversity of an area is the number of species found there. However, the range of different life forms (bacteria, fungi, ferns, flowering plants, nematodes, insects, birds, reptiles and so on) is also important.

2. One of the main aims of conservation is to try and preserve the biodiversity we have inherited. Experiments and field studies suggest that a loss of diversity may lead to a loss of ecological stability.

3. When we consider the conservation of forests, we need to look at the stakeholders who are –

(i) the people who live in or around forests are dependent on forest produce for various aspects of their life.

(ii) the Forest Department of the Government which owns the land and controls the resources from forests.

(iii) the industrialists – from those who use ‘tendu’ leaves to make bidis to the ones with paper mills – who use various forest produce but are not dependent on the forests in any one area.

(iv) the wildlife and nature enthusiasts who want to conserve nature in its pristine form.

4. There have been enough instances of local people working traditionally for conservation of forests-

(i) For example, the Bishnoi community in Rajasthan, for whom conservation of forest and wildlife has been a religious tenet.

(ii) Amrita Devi Bishnoi, in 1731 sacrificed her life along with 363 others for the protection of ‘khejri’ trees in Khejrali village near Jodhpur in Rajasthan.

(iii) The Government of India has recently instituted an ‘Amrita Devi Bishnoi National Award for Wildlife Conservation’.

Amrita Devi Bishnoi


5. The Chipko Andolan (‘Hug the Trees Movement’) was the result of a grassroot level effort to end the alienation of people from their forests.

(i) The movement originated from an incident in a remote village called Reni in Garhwal, high-up in the Himalayas during the early 1970s.

(ii) There was a dispute between the local villagers and a logging contractor who had been allowed to fell trees in a forest close to the village.

(iii) The contractor’s workers appeared one day in the forest to cut the trees while the menfolk were absent.

(iv) Undeterred, the women of the village reached the forest quickly and clasped the tree trunks thus preventing the workers from felling the trees. The contractor had to withdraw.

(v) The Chipko movement quickly spread and forced the government to rethink their priorities in the use of forest produce.

6. An Example of People’s Participation in the Management of Forests –

(i) In 1972, the West Bengal Forest Department recognised its failures in reviving the degraded Sal forests in the southwestern districts of the state.

(ii) Thereafter, Department involved villagers in the protection of 1,272 hectares of badly degraded sal forest. In return for help in protection, villagers were given employment in both silviculture and harvesting operations, 25 per cent of the final harvest, and allowed fuelwood and fodder collection on payment of a nominal fee.

(iii) With the active and willing participation of villagers the sal forests of Arabari underwent a remarkable recovery.


1. Irrigation methods like dams, tanks and canals have been used in various parts of India since ancient times.

2. Of the fresh water, available there are three main sources.

(i) First one is rainwater (or precipitation).

(ii) Second source is surface water; there are 14 major river systems such as Ganga, Brahmaputra, Godavari, Krishna, and Cauvery.

(iii) Third is groundwater or underground aquifers which is the water that percolates down the surface soil into pore spaces of rocks.

3. It is evident that human intervention can change the availability of water in various regions; it can also lead to pollution of water sources.

4. Dams – Large dams can ensure the storage of adequate water not just for irrigation, but also for generating electricity. Canal systems leading from these dams can transfer large amounts of water great distances. For example, the Indira Gandhi Canal has brought greenery to considerable areas of Rajasthan.

5. Criticisms about large dams address three problems in particular –

(i) Social problems because they displace large number of peasants and tribals without adequate compensation or rehabilitation,

(ii) Economic problems because they swallow up huge amounts of public money without the generation of proportionate benefits,

(iii) Environmental problems because they contribute enormously to deforestation and the loss of biological diversity.

6. Water HarvestingWatershed management emphasizes scientific soil and water conservation to increase the biomass production. The aim is to develop primary resources of land and water, to produce secondary resources of plants and animals for use in a manner which will not cause ecological imbalance.

7. Watershed management not only (i) increases the production and income of the watershed community, but also (ii) mitigates droughts and floods and increases the life of the downstream dam and reservoirs.

