Class 8 Science Chapter 2 Microorganisms : Friend and Foe

Key Concept: Microorganisms and Their Types

d) Viruses

[Solution Description]

Viruses are unique among microorganisms as they require a host organism to reproduce. This is a defining characteristic of viruses, which differentiates them from bacteria, fungi, and protozoa, which can reproduce independently.

Key Concept: Microorganisms – Definition

b) Tiny living organisms visible only under a microscope

[Solution Description]

Microorganisms are tiny living organisms that cannot be seen with the unaided eye. They include bacteria, fungi, protozoa, and some algae. These organisms are found in various environments such as soil, water, and air.

Key Concept: Microorganisms and their visibility

d) Assertion is false, but Reason is true.

[Solution Description]

The syllabus mentions that microorganisms are too small to be seen with the unaided eye, and some can be observed with a magnifying glass, while others require a microscope. Therefore, the Assertion that microorganisms can only be observed using a microscope is partially correct, as some can be seen with a magnifying glass. However, the Reason that microorganisms are too small to be seen with the naked eye is entirely true. Hence, the Assertion is false, but the Reason is true.

Key Concept: Microorganisms in Soil and Water

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description]

The assertion states that microorganisms can be observed in soil and water samples under a microscope, which is true as demonstrated by Activities 2.1 and 2.2 in the syllabus. The reason given is that microorganisms are too small to be seen with the unaided eye, which is also true. However, the reason does not directly explain why they can be observed in soil and water samples under a microscope; it only explains their size. Therefore, both the assertion and reason are true, but the reason is not the correct explanation of the assertion.

Key Concept: Reproduction of Viruses

c) Inside the host organism’s cells

[Solution Description]

Viruses are unique among microorganisms in that they cannot reproduce on their own. They require a host cell to replicate. This host cell can be a bacterium, plant, or animal cell.

Key Concept: Role of Microorganisms

a) Fixing nitrogen in the soil

[Solution Description]

Bacteria play a crucial role in agriculture by fixing nitrogen from the atmosphere into the soil, which increases soil fertility. This process is essential for plant growth as it converts atmospheric nitrogen into a form that plants can absorb and use.

Key Concept: Fungi

c) Assertion is true, but Reason is false.

[Solution Description]

Fungi are indeed microorganisms that can be seen with a magnifying glass, as mentioned in the syllabus. However, the reason provided that fungi are larger in size compared to bacteria and protozoa is not entirely accurate. While fungi are generally larger than bacteria, some protozoa can be similar in size or even larger than certain fungi. Therefore, the assertion is true, but the reason is false.

Key Concept: Protozoa, Disease Causation

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description] The assertion states that protozoa cause diseases like malaria and dysentery, which is true according to the syllabus. The reason explains that protozoa are microorganisms that thrive in the human body and disrupt normal bodily functions. While this is also true, it does not specifically explain why protozoa cause diseases like malaria and dysentery. Therefore, both statements are true, but the reason does not correctly explain the assertion.

Key Concept: Where Microorganisms Live

d) All of the above

[Solution Description]

Microorganisms are highly adaptable and can live in a variety of environments. They can survive in ice-cold climates, hot springs, deserts, marshy lands, and even inside the bodies of animals. This adaptability allows them to exist in almost all types of environments on Earth.

Key Concept: Viruses

c) Chicken pox

[Solution Description]

Viruses are responsible for causing various diseases in humans, including common ailments like cold, influenza (flu), and serious diseases like polio and chicken pox.

Key Concept: Disease Causing Microorganisms

c) Chicken Pox

[Solution Description]

According to the syllabus, several diseases are caused by viruses, including measles, chicken pox, polio, and hepatitis A. Among the given options, chicken pox is caused by a virus.

Key Concept: Common cold and vaccination

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Polio drops given to children are indeed a type of vaccine. Vaccines work by introducing dead or weakened microbes into the body, which stimulates the immune system to produce antibodies. These antibodies remain in the body, providing immunity against future infections by those specific microbes. Therefore, both the Assertion and Reason are true, and the Reason correctly explains why polio drops are considered a vaccine.

Key Concept: Dysentery, Malaria

d) Assertion is false, but Reason is true.

[Solution Description]

The assertion states that the protozoan parasite Plasmodium causes dysentery. However, this is incorrect because Plasmodium is responsible for causing malaria, not dysentery. Dysentery is caused by another protozoan parasite called Entamoeba histolytica. The reason states that dysentery is characterized by severe diarrhea with blood and mucus, which is correct, but it incorrectly attributes this symptom to the infection of red blood cells by Plasmodium. Instead, these symptoms are due to the infection of the intestines by Entamoeba histolytica. Therefore, the assertion is false, but the reason is true.

Key Concept: Pathogens and Communicable Diseases

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Tuberculosis is indeed a communicable disease caused by bacteria, as stated in the syllabus. It spreads through the air when an infected person sneezes or coughs, releasing bacteria into the air which can then be inhaled by a healthy person. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Habitat of Microorganisms

c) Outer space without protection

[Solution Description]

Microorganisms are highly adaptable and can survive in extreme environments such as ice cold climates, hot springs, deserts, and marshy lands. They are found in air, water, and the bodies of plants and animals. However, they cannot survive in environments that are completely devoid of any form of life-sustaining conditions, such as outer space without any protection.

Key Concept: Habitats of Microorganisms, Extreme Environments

d) Absolute zero vacuum

[Solution Description]

Microorganisms are known for their ability to survive in extreme environments, including boiling mudpots, icy waters, and even concentrated sulphuric acid. However, environments with conditions that completely inhibit biochemical reactions, such as absolute zero temperatures or complete absence of water, would be least likely to support their survival.

