Science Textbook for Class 8 (2018-19)

SCIENCE TEXTBOOK FOR CLASS VIII 2018-19  ISBN 978-81-7450-812-6 First Edition January 2008 Magha 1929 ALL RIGHTS RESERVED Reprint Edition...

SCIENCE TEXTBOOK FOR CLASS VIII 2018-19  ISBN 978-81-7450-812-6 First Edition January 2008 Magha 1929 ALL RIGHTS RESERVED Reprint Edition q No part of this publication may be reproduced, stored in a retrieval December 2008 Pausa 1930 system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior January 2010 Magha 1931 permission of the publisher. November 2010 Kartika 1932 q This book is sold subject to the condition that it shall not, by way of January 2012 Magha 1933 trade, be lent, re-sold, hired out or otherwise disposed of without the publisher’s consent, in any form of binding or cover other than that in November 2012 Kartika 1934 which it is published. October 2013 Asvina 1935 q The correct price of this publication is the price printed on this page, December 2014 Pausa 1936 Any revised price indicated by a rubber stamp or by a sticker or by any other means is incorrect and should be unacceptable. December 2015 Agrahayana 1937 February 2017 Magha 1938 December 2017 Agrahayana 1939 OFFICES OF THE PUBLICATION DIVISION,, NCERT PD 750T+100T RPS NCERT Campus Sri Aurobindo Marg © National Council of Educational New Delhi 110 016 Phone : 011-26562708 Research and Training, 2008 108, 100 Feet Road Hosdakere Halli Extension Banashankari III Stage Bangaluru 560 085 Phone : 080-26725740 Navjivan Trust Building P.O.Navjivan Ahmedabad 380 014 Phone : 079-27541446 CWC Campus Opp. Dhankal Bus Stop Panihati Kolkata 700 114 Phone : 033-25530454 CWC Complex Maligaon Guwahati 781 021 Phone : 0361-2674869 ` 55.00 Publication Team Head, Publication : M. Siraj Anwar Division Chief Editor : Shveta Uppal Chief Business : Gautam Ganguly Manager Chief Production : Arun Chitkara Officer (In-charge) Printed on 80 GSM paper with NCERT watermark Assistant Editor : Shashi Chadha Published at the Publication Division, Production Assistant : Om Prakash by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi 110 016 and Cover, Layout and Illustrations printed at Educational Stores, S-5, Ashwani Tyagi Bulandshahar Road, Industrial Area Site-I (Near RTO Office) Ghaziabad (U.P.) 2018-19  Foreword The National Curriculum Framework (NCF), 2005, recommends that children’s life at school must be linked to their life outside the school. This principle marks a departure from the legacy of bookish learning which continues to shape our system and causes a gap between the school, home and community. The syllabi and textbooks developed on the basis of NCF signify an attempt to implement this basic idea. They also attempt to discourage rote learning and the maintenance of sharp boundaries between different subject areas. We hope these measures will take us significantly further in the direction of a child-centred system of education outlined in the National Policy on Education (1986). The success of this effort depends on the steps that school principals and teachers will take to encourage children to reflect on their own learning and to pursue imaginative activities and questions. We must recognise that, given space, time and freedom, children generate new knowledge by engaging with the information passed on to them by adults. Treating the prescribed textbook as the sole basis of examination is one of the key reasons why other resources and sites of learning are ignored. Inculcating creativity and initiative is possible if we perceive and treat children as participants in learning, not as receivers of a fixed body of knowledge. These aims imply considerable change in school routines and mode of functioning. Flexibility in the daily time-table is as necessary as rigour in implementing the annual calendar so that the required number of teaching days are actually devoted to teaching. The methods used for teaching and evaluation will also determine how effective this textbook proves for making children’s life at school a happy experience, rather than a source of stress or boredom. Syllabus designers have tried to address the problem of curricular burden by restructuring and reorienting knowledge at different stages with greater consideration for child psychology and the time available for teaching. The textbook attempts to enhance this endeavour by giving higher priority and space to opportunities for contemplation and wondering, discussion in small groups and activities requiring hands-on experience. The National Council of Educational Research and Training (NCERT) appreciates the hard work done by the textbook development committee responsible for this book. We wish to thank the Chairperson of the advisory group in science and mathematics, Professor J.V. Narlikar and the Chief Advisor for this book, Professor V.B. Bhatia for guiding the work of this committee. Several teachers contributed to the development of this textbook. We are grateful to their principals for making this possible. We are indebted to the institutions and organisations which have generously permitted us to draw upon their resources, material and personnel. We are especially grateful to the members of the National Monitoring Committee, appointed by the Department of Secondary and Higher Education, 2018-19  iv Ministry of Human Resource Development under the Chairpersonship of Professor Mrinal Miri and Professor G.P. Deshpande, for their valuable time and contribution. As an organisation committed to systemic reform and continuous improvement in the quality of its products, NCERT welcomes comments and suggestions which will enable us to undertake further revision and refinement. Director New Delhi National Council of Educational 30 November 2007 Research and Training 2018-19  Preface This book is the outcome of the efforts of the textbook development committee appointed by the NCERT. The committee met a few times to interact with one another to improve the draft. Then there was a review meeting in which many experts and practicing school teachers were invited to review the draft and suggest improvements. By and large we have stuck to the format of the Class VII book. By now famous characters, Boojho and Paheli, have been used to make the text interactive. Attempt has been made to recall children’s own experiences and build concepts around them. This is designed to connect science that they study in the school with their everyday life. Many activities have been suggested to clarify concepts. Some of these activities are so simple that children can perform them on their own. The requirement of the apparatus required for the activities is minimal. We performed all the activities ourselves to ensure that there was no difficulty in performing them in the school situation. The activities should also help children in developing skills such as presentation of data in tabular and graphical forms, reasoning and drawing inference from the given data. The language of the book has been kept as simple as possible. A large number of photographs, illustrations, cartoons, etc. have been included to make the book attractive. To help teachers evaluate children effectively, a large number of exercises have been given at the end of each chapter. The teachers are encouraged to frame additional exercises to test children’s understanding. Some challenging exercises have also been devised for those children who would like to appear for the National Talent Search Examination conducted by the NCERT. We are conscious of the fact that there is a paucity of additional reading material for children. We have tried to address this problem by providing non-evaluative boxes. These boxes, in light orange, contain additional information, anecdotes, stories, strange facts and other such interesting materials. We all know that children are mischievous and playful by nature. Therefore, in order to prevent any untoward incident during the performance of the activities in the school or outside, necessary cautions, in magenta, have been inserted at