8. Water harvesting is an age-old concept in India. Some of the ancient water harvesting, including water conveyance, structures still in use today are: –

(i) Khadins, Tanks and Nadis in Rajasthan,
(ii) Bandharas and Tals in Maharashtra,
(iii) Bundhis in Madhya Pradesh and Uttar Pradesh,
(iv) Ahars and Pynes in Bihar,
(v) Kulhs in Himachal Pradesh,
(vi) Ponds in the Kandi belt of Jammu region, and
(vii) Eris (tanks) in Tamil Nadu,
(viii) Surangams in Kerala, and
(ix) Kattas in Karnataka

9. In level terrain, the water harvesting structures are mainly crescent-shaped earthen embankments or low, straight concrete-and rubble “check dams”. Their main purpose is to recharge the groundwater beneath. The advantages of groundwater are:-

(i) It does not evaporate, but spreads out to recharge wells and provides moisture for vegetation over a wide area.

(ii) It does not provide breeding grounds for mosquitoes like stagnant water collected in ponds or artificial lakes. The ground-water is also relatively protected from contamination by human and animal waste.

10. In conclusion: –

(i) the management of water resources of a country does require government interventions like the national river commission under which the Ganga action plan I and II and major irrigation projects are managed.

(ii) utilize the local people and resources for water regeneration and harvesting.


1. Fossil fuels are coal and petroleum

2. Coal and petroleum were formed from the degradation of bio-mass millions of years ago. Both have an organic origin and are called hydrocarbon fuels.

3. Coal was formed in nature as a solid from the remains of the trees buried deep inside the earth, some 500 million years ago.

4. Petroleum also occurs deep inside the earth’s crust, as a liquid, and is formed by the bacterial decomposition of marine plant and animal matter and prehistoric forests in the absence of air buried at the bottom of the seas. Petroleum is often referred to as liquid gold, due to its importance as a fuel in transportation (Petrol, Diesel, kerosene, gas oil, fuel oil).

5. Fossil fuels, like, coal, lignite, petroleum and natural gas are non-renewable natural resources.

6. Since coal and petroleum have been formed from biomass, in addition to carbon, these contain hydrogen, nitrogen, and sulphur.

7. When these are burnt, the products are carbon dioxide, water, oxides of nitrogen and oxides of sulphur. When combustion takes place in insufficient air (oxygen), then carbon monoxide is formed instead of carbon dioxide.

8. Of these products, the oxides of sulphur and nitrogen and carbon monoxide are poisonous at high concentrations and carbon dioxide is a greenhouse gas leading to intense global warming.

9. Conservation and managing of these resources are to: –

(i) substitute existing technologies so that hydrocarbon fuels are more efficiently used or used less.
(ii) is to affect the policy of finding non-conventional and renewable energy sources.

10. Some simple environment-friendliness steps would be: –

(i) Taking a bus, using your personal vehicle or walking/ cycling.
(ii) Using bulbs or fluorescent tubes in your homes.
(iii) Using the lift or taking the stairs.
(iv) Wearing an extra sweater or using a heating device (heater or ‘sigri’) on cold days.

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Class X Science: Sources of Energy


1. a good source of energy would be one

(i) A large amount of work per unit volume or mass,
(ii) easily accessible,
(iii) easy to store and transport, and
(iv) economical.

2. We can classify energy sources in different ways. One classification is based on:-

(i) the reserves (supply) of an energy source [ Conventional and non-conventional sources of energy]

(ii) Another is based on how long we have been using it. [Renewable and non-renewable sources of energy]

3. Renewable sources of energy are those which can be generated by us or which are constantly being generated by natural processes or whose supply is unlimited. Wood is a renewable source of energy although its supply is limited. Nuclear fuels such as uranium are required in very small quantities to generate electricity. So, their reserves will last for a long, long time. Therefore, they are classified as renewable sources of energy.

4. Non-renewable sources of energy are those which were produced in the past by natural processes, whose supply is limited and which we cannot generate ourselves. Coal, natural gas and petroleum are non-renewable sources of energy.

5. Sources of energy that have been in use for centuries are called conventional sources of energy. Conventional sources include wood, coal, petroleum and flowing water. Conventional sources like coal and petroleum are non-renewable, while sources like flowing water are renewable.

6. Sources of energy that we have started using in new ways or only in recent times are called non-conventional or alternate sources of energy. These include energy from the sun, the heat inside the earth (geothermal energy), tides, ocean waves, etc. Nuclear energy is also a nonconventional source. Noon-conventional energy sources are renewable. We have been using wind and biomass (like cow dung) for energy for ages. However, they were not used conventionally to do tasks like electricity generation, which has now been made possible with improvement in technology. In that sense, they can also be called nonconventional sources of energy.