Key Concept: Microorganisms in diverse environments, survival strategies

c) Use of anaerobic respiration or fermentation for energy

[Solution Description] In low-oxygen environments like marshlands, some microorganisms use anaerobic respiration or fermentation to produce energy. These processes do not require oxygen and allow microorganisms to break down organic matter to generate ATP (energy) in the absence of oxygen. This adaptation is crucial for their survival in such habitats.

Key Concept: Types of Microorganisms

b) Bacteria

[Solution Description]

Single-celled microorganisms include bacteria, some algae, and protozoa. These organisms consist of only one cell and perform all life functions within that single cell.

Key Concept: Microorganisms, Host Dependency

d) Host-dependent microorganism

[Solution Description]

The microorganism described in the question depends on the human gut for survival and cannot exist freely outside this environment. This indicates that it is a host-dependent microorganism. Such microorganisms rely on the host for nutrients, shelter, or other essential conditions for survival.

Key Concept: Microorganisms in Food Preparation

c) They help in the rising of the batter

[Solution Description]

Microorganisms, particularly bacteria and yeast, are involved in the fermentation process of idli and dosa batter. They break down carbohydrates into simpler sugars, producing carbon dioxide gas that helps the batter to rise and become fluffy.

Key Concept: Friendly Microorganisms

c) Lactobacillus

[Solution Description]

The process of converting milk into curd involves the action of bacteria, specifically Lactobacillus. This bacterium multiplies in milk and converts lactose into lactic acid, which causes the milk to curdle and form curd.

Key Concept: Friendly Microorganisms, Increasing Soil Fertility

b) Microorganisms in compost break down organic matter to increase soil fertility

[Solution Description]

Curd contains Lactobacillus, which is not directly involved in nitrogen fixation but helps in converting milk into curd. However, composting involves breaking down organic wastes like vegetable peels and animal remains into harmless substances by bacteria, which enriches the soil with nutrients. Nitrogen-fixing bacteria, such as Rhizobium, are responsible for converting atmospheric nitrogen into a form usable by plants, thereby increasing soil fertility.

Therefore, the accurate statement is that microorganisms in compost break down organic matter to increase soil fertility, while nitrogen-fixing bacteria convert atmospheric nitrogen to enrich the soil.

Key Concept: Microorganisms in Food Preparation

b) Lactobacillus

[Solution Description]

The primary microorganism involved in the conversion of milk into curd is Lactobacillus. This bacterium multiplies in milk and converts it into curd by promoting fermentation.

Key Concept: Fermentation, Microorganisms in food preparation

b) Saccharomyces cerevisiae

[Solution Description]

The fermentation process of idli batter involves the action of microorganisms such as bacteria and yeast. Among these, yeast (specifically Saccharomyces cerevisiae) plays a crucial role in producing carbon dioxide gas during fermentation. This gas gets trapped in the batter, causing it to rise and become fluffy. Therefore, the primary microorganism responsible for this process is yeast.

Key Concept: Role of Microorganisms in Cleaning the Environment

b) Degrading harmful and smelly substances into simpler reusable ones

[Solution Description] Microorganisms play a crucial role breaking down dead organic waste into simple substances that can be reused by other plants and animals. This process helps in degrading harmful and smelly substances, thereby cleaning the environment.

Key Concept: Nitrogen cycle, Nitrogen fixation, Atmospheric nitrogen

c) Release of nitrogen gas through denitrification

[Solution Description]

The nitrogen cycle involves several processes, including nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. Denitrification is the process where certain bacteria convert nitrogen compounds back into nitrogen gas, which is released into the atmosphere. This helps maintain the balance of nitrogen in the atmosphere.

Key Concept: Nitrogen Fixation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that Rhizobium bacteria are essential for increasing soil fertility by fixing atmospheric nitrogen. This is true because Rhizobium is a biological nitrogen fixer that converts atmospheric nitrogen into compounds that plants can use, thereby enriching the soil with nitrogen.

The reason states that Rhizobium forms a symbiotic relationship with leguminous plants and resides in their root nodules. This is also true because Rhizobium lives in the root nodules of leguminous plants like beans and peas, forming a mutualistic relationship where the bacteria provide nitrogen to the plant, and the plant provides carbohydrates to the bacteria.

Furthermore, the reason correctly explains the assertion because it is the symbiotic relationship between Rhizobium and leguminous plants that enables the bacteria to fix atmospheric nitrogen, which in turn increases soil fertility.

Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Role of Lactobacillus in curd formation

b) Lactobacillus

[Solution Description]

The bacterium Lactobacillus is primarily responsible for converting milk into curd. It multiplies in milk and promotes the formation of curd by fermenting lactose (milk sugar) into lactic acid, which causes the milk to coagulate.

Key Concept: Microorganisms, Nutrient Recycling, Environmental Cleaning

d) Microorganisms decomposing dead organic waste in a forest

[Solution Description]

Microorganisms play a crucial role in breaking down dead organic matter into simpler substances, which can then be reused by plants and animals. This process not only cleans the environment by removing harmful and smelly substances but also recycles essential nutrients back into the ecosystem. Scenario d) exemplifies this process, where microorganisms decompose dead organic waste in a forest, converting it into nutrients that enrich the soil and support plant growth. This dual role of cleaning the environment and recycling nutrients is a key function of microorganisms.

Key Concept: Role of Lactobacillus in curd formation

c) $Lactobacillus$

[Solution Description]

The bacterium $Lactobacillus$ is responsible for converting milk into curd. It multiplies in milk and helps in the curd formation process.

Key Concept: Role of yeast in bread and cake making, Fermentation process

d) Assertion is false, but Reason is true.