various places in the book. To prepare children to assume their roles as responsible citizens of tomorrow, attempt has been made to sensitise them to the issues concerning gender, religion, environment, health and hygiene, water scarcity and energy conservation. We have sought to weave into the text the value of cooperation and the importance of peer learning. An important feature of the book is what we call Extended Learning. These are totally non-evaluative, and purely voluntary activities and projects. Some of the projects in this section have been designed to enhance children’s interaction with the experts, teachers, even parents, and society at large. The children are required to collect information of various kinds and draw conclusions of their own. My request to teachers and parents is to use the book in the spirit in which it has been written. Encourage children to perform activities and learn by doing, rather than by rote. You can supplement, or even replace, the activities given here. If you 2018-19  vi feel that you have better alternatives, especially with your local/regional flavour, please write to us so that these activities could be used in the future editions of the book. We have been able to include only a small subset of children’s experiences. You have a better knowledge of their experiences because you are in touch with them. Use them to illustrate the concepts being taught. Above all, please do not stifle children’s natural curiosity. Encourage them to ask questions, even if sometimes you feel uncomfortable. If you do not know the answer to a question on the spot, do not feel embarrassed. You can promise them to find the answer and deal with it later. Make a genuine attempt to get the answer from whatever resources are within your reach, such as senior school or college teachers, experts, libraries, internet etc. If, inspite of your efforts, you cannot get the answer to some question, you could write to NCERT. I must thank the NCERT for enabling us to talk to children through the medium of this book. Every member of the NCERT has been courteous and helpful to us. In the end, I must express my gratitude to the members of the Editing Team, who worked tirelessly to help me bring the book to the present form. If you and your students find this book useful and enjoy teaching/learning science through this book, the Editing Team and I shall consider ourselves well-rewarded. V.B. BHATIA Chief Advisor Textbook Development Committee 2018-19 Textbook Development Committee CHAIRPERSON, ADVISORY GROUP FOR TEXTBOOKS IN SCIENCE AND MATHEMATICS J.V. Narlikar, Emeritus Professor, Inter University Centre for Astronomy and Astrophysics (IUCCA), Ganeshkhind, Pune University, Pune CHIEF ADVISOR V.B. Bhatia, Professor (Retd.) (Physics), Delhi University, Delhi MEMBERS Bharati Sarkar, Reader (Retd.) (Zoology), Maitreyi College, Delhi University, Delhi C. V. Shimray, Lecturer, Department of Education in Science and Mathematics, (DESM), NCERT, Sri Aurobindo Marg, New Delhi Gagandeep Bajaj, Lecturer, Department of Education, SPM College, Delhi University, Delhi H.C. Jain, Principal, (Retd.) Regional Institute of Education, Ajmer Harsh Kumari, Headmistress, CIE Experimental Basic School, Department of Education, Delhi University, Delhi J. S. Gill, Professor (Retd.), DESM, NCERT, Sri Aurobindo Marg, New Delhi Kanhiya Lal, Principal (Retd.), Directorate of Education, Delhi Lalita S. Kumar, Reader (Chemistry), School of Sciences, Indira Gandhi National Open University (IGNOU), Maidan Garhi, New Delhi P.C. Agarwal, Reader, Regional Institute of Education, Ajmer P.S. Yadava, Professor, Department of Life Sciences, Manipur University, Imphal Puranchand, Professor and Joint Director (Retd.), Central Institute of Educational Technology (CIET), NCERT, Sri Aurobindo Marg, New Delhi R. Joshi, Lecturer (SG), DESM, NCERT, Sri Aurobindo Marg, New Delhi Rachna Garg, Lecturer, Central Institute of Educational Technology (CIET), NCERT, Sri Aurobindo Marg, New Delhi Rashmi Sharma, Lecturer, North-East Regional Institute of Education, Shillong R.K. Parashar, Reader, Regional Institute of Education, Bhubaneshwar Ruchi Verma, Lecturer, DESM, NCERT, Sri Aurobindo Marg, New Delhi Shashi Prabha, Lecturer, DESM, NCERT, Sri Aurobindo Marg, New Delhi Sunila Masih, Teacher, Mitra GHS School, Suhagpur, P.O. Hoshangabad, Madhya Pradesh MEMBER-COORDINATOR R. S. Sindhu, Professor, DESM, NCERT, Sri Aurobindo Marg, New Delhi 2018-19  Acknowledgements The National Council of Educational Research and Training (NCERT) acknowledges the valuable contribution of the individuals and organisations involved in the development of this book. The Council acknowledges the valuable contribution of the following academics for reviewing and refining the manuscript of this book: K.C. Sharma, Reader (Physics), Regional Institute of Education, Ajmer; Charu Verma, Lecturer (Science), DIET, Pitampura, Delhi; Geeta Bajaj, TGT (Science), K.V. No. 3, Delhi Cantt., New Delhi; K.D. Sharma, TGT (Science), R.P.V.V. Civil Lines, Delhi; Manohar Lal Patel, Teacher, Govt. R.N.A. Higher Secondary School, Pipariya, Madhya Pradesh; Reeta Sharma, Reader (Botany), Regional Institute of Education, Bhopal; Kamal Deep Peter, OEI, Oracle India, Bangalore; Puneeta Sharma, TGT (Science), L.D. Jain Girls Senior Secondary School, Sadar Bazar, Delhi; M.C. Das, Teacher (Science), Govt. Secondary School, Zoom, West Sikkim; Deepti Kohli, P.D. Public School, Shalimar Bagh, Delhi; Sulekha Chandra, Reader (Chemistry), Zakir Husain College, Delhi University, Delhi; R.S. Tomar, TGT (Science), J.N.V. Mothuka, Faridabad (Haryana); Anjali Khirwadkar, Lecturer, Department of Education, M.S. University, Baroda (Gujrat); Suresh Chand, TGT (Science), J.N.V. Ghaziabad Uttar Pradesh; Satheesh H.L., TGT (Science), Demonstration School, Regional Institute of Education, Mysore; Simminder Kaur Thukral, NIIT, Kalkaji, New Delhi; M.M. Kapoor, Professor (Retd.) (Chemistry), Delhi University, Delhi; Sarita Kumar, Reader (Zoology), Acharya Narendra Dev College, Delhi University, Delhi. The contributions of Pramila Tanvar and Ashish K. Srivastava, Assistant Professors, are acknowledged for being a part of the review of this textbook. The Council gratefully acknowledges the valuable suggestions received from the National Monitoring Committee in the development of the manuscript of this textbook. The dynamic leadership of Professor Hukum Singh, Head, DESM, for providing guidance at different stages of development of the textbook and extending infrastructure facilities is highly acknowledged. Special thanks are due to Shveta Uppal, Chief Editor and Shashi Chadha, Assistant Editor for going through the manuscript and suggesting relevant changes. The Council also acknowledges the efforts of Deepak Kapoor, Incharge Computer Station, Inder Kumar, DTP Operator; K.T. Chitralekha, Copy Editor and Ritu Jha, Proof Reader. The contributions of APC-office, administration of DESM, Publication Department and Secretariat of NCERT is also acknowledged. 