1. Fossil Fuels are Coal and Petroleum.

(i) about 25% of our total energy consumption comes from coal. More than 90% of this coal is used for electricity generation in thermal power plants.

(ii) In most thermal power plants, coal is burnt to heat water for producing steam. The steam falls on the blades of a special kind of wheel called a turbine. A turbine is a device that rotates when steam, water or windfalls on its blades. The turbine turns the shaft of an electric generator to which it is connected. In this way, electricity is produced.

2. The burning of most fossil fuels causes air pollution. The pollutants produced include carbon dioxide, carbon monoxide, oxides of nitrogen, sulphur dioxide and unburnt particles.

(i) The burning of fossil fuels produces carbon dioxide leading to the greenhouse effect and global warming.

(ii) carbon monoxide is produced by incomplete combustion of coal and petrol. Excessive inhalation of this poisonous gas can cause death.

(iii) During the combustion of coal sulphur dioxide and oxides of nitrogen are formed combine with oxygen at the high temperatures inside a running automobile engine to form sulphuric acid and nitric acid. These fall on the earth with rain, which we call acid rain. Acid rain damages soil, water bodies, crops, living tissues and structures like the Taj Mahal.

(iv) The unburnt particles produced during the combustion of fossil fuels are carried by smoke. They affect our lungs, and blacken clothes and buildings.

3. Burning fossil fuels has other disadvantages too. The oxides of carbon, nitrogen and sulphur that are released on burning fossil fuels are acidic oxides. These lead to acid rain which affects our water and soil resources.

4. Hydro Power Plants – Another conventional source of energy was the kinetic energy of flowing water or the potential energy of water at a height. Hydropower plants convert the potential energy of falling water into electricity. A quarter of our energy requirement in India is met by hydropower plants.

5. Advantages of hydroelectricity: –

(i) The source of energy (water) is free and renewable.

(ii) Harnessing the energy of flowing water is a pollution-free process, with no smoke, chemicals, etc., being produced.

(iii) The cost of electricity generation is low as compared to electricity generated from other types of power plants.

(iv) Flowing water is a more reliable source of energy than wind.

(v) Hydroelectricity can be generated on a large scale from a single plant.

(vi) Dams built for hydroelectric plants also help in flood control and irrigation.

(vii) Small hydroelectric stations are one of the best options for generating electricity from renewable sources, as they do not affect the environment much.

Srisailam Dam

6. To produce hydel electricity, high-rise dams are constructed on the river to obstruct the flow of water and thereby collect water in larger reservoirs. The water in the reservoir would be refilled each time it rains (hydropower is a renewable source of energy). But, constructions of big dams have certain problems associated with it.

(i) Large areas of agricultural land and human habitation are to be sacrificed as they get submerged.

(ii) Large eco-systems are destroyed when submerged under the water in dams.

(iii) The vegetation which is submerged rots under anaerobic conditions and gives rise to large amounts of methane which is also a greenhouse gas.

(iv) It creates the problem of satisfactory rehabilitation of displaced people.

Improvements in the Technology for using Conventional Sources of Energy

7. Bio-Mass – When fuels are plant and animal products the source is said to be bio-mass. These fuels do not produce much heat on burning and a lot of smoke is given out when they are burnt. Therefore, technological inputs to improve the efficiency of these fuels are necessary.

(i) Cow-dung, various plant materials like the residue after harvesting the crops, vegetable waste and sewage are decomposed in the absence of oxygen to give bio-gas. Since the starting material is mainly cow-dung, it is popularly known as ‘gobar-gas’.

(ii) Bio-gas is produced in a plant. The plant has a dome-like structure built with bricks. A slurry of cow-dung and water is made in the mixing tank from where it is fed into the digester.

(iii) The digester is a sealed chamber in which there is no oxygen. Anaerobic micro-organisms that do not require oxygen decompose or break down complex compounds of the cow-dung slurry.

(iv) It takes a few days for the decomposition process to be complete and generate gases like methane, carbon dioxide, hydrogen and hydrogen sulphide.

(v) The bio-gas is stored in the gas tank above the digester from which they are drawn through pipes for use.

(vi) Bio-gas is an excellent fuel as it contains up to 75% methane. It burns without smoke, leaves no residue like ash in wood, charcoal and coal burning. Its heating capacity is high. Bio-gas is also used for lighting.