[Solution Description]

The assertion states that yeast is used in bread making primarily to enhance the flavor of the bread. However, the primary role of yeast in bread making is not flavor enhancement but rather the production of carbon dioxide gas during respiration. This gas causes the dough to rise, leading to the light and airy texture of bread. The reason provided is correct as it accurately describes the process by which yeast causes dough to rise through the production of carbon dioxide. Therefore, the assertion is false, but the reason is true.

Key Concept: Yeast fermentation, Carbon dioxide production

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that the dough rises due to yeast converting sugar into carbon dioxide and alcohol through fermentation. This is true as yeast undergoes fermentation in the presence of sugar, producing carbon dioxide gas, which fills the dough and increases its volume. The reason states that yeast reproduces rapidly in warm conditions and produces carbon dioxide during respiration. This is also true as yeast thrives in warm environments and releases carbon dioxide as a byproduct of respiration. Furthermore, the reason correctly explains why the dough rises, making it the correct explanation of the assertion.

Key Concept: Commercial Use of Microorganisms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that yeast is used in the production of bread because it produces carbon dioxide during respiration. This is true as yeast indeed respires anaerobically, producing carbon dioxide and alcohol. The reason explains that the carbon dioxide produced by yeast increases the volume of dough, making it suitable for baking bread. This is also true and directly explains why yeast is used in bread production. Therefore, both the assertion and the reason are true, and the reason correctly explains the assertion.

Key Concept: Fermentation, Alcohol Production

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description]

The assertion states that yeast is used in the production of alcohol because it converts sugar into alcohol and carbon dioxide through fermentation. This is true as yeast performs fermentation, which is a metabolic process where sugar is converted into alcohol and carbon dioxide ($\text{sugar} \rightarrow \text{alcohol} + \text{carbon dioxide}$). The reason states that fermentation is a process discovered by Louis Pasteur that involves the breakdown of sugar into alcohol and carbon dioxide by microorganisms like yeast. This is also true as Louis Pasteur did discover fermentation in 1857, and it does involve the breakdown of sugar into alcohol and carbon dioxide by yeast. However, the reason does not directly explain why yeast is specifically used for alcohol production; it only describes the process of fermentation. Therefore, while both statements are true, the reason does not correctly explain the assertion.

Key Concept: Fermentation, Yeast

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that yeast is used in the commercial production of alcohol due to its ability to convert sugar into alcohol and carbon dioxide through fermentation. This is true as fermentation is the biochemical process where yeast metabolizes sugar to produce alcohol and carbon dioxide. The reason provided is also true as Louis Pasteur did discover fermentation in 1857, and it involves the breakdown of sugar into alcohol and carbon dioxide by yeast in anaerobic conditions. Moreover, the reason correctly explains the assertion since the process of fermentation is the basis for yeast’s role in alcohol production.

Key Concept: Fermentation Process

c) Carbon dioxide

[Solution Description]

During fermentation, yeast converts sugar into alcohol and carbon dioxide. The equation for this process is $\text{sugar} \rightarrow \text{alcohol} + \text{carbon dioxide}$. The primary gas produced is carbon dioxide, which causes the dough to rise in baking.

Key Concept: Medicinal Use of Microorganisms

d) Assertion is false, but Reason is true.

[Solution Description]

The assertion states that antibiotics are effective against all types of microorganisms, including viruses. This is false because antibiotics are specifically designed to target bacteria and fungi, not viruses. The reason states that antibiotics work by inhibiting the growth of or killing bacteria and fungi, which is true. Therefore, the assertion is false, but the reason is true.

Key Concept: Antibiotics, Microorganisms, Viral Infections

b) Antibiotics should be taken only on the advice of a qualified doctor

[Solution Description]

Antibiotics are effective against bacterial infections but not viral infections like cold and flu. Taking antibiotics unnecessarily can lead to antibiotic resistance and kill beneficial bacteria in the body. Therefore, antibiotics should only be taken when prescribed by a qualified doctor.

Key Concept: Effectiveness of Antibiotics

b) Cold and flu arecaused by viruses

[Solution Description]

Antibiotics are designed to kill or inhibit the growth of bacteria, but they have no effect on viruses. Since cold and flu are caused by viruses, antibiotics are ineffective against them.

Key Concept: Antibiotics and Viruses, Precautions for Antibiotic Use

c) Antibiotics only work on bacterial infections

[Solution Description]

Cold and flu are caused by viruses, not bacteria. Antibiotics are designed to target bacterial infections by either killing bacteria or inhibiting their growth. Since viruses have a different structure and life cycle compared to bacteria, antibiotics have no effect on them. Using antibiotics for viral infections is ineffective and contributes to antibiotic resistance.

Key Concept: Antibiotics Usage

c) To avoid killing beneficial bacteria and prevent antibiotic resistance

[Solution Description]

Antibiotics should be taken only on the advice of a qualified doctor because taking them unnecessarily may kill beneficial bacteria in the body and make the drug less effective in the future. Completing the prescribed course ensures that all harmful bacteria are completely eradicated, preventing the development of antibiotic resistance.

Key Concept: Antibiotics, Microbial Infections

a) The bacteria become resistant to the antibiotic

[Solution Description]

The patient did not complete the prescribed antibiotic course, which can lead to the survival of some bacteria. These surviving bacteria may develop resistance to the antibiotic, making it less effective in the future. Additionally, the incomplete course may not fully eradicate the infection, leading to a potential relapse. Therefore, the most likely consequence is that the bacteria become resistant to the antibiotic.

Key Concept: Antibiotics and Viruses

c) Cold and flu are caused by viruses

[Solution Description]

Antibiotics are designed to target and kill bacteria. Cold and flu are caused by viruses, not bacteria. Since antibiotics do not affect viruses, they are ineffective against cold and flu.