2018-19  A Note for the Students The team of Paheli and Boojho will be with you as you journey through this textbook. They love to ask questions. All kinds of questions come to their minds and they collect them in their sacks. Sometimes, they may share some of the questions with you, as you read through the chapters. Paheli and Boojho are also on the lookout for answers to many questions — sometimes the questions seem answered after they discuss them with each other, sometimes through discussions with other classmates, teachers or their parents. Answers to some questions do not seem available even after all these. They might need to experiment on their own, read books in the library, send questions to scientists. Just dig and dig and dig into all possibilities and see if the questions can be answered. Perhaps, they would carry some of the unanswered questions in their sacks to higher classes. What will really thrill them would be your adding questions to their sacks or answers to their questions. Sometimes, activities are suggested in the textbook, results or findings of these by different groups of students would be of interest to other students and teachers. You can complete the suggested activities and send your results or findings to Paheli and Boojho. Do keep in mind that activities that involve using blades, scissors or fire need to be done strictly under the care of your teachers. Stick to the precautions given and then enjoy doing all the suggested activities. Mind, the book will not be able to help you much, if the activities are not completed! We would like to advise you that you must make observations yourself and record whatever results you get. Keen and true observations are necessary for exploring any subject of study. For some reason your results may turn out to be different from those of your classmates. Do not worry. Try to find out the reason for these results instead of disregarding them. Do not ever copy results from your classmate. You can send your feedback for Paheli and Boojho at: To The Head Department of Education in Science and Mathematics, NCERT, Sri Aurobindo Marg, New Delhi 110016 2018-19  THE CONSTITUTION OF INDIA PREAMBLE WE, THE PEOPLE OF INDIA, having solemnly resolved to constitute India into a 1 [SOVEREIGN SOCIALIST SECULAR DEMOCRATIC REPUBLIC] and to secure to all its citizens : JUSTICE, social, economic and political; LIBERTY of thought, expression, belief, faith and worship; EQUALITY of status and of opportunity; and to promote among them all FRATERNITY assuring the dignity of the individual and the 2[unity and integrity of the Nation]; IN OUR CONSTITUENT ASSEMBLY this twenty-sixth day of November, 1949 do HEREBY ADOPT, ENACT AND GIVE TO OURSELVES THIS CONSTITUTION. 1. Subs. by the Constitution (Forty-second Amendment) Act, 1976, Sec.2, for "Sovereign Democratic Republic" (w.e.f. 3.1.1977) 2. Subs. by the Constitution (Forty-second Amendment) Act, 1976, Sec.2, for "Unity of the Nation" (w.e.f. 3.1.1977) 2018-19 CONTENTS FOREWORD PREFACE Chapter 1 iii v CROP PRODUCTION AND MANAGEMENT 1 Chapter 2 MICROORGANISMS : FRIEND AND FOE 17 Chapter 3 SYNTHETIC FIBRES AND PLASTICS 32 Chapter 4 MATERIALS : METALS AND NON-METALS 44 Chapter 5 COAL AND PETROLEUM 56 Chapter 6 COMBUSTION AND FLAME 64 Chapter 7 CONSERVATION OF PLANTS AND ANIMALS 77 Chapter 8 CELL — STRUCTURE AND FUNCTIONS 90 Chapter 9 REPRODUCTION IN ANIMALS 100 Chapter 10 REACHING THE AGE OF ADOLESCENCE 113 2018-19  xii CONTENTS Chapter 11 FORCE AND PRESSURE Chapter 12 127 FRICTION 146 Chapter 13 SOUND 157 Chapter 14 CHEMICAL EFFECTS OF ELECTRIC CURRENT 172 Chapter 15 SOME NATURAL PHENOMENA 184 Chapter 16 LIGHT 199 Chapter 17 STARS AND THE SOLAR SYSTEM 215 Chapter 18 POLLUTION OF AIR AND WATER 239 INDEX 253 2018-19  TheRoleofBegums ofBhopalinGir ls’ Educat ion Go uriSr iva sta va `40. 00/pp.83/2006 I SBN:81-7450- 598-9 Forf urt herenqui ri es,pl e as evisitwww.nc ert.ni c.i norcont a c ttheBus ine ssManage rsatt he addr ess esofther egionalcent resgive nonthecopyri ghtpage.  CROP PRODUCTION AND MANAGEMENT P aheli and Boojho went to their uncle’s house during the summer vacation. Their uncle is a farmer. One day they saw some tools like khurpi, Food has to be produced on a large sickle, shovel, plough, etc., in the field. scale. In order to provide food for a large population— regular production, proper I want to know management and distribution is where and how we necessary. use these tools. 1.1 Agricultural Practices You have learnt that all living organisms require food. Plants can make T ill 10,000 B.C.E. people were their food themselves. Can you recall nomadic. They were wandering in how green plants synthesise their own groups from place to place in search food? Animals including humans can of food and shelter. They ate raw not make their own food. So, where do fruits and vegetables and started animals get their food from? hunting animals for food. Later, they But, first of all why do we have to eat could cultivate land and produce rice, food? wheat and other food crops. Thus, You already know that energy from was born ‘Agriculture’. the food is utilised by organisms for carrying out their various body When plants of the same kind are functions, such as digestion, respiration cultivated at one place on a large scale, and excretion. We get our food from it is called a crop. For example, crop of plants, or animals, or both. wheat means that all the plants grown in a field are that of wheat. You already know that crops are of different types like cereals, vegetables and fruits. These can be classified on the Since we all need food, basis of the season in which they grow. how can we provide India is a vast country. The climatic food to a large number conditions like temperature, humidity of people in our and rainfall vary from one region to country? another. Accordingly, there is a rich 2018-19 variety of crops grown in different parts to as agricultural practices which are of the country. Despite this diversity, listed below: two broad cropping patterns can be (i) Preparation of soil identified. These are: (ii) Sowing (i) Kharif Crops : The crops which are (iii) Adding manure and fertilisers sown in the rainy season are called (iv) Irrigation kharif crops. The rainy season in India (v) Protecting from weeds is generally from June to September. (vi) Harvesting Paddy, maize, soyabean, groundnut and (vii) Storage cotton are kharif crops. (ii) Rabi Crops : The crops grown in the 1.3 Preparation of Soil winter season (October to March) are called rabi crops. Examples of rabi The preparation of soil is the first step crops are wheat, gram, pea, mustard before growing a crop. One of the most and linseed. important tasks in agriculture is to turn Besides these, pulses and vegetables the soil and loosen it. This allows the are grown during summer at many roots to penetrate deep into the soil. The places. loose soil allows the roots to breathe easily even when they go deep into the 1.2 Basic Practices of Crop soil. Why does the loosening of soil allow Production the roots to breathe easily? The loosened soil helps in the growth of earthworms and microbes present in the soil. These organisms are friends of the farmer since they further turn and loosen the soil and add humus to it. Why paddy can not be grown in the winter season? But why the soil needs to be turned and loosened? You have learnt in the previous classes that soil contains minerals, water, air and some living organisms. Paddy requires a lot of In addition, dead plants and animals water. Therefore, it is grown get decomposed by soil organisms. In only in the rainy season. this way, various nutrients in the dead organisms are released back into the Cultivation of crops involves several soil. These nutrients are again absorbed activities undertaken by farmers over a by plants. period of time. You may find that these Since only a few centimetres of the activities are similar to those carried out top layer of soil supports plant growth, by a gardener or even by you when you turning and loosening of soil brings the grow ornamental plants in your house. nutrient-rich soil to the top so that These activities or tasks are referred plants can use these nutrients. Thus, 2 SCIENCE 2018-19 turning and loosening of soil is very Plough : This is being used since important for cultivation of crops. ancient times for tilling the soil, adding The process of loosening and turning fertilisers to the crop, removing the weeds of the soil is called tilling or ploughing. and turning the soil. This is made of This is done by using a plough. Ploughs wood and is drawn by a pair of bulls or are made of wood or iron. If the soil is other animals (horses and camels). It very dry, it may need watering before contains a strong triangular iron strip ploughing. The ploughed field may have called ploughshare. The main part of the big clumps of soil called crumbs. It is plough is a long log of wood which is necessary to break these crumbs. called a ploughshaft. There is a handle Levelling the field is