(vii) The slurry left behind is removed periodically and used as excellent manure, rich in nitrogen and phosphorous. The large-scale utilisation of bio-waste and sewage material provides a safe and efficient method of waste-disposal besides supplying energy and manure.

biogas plants

8. Advantages of Biogas Plants

(i) Reduces burden on forests and fossil fuels
(ii) Produces a clean fuel – helps in controlling air pollution
(iii) Provides nutrient-rich (N & P) manure for plants
(iv) Controls water pollution by decomposing sewage, animal dung and human excreta

9. Limitations of Biogas Plants

(i) Initial cost of installation of the plant being high, and is beyond the reach of an average rural Indian

(ii) Number of cattle owned by an average family of farmers is inadequate to feed a biogas plant


10. Wind Energy – Unequal heating of the landmass and water bodies by solar radiation generates air movement and causes winds to blow.

(i) The kinetic energy of the wind can be used to do work. This energy was harnessed by windmills in the past to do mechanical work. Today, wind energy is also used to generate electricity.

(ii) The output of a single windmill is quite small and cannot be used for commercial purposes. Therefore, a number of windmills are erected over a large area, which is known as wind energy farm. The energy output of each windmill in a farm is coupled together to get electricity on a commercial scale.

Wind Energy Farm

11. Advantages of wind energy

(i) The source of energy (wind) is free.

(ii) Harnessing wind energy is a pollution-free process, with no smoke, chemicals, etc., being produced.

(iii) A small wind-electric plant can be set up near a factory to provide pollution-free power for its use.

12. Limitations of wind energy

(i) Wind energy cannot be harnessed at places where wind does not blow regularly. A wind-electric generator works only on winds of at least 15 km/h.

(ii) Wind is not a dependable source of energy because sometimes the air is absolutely still and at other times there are storms.

(iii) It is expensive to set up a wind farm for generating electricity because wind farms need a large area.

(iv) Moreover, since the tower and blades are exposed to the vagaries of nature like rain, Sun, storm and cyclone, they need a high level of maintenance.


13. Solar Energy is radiant light and heat from the Sun.

14. A solar heating device is one which allows collection of a large amount of heat from the sunlight in a given region and restricts the loss of heat to the surrounding in the form of radiation.

15. The following facts help us to design an efficient solar cooker and solar water heaters:

(i) A black body absorbs maximum heat

(ii) Loss of heat by convection can be restricted in a closed system

(iii) Radiation losses can be avoided by reflecting the rays inside the body with the help of reflecting object like a plane mirror

(iv) Conduction can be reduced by using poor conducting materials


16. Advantages of Solar Cooker-

(i) high performance
(ii) no fuel
(iii) eco-friendly
(iv) one-time investment

17. Disadvantages of Solar Cooker-

(i) can’t be used in cloudy weather
(ii) take longer to cook
(iii) need to learn special methods to cook

solar cooker

18. Solar cells are the devices where solar energy is directly converted into electricity. A large number of solar cells are, combined in an arrangement called solar cell panel that can deliver enough electricity for practical use.


19. Advantages of using Solar Cells

(i) They have no moving parts and hence require little maintenance and work quite satisfactorily without any focusing device

(ii) It does not cause any environmental pollution like the fossil fuels and nuclear power

(iii) Solar cells last a long time and have low running costs.

(iv) They can be set up in remote and inaccessible hamlets or very sparsely inhabited areas

20. Disadvantages using Solar Cells

(i) The entire process of manufacture is still very expensive as silver is used for interconnection of these cells in the panel, which is a very expensive metal.

(ii) A practical problem linked with the use of solar cell panels is regarding the storage of electricity storage batteries which give us only direct current. But to operate our devices we need alternating current. Therefore, we need to convert DC to AC before using any appliance and thus it increases the cost of such solar panels as the sources of electricity.

21. Uses of Solar Cells: –

(i) Artificial satellites and in space probes like Mars orbiters
(ii) Wireless transmission systems or TV relay stations in remote locations
(iii) Traffic signals, calculators and in toys


22. Sea Energy can be harnessed through:

(i) Tides
(ii) Sea-waves
(iii) Temperature differences in the water

23. Tidal Energy – Due to the gravitational pull of mainly the moon on the spinning earth, the level of water in the sea rises and falls. This phenomenon is called high and low tides and the difference in sea-levels gives us tidal energy.

(i) Tidal energy is harnessed by constructing a dam across a narrow opening to the sea.

(ii) During high tide, when the level of water in the sea is high, sea-water flows into the reservoir of the barrage and turns the turbines. The turbines then turn the generator shaft to produce electricity.

