Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eye (New Course)

Key Concept: Microorganisms, Unicellular vs Multicellular

c) Unicellular microorganisms consist of a single cell, while multicellular microorganisms have many cells.
[Solution Description]
Unicellular microorganisms consist of a single cell that performs all life functions independently (e.g., bacteria and Amoeba). Multicellular microorganisms, like some fungi and algae, have multiple cells working together to perform life processes. Viruses are not considered living organisms as they cannot reproduce or carry out metabolic activities outside a host cell.

Key Concept: Microorganisms

d) Assertion is false, but Reason is true.
[Solution Description]
Microorganisms are very tiny organisms, usually less than 0.1 mm in size, and cannot be seen with the naked eye. The human eye has limited resolution and cannot resolve objects smaller than about 0.1 mm. While the Assertion states that microorganisms can be seen if they are large enough, this is false because most microorganisms are much smaller than what the naked eye can perceive. The Reason is correct as the human eye indeed cannot see objects smaller than approximately 0.1 mm. Thus, the Assertion is false, but the Reason is true.

Key Concept: Microscopy History

c) Robert Hooke
[Solution Description]
Robert Hooke was the first scientist to observe and name “cells” while examining a thin slice of cork under a microscope in 1665. His observations were published in his book *Micrographia*.

Key Concept: Discovery of cells

d) Robert Hooke
[Solution Description]
Robert Hooke observed cork under a microscope in 1665 and described its tiny compartments as “cells,” resembling a honeycomb structure.

Key Concept: Role of lenses

c) To bend light inward and magnify objects.
[Solution Description]
Early lenses were shaped like lentil seeds because this convex shape allowed them to bend light rays inward, converging them to a focal point. This convergence magnified the image of small objects, making them appear larger when viewed through the lens.

Key Concept: Microscopy, Cell Theory

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion is true because the microscope enabled the observation of tiny organisms like bacteria and protozoa, which were previously invisible to the naked eye. The Reason is also true, as Robert Hooke’s discovery of cells in cork laid the foundation for the cell theory, confirming that all living things are made up of cells. However, while both statements are true, the Reason does not explain the Assertion directly, as Hooke’s work was more about cell structure rather than the initial observation of microorganisms.

Key Concept: Microscope functionality, Historical context

c) Modern light microscopes can achieve about 3-5 times higher magnification than Hooke’s microscope.
[Solution Description]
Robert Hooke’s microscope could magnify objects 200-300 times, while modern light microscopes can typically achieve magnifications up to 1000-1500 times. The improvement is due to better lens quality, multiple lens systems, and advanced illumination techniques in modern instruments.

Key Concept: Lens improvement, Impact on microbiology

b) He developed glass grinding techniques to produce superior single-lens microscopes.
[Solution Description]
Antonie van Leeuwenhoek improved lens-making techniques to create more powerful microscopes. His advancements allowed him to observe microorganisms like bacteria and blood cells with greater clarity, earning him the title “Father of Microbiology.”

Key Concept: Microscopic Discoveries

c) First to observe and describe bacteria
[Solution Description]
Antonie van Leeuwenhoek improved microscope lenses, enabling him to observe microorganisms like bacteria and blood cells clearly. His detailed observations earned him the title “Father of Microbiology.”

Key Concept: Key Concepts

b) Micrographia
[Solution Description]
Robert Hooke published Micrographia in 1665, which included intricate illustrations of microscopic observations, such as cork cells. This was among the earliest scientific works documenting microorganisms.

Key Concept: Microscope magnification and lens improvement

d) They provided clearer images at higher magnifications (up to 300x).
[Solution Description]
Antonie van Leeuwenhoek’s microscopes had superior lenses compared to Robert Hooke’s, allowing for higher magnification (up to 300x). This enabled him to observe and describe microorganisms like bacteria and blood cells with unprecedented clarity, earning him the title “Father of Microbiology.”

Key Concept: Robert Hooke’s Microscope

b) 200–300 times
[Solution Description]
According to the syllabus, Robert Hooke’s microscope magnified objects 200–300 times.

Key Concept: Title of Leeuwenhoek

d) He was the first to observe and describe microorganisms
[Solution Description]
Antonie van Leeuwenhoek is called the “Father of Microbiology” because he was the first to observe and describe microorganisms such as bacteria and blood cells, laying the foundation for microbiology.

Key Concept: Microscopes, Leeuwenhoek’s contributions

d) He crafted superior quality lenses for better magnification and clarity
[Solution Description]
Antonie van Leeuwenhoek’s key advantage was his ability to create high-quality lenses with superior magnification and clarity. Unlike others, he meticulously ground and polished his lenses, achieving up to 300x magnification. This allowed him to see tiny organisms like bacteria and blood cells clearly, which were invisible to other microscopes of his time.

Key Concept: Hooke’s Micrographia and discovery of cells

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Hooke named the compartments in cork as “cells” due to their resemblance to small rooms. This is true, as documented in *Micrographia*. The reason explains that Hooke’s microscope enabled him to see microscopic structures, which facilitated his discovery. Both statements are correct, and the reason directly supports why Hooke could observe and name these structures.