Key Concept: Vaccination

c) To provide immunity against specific diseases

[Solution Description] A vaccine introduces dead or weakened microbes into the body. The body then produces antibodies to fight these microbes and remembers how to fight them if they enter again. This provides long-term protection against the disease-causing microbes. Therefore, the primary purpose of a vaccine is to provide immunity against specific diseases.

Key Concept: Disease Prevention

c) Smallpox

[Solution Description]

Vaccines are available for several diseases, including cholera, tuberculosis, smallpox, and hepatitis. Polio drops given to children are also a form of vaccine.

Key Concept: Vaccination Process

c) To prevent future infections by building immunity

[Solution Description]

The primary function of introducing dead or weakened microbes into the body through vaccination is to stimulate the immune system to produce antibodies. These antibodies help the body recognize and fight the actual disease-causing microbe if it enters the body in the future. This process builds immunity and provides long-term protection against specific diseases.

Key Concept: Diseases Prevented by Vaccination

d) Smallpox

[Solution Description]

Smallpox is a disease that has been eradicated worldwide due to successful vaccination campaigns. Edward Jenner discovered the smallpox vaccine in 1798, and widespread vaccination programs led to its eradication.

Key Concept: Communicable Diseases and Vaccination

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that cholera is a communicable disease transmitted through contaminated water or food. This is correct as per the syllabus, which mentions that cholera is caused by bacteria and spreads through water or food. The reason states that cholera can be prevented by vaccination, which introduces dead or weakened bacteria to stimulate antibody production. This is also correct as the syllabus explains that vaccines work by introducing dead or weakened microbes to trigger antibody production, providing long-term protection. Moreover, the reason correctly explains how vaccination prevents cholera, which is directly related to the assertion. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Pulse Polio Programme

c) To develop immunity against the poliovirus

[Solution Description]

The Pulse Polio Programme aims to eradicate polio by immunizing children against the poliovirus. Polio drops contain a vaccine that helps the body develop immunity to the virus, thereby preventing the disease. This is crucial in reducing and eventually eliminating the risk of polio in the population.

Key Concept: Biological Nitrogen Fixers

b) They convert atmospheric nitrogen into nitrogenous compounds, enriching the soil.

[Solution Description]

Biological nitrogen fixers, such as certain bacteria, convert atmospheric nitrogen into nitrogenous compounds that enrich the soil. This process increases soil fertility, making it more suitable for plant growth. These bacteria are often found in the \textbf{root nodules of leguminous plants} and play a vital role in sustainable agriculture by reducing the need for chemical fertilizers.

Key Concept: Soil fertility

b) They convert nitrogen gas into usable compounds

[Solution Description]

Nitrogen-fixing bacteria convert atmospheric nitrogen into nitrogenous compounds that plants can absorb through their roots. This process enriches the soil with essential nutrients, thereby increasing its fertility and promoting plant growth.

Key Concept: Decomposition, Nitrogen Cycle

b) 3 kg

[Solution Description]

The total nitrogen in the dead organic matter is 10 kg. According to the question, 30\% of this nitrogen is converted into nitrogen gas. To find the amount of nitrogen returned to the atmosphere, calculate 30\% of 10 kg:

$\text{Nitrogen returned to atmosphere} = 10 \, \text{kg} \times 0.30 = 3 \, \text{kg}$

Therefore, 3 kg of nitrogen is returned to the atmosphere.

Key Concept: Disease causing Microorganisms in Humans

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that communicable diseases can spread through air, water, food, or physical contact, which is true as per the syllabus. The reason explains that pathogens enter the body through the air we breathe, the water we drink, or the food we eat, which is also true and directly explains why communicable diseases can spread through these mediums.

Key Concept: Mode of Transmission

b) Malaria

[Solution Description]

The female Anopheles mosquito carries the parasite $Plasmodium$, which causes malaria. It transfers the parasite to humans when it bites and sucks blood. Therefore, the disease transmitted through the bite of a female Anopheles mosquito is malaria.

Key Concept: Communicable Diseases

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description]

The Assertion states that cholera is a communicable disease spread through contaminated water and food. This is true as cholera is caused by the bacterium Vibrio cholerae, which is transmitted through ingestion of contaminated water or food. The Reason explains that pathogens can enter the human body through air, water, or food, which is also true. However, the Reason does not specifically explain why cholera spreads through water and food, but it provides a general mode of transmission for pathogens. Therefore, both Assertion and Reason are true, but the Reason is NOT the correct explanation of the Assertion.

Key Concept: Spread through air, water, food, or direct contact

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that cholera is a communicable disease that spreads through contaminated food and water, which is true according to the syllabus. The reason explains that pathogens responsible for cholera enter the body when a person consumes contaminated food or water, which is also true. Additionally, the reason correctly explains the assertion as it directly connects the mode of transmission with the entry of pathogens into the body.

Key Concept: Carriers of Disease

c) Female Aedes mosquito

[Solution Description]

The Aedes mosquito, specifically the female Aedes mosquito, is known to carry the dengue virus. When a female Aedes mosquito bites a person infected with dengue, it becomes a carrier of the virus. It then transmits the virus to other humans when it bites them. Therefore, the female Aedes mosquito is the correct answer.

Key Concept: Carriers of Microbes

c) To prevent the transfer of pathogens by houseflies

[Solution Description]

Houseflies sit on garbage and animal excreta, where pathogens stick to their bodies. When these flies sit on uncovered food, they transfer the pathogens. Consuming this contaminated food can lead to illness. Therefore, keeping food covered prevents the transfer of pathogens by houseflies.

Key Concept: Houseflies and Disease Spread

c) By transferring pathogens from garbage to food

[Solution Description]

Houseflies sit on garbage and animal excreta where pathogens stick to their bodies. When they land on uncovered food, they transfer these pathogens, contaminating the food. Consuming such contaminated food can lead to illnesses.