(iii) During low tide, the sea-water stored in the barrage reservoir flows out into the sea. This flowing water also turns the turbines and generates electricity.

tidal energy2

24. Limitations of Tidal Energy

(i) The tides do not possess enough energy to generate electricity on a large scale

(ii) Only few sites are suitable for building such barrages (or tidal dams)

25. Wave Energy – The waves are generated by strong winds blowing across the sea.

(i) Kinetic energy possessed by huge waves near the seashore can be trapped to generate electricity.

(ii) A wide variety of devices have been developed to trap wave energy for rotation of turbine and production of electricity.

(iii) Wave energy would be a viable proposition only where waves are very strong.

wave energy devices


26. Ocean Thermal Energy – there is always a temperature difference between the water “at the surface of ocean” and “at deeper levels.” At many places in the ocean, this difference in temperatures of water is up to 20oC. The energy available due to the difference in the temperature of water at the surface of the ocean and at deeper levels is called ocean thermal energy(OTC).

(i) These plants can operate if the temperature difference between the water at the surface and water at depths up to 2 km is 293 K (20°C) or more.

(ii) In one type of OTEC [Ocean Thermal Energy Conversion] power plants, the warm surface-water is used to boil a volatile liquid like ammonia. The vapours of the liquid are then used to run the turbine of generator. The cold water from the depth of the ocean is pumped up and condense vapour again to liquid.


27. Advantages of OETC are

(i) A greater advantage of the ocean thermal energy is that it can be used continuously for 24 hours throughout the year.

(ii) Ocean thermal energy is a renewable source of energy and its use does not cause any pollution.

28. Limitation – The installation and maintenance of an OTEC power plant is very high and efficient commercial exploitation is difficult.

29. Geothermal Energy is that heat energy obtained from hot rocks present inside the earth. This heat can be used as a source of energy to produce electricity. When underground water comes in contact with the hot spot, steam is generated. Such outlets are known as hot springs. The steam trapped in rocks is routed through a pipe to a turbine and used to generate electricity.

30. Advantages of Geothermal Energy

(i) Geothermal plants can operate round the clock, unlike those based on solar and tidal energy

(ii) Geothermal energy, is almost pollution-free

(iii) The cost of a geothermal plant is high as compared to a coal-based plant, but operating costs are lower

Geothermal Power
31. Disadvantages of Geothermal Energy

(i) Not all areas are suitable for production of electricity in this way
(ii) It depends on the structure of rocks
(iii) It creates noise pollution
(iv) very few commercially viable sites where such energy can be exploited.

32. There are a number of power plants based on geothermal energy operational in New Zealand and the United States of America.


1. Nuclear Energy – In a process called nuclear fission, the nucleus of a heavy atom (such as uranium, plutonium or thorium), when bombarded with low-energy neutrons, can be split apart into lighter nuclei. When this is done, a tremendous amount of energy is released

2. The released energy can be used to produce steam and further generate electricity.

Nuclear Power Plant

3. Advantages of Nuclear Power –

(i) Nuclear power is a viable option where fossil fuels like coal are not available
(ii) If operated properly, nuclear power plants produce less atmospheric pollution than thermal power plants

(iii) It consumes very little fuel. It can operate for more than a year without needing new fuel elements

(iv) A sizeable amount of fuel (uranium and plutonium) can be reclaimed by processing the spent fuel material in contrast to fuels like coal which cannot be reclaimed

(v) Some radioactive by-products in the process are used in medicine and industry

4. Disadvantages-

(i) The high cost of installation of a nuclear power plant

(ii) high risk of environmental contamination

(iii) limited availability of uranium makes large-scale use of nuclear energy prohibitive

(iv) The disposal of radioactive wastes in the fission process is a major problem. Expensive long-term storage areas have to be built

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Class X Science: Our Environment

1. Environment is defined as the surroundings or conditions in which a person, animal, or plant lives or operates.

2. The environment may be the physical environment, the chemical environment or the biological environment. Thus, the environment has two components – abiotic and biotic.


3. The abiotic environment includes the air (atmosphere), water (hydrosphere) and land (lithosphere). The biotic environment includes the plants, animals and the microbes.

4. A healthy environment is an absolute necessity for the well-being of all organisms, including man. Accumulation of wastes due to its improper disposal is a major problem in our country.


1. We generate a lot of material that is thrown away called waste materials.

(i) Substances that are broken down by biological processes are said to be biodegradable.