beneficial for at one end of the shaft. The other end is sowing as well as for irrigation. Levelling attached to a beam which is placed on of soil is done with the help of a leveller. the bulls’ necks. One pair of bulls and a Sometimes, manure is added to the man can easily operate the plough soil before tilling. This helps in proper [Fig. 1.1 (a)]. mixing of manure with soil. The soil is The indigenous wooden plough is moistened before sowing. increasingly being replaced by iron Agricultural Implements ploughs nowadays. Before sowing the seeds, it is necessary to Hoe : It is a simple tool which is used break soil clumps to get better yield. This for removing weeds and for loosening is done with the help of various tools. the soil. It has a long rod of wood or The main tools used for this purpose are iron. A strong, broad and bent plate of the plough, hoe and cultivator. iron is fixed to one of its ends and beam ploughshaft ploughshare Fig. 1.1 (a) : The plough CROP PRODUCTION AND MANAGEMENT 3 2018-19 works like a blade. It is pulled by Activity 1.1 animals [Fig. 1.1 (b)]. Take a beaker and fill half of it grip beam with water. Put a handful of wheat handle seeds and stir well. Wait for some time. bent plate Are there seeds which float on rod Fig. 1.1 (b) : A hoe water? Would those be lighter or heavier than those which sink? Why Cultivator : Nowadays ploughing is would they be lighter? Damaged seeds done by tractor-driven cultivator. The become hollow and are thus lighter. use of cultivator saves labour and time. Therefore, they float on water. [Fig. 1.1 (c)]. This is a good method for separating good, healthy seeds from the damaged ones. Before sowing, one of the important tasks is to know about the tools used for sowing seeds [Fig. 1.2 (a), (b)]. Traditional tool : The tool used traditionally for sowing seeds is shaped like a funnel [Fig. 1.2 (a)]. The Fig. 1.1 (c) : Cultivator driven by a tractor seeds are filled into the funnel, passed down through two or three 1.4 Sowing pipes having sharp ends. These Sowing is an important part of crop ends pierce into the soil and place production. Before sowing, good quality, seeds there. clean and healthy seeds of a good variety—are selected. Farmers prefer to use seeds which give high yield. Selection of Seeds One day I saw my mother put some gram seeds in a vessel and pour some water on them. After a few minutes some seeds started to float on top. I wonder why some seeds float on water! Fig. 1.2 (a) : Traditional method of sowing 4 SCIENCE 2018-19  sufficient sunlight, nutrients and water from the soil. At times a few plants may have to be removed to prevent overcrowding. 1.5 Adding Manure and Fertilisers The substances which are added to the soil in the form of nutrients for the healthy growth of plants are called manure and fertilisers. Fig. 1.2 (b) : A seed drill Seed drill : Nowadays the seed drill [Fig. I saw a healthy crop growing 1.2 (b)] is used for sowing with the help in a farm. In the neighbouring of tractors. This sows the seeds farm, the plants were weak. uniformly at equal distance and depth. Why do some plants grow It ensures that seeds get covered by the better than others? soil after sowing. This protects seeds from being eaten by birds. Sowing by Soil supplies mineral nutrients to the using a seed drill saves time and labour. crop plants. These nutrients are essential for the growth of plants. In certain areas, farmers grow crop after There is a nursery near my crop in the same field. The field is never school. I found that little left uncultivated or fallow. Imagine what plants were kept in small happens to the nutrients? bags. Why are they Continuous cultivation of crops kept like this? makes the soil poor in nutrients. Therefore, farmers have to add manure to the fields to replenish the soil with nutrients. This process is called Seeds of a few plants such as paddy are first manuring. Improper or insufficient grown in a nursery. When manuring results in weak plants. they grow into seedlings, Manure is an organic substance they are transplanted to obtained from the decomposition of the field manually. Some plant or animal wastes. Farmers dump forest plants and flowering plant and animal waste in pits at open plants are also grown in places and allow it to decompose. The the nursery. decomposition is caused by some microorganisms. The decomposed Appropriate distance between the matter is used as organic manure. seeds is necessary to avoid overcrowding You have alr eady learnt about of plants. This allows plants to get vermicomposting in Class VI. CROP PRODUCTION AND MANAGEMENT 5 2018-19 Activity 1.2 better growth of plants? In which glass was the growth fastest? Take moong or gram seeds and Fertilisers are chemicals which are germinate them. Select three equal rich in a particular nutrient. How are sized seedlings. Take three empty they different from manure? Fertilisers glasses or similar vessels. Mark them are produced in factories. Some A, B and C. To glass A add little amount of soil mixed with a little cow examples of fertilisers are— urea, dung manure. In glass B put the ammonium sulphate, super same amount of soil mixed with a phosphate, potash, NPK (Nitrogen, little urea. Take the same amount Phosphorus, Potassium). of soil in glass C without adding The use of fertilisers has helped anything [Fig. 1.3(a)]. Now pour the farmers to get better yield of crops same amount of water in each glass such as wheat, paddy and maize. But and plant the seedlings in them. excessive use of fertilisers has made Keep them in a safe place and water the soil less fertile. Fertilisers have also them daily. After 7 to 10 days become a source of water pollution. observe their growth [Fig. 1.3(b)]. Therefore, in order to maintain the fertility of the soil, we have to substitute fertilisers with organic manure or leave the field uncultivated (fallow) in between two crops. The use of manure improves soil texture as well as its water retaining capacity. It replenishes the soil with nutrients. Another method of replenishing Fig. 1.3 (a) : Preparation of the experiment the soil with nutrients is through crop rotation. This can be done by growing different crops alternately. Earlier, farmers in northern India used to grow legumes as fodder in one season and wheat in the next season. This helped in the replenishment of the soil with nitrogen. Farmers are being encouraged to adopt this practice. In the previous classes, you have Fig. 1.3 (b) : Growing seedlings with manure and fertiliser learnt about Rhizobium bacteria. These are present in the nodules of Did plants in all the glasses grow at roots of leguminous plants. They fix the same pace? Which glass showed atmospheric nitrogen. 6 SCIENCE 2018-19  Table 1.1 : Differences between Fertiliser and Manure S. No. Fertiliser Manure 1. Fertiliser is a man-made Manure is a natural substance obtained inorganic salt. by the decomposition of cattle dung and plant residues. 2. Fertiliser is prepared in Manure can be prepared in the fields. factories. 3. Fertiliser does not provide Manure provides a lot of humus to the soil. any humus to the soil. 4. Fertilisers are very rich in Manure is relatively less rich in plant plant nutrients like nitrogen, nutrients. phosphorus and potassium. Table 1.1 gives the differences protects the crop from both frost and between a fertiliser and manure. hot air currents. To maintain the moisture of the soil for healthy crop Advantages of Manure : The organic growth, fields have to be watered manure is considered better than regularly. fertilisers. This is because The supply of water to crops at l it enhances the water holding regular intervals is called irrigation. The capacity of the soil. time and frequency of irrigation varies l it makes the soil porous due to which from crop to crop, soil to soil and season exchange of gases becomes easy. to season. In summer, the frequency of l it increases the number of friendly watering is higher. Why is it so? Could microbes. it be due to the increased rate of l it improves the texture of the soil. evaporation of water from the soil and the leaves? 