Key Concept: Female Anopheles Mosquito and Malaria

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that the female Anopheles mosquito is the carrier of the malaria parasite ($Plasmodium$), which is true as per the syllabus. The reason states that the female Anopheles mosquito spreads malaria by biting humans, which is also true. Moreover, the reason correctly explains the assertion because the transmission of malaria occurs when the infected mosquito bites a human, thereby transferring the parasite into the human bloodstream.

Key Concept: Female Aedes mosquito spreads dengue

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that the female Aedes mosquito is the primary vector for the transmission of the dengue virus, which is true according to the syllabus. The reason explains that the female Aedes mosquito requires a blood meal to produce eggs, and during this process, it can transmit the dengue virus to humans. This is also true and directly explains why the female Aedes mosquito is the primary vector for dengue transmission. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Vector-borne Diseases, Prevention

a) Consume uncovered food

[Solution Description]

Dengue is a vector-borne disease transmitted by the female Aedes mosquito. To prevent its spread, it is essential to eliminate stagnant water where mosquitoes breed. Spraying insecticides, using mosquito nets, and keeping surroundings clean are effective measures. However, consuming uncovered food does not directly relate to mosquito breeding or dengue prevention, making it the least effective action.

Key Concept: Maintaining hygiene, Vaccination, Controlling mosquito breeding

d) Assertion is false, but Reason is true.

[Solution Description]

The assertion states that vaccination is the \textbf{most effective} method to prevent malaria and dengue. While vaccines are indeed a powerful tool for preventing many diseases, currently, there is no widely available vaccine for dengue that is universally effective, and malaria vaccines (e.g., RTS,S/AS01) have limited efficacy and are not widely deployed. Therefore, the assertion is false. The reason, however, is true. Vaccines work by introducing dead or weakened microbes, prompting the immune system to produce antibodies and develop memory against the pathogen. Thus, while the reason correctly explains how vaccines work, it does not support the assertion because vaccination is not the only or most effective method for controlling these diseases. Instead, measures like controlling mosquito breeding and maintaining hygiene are crucial.

Key Concept: Disease causing Microorganisms in Animals, Anthrax

d) It is caused by a bacterium that infects both humans and cattle

[Solution Description]

Anthrax is caused by the bacterium $Bacillus anthracis$, which can infect both humans and cattle. The disease is highly contagious and can spread rapidly among animals, posing a significant risk to livestock. In humans, anthrax can cause severe illness and even death if not treated promptly. Its ability to affect multiple species and its severity make it a significant threat.

Key Concept: Anthrax in cattle caused by bacteria

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Anthrax is indeed a disease that affects both humans and cattle, making the Assertion true. The Reason is also correct as Robert Köch discovered the bacterium $Bacillus\ anthracis$ in 1876, which is the causative agent of anthrax. Furthermore, the Reason correctly explains the Assertion because it identifies the specific bacterium responsible for the disease.

Key Concept: Foot and Mouth Disease

c) Virus

[Solution Description]

Foot and Mouth Disease in cattle is caused by a virus, as mentioned in the syllabus. Viruses are the microorganisms responsible for this disease.

Key Concept: Disease-causing Microorganisms in Plants

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that citrus canker is caused by bacteria and transmitted through air, which is correct as per the syllabus. The reason states that bacteria are microorganisms that can spread through air and cause diseases in plants, which is also true. Furthermore, the reason correctly explains why citrus canker is transmitted through air, as it is caused by bacteria, which can be airborne.

Key Concept: Citrus Canker

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description] Citrus canker is indeed caused by bacteria, specifically the bacterium Xanthomonas axonopodis. The disease spreads through air as the bacteria can be carried by wind or rain splash to infect other plants. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Microorganism Type

c) Fungi

[Solution Description]

According to the syllabus, the rust of wheat disease is caused by fungi. This is a direct recall question based on the given information.

Key Concept: Yellow Vein Mosaic in Bhindi, Disease Symptoms

c) Yellowing of the leaf veins with a mosaic pattern

[Solution Description]

Yellow vein mosaic disease in bhindi is caused by a virus and its primary symptoms include yellowing of the veins in the leaves, leading to a mosaic-like pattern. This discoloration is distinct and different from other diseases that might cause wilting, spots, or curling of leaves. Therefore, the presence of yellow veins in the leaves is the most indicative symptom of this disease.

Key Concept: Use of Chemicals to Control Plant Diseases

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Sodium benzoate and sodium metabisulphite are common preservatives used in the food industry to prevent spoilage by inhibiting the growth of microorganisms. The Assertion correctly states their use in preserving jams and squashes, which aligns with the syllabus on food preservation methods. The Reason accurately explains that these chemicals work by inhibiting microbial growth, which is directly related to the prevention of food spoilage. Therefore, both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description] Food poisoning is caused by consuming food that has been spoilt by microorganisms. These microorganisms often produce toxic substances, which make the food unsafe to eat. Therefore, both the assertion and the reason are true, and the reason correctly explains why the assertion is true.

Key Concept: Food Poisoning, Food Preservation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that preserving food by adding salt or sugar prevents the growth of microorganisms due to these substances creating an inhospitable environment. This is true because salt and sugar act as preservatives by creating a hypertonic environment that draws water out of microbial cells through osmosis, leading to dehydration and eventual death of the microorganisms.

The reason given is that microorganisms require a specific moisture level to grow, and salt or sugar absorbs moisture, thereby inhibiting their growth. This is also true because microorganisms thrive in moist environments, and the presence of salt or sugar reduces the available moisture, making it difficult for them to survive and reproduce.