(ii) Substances that are not broken down by biological processes are said to be non-biodegradable.


1. All organisms such as plants, animals, microorganisms and human beings as well as the physical surroundings interact with each other and maintain a balance in nature. All the interacting organisms in an area together with the non-living constituents of the environment form an ecosystem.

2. An ecosystem consists of biotic components comprising living organisms and abiotic components comprising physical factors like temperature, rainfall, wind, soil and minerals.

3. For example, if you visit a garden you will find different plants, such as grasses, trees; flower-bearing plants like rose, jasmine, sunflower; and animals like frogs, insects and birds. All these living organisms interact with each other and their growth, reproduction and other activities are affected by the abiotic components of the ecosystem. So, a garden is an ecosystem.

4. Other types of ecosystems are forests, ponds and lakes. These are natural ecosystems while gardens and crop-fields are human-made (artificial) ecosystems.

5. Organisms can be grouped as producers, consumers and decomposers according to which they obtain their food from the environment.

6. All green plants and certain blue-green algae which can produce food by photosynthesis come under this category and are called the producers.

7. These organisms which consume the food produced, either directly from producers or indirectly by feeding on other consumers are the consumers. Consumers can be classed variously as herbivores, carnivores, omnivores and parasites.

8. The microorganisms, comprising bacteria and fungi, break-down the dead remains and waste products of organisms. These microorganisms are the decomposers as they break-down the complex organic substances into simple inorganic substances.

9. The organisms, in addition to being dependent on the environment for their needs, are also dependent on each other. This dependency is especially for food. This results in the presence of food chains and food webs.


1. Food chain – A succession of organisms in an ecological community that are linked to each other through the transfer of energy and nutrients, beginning with an autotrophic organism such as a plant and continuing with each organism being consumed by one higher in the chain.

trophic levels3

2. Each step or level of the food chain forms a trophic level.

(i) The autotrophs or the producers are at the first trophic level. They fix up the solar energy and make it available for heterotrophs or the consumers.

(ii) The herbivores or the primary consumers come at the second trophic Level.
(iii) Small carnivores or the secondary consumers are the third Level.

(iv) Larger carnivores or the tertiary consumers form the fourth trophic level.

3. The study of food chains helps in understanding of the important aspects of the ecosystem in particular and environment in general:-

(i) The food relationship among the different organisms in an ecosystem

(ii) The food chains are the living components of the biosphere

(iii) These are the vehicles of transfer of energy from one level to another

(iv) Through the food chains, transfer of materials and nutrients also takes place

4. Autotrophs capture the energy present in sunlight and convert it into chemical energy.

foof web2

5. From autotrophs, the energy goes to the heterotrophs and decomposers. When one form of energy is changed to another, some energy is lost to the environment in forms which cannot be used again. It is noted that:-

(i) Green plants in a terrestrial ecosystem capture about 1% of the energy of sunlight that falls on their leaves and converts it into food energy.

(ii) An average of 10% of the food eaten is available for the next level of consumers. A great deal of energy is lost as heat to the environment, some amount goes into digestion and in doing work and the rest goes towards growth and reproduction.

(iii) Since so little energy is available for the next level of consumers, food chains generally consist of only three or four steps. The loss of energy at each step is so great that very little usable energy remains after four trophic levels.

(iv) There are generally a greater number of individuals at the lower trophic levels of an ecosystem, the greatest number is the producers.

6. Food web is a system of interlocking and interdependent food chains. Each organism is generally eaten by two or more other kinds of organisms which in turn are eaten by several other organisms. So instead of a straight line food chain, the relationship can be shown as a series of branching lines called a food web.

7. The interactions among various components of the environment involve flow of energy from one component of the system to another.

8. The significance of Eco-System:

(i) Firstly, the flow of energy is unidirectional. The energy that is captured by the autotrophs passes to the herbivores does not come back to autotrophs. As it moves progressively through the various trophic levels it is no longer available to the previous level.

(ii) Secondly, the shorter the food chain, the greater is the available energy.

(iii) Thirdly, the nutrient movement goes on side by side along with a unidirectional flow of energy. The nutrient movement is a cyclic movement where the nutrients revolve around with an ecosystem, hence cyclic. This cyclic movement of nutrients is also called as biogeochemical cycle.

9. The phenomenon of passing the toxic substances through different levels is termed as biological magnification. The movement of some toxic substances (like DDT) in the ecosystem, sprayed to kill the pests and insects passes through the various trophic levels and it accumulates at the highest trophic level. Man being an omnivore, man eats food of every type and in the process receives these poisons at every trophic level leading to accumulation of toxic substance.