Key Concept: Concept: Discovery of cells by Robert Hooke

b) Tiny, empty compartments resembling a honeycomb
[Solution Description]
Robert Hooke observed tiny, empty spaces in the cork that resembled a honeycomb. He named these compartments “cells.” This was the first documented observation of cells, although they were actually dead plant cell walls.

Key Concept: Function of Cells

c) Transmitting messages
[Solution Description]
Nerve cells, or neurons, are specialized to transmit messages in the body. Their elongated shape and branched structure allow them to quickly pass signals between different parts of the body.

Key Concept: Differences Between Plant and Animal Cells

c) Chloroplasts
[Solution Description]
Plant cells have a rigid cell wall made of cellulose, chloroplasts for photosynthesis, and large central vacuoles, which are generally absent or small in animal cells. Chloroplasts contain chlorophyll and are essential for photosynthesis.

Key Concept: Definition and structure of a cell

c) Controls all cellular activities and hereditary transmission
[Solution Description]
The nucleus is the control center of the cell. It regulates all activities within the cell, including growth and reproduction. Additionally, it is responsible for transmitting hereditary characters from parents to offspring. Therefore, the correct answer is the option that states the nucleus controls all cellular activities and hereditary transmission.

Key Concept: Definition and structure of a cell

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that the nucleus regulates all activities within the cell and controls growth, which is true as per the syllabus. The reason explains that the nucleus is covered by a thin membrane and contains genetic material, which is also correct. However, while the reason describes the nucleus’s structure, it does not fully explain why the nucleus regulates activities and controls growth (genetic material controls these functions). Thus, both are true, but the reason is not the correct explanation of the assertion.

Key Concept: Shape and Structure

d) The rigid cell wall in plant cells maintains their shape.
[Solution Description]
The rigid cell wall in plant cells provides structural support, giving them a fixed, rectangular shape. Animal cells lack a cell wall, allowing them to be flexible and vary in shape.

Key Concept: Cell wall, vacuole

c) The cell loses its structural integrity and becomes flaccid
[Solution Description]
The large central vacuole in plant cells maintains turgidity and provides structural support by exerting pressure against the cell wall. If the vacuole shrinks or disappears due to water loss, the cell will lose its turgor pressure, causing the cell to become flaccid and potentially leading to wilting. The cell wall remains intact but without the internal pressure from the vacuole, it cannot maintain rigidity.

Key Concept: Cell Membrane Function

c) Regulates the movement of substances in and out of the cell
[Solution Description]
The cell membrane encloses the cytoplasm and nucleus, separates one cell from another, and allows the entry of essential materials and exit of waste material due to its porous nature. Thus, it regulates the movement of substances in and out of the cell.

Key Concept: Nucleus Function

c) Nucleus
[Solution Description]
The nucleus regulates all activities occurring within the cell, including growth and other cellular functions. It is often referred to as the control center of the cell.

Key Concept: Vacuole Role

c) Stores substances and maintains cell shape
[Solution Description]
The vacuole stores important substances, removes waste, and maintains cell shape, providing support to the plant.

Key Concept: Plastids, chloroplast distribution

b) Root plastids lack chlorophyll as they don’t perform photosynthesis
[Solution Description]
Plastids are present in all plant cells, including roots. However, chloroplasts (which contain chlorophyll) are only found in green parts where photosynthesis occurs. Roots are non-photosynthetic, so their plastids store substances rather than containing chlorophyll. This explains why roots aren’t green despite having plastids.

Key Concept: Cell Structure, Microscope Observation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the rectangular shape of onion peel cells is maintained by the rigid cell wall, which is true as plant cells have a well-defined cell wall made of cellulose. The reason explains that cellulose provides structural support and prevents bursting, which is also true and directly explains why the cell wall maintains the shape (rectangular structure). Hence, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Cell structure, microscopy technique

c) It will create distortions and obscured views of cells
[Solution Description]
Air bubbles can interfere with the microscopic observation in several ways: First, they distort the image by blocking or refracting light differently, creating dark spots or halos in the field of view. Second, they may prevent the glycerin from properly covering the specimen, leading to drying or poor clarity. Third, they can obscure important cellular details or even push the specimen out of the focal plane, making it difficult to observe the cell structures clearly. Proper mounting without air bubbles ensures uniform distribution of glycerin and optimal viewing conditions.

Key Concept: Function-specific cell structure

d) To enable contraction and relaxation for food movement
[Solution Description]
Muscle cells in the stomach are spindle-shaped because this structure allows them to contract and relax effectively, creating wave-like movements that churn food. Their flexibility and thinness ensure smooth contractions required for digestion.

Key Concept: Plant cell shapes

d) Branched
[Solution Description]
Plant cells can be rectangular, elongated, oval, or tube-like but not branched like nerve cells.