Moreover, the reason correctly explains why the assertion is true. Both statements are factually correct, and the reason provides a valid explanation for the assertion.

Key Concept: Disease-causing Microorganisms in Humans

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that houseflies can transmit disease-causing microorganisms to humans by sitting on uncovered food. This is true because houseflies often sit on garbage and animal excreta, where they pick up pathogens. When they land on uncovered food, these pathogens can be transferred to the food, which, when consumed, can cause illness. The reason explains that houseflies carry pathogens on their bodies after sitting on garbage and animal excreta, which is also true. This reason correctly explains why houseflies can transmit diseases to humans, as it describes the mechanism by which pathogens are carried and transferred.

Key Concept: Food Poisoning

b) Bacterial contamination from uncovered food

[Solution Description]

The symptoms of vomiting, stomach pain, and diarrhea are commonly associated with food poisoning. Food poisoning occurs when food is contaminated with harmful microorganisms, often due to being left uncovered. Houseflies, which carry pathogens from garbage and animal excreta, can transfer these pathogens to uncovered food, leading to contamination and subsequent illness when consumed.

Key Concept: Food Preservation

d) Using sugar as a preservative

[Solution Description]

One of the common methods of food preservation is using sugar. Sugar reduces the moisture content in food, which inhibits the growth of bacteria that spoil the food. This method is commonly used for preserving jams, jellies, and squashes. By reducing the moisture, bacteria cannot thrive, thus preserving the food for a longer period.

Key Concept: Chemical Method

b) Sodium benzoate

[Solution Description]

Sodium benzoate and sodium metabisulphite are common preservatives used in jams and squashes to prevent spoilage by microorganisms.

Key Concept: Preservation by Sugar

c) It reduces the moisture content

[Solution Description]

Sugar is used in preserving jams and jellies because it reduces the moisture content in the food. When the moisture content is reduced, it becomes difficult for bacteria to grow, thereby preventing spoilage. This is why sugar acts as a preservative in jams and jellies.

Key Concept: Chemical Method, Preservation by Sugar

c) Assertion is true, but Reason is false.

[Solution Description] Sodium benzoate is indeed a common preservative used in jams and squashes to inhibit microbial growth, making the Assertion true. However, the Reason states that sodium benzoate reduces moisture content to prevent bacterial growth, which is false. Sodium benzoate works by altering the pH of the food, creating an environment unsuitable for microbial growth, not by reducing moisture content. Therefore, the Assertion is true, but the Reason is false.

Key Concept: Preservation by Common Salt

b) Meat

[Solution Description]

Dry salt is used to preserve meat and fish by checking the growth of bacteria. It is also used for preserving amla, raw mangoes, and tamarind.

Key Concept: Preservation by Common Salt

c) To inhibit the growth of bacteria by creating a hypertonic environment

[Solution Description]

Common salt is used to preserve meat and fish because it inhibits the growth of bacteria by creating a hypertonic environment that draws water out of microbial cells, leading to dehydration and preventing their growth.

Key Concept: Chemical Preservatives

c) Sodium benzoate

[Solution Description]

Key Concept: Preservation by Common Salt

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that common salt is used to preserve meat and fish by inhibiting the growth of bacteria, which is true as salt is a well-known preservative. The reason explains that the high concentration of salt creates a hypertonic environment, causing dehydration of bacterial cells and preventing their growth, which is also true and correctly explains the assertion. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Preservation by Common Salt, Application in Fruits

c) 100 grams

[Solution Description]

To find the amount of salt in a 20\% solution for 500 grams of raw mangoes, we calculate 20\% of 500 grams:

$\text{Salt Required} = \frac{20}{100} \times 500 = 100 \text{ grams}$

Thus, 100 grams of salt are needed in the solution.

Key Concept: Preservation by Sugar

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description] Sugar acts as a preservative in jams and jellies by reducing the moisture content of the food. When sugar is added to these products, it binds with water molecules, making less water available for microbial growth. This reduction in moisture content inhibits the growth of bacteria, which require water to survive and multiply. Thus, both the assertion and reason are true, and the reason correctly explains why sugar is used as a preservative.

Key Concept: Preservation by Sugar

c) Moisture content decreases

[Solution Description]

When sugar is added to jams and squashes, it absorbs the moisture present in the food. This absorption leads to a reduction in the overall moisture content. The reduced moisture creates an environment that is less favorable for the growth of bacteria and other spoilage-causing microorganisms, thus helping to preserve the food.

Key Concept: Preservation by Oil and Vinegar

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description]

The assertion states that oil and vinegar prevent bacterial growth in pickling, which is true because vinegar creates an acidic environment that inhibits bacterial activity. The reason states that bacteria cannot survive in high acidity and low moisture, which is also true. However, the reason does not fully explain the assertion because the role of oil in reducing moisture content is not mentioned. Thus, while both are true, the reason is not the correct explanation of the assertion.

Key Concept: Food Preservation by Common Salt

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that common salt is used to preserve meat and fish, which is true as per the syllabus. The reason given is that common salt inhibits the growth of bacteria by drawing out moisture. This is also true and directly explains why common salt is effective in preserving meat and fish. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Heat Treatment, Pasteurisation

a) 100 CFU/mL

[Solution Description]

The initial bacterial load is $10^6$ CFU/mL. The pasteurisation process achieves a 99.99\% kill rate, meaning that 99.99\% of the bacteria are killed. To find the reduction in bacterial load, we calculate 99.99\% of $10^6$ CFU/mL, which is $0.9999 \times 10^6 = 999900$ CFU/mL. The remaining bacterial load is $10^6 – 999900 = 100$ CFU/mL.

Key Concept: Heat and Cold Treatments

d) Assertion is false, but Reason is true.