1.6 Irrigation All living beings need water to live. Water is important for proper growth I am very careful this and development. Water is absorbed by year about watering the plant roots. Along with water, the plants. Last minerals and fertilisers are also summer my plants absorbed. Plants contain nearly 90% dried up and died. water. Water is essential because germination of seeds does not take place Sources of irrigation : The sources of under dry conditions. Nutrients water for irrigation are— wells, dissolved in water are transported to tubewells, ponds, lakes, rivers, dams each part of the plant. Water also and canals. CROP PRODUCTION AND MANAGEMENT 7 2018-19  Traditional Methods of Irrigation The water available in wells, lakes and canals is lifted up by different methods in different regions, for taking it to the fields. Cattle or human labour is used in these methods. So these methods are cheaper, but less efficient. The various traditional ways are: (i) moat (pulley -system) (ii) chain pump Fig. 1.4 (a) : Moat Fig. 1.4 (b) : Chain pump Fig. 1.4 (c) : Dhekli (iii) dhekli, and (iv) rahat (Lever system) [Figs. 1.4 (a)- (d)]. Pumps are commonly used for lifting water. Diesel, biogas, electricity and solar energy is used to run these pumps. Fig. 1.4 (d) : Rahat 8 SCIENCE 2018-19 Modern Methods of Irrigation Modern methods of irrigation help us to use water economically. The main methods used are as follows: (i) Sprinkler System: This system is more useful on the uneven land where sufficient water is not available. The perpendicular pipes, having rotating nozzles on top, are joined to the main pipeline Fig. 1.5 (a) : Sprinkler system at regular intervals. When water is allowed to flow through the main pipe under pressure (ii) Drip system : In this system, the with the help of a pump, it escapes from water falls drop by drop directly near the rotating nozzles. It gets sprinkled the roots. So it is called drip system. It on the crop as if it is raining. is the best technique for watering fruit Sprinkler is very useful for lawns, coffee plants, gardens and trees [Fig. 1.5(b)]. plantation and several other crops Water is not wasted at all. It is a boon in [Fig. 1.5 (a)]. regions where availability of water is poor. Fig. 1.5 (b) : Drip System CROP PRODUCTION AND MANAGEMENT 9 2018-19 1.7 Protection from Weeds Boojho and Paheli went to a nearby wheat field and saw that there were some other plants in the field, growing along with wheat plants. Have these other plants been planted purposely? In a field many other undesirable plants may grow naturally along with Fig. 1.6 : Spraying weedicide the crop. These undesirable plants are called weeds. The removal of weeds is called weeding. Weeding is necessary since Do weedicides have any weeds compete with the crop plants for effect on the person handling the weedicide sprayer? water, nutrients, space and light. Thus, they affect the growth of the crop. Some As already mentioned, the weedicides weeds interfere even in harvesting and are sprayed during the vegetative growth may be poisonous for animals and of weeds before flowering and seed human beings. formation. Spraying of weedicides may Farmers adopt many ways to remove affect the health of farmers. So they weeds and control their growth. Tilling should use these chemicals very carefully. before sowing of crops helps in They should cover their nose and mouth uprooting and killing of weeds, which with a piece of cloth during spraying of may then dry up and get mixed with these chemicals. the soil. The best time for the removal of weeds is before they produce flowers 1.8 Harvesting and seeds. The manual removal includes physical removal of weeds by uprooting Harvesting of a crop is an important or cutting them close to the ground, task. The cutting of crop after it is from time to time. This is done with the mature is called harvesting. In help of a khurpi. A seed drill [Fig. 1.2(b)] harvesting, crops are pulled out or cut is also used to uproot weeds. close to the ground. It usually takes 3 Weeds are also controlled by using to 4 months for a cereal crop to mature. certain chemicals, called weedicides, Harvesting in our like 2,4-D. These are sprayed in the country is either done fields to kill the weeds. They do not manually by sickle damage the crops. The weedicides are (Fig. 1.7) or by a machine diluted with water to the extent required called harvester. In the and sprayed in the fields with a sprayer. harvested crop, the grain Fig. 1.7 : (Fig. 1.6). seeds need to be separated Sickle 10 SCIENCE 2018-19 from the chaff. This process is called Harvest Festivals threshing. This is carried out with the help of a machine called ‘combine’ which After three or four months of hard work there comes the day of the is in fact a harvester as well as a thresher harvest. The sight of golden fields (Fig. 1.8). of standing crop, laden with grain, fills the hearts of farmers with joy a n d a s e n s e o f w e l l -b e i n g. T h e efforts of the past season have borne fruit and it is time to relax and enjoy a little. The period of harvest is, thus, of great joy and happiness in all parts of India. Men and women celebrate it with great enthusiasm. Special festivals associated with the h a r v e s t s e a s o n are P o n g a l , Baisakhi, Holi, Diwali, Nabanya Fig. 1.8 : Combine and Bihu. 1.9 Storage After harvesting, sometimes Storage of produce is an important task. stubs are left in the field, which are burnt by farmers. Paheli is If the harvested grains are to be kept worried. She knows that it for longer time, they should be safe causes pollution. It may also from moisture, insects, rats and catch fire and damage the crops microorganisms. Harvested grains have lying in the fields. more moisture. If freshly harvested grains (seeds) are stored without drying, Farmers with small holdings of land they may get spoilt or attacked by do the separation of grain and chaff by organisms, making them unfit for use winnowing (Fig. 1.9). You have already studied this in Class VI. or for germination. Hence, before storing them, the grains are properly dried in the sun to reduce the moisture in them. This prevents the attack by insect pests, bacteria and fungi. I saw my mother putting some dried neem leaves in an iron drum containing wheat. I wonder why? Fig. 1.9 : Winnowing machine CROP PRODUCTION AND MANAGEMENT 11 2018-19  Fig. 1.10 (a) : Silos for storage of grains 1.10 Food from Animals Activity 1.3 Make the following Table in your note book and complete it. S.No. Food Sources 1. Milk Cow, Buffalo, She- goat, She-camel... 2. 3. 4. After completing this Table, you must have seen that, like plants, animals also provide us with different kinds of food. Many people living in the coastal areas consume fish as a major Fig. 1.10 (b) : Storage of grains in part of their diet. In the previous classes gunny bags in granaries you have learnt about the food that we obtain from plants. We have just seen Farmers store grains in jute bags or that the process of crop production metallic bins. However, large scale involves a number of steps like selection storage of grains is done in silos and of seeds, sowing, etc. Similarly, animals granaries to protect them from pests reared at home or in farms, have to be like rats and insects [Fig. 1.10 (a) provided with proper food, shelter and and (b)]. care. When this is done on a large scale, Dried neem leaves are used for it is called animal husbandry. storing food grains at home. For storing large quantities of grains in big godowns, specific chemical treatments Fish is good for health. are required to protect them from pests We get cod liver oil from fish and microorganisms. which is rich in vitamin D. 12 SCIENCE 2018-19 KEYWORDS WHAT Y WHAT OU HA YOU VE LEARNT HAVE AGRICULTURAL Ü In order to provide food to our growing PRACTICES population, we need to adopt certain agricultural practices. ANIMAL HUSBANDRY Ü Same kind of plants cultivated