Key Concept: Hierarchy of Organisation, Multicellular Organisms

b) Cell $\rightarrow$ Tissue $\rightarrow$ Organ $\rightarrow$ Organ System $\rightarrow$ Organism
[Solution Description]
The correct hierarchical order of organisation in multicellular organisms is: Cell $\rightarrow$ Tissue $\rightarrow$ Organ $\rightarrow$ Organ System $\rightarrow$ Organism. This sequence starts from the basic unit (cell) and progresses to the complete organism.

Key Concept: Levels of Organisation, Organ Systems

c) Nervous system
[Solution Description]
The nervous system is responsible for coordinating the activities of all other organ systems by transmitting signals between different parts of the body. It also processes sensory information and responds to external stimuli. This makes it the correct answer.

Key Concept: Levels of Organisation

d) Assertion is false, but Reason is true.
[Solution Description]
Assertion (A) states that the organ system is the highest level of organisation, which is incorrect because the organism itself is the highest level of organisation, formed by all organ systems combined. However, Reason (R) correctly explains the relationship between organs, organ systems, and organisms. Therefore, Assertion is false, but Reason is true.

Key Concept: Levels of Organisation, Multicellular Organisms

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that an egg (a single cell) can develop into a complete organism through repeated cell divisions, which is true as per the syllabus (“The life of complex living organisms begins with a single cell—‘egg’… divides repeatedly to form a complete living being”). The reason states that all levels of organisation (cells → tissues → organs → organ systems) must form correctly for survival, which is also true because these levels ensure proper functioning of the organism. However, the reason does not explain why the egg can develop into a complete organism; it only provides additional factual information about multicellular organisms. Thus, both are true, but the reason does not explain the assertion.

Key Concept: Concept: Organ Systems in Plants

b) Transport System
[Solution Description]
The transport system in plants consists of vascular tissues such as xylem and phloem. Xylem transports water and dissolved minerals upward from roots, while phloem distributes sugars and other nutrients.

Key Concept: Concept: Levels of Organisation in Plants

b) Cell $\rightarrow$ Tissue $\rightarrow$ Organ $\rightarrow$ Organ System $\rightarrow$ Organism
[Solution Description]
The correct sequence of levels of organisation in plants is: Cell $\rightarrow$ Tissue $\rightarrow$ Organ $\rightarrow$ Organ System $\rightarrow$ Organism. For example, cells form tissues like xylem, tissues form organs like leaves, and organs work together in systems like the transport system.

Key Concept: Microorganisms Characteristics

c) Presence of chlorophyll
[Solution Description]
The question asks for a common characteristic between algae found in pond water and soil suspension. From Tables 2.1 and 2.2, both mention the presence of chlorophyll, a green pigment, which gives algae their green color.

Key Concept: Types of Microorganisms

c) Amoeba (Protozoa)
[Solution Description]
Unicellular microorganisms consist of a single cell, like Amoeba and bacteria.

Key Concept: Unicellular vs Multicellular

a) Bacteria
[Solution Description]
Unicellular organisms consist of a single cell that performs all functions necessary for survival. Examples include bacteria and protozoans, while multicellular organisms like plants and animals have many cells. Among the options, yeast is a unicellular fungus, whereas humans, mushrooms, and roses are multicellular.

Key Concept: Unicellular vs Multicellular, Cell Specialization

b) They must be multicellular fungi or plants
[Solution Description]
The observation indicates that the organisms have specialized cells performing different functions, which is a characteristic feature of multicellular organisms. Unicellular organisms perform all functions within a single cell and do not have specialized cells.

Key Concept: Cell Structure of Microorganisms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that bacteria lack a well-defined nucleus and nuclear membrane, which is true as per the syllabus (page 24). The Reason explains that bacteria possess a nucleoid instead of a true nucleus, which is also correct and directly explains why bacteria do not have a well-defined nucleus. Hence, both Assertion and Reason are true, and Reason correctly justifies the Assertion.

Key Concept: Types of Microorganisms: Fungi

c) Bread mould
[Solution Description]
Fungi do not have chloroplasts or chlorophyll, so they cannot make their own food through photosynthesis.

Key Concept: Microorganisms and Cell Structure

d) Assertion is false, but Reason is true.
[Solution Description]
The assertion states that bacteria have a well-defined nucleus enclosed by a nuclear membrane, which is false. Bacterial cells lack a well-defined nucleus and nuclear membrane; they instead have a nucleoid (a region where genetic material is located). The reason correctly explains this distinction, stating that bacterial cells contain a nucleoid instead of a true nucleus.

Key Concept: Microbial fermentation in food production

c) It converts lactose into lactic acid, causing milk to thicken.
[Solution Description]
*Lactobacillus* converts lactose present in milk into lactic acid through the fermentation process. The chemical reaction can be represented as: $\text{Lactose} \rightarrow \text{Lactic Acid}$. This acidification causes the milk proteins to coagulate, forming curd.

Key Concept: Microbial Fermentation

c) It converts milk into yogurt by producing lactic acid
[Solution Description]
Lactobacillus ferments milk by producing lactic acid, which turns milk into curd under warm conditions.

Key Concept: Decomposition