[Solution Description]

Boiling milk is a common method to kill many microorganisms, but it does not necessarily kill all types of bacteria. Some heat-resistant bacteria may survive boiling. The reason states that high temperature denatures proteins in bacteria, which is true and leads to their death. However, since the assertion that boiling kills all types of bacteria is false, the correct answer is that the Assertion is false, but the Reason is true.

Key Concept: Refrigeration and Microbial Growth

c) To inhibit the growth of microbes

[Solution Description]

The primary reason for storing food in a refrigerator is to inhibit the growth of microbes. Low temperatures slow down the metabolic activities of microorganisms, thereby preserving the food for a longer period.

Key Concept: Pasteurisation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that pasteurised milk does not require boiling before consumption because it is free from harmful microbes. This is true because pasteurisation is a process designed to kill harmful microorganisms in milk, making it safe for consumption without further boiling. The reason explains that pasteurisation involves heating milk to about $70^\circ C$ for 15 to 30 seconds and then suddenly chilling it to inhibit microbial growth. This is also true and directly explains why pasteurised milk is safe to consume without boiling. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Commercial Use of Yeast

b) Baking Industry

[Solution Description]

Yeast is used in the baking industry for making bread, pastries, and cakes. During respiration, yeast produces carbon dioxide gas, which causes the dough to rise by filling it with bubbles of gas.

Key Concept: Pasteurisation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Pasteurised milk is indeed safe to consume without boiling because the pasteurisation process eliminates harmful microbes. The process involves heating the milk to $70^\circ C$ for 15 to 30 seconds followed by sudden chilling, which effectively prevents microbial growth. Therefore, both the assertion and reason are true, and the reason correctly explains why pasteurised milk can be consumed without boiling.

Key Concept: Storage and Packing, Shelf Life Extension

d) Air-tight containers in a refrigerator

[Solution Description] Extending the shelf life of dried vegetables requires reducing factors that promote spoilage, such as moisture, light, and temperature fluctuations. Storing the vegetables in air-tight containers in a cool, dark place minimizes exposure to these factors. Refrigeration further slows down microbial activity and chemical reactions that cause spoilage. Therefore, the most effective storage condition involves keeping the dried vegetables in air-tight containers in a refrigerator.

Key Concept: Air-tight packaging, Microbial growth

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description]

Air-tight packaging significantly reduces the risk of microbial growth by limiting the oxygen supply, which is essential for the growth of aerobic microbes. However, it does not completely eliminate the risk because some microbes can grow anaerobically (without oxygen). Therefore, while the Assertion that air-tight packaging prevents microbial growth is true, it does not completely eliminate the risk, making the Reason partially correct but not a complete explanation for the Assertion.

Key Concept: Symbiotic Relationship

c) Symbiotic relationship

[Solution Description]

Rhizobium lives in the root nodules of leguminous plants such as beans and peas. It fixes atmospheric nitrogen into a form that the plant can use, while the plant provides nutrients to the bacteria. This mutually beneficial relationship is called symbiosis.

Key Concept: Nitrogen Fixation

c) Rhizobium bacteria

[Solution Description]

Certain bacteria and blue-green algae are responsible for fixing atmospheric nitrogen into compounds that can be utilized by plants. Rhizobium is a well-known bacterium that fixes nitrogen in leguminous plants.

Key Concept: Nitrogen Fixation, Nitrogen Cycle

c) Nitrogen-fixing bacteria in root nodules enrich soil nitrogen

[Solution Description]

Leguminous plants have root nodules that contain nitrogen-fixing bacteria such as Rhizobium. These bacteria convert atmospheric nitrogen ($N_2$) into ammonia ($NH_3$), which is then used by the plant to synthesize proteins and other nitrogenous compounds. This process enriches the soil with nitrogen, making it more fertile for leguminous plants. Non-leguminous plants do not have this symbiotic relationship and rely solely on nitrogen already present in the soil.

Key Concept: Biological Nitrogen Fixation

b) Symbiotic

[Solution Description]

Rhizobium has a symbiotic relationship with leguminous plants. This means that both the bacteria and the plants benefit from each other. The bacteria fix atmospheric nitrogen into usable compounds for the plant, and in return, the plant provides nutrients to the bacteria.

Key Concept: Nitrogen Fixation

b) Fix atmospheric nitrogen

[Solution Description]

The primary role of Rhizobium bacteria is to fix atmospheric nitrogen into nitrogen compounds that can be utilized by plants. This process enriches the soil with essential nutrients and improves its fertility.

Key Concept: Role of blue-green algae in soil fertility

b) Fixing atmospheric nitrogen

[Solution Description]

Blue-green algae fix atmospheric nitrogen into nitrogenous compounds, which enrich the soil with nitrogen. This increased nitrogen content enhances soil fertility, allowing plants to grow better by absorbing these nutrients through their roots.

Key Concept: Nitrogen Fixation by Lightning, Chemical Reactions

b) 672 grams

[Solution Description]

First, calculate the moles of $N_2$: Molar mass of $N_2$ is 28 g/mol. So, 224 grams of $N_2$ is $\frac{224}{28} = 8$ moles. From the reaction $N_2 + O_2 \rightarrow 2NO$, 1 mole of $N_2$ produces 2 moles of $NO$. Therefore, 8 moles of $N_2$ will produce $8 \times 2 = 16$ moles of $NO$.

Next, from the reaction $2NO + O_2 \rightarrow 2NO_2$, 2 moles of $NO$ produce 2 moles of $NO_2$. Therefore, 16 moles of $NO$ will produce 16 moles of $NO_2$.

