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Q Describe two methods of irrigation which conserve water Two methods of irrigation which help in conservation of water are: a) Sprinkler system: In this method of irrigation, water is supplied using pipes to one or more central locations within the field. When water is allowed to flow under high pressure with the help of a pump, it gets sprinkled on the crops. This system is more useful on uneven land and sandy soil , having fewer water supplies. (b) Drip system: In this system of irrigation, water is delivered at or near the roots of plants, drop by drop. This is the most efficient method of irrigation as there is no wastage of water at all. This method is important in areas where water availability is poor   Class 08 Basic sciences Solution Download File
Constant : A symbol having a fixed numerical value is called a constant. Variable: A symbol which takes various numerical values is called a variable. The perimeter P of a rectangle of sides l and b is given by P = 2 (l + b). Here, 2 is a constant and l and b are variables. Algebraic Expressions: A combination of constants and variables with four fundamental operations of addition, subtraction, multiplication and division is called an algebraic expression. Terms: Various parts of an algebraic expression which are separated by the signs of + or – are called the ‘terms’ of the expression. Ex. 2x2 – 3xy + 5y2 is an algebraic expression consisting of three terms, namely, 2x2,–3xy and 5y2. Kinds of algebraic expression: Monomial : An algebraic expression containing only one term is called a monomial. Ex. – 5, 3y, 7xy, 3/2 x2yz, etc. are all monomials. Binomial : An algebraic expression containing two terms is called a binomial. Ex. The expression 2x – 3, 3x + 2y, xyz –5 etc. are all binomials. Trinomial: An algebraic expression containing three terms is called a trinomial. Ex. The expressions a – b + c , 2x2 + y2 – xy, etc. are trinomial. Factors of term: Each terms in an algebraic expression is a product of constant and variable . These number(s) and variables are known as the factors of those terms. A constant factor is called a numerical factor, while a variable factor is known as a literal factor.     Class 8 Algebraic Expressions and Identities - 1
Class 8 Algebraic Expressions and Identities - 2 Class 8 Algebraic Expressions and Identities - 3 Class 8 Algebraic Expressions and Identities - 4 DPS Algebric Expression class 8 work sheet - 5 Download File  Learnwell science chapter the cell Questions answer
Q.1. what are the various level of organisation in multicellular organism? Ans: Cell performs similar functions are grouped into Tissue. A group of tissue that perform special functions are called organ. A group of organ is called organ system. Organ system work together and form organism Q.2. Give two examples to explain there exist relation between cell shape and function? Ans: The nerve cell cells are branched that help them to send message over longer distances. White blood cells have irregular shape as they have to move in different parts of tissue. Q.3. Why mitochondria is called power house of cell? Ans: Mitochondria is called power house of cell because it release energy by the oxidation of food. Q.4. Why is Lysosomes called suicide bag? Ans: When cell get damaged Lysosomes may burst and destroy the cell. For this activity Lysosomes are called suicide bag. Q.5.Write the functions of: cell wall, cell membrane, chromosomes and cytoplasm Ans: cell wall – Provide rigidity, shape and protection to plant cells. cell membrane: It separate cell from surrounding and allow the moment of some substance. Chromosomes: They carry hereditary information from parent to children Cytoplasm : All chemical rection and function takes place here. Q.6. Describe the part of the nucleus of cell? Ans: Nucleus is a dense round spherical body in the centre of a cell. It was discovered by Robert brown in 1831. Nucleus is covered with a double layer porous covering called nuclear membrane. The dense fluid like substance present inside the nucleus i is called Nucleoplasm. There is a dense round spherical body in the centre of Nucleus is called the nucleolus which contains RNA. Chromosomes are condensed form of the fine thread like structure in side nucleus called chromatin material (thread) which are seen only cell undergoing cell division. The chromatin thread are made up of DNA which organize into gene Q.7. Distinguish between prokaryotic and eukaryotic? Ans: Prokaryotes
Eukaryotes
Q.8. What are cell organelles. Give three examples? Ans: Membrane bound structures present in the cytoplasm that perform specific functions are called cell organelles like mitochondria, Lysosomes, Golgi bodies. Q.9. Why was cell discovered after the discovery of microscope? Ans: cell was discovered after the discovery of microscope because cell is microscopic and cannot be seen by naked eyes. Q.10. why plant cell have call wall but animal cell do not? Ans: plants do not move from one place to another. Plant cells have cell wall that helps the plant to have a fixed shape and protect it from injury. The cell wall if present in the animal cell, they will not show movement. Q11. Raghav viewed a slide under microscope: it had a cell wall and chloroplast. Was he viewing a slide of animal or plant cell? Ans: plant cell Fore more chapters questions Excel in your exam  class 8 chapter microorganism friend and foe solution Q. Why do you think: a. curd set faster in summer than in winter. b. Curd turns sour faster in summer than in winter Answer: (a) Bacteria grow faster in summer, as they require warm temperatures to grow. Thus, they set curd faster in summer than in winter. (b) Curd turns sour faster in summer than in winter because during summer, the temperature is high, which allows the bacteria to grow and multiply faste Q. The sealed packets in which food items such as chips are sold are usually filled with nitrogen. How do you think that help? Answer : Nitrogen prevents the growth of microorganisms as microorganisms need oxygen for their growth and survival. In this way, the