at a place CROP constitute a crop. Ü In India, crops can be broadly categorised into FERTILISER two types based on seasons - rabi and kharif GRANARIES crops. Ü It is necessary to prepare soil by tilling and HARVESTING levelling. Ploughs and levellers are used for this purpose. IRRIGATION Ü Sowing of seeds at appropriate depths and KHARIF distances gives good yield. Good variety of seeds are sown after selection of healthy seeds. MANURE Sowing is done by seed drills. Ü Soil needs replenishment and enrichment PLOUGH through the use of organic manure and RABI fertilisers. Use of chemical fertilisers has increased tremendously with the introduction SEEDS of new crop varieties. Ü Supply of water to crops at appropriate SILO intervals is called irrigation. SOWING Ü Weeding involves removal of unwanted and uncultivated plants called weeds. STORAGE Ü Harvesting is the cutting of the mature crop manually or by machines. THRESHING Ü Separation of the grains from the chaff is called WEEDS threshing. Ü Proper storage of grains is necessary to protect WEEDICIDE them from pests and microorganisms. Ü Food is also obtained from animals for which WINNOWING animals are reared. This is called animal husbandry. Exercises 1. Select the correct word from the following list and fill in the blanks. float, water, crop, nutrients, preparation (a) The same kind of plants grown and cultivated on a large scale at a place is called _____________. (b) The first step before growing crops is _____________ of the soil. CROP PRODUCTION AND MANAGEMENT 13 2018-19  (c) Damaged seeds would _____________ on top of water. EXERCISES (d) For growing a crop, sufficient sunlight and _____________ and _____________ from the soil are essential. 2. Match items in column A with those in column B. A B (i) Kharif crops (a) Food for cattle (ii) Rabi crops (b) Urea and super phosphate (iii) Chemical fertilisers (c) Animal excreta, cow dung urine and plant waste (iv) Organic manure (d) Wheat, gram, pea (e) Paddy and maize 3. Give two examples of each. (a) Kharif crop (b) Rabi crop 4. Write a paragraph in your own words on each of the following. (a) Preparation of soil (b) Sowing (c) Weeding (d) Threshing 5. Explain how fertilisers are different from manure. 6. What is irrigation? Describe two methods of irrigation which conserve water. 7. If wheat is sown in the kharif season, what would happen? Discuss. 8. Explain how soil gets affected by the continuous plantation of crops in a field. 9. What are weeds? How can we control them? 10. Arrange the following boxes in proper order to make a flow chart of sugarcane crop production. Sending crop to Irrigation Harvesting Sowing sugar factory 1 2 3 4 Preparation of Ploughing the Manuring soil field 5 6 7 14 SCIENCE 2018-19  11. Complete the following word puzzle with the help of clues given below. EXERCISES Down 1. Providing water to the crops. 2. Keeping crop grains for a long time under proper conditions. 5. Certain plants of the same kind grown on a large scale. Across 3. A machine used for cutting the matured crop. 4. A rabi crop that is also one of the pulses. 6. A process of separating the grain from chaff. Extended Learning — Activities and Projects 1. Sow some seeds in the soil and arrange to water them by drip irrigation. Observe daily. (i) Do you think it can save water? (ii) Note the changes in the seed. 2. Collect different types of seeds and put them in small bags. Label them. 3. Collect pictures of some other agricultural machines and paste them in a file. Write their names and uses. 4. Project Work Visit a farm, nursery or a garden nearby. Gather information about (i) importance of seed selection. (ii) method of irrigation. CROP PRODUCTION AND MANAGEMENT 15 2018-19  (iii) effect of extreme cold and extreme hot weather on the plants. (iv) effect of continuous rain on the plants. (v) fertilisers/manure used. An Example for Field Trip Work Himanshu and his friends were very anxious and curious to go to Thikri village. They went to Shri Jiwan Patel’s farmhouse. They had taken bags to collect some seeds and other things. Himanshu : Sir namaskar, I am Himanshu. Here are my friends Mohan, David and Sabiha. We want some information about crops. Please guide us. Shri Patel : Namaskar and welcome all of you. What are your queries? Sabiha : When did you start this work and what are the main crops that you grow? Shri Patel : About 75 years ago, my grandfather started this work. The main crops that we grow are wheat, gram, soyabean and moong. David : Sir, can you tell us the difference between traditional and modern agricultural practices? Shri Patel : Earlier we used traditional tools like sickle, bullock plough, trowel, etc., and depended on rain water for irrigation. But now we use modern methods of irrigation. We use implements like tractors, cultivators, seed drill and harvester. We get good quality seeds. We carry out soil testing and use manure and fertilisers. New information about agriculture is obtained through radio, T.V. and other sources. As a result we are able to get good crops on a large scale. This year we got 9 to 11 quintals of gram crop/acre and 20 to 25 quintals of wheat/acre. In my opinion awareness of new technology is important for better crop yield. Mohan : Sabiha, come here and see some earthworms. Are they helpful to the farmers? Sabiha : Oh Mohan! we learnt about it in Class VI. Shri Patel : Earthworms turn the soil and loosen it for proper aeration, so they help the farmer. David : Can we have some seeds of the crops you grow here? [They put some seeds, fertilisers and soil sample in the bags.] Himanshu : Sir, we are thankful to you for making this visit pleasant and for providing useful information. 16 SCIENCE 2018-19  MICROORGANISMS : FRIEND AND FOE Y ou have seen several kinds of These observations show that water plants and animals. However, and soil are full of tiny organisms, there are other living organisms though not all of them fall into the around us which we normally cannot category of microbes. These see. These are called microorganisms microorganisms or microbes are so small in size that they cannot be seen or microbes. For example, you might with the unaided eye. Some of these, have observed that during the rainy such as the fungus that grows on bread, season moist bread gets spoilt and its can be seen with a magnifying glass. surface gets covered with greyish white Others cannot be seen without the help patches. Observe these patches through of a microscope. That is why these are a magnifying glass. You will see tiny, called microorganisms or microbes. black rounded structures. Do you know Microorganisms are classified into what these structures are and where do four major groups. These groups are these come from? bacteria, fungi, protozoa and some algae. Some of these common 2.1 Microorganisms microorganisms are shown in Figs. 2.1 - 2.4. Activity 2.1 Viruses are also microscopic but are Collect some moist soil from the different from other microorganisms. field in a beaker and add water to They, however, reproduce only inside the it. After the soil particles have cells of the host organism, which may be a bacterium, plant or animal. Some of the settled down, observe a drop of viruses are shown in Fig. 2.5. Common water from the beaker under a ailments like cold, influenza (flu) and microscope. What do you see ? most coughs are caused by viruses. Serious diseases like polio and chicken Activity 2.2 pox are also caused by viruses. Diseases like dysentery and malaria Take a few drops of water from a are caused by protozoa(protozoans) pond. Spread on a glass slide and whereas typhoid and tuberculosis (TB) observe through a microscope. are bacterial diseases. You have lear nt about some of Do you find tiny organisms moving these microorganisms in Classes VI around? and VII. 2018-19  Fig. 2.1: Bacteria Amoeba Chlamydomonas Spirogyra Paramecium Fig. 2.2 : Algae Fig. 2.3 : Protozoa Bread mould Penicillium Aspergillus Fig. 2.4 : Fungi 18 SCIENCE 2018-19  Friendly Microorganisms Microorganisms