Finally, from the reaction $3NO_2 + H_2O \rightarrow 2HNO_3 + NO$, 3 moles of $NO_2$ produce 2 moles of $HNO_3$. Therefore, 16 moles of $NO_2$ will produce $\frac{16 \times 2}{3} = \frac{32}{3} = 10.\overline{6}$ moles of $HNO_3$.

The molar mass of $HNO_3$ is $1 + 14 + (3 \times 16) = 63$ g/mol. So, $10.\overline{6}$ moles of $HNO_3$ is $10.\overline{6} \times 63 = 672$ grams.

Key Concept: Nitrogen Fixation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that nitrogen combines with oxygen to form nitrogen oxides during lightning, which is true. The reason explains that the action of lightning provides the energy needed for this reaction, which is also true. Additionally, the reason correctly explains the assertion because the high energy from lightning facilitates the combination of nitrogen and oxygen.

Key Concept: Decomposition, Nitrogen Cycle

c) The nitrogen cycle would slow down due to the accumulation of nitrogenous wastes.

[Solution Description]

Decomposers play a crucial role in converting nitrogenous wastes from dead plants and animals back into nitrogenous compounds, which can be reused by plants. Without decomposers, this conversion process would halt, leading to an accumulation of nitrogenous wastes in the ecosystem and a disruption in the nitrogen cycle. Plants would struggle to obtain nitrogen compounds, affecting their growth and the overall ecosystem balance.

Key Concept: Nitrogen Fixation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that Rhizobium bacteria are found in the root nodules of leguminous plants, which is true as Rhizobium forms a symbiotic relationship with these plants. The reason states that Rhizobium fixes atmospheric nitrogen into a form that plants can use, which is also true. The reason correctly explains the assertion because the role of Rhizobium in nitrogen fixation is directly related to its presence in root nodules of leguminous plants.

Key Concept: Nitrogen Fixation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia, which is true as these bacteria are capable of fixing nitrogen from the atmosphere into ammonia. The reason states that ammonia is a compound that plants can absorb and utilize for their growth, which is also true because ammonia is one of the primary nitrogenous compounds that plants can take up through their roots to synthesize proteins and other essential compounds. Additionally, the reason correctly explains the assertion because the conversion of atmospheric nitrogen into ammonia by bacteria makes it available for plant use.

Key Concept: Nitrogen Cycle, Absorption by Plants

d) Assertion is false, but Reason is true.

[Solution Description]

The assertion states that plants can directly absorb nitrogen gas from the atmosphere for growth. This is incorrect because atmospheric nitrogen (\$N_2\$) is chemically inert and cannot be directly used by plants. The reason explains that nitrogen gas must first be converted into nitrogen compounds by bacteria or blue-green algae before plants can absorb it through their roots. The reason is true and correctly explains why the assertion is false.

Key Concept: Nitrogen Cycle – Absorption by Plants

d) Assertion is false, but Reason is true.

[Solution Description]

Plants cannot directly absorb atmospheric nitrogen because it is inert and not in a usable form. Certain bacteria and blue-green algae fix atmospheric nitrogen into compounds like nitrates and ammonium, which plants can absorb through their roots. Therefore, the assertion is false. The reason is also false because atmospheric nitrogen is not in a form that can be readily used by plants.

Key Concept: Nitrogen Cycle – Role of Microorganisms

c) They convert nitrogenous wastes into plant-usable compounds

[Solution Description]

When plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into nitrogenous compounds that can be used by plants again.

Key Concept: Nitrogen Cycle

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that animals obtain nitrogen by consuming plants, which is true because plants absorb nitrogen from the soil in the form of nitrates and ammonium ions, which are then used to synthesize proteins and other nitrogen compounds. When animals consume these plants, they ingest these nitrogen compounds, thereby obtaining nitrogen. The reason explains that plants synthesize proteins and other nitrogen compounds from the nitrogen they absorb from the soil, which is also true. Moreover, the reason correctly explains why animals obtain nitrogen by consuming plants, as it is through these plant-synthesized compounds that animals acquire nitrogen. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Decomposition, Nitrogen Cycle

b) 2 grams

[Solution Description]

The total nitrogen content in the dead plant is 5 grams. 60\% of this nitrogen is converted into nitrogen gas and released into the atmosphere. This means 40\% of the nitrogen is converted into nitrates for plant use. To calculate the amount of nitrogen available as nitrates:

$\text{Nitrogen available as nitrates} = 5 \text{ grams} \times 0.40 = 2 \text{ grams}$

Therefore, 2 grams of nitrogen are available for plants to use as nitrates.

Key Concept: Decomposition

c) It is transformed into usable compounds by microorganisms

[Solution Description] During decomposition, microorganisms break down the nitrogen in dead organisms into simpler nitrogenous compounds, which are then returned to the soil for use by other plants and animals. This ensures the continuous recycling of nitrogen in the ecosystem.

Key Concept: Denitrification, Nitrogen Cycle

c) Decrease in nitrogen content

[Solution Description]

Denitrifying bacteria convert nitrates ($NO_3^-$) into nitrogen gas ($N_2$), which then escapes into the atmosphere. When these bacteria are introduced into a soil sample with high nitrate concentration, they will reduce the amount of nitrates in the soil by converting them into nitrogen gas. This process decreases the nitrogen content in the soil as the nitrogen is released back into the atmosphere.

Key Concept: Denitrification Process

b) Convert nitrates into nitrogen gas

[Solution Description]

Denitrifying bacteria play a crucial role in the nitrogen cycle by converting nitrogen compounds, such as nitrates ($NO_3^{-}$), back into nitrogen gas ($N_2$). This process helps in maintaining the balance of nitrogen in the atmosphere. The bacteria perform this conversion under anaerobic conditions, releasing nitrogen gas into the atmosphere, which can then be used by other organisms or remain as free nitrogen.