food items are prevented from being spoilt by microorganisms. Q. Why should you always wash our hands before handling food and after using the toilet Answer: We should always wash our hands before handling food and after using the toilet to remove the germs from our hands. If we eat food without washing our hands, there is a chance that the germs may enter our body, causing infections. Q. why does bacterias become resistant to antibiotics within our own life time Answer : Evolution occurs quickly in bacteria, as bacteria reproduce faster than other organisms. Some bacteria may reproduce as fast as every 20 minutes, which is halfamillion times faster than humans. This is the reason why bacteria become resistant to antibiotics within our own life time. Q. How do viruses caused diseses? Answer : A virus enters the living cell of an organism and uses the resources inside the host cell for its reproduction. It duplicates itself and then the host cell dies and finally bursts. The newly formed virus released from that cell starts invading the other cells. This process is repeated and in this way, many cells of the host organism are destroyed. As large numbers of host cells are destroyed, the person becomes ill. Q. Which organism act as decomposer? How is this activity helpful to us? Answer : Bacteria, protozoa and fungi act as decomposers. These decomposers break down organic matter present in the bodies of dead plants and animals into simple substances and mix them in the soil. A plant growing in this soil absorbs the nutrients and passes them to the animals that consume the plant products. Thus, these microorganisms act as decomposers and help in recycling the nutrients in the soil. More solved Questions Visit Micro organism class 8 laboratory experiment   Learnwell science class 8 Solution Question: Define these terms: Micro organism, Immunity, Fermentation, Pasteurization Answer: Micro organisms are tiny living organism that cannot be seen with naked eyes. For example: Bacteria, fungi, algae, virus and protozoa Immunity: The body of living organisms have ability to resist fight against disease. This is known as Immunity. Fermentation: The conversion of sugar into alcohol in absence of oxygen using yeast is called Fermentation. Pasteurization: Pasteurization is a heat-treatment process that destroys pathogenic microorganisms in certain foods and beverages. Question: How do bacteria help in curd formation? Answer: Lactobacillus bacteria present in curd that convert lactose present in milk to lactic acid Question: Why does dough of bread rise when yeast is added to it? Answer: When yeast is added to dough of bread, yeas reproduce rapidly and produce CO2 during respiration. The bubbles of CO2 fill the dough this make the bread porous and spongy. Question: Viruses are considered as being on the border line between living and non-living organisms, Why? Ans: Viruses is nonliving outside living cells but grows and multiplies inside living cell so, viruses are considered as being on the border line between living and non-living organisms. The study of viruses is called Virology. Question: Write any two ways in which micro organism are useful and harmful for us? Answer: Micro organisms are helpful in for us in following ways : Making curd, cheese from milk, bread from drought of bread and vinegar from alcohol. Micro organisms are harmful for us in following ways: They can cause harmful diseases like polio, chicken pox etc They can spoil our food called rancidity. Question: Describe the role of micro organism in (i) cleaning environment (ii) sewage tratment Answer : The role of micro organism in (i) Cleaning environment: Bacteria and fungi breakdown dead and decaying plants and animals and help in cleaning environment. (ii) Sewage treatment: Bacteria decompose municipal sewage into useful substance like methane gas or biogas. Question: What are vaccines? Answer : A small dose of dead and weekend microbes that produce immunity to diseases in the body is called vaccine. Question: What are antibiotic? Answer: Antibiotic are chemical prepared from bacteria and fungi Antibiotic either kill or stop the growth of disease causing microbes like penicillin. Question: What is food poisoning? Answer: Food poisoning is an illness caused by eating contaminated food. It's not usually serious and most people get better within a few days without treatment. Question: Name the different methods of food preservation. Give one example of each? Answer: The different methods of food preservation are: Drying or dehydration: vegetable Freezing: Milk,fruit,meat etc Pasteurization: milk Salting and adding sugar: pickles, jam, Question: what are the advantages of food preservation? Answer : The advantages of food preservation are: (i) It reduces the wastage of food by avoiding its spoilage. (ii) It help in retaining nutritive value of food. (ii) It help to ensure avaibility of food item during off season and at distant palces. Question: Why should you empty your water cooler during the rainy season? Answer : We should empty our water cooler during the rainy season to prevent the breeding of mosquito in it. Question: Why do sufficient amount of salt and oil added to pickles? Answer: when salt and oil are added to pickles loose water that prevent their growth. This is why sufficient amount of salt and oil added to pickles to preserve it. Question: Why are we preserving dead body in ice for long periods? Answer: we preserving dead body in ice for long periods because freezing or lowering temperature stop growth and multiplication of micro organism. Question: Why does sugar solution with yeast powder become alcoholic in taste? Answer: Sugar solution with yeast powder become alcoholic in taste because yeast convert sugar solution into alcohol during anaerobic respiration. Question: Why milk in polypacks does not get spoiled? Answer: Milk in polypacks does not get spoiled because before storage milk is heated to about 70 degree C for 15 min then cool down quickly this sudden change in temperature kill microbes and prevents them from growth. More solved Questions Visit   |