are used for various purposes. They are used in the preparation of curd, bread and cake. Microorganisms have been used for the production of alcohol since ages. They are also used in cleaning up of the environment. For example, the organic wastes (vegetable peels, remains of animals, faeces, etc.) are broken down into harmless and usable substances by bacteria. Recall that bacteria are also used in the preparation of medicines. In agriculture they are used to increase soil fertility by fixing nitrogen. Fig. 2.5 : Viruses Making of Curd and Bread You have learnt in Class VII that milk is 2.2 Where do Microorganisms turned into curd by bacteria. Live? Microorganisms may be single-celled like bacteria, some algae and protozoa, I saw that my mother or multicellular, such as many algae and added a little curd to warm fungi. They live in all types of milk to set curd for the environment, ranging from ice cold next day. I wonder why? climate to hot springs; and deserts to marshy lands. They are also found Curd contains several micro- inside the bodies of animals including organisms. Of these, the bacterium, humans. Some microorganisms Lactobacillus promotes the formation grow on other organisms while others of curd. It multiplies in milk and exist freely. converts it into curd. Bacteria are also involved in the making of cheese, 2.3 Microorganisms and Us pickles and many other food items. An Microorganisms play an important role important ingredient of rava (sooji ) in our lives. Some of them are beneficial idlis and bhaturas is curd. Can you in many ways whereas some others are guess why? Bacteria and yeast are also harmful and cause diseases. Let us helpful for fermentation of rice idlis study about them in detail. and dosa batter. MICROORGANISMS : FRIEND AND FOE 19 2018-19 Activity 2.3 spoon of yeast powder to the sugar Take ½ kg flour (atta or maida), solution. Keep it covered in a warm add some sugar and mix with place for 4-5 hours. Now smell the warm water. Add a small amount solution. Can you get a smell? of yeast powder and knead to This is the smell of alcohol as sugar make a soft dough. What do you has been converted into alcohol by yeast. observe after two hours? Did you This process of conversion of sugar into find the dough rising? alcohol is known as fermentation. Louis Pasteur discovered Maida with yeast powder fermentation in 1857. Medicinal Use of Microorganisms Raised maida Whenever you fall ill the doctor may Fig. 2.6 give you some antibiotic tablets, Yeast r eproduces rapidly and capsules or injections such as of produces carbon dioxide during penicillin. The source of these respiration. Bubbles of the gas fill the medicines is microorganisms. These dough and increase its volume (Fig. 2.6). medicines kill or stop the growth of the This is the basis of the use of yeast in disease-causing microorganisms. Such the baking industry for making breads, medicines are called antibiotics. These pastries and cakes. days a number of antibiotics are being produced from bacteria and fungi. Commercial Use of Microorganisms Streptomycin, tetracycline and Microorganisms are used for the large scale erythromycin are some of the production of alcohol, wine and acetic acid (vinegar). Yeast is used for commercial production of alcohol and wine. For this In 1929, Alexander Fleming was working purpose yeast is grown on natural sugars on a culture of disease- present in grains like barley, wheat, rice, causing bacteria. crushed fruit juices, etc. Suddenly he found the spores of a little green mould in one of Activity 2.4 his culture plates. He observed that the presence of mould prevented the Take a 500 mL beaker filled upto growth of bacteria. In fact, it also killed ¾ with water. Dissolve 2-3 many of these bacteria. From this the teaspoons of sugar in it. Add half a mould penicillin was prepared. 20 SCIENCE 2018-19 commonly known antibiotics which are from the disease-causing microbes for made from fungi and bacteria. The ever. This is how a vaccine works. antibiotics are manufactured by Several diseases, including cholera, growing specific microorganisms and tuberculosis, smallpox and hepatitis can are used to cure a variety of diseases. be prevented by vaccination. Antibiotics are even mixed with the feed of livestock and poultry to check microbial infection in animals. They are Edward Jenner also used to control many plant discovered the vaccine for small- diseases. pox in 1798. It is important to remember that antibiotics should be taken only on the advice of a qualified doctor. Also In your childhood, you must have you must complete the course been given injections to protect prescribed by the doctor. If you take yourself against several diseases. antibiotics when not needed or in Can you prepare a list of these wrong doses, it may make the drug diseases? You may take help from less effective when you might need your parents. it in future. Also antibiotics taken unnecessarily may kill the beneficial It is essential to protect all children bacteria in the body. Antibiotics, against these diseases. Necessary however, are not effective against vaccines are available in the nearby cold and flu as these are caused by hospitals. You might have seen the viruses. advertisement on TV and newspapers regarding protection of children against Vaccine polio under the Pulse Polio Programme. Polio drops given to children are actually a vaccine. A worldwide campaign against Why are children/infants smallpox has finally led to its eradication given vaccination? from most parts of the world. These days vaccines are made on a When a disease-carrying microbe enters large scale from microorganisms to our body, the body produces antibodies protect humans and other animals from to fight the invader. The body also several diseases. remembers how to fight the microbe if it enters again. If dead or weakened Increasing Soil Fertility microbes are introduced into a healthy Some bacteria (Fig. 2.7) are able to fix body, the body fights and kills nitrogen from the atmosphere to enrich the invading bacteria by producing soil with nitrogen and increase its suitable antibodies. The antibodies fertility. These microbes are commonly remain in the body and we are protected called biological nitrogen fixers. MICROORGANISMS : FRIEND AND FOE 21 2018-19  Fig. 2.7 : The Nitrogen fixing cyanobacteria (blue green algae) Cleaning the Environment released in the process can be used by Boojho and Paheli had observed the the plants again. school gardener making manure. Along Did you notice that in pot B, the with their friends, they collected wastes polythene bags, empty glasses, bottles of plants, vegetables and fruits from and broken toy parts did not undergo nearby houses and gardens. They put any such change? The microbes could them in a pit meant for waste disposal. not ‘act’ on them and convert them into After some time, it decomposed and got manure. converted to manure. Boojho and You often see large amounts of Paheli wanted to know how this had dead organic matter in the form of happened. decaying plants and sometimes dead animals on the ground. You find Activity 2.5 that they disappear after some time. This is because the microorganisms Take two pots and fill each pot half decompose dead organic waste of plants with soil. Mark them A and B. Put plant waste in pot A and things like and animals converting them into polythene bags, empty glass bottles simple substances. These substances and broken plastic toys in pot B. are again used by other plants and Put the pots aside. Observe them animals. Thus, microorganisms can after 3-4 weeks. be used to degrade the harmful and smelly substances and thereby clean Do you find any difference in the up the environment. contents of the two pots? If so, what is the difference? You will find that plant 2.4 Harmful Microorganisms waste in pot A, has been decomposed. Microorganisms are harmful in many How did this happen? The plant waste ways. Some of the microorganisms has been converted into manure by the cause diseases