1. We look at two of the environmental problems in detail, depletion of the ozone layer and waste disposal.

2. Ozone (O3) is a molecule formed by three atoms of oxygen. While O2, which we normally refer to as oxygen, is essential for all aerobic forms of life. Ozone is a deadly poison. However, at the higher levels of the atmosphere, ozone performs an essential function. It shields the surface of the earth from ultraviolet (UV) radiation from the Sun. This radiation is highly damaging to organisms, for example, it is known to cause skin cancer in human beings.


3. Ozone at the higher levels of the atmosphere is a product of UV radiation acting on oxygen (O2) molecule. The higher energy UV radiations split apart some molecular oxygen (O2) into free oxygen (O) atoms.

O2 ⎯⎯UV⎯→ O +O
O2 +O ⎯⎯UV⎯→ O3 (Ozone)

4. The amount of ozone in the atmosphere began to drop sharply in the 1980s. This decrease has been linked to synthetic chemicals like chlorofluorocarbons (CFCs) which are used as refrigerants and in fire extinguishers. In 1987, the United Nations Environment Programme (UNEP) succeeded in forging an agreement to freeze CFC production at 1986 levels.


5. In our daily activities, we generate a lot of materials that are thrown away. These accumulated wastes can be classified as biodegradable and non-biodegradable.

(i) Substances that are broken down by biological processes are said to be biodegradable.

(ii) Substances that are not broken down by biological processes are said to be non-biodegradable.

biodegradable and non biodegrable

6. It is necessary to have awareness regarding methods of disposing of different types of waste.

7. Waste management involves the collection, storage, transportation, recycling and disposal.

8. E-waste- Discarded electronic devices like televisions, personal computers, floppies, audio-video CDs, batteries, etc. contribute to the growing pile of e-waste in the city. At least, 30,000 personal computers are sent for dismantling every year in the city. These products contain components that contain toxic substances like lead, cadmium, mercury, hexavalent chromium, plastic, PVC, BFRs, barium, beryllium, and carcinogens like carbon black and heavy metals. The deadly mix can cause severe health problems for those handling the waste. It could even prove fatal.

9. Sewage Treatment – The ultimate return of used water to the environment is done by treating sewage.

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Class X Biology: EVOLUTION

1. Evolution is the process by which different kinds of living organism are believed to have developed from earlier forms during the history of the earth.

2. Evolutionary History – Consider a group of twelve red beetles. They live in some bushes with green leaves. Their population will grow by sexual reproduction, and therefore, can generate variations. Crows eat these beetles. The more beetles the crows eat, the fewer beetles are available to reproduce. Now –

(i) Natural Selection – In the first situation, a colour variation arises during reproduction and one beetle that is green in colour instead of red (due to mutation). This beetle passes the colour on to its progeny. Crows cannot see green-coloured beetles on the green leaves and therefore cannot eat them. As a result, there are more and more green beetles than red ones in the beetle population. This is It results in adaptations in the beetle population to fit their environment better

(ii) Genetic Drift – In a second situation a colour variation arises during reproduction resulting in a beetle blue in colour instead of red. Progeny of this beetle is blue. Crows can see blue-coloured beetles in the green leaves of the bushes as well as they can see red ones, and therefore can eat them. As population expands, there are a few blue beetles, but most are red. But at this point, an elephant comes by, and stamps on the bushes where the beetles live. This kills most of the beetles. By chance, the few beetles that have survived are mostly blue. The blue beetle population slowly expands again, but now, the beetles in the population are mostly blue. It was simply a matter of accidental survival of beetles of one colour that changed the common characteristic of the resultant population. This is the notion of genetic drift, which provides diversity without any adaptations.

(iii) Mutation – Some “green genes” randomly mutated to “brown genes”. But this process alone can account for the change in the gene frequency in just one generation because mutation is rare and occurs under severe conditions only.

(iv) Gene Flow –(also known as gene migration) refers to the transfer of genes from the gene pool of one population to another. The introduction of new alleles increases variability within a population and allows for new combinations of traits.

3. Somatic Variations– bushes start suffering from a plant disease. The amount of leaf material for the beetles is reduced. The beetles are poorly nourished as a result. The average weight of adult beetles decreases. After a few years, the plant disease is eliminated. There is a lot of leaf food. The beetles return to their original weight. If the weight of the beetle is reduced because of starvation, that will not change the DNA of the germ cells. Therefore, even if some generations of beetles are low in weight because of starvation, that is not an example of evolution, since the change is not inherited over generations.

4. Inherited traits are traits that are passed down from parents or ancestors. For example, hair colour, eye colour, bone structure.

5. Acquired traits are traits that are not passed genetically from one organism to another. These traits cannot be inherited. For example, a wrestler develops large muscles due to his training program. but this doesn’t mean it’ll be passed on to his offspring. Acquired traits are skills, knowledge or memory that an individual develops during his/her lifetime.

Acquired and Inherited Traits differences

6. Speciation is the evolutionary process by which new biological species arise from existing species. This happens when the original group will not or cannot reproduce with the new species. When this happens, they can be called two independent species. There can be a number of ways by which this can happen. If the DNA changes are severe enough, such as a change in the number of chromosomes, eventually the germ cells of the two groups cannot fuse with each other. Effectively, new species are being generated.

7. The more characteristics two species will have in common, the more closely they are related. And the more closely they are related, the more recently they will have had a common ancestor. An example will help. Classification of species is, in fact, a reflection of their evolutionary relationship.

8. A comparison of the common features shows that an evolutionary process and a pattern of relationships exist between species. As lineages evolve and split and modifications are inherited, the evolutionary paths of species diverge. This produces a branching pattern of evolutionary relationships. Such evolutionary change and relationships are represented in “family trees,” or ‘phylogenetic‘ trees.

9. Homologous Organs are those organs which have the same basic structural design and developmental origin but have different functions and appearance. Consider the forelimb of a frog, a lizard, a bird, and a man seem to be built from the same basic design of bones but they perform different functions. Such a homologous characteristic helps to identify an evolutionary relationship between apparently different species. However, all similarities simply in organ shape are not necessarily because of common ancestry.

homologous organs

10. Analogous Organs are those organs which have different basic structural design and developmental origin but have a similar appearance and perform similar functions. For. Eg. The wings of birds and bats look similar but have a different design in their structure. Wings of bats are skin folds stretched between elongated fingers but wings of birds are covered by feathers all along the arm.

Analogous Organs2

11. Fossils – Fossils are defined as preserved remains of a living organism that existed on earth a long time ago. Fossils are formed by the preservation of the remains of the organisms that existed in the earlier days.


12. There are two components to estimation the age of fossils-

(i) One is relative. If we dig into the earth and start finding fossils, it is reasonable to suppose that the fossils we find closer to the surface are more recent than the fossils we find in deeper layers.

(ii) The second way of dating fossils is by detecting the ratios of different isotopes of the same element in the fossil material. It is called Radioactive dating.

13. Evolution by Stages –

(i) An intermediate stage, such as a rudimentary eye, can be useful to some extent. A change that is useful for one property to start with can become useful later for quite a different function.

(ii) Feathers, for example, can start out as providing insulation in cold weather. But later, they might become useful for flight. Some dinosaurs had feathers, although they could not fly. Birds seem to have later adapted the feathers to fly. This means that birds are very closely related to reptiles since dinosaurs were reptiles!

(iii) It is true that analysis of the organ structure in fossils allows us to make estimates of how far back evolutionary relationships go.

14. Artificial Selection – The wild cabbage plant is a good example to study selective breeding/artificial selection. Humans have, over more than two thousand years, cultivated wild cabbage as a food plant, and generated different vegetables from it by selection. Some farmers wanted to select for arrested flower development, and have bred broccoli, or for sterile flowers, and have made the cauliflower. Some have selected for swollen parts, and come up with kohlrabi. Some have simply looked for slightly larger leaves, and come up with a leafy vegetable called kale.

artificial selection

15. Determining the DNA – Changes in DNA during reproduction are the basic events in evolution. Therefore, comparing the DNA of different species should give us a direct estimate of how much the DNA has changed during the formation of these species. This method is now used to define evolutionary relationships.

16. Evolution is simply the generation of diversity and the shaping of the diversity by environmental selection. The only progressive trend in evolution seems to be that more and more complex body designs have emerged over time.

17. The same tools for tracing evolutionary relationships – excavating, time-dating and studying fossils, as well as determining DNA sequences – have been used for studying human evolution.

18. Regardless of where we have lived for the past few thousand years, we all come from Africa. The earliest members of the human species, Homo sapiens, can be traced there.

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