in human beings, plants action of microbes. The nutrients and animals. Such disease-causing 22 SCIENCE 2018-19 microorganisms are called pathogens. There are some insects and animals Some microorganisms spoil food, which act as carriers of disease- clothing and leather. Let us study more causing microbes. Housefly is one such about their harmful activities. carrier. The flies sit on the garbage and animal excreta. Pathogens stick to their Disease causing Microorganisms bodies. When these flies sit on uncovered in Humans food they may transfer the pathogens. Pathogens enter our body through the Whoever eats the contaminated food is air we breathe, the water we drink or likely to get sick. So, it is advisable to the food we eat. They can also get always keep food covered. Avoid transmitted by direct contact with an consuming uncovered items of food. infected person or carried by an animal. Another example of a carrier is the Microbial diseases that can spread from female Anopheles mosquito (Fig. 2.8), an infected person to a healthy person which carries the parasite of malaria through air, water, food or physical (Plasmodium). Female Aedes mosquito contact are called communicable acts as carrier of dengue virus. diseases. Examples of such diseases How can we control the spread of malaria include cholera, common cold, chicken or dengue? pox and tuberculosis. When a person suffering from common cold sneezes, fine droplets of moisture carrying thousands of viruses are spread in the air. The virus may enter the body of a healthy person while breathing and cause infection. Fig. 2.8 : Female Anopheles mosquito Then how do you prevent the spread of Why does the teacher communicable keep telling us not diseases? to let water collect anywhere in the neighbourhood? All mosquitoes breed in water. Hence, one should not let water collect We should keep a anywhere, in coolers, tyres, flower pot, handkerchief on the etc. By keeping the surroundings clean nose and mouth while and dry we can prevent mosquitoes from sneezing. It is better to breeding. Try to make a list of measures keep a distance from which help to avoid the spread of infected persons. malaria. MICROORGANISMS : FRIEND AND FOE 23 2018-19  Table 2.1: Some Common Human Diseases caused by Microorganisms Human Disease Causative Mode of Preventive Measures Microorganism Transmission (General) Tuberculosis Bacteria Air Keep the patient in complete isolation. Keep the personal Measles Virus Air belongings of the patient away from those of the others. Chicken Pox Virus Air/Contact Vaccination to be given at Polio Virus Air/Water suitable age. Cholera Bacteria Water/Food Maintain personal hygiene and good sanitary habits. Typhoid Bacteria Water Consume properly cooked food and boiled drinking water. Vaccination. Hepatitis A Virus Water Drink boiled drinking water. Vaccination. Use mosquito net and Malaria Protozoa Mosquito repellents. Spray insecticides and control breeding of mosquitoes by not allowing water to collect in the surroundings. Some of the common diseases in other animals. For example, anthrax affecting humans, their mode of is a dangerous human and cattle transmission and few general methods disease caused by a bacterium. Foot of prevention are shown in Table 2.1. and mouth disease of cattle is caused by a virus. Disease causing Microorganisms in Animals Disease causing Microorganisms Several microorganisms not only cause in Plants diseases in humans and plants, but also Several microorganisms cause diseases in plants like wheat, rice, potato, Robert Köch (1876) sugarcane, orange, apple and others. discovered the bacterium The diseases reduce the yield of crops. (Bacillus anthracis) which See Table 2.2 for some such plant causes anthrax disease. diseases. They can be controlled by the 24 SCIENCE 2018-19  Table 2.2: Some Common Plant Diseases caused by Microorganisms Plant Micro- Mode of Diseases organism Transmission Citrus Bacteria Air canker Rust of Fungi Air, wheat seeds Yellow vein Virus Insect mosaic of bhindi (Okra) use of certain chemicals which kill the make the food poisonous causing microbes. serious illness and even death. So, it is very important that we preserve Food Poisoning food to prevent it from being spoilt. Boojho was invited by his friend to a party and he ate a variety of foodstuff. 2.5 Food Preservation On reaching home he started vomiting In Chapter 1, we have learnt about the and had to be taken to a hospital. The methods used to preserve and store food doctor said that this condition could be grains. How do we preserve cooked food due to food poisoning. at home? You know that bread left unused under moist conditions is attacked by fungus. Microorganisms spoil our food. Spoiled food emits bad Paheli wonders how smell and has a bad taste and changed food can become a colour. Is spoiling of food a chemical ‘poison’. reaction? Paheli bought some mangoes but she Food poisoning could be due to the could not eat them for a few days. Later consumption of food spoilt by some she found that they were spoilt and microorganisms. Microorganisms rotten. But she knows that the mango that grow on our food sometimes pickle her grandmother makes does not produce toxic substances. These spoil for a long time. She is confused. MICROORGANISMS : FRIEND AND FOE 25 2018-19  Let us study the common methods Similarly, we keep our food in the of preserving food in our homes. We refrigerator. Low temperature inhibits have to save it from the attack of the growth of microbes. microorganisms. Chemical Method Salts and edible oils are the common Why does the milk that chemicals generally used to check the comes in packets not spoil? growth of microorganisms. Therefore My mother told me that the they are called preservatives. We add milk is ‘pasteurised’. What salt or acid preservatives to pickles to is pasteurisation? prevent the attack of microbes. Sodium benzoate and sodium metabisulphite are Pasteurised milk can be consumed common preservatives. These are also without boiling as it is free from harmful used in jams and squashes to check microbes. The milk is heated to about their spoilage. 700C for 15 to 30 seconds and then suddenly chilled and stored. By doing Preservation by Common Salt so, it prevents the growth of microbes. Common salt has been used to preserve This process was discovered by Louis meat and fish for ages. Meat and fish Pasteur. It is called pasteurisation. are covered with dry salt to check the growth of bacteria. Salting is also Storage and Packing used to preserve amla, raw mangoes, These days dry fruits and even tamarind, etc. vegetables are sold in sealed air tight packets to prevent the attack of Preservation by Sugar microbes. Jams, jellies and squashes are preserved by sugar. Sugar reduces the moisture 2.6 Nitrogen Fixation content which inhibits the growth of You have learnt about the bacterium bacteria which spoil food. Rhizobium in Classes VI and VII. It is involved in the fixation of nitrogen in Preservation by Oil and Vinegar leguminous plants (pulses). Recall that Use of oil and vinegar prevents spoilage Rhizobium lives in the root nodules of of pickles because bacteria cannot live leguminous plants (Fig. 2.9), such as in such an environment. Vegetables, beans and peas, with which it has a fruits, fish and meat are often preserved symbiotic relationship. Sometimes by this method. nitrogen gets fixed through the action of lightning. But you know that the Heat and Cold Treatments amount of nitrogen in the atmosphere You must have observed your mother remains constant. You may wonder boiling milk before it is stored or used. how? Let us understand this in the next Boiling kills many microorganisms. section. 26 SCIENCE 2018-19  nitrogen cannot be taken directly by plants and animals. Certain bacteria and blue green algae present in the soil fix nitrogen from the atmosphere and convert it into compounds of nitrogen. Once nitrogen is converted into these usable compounds, it can be utilised by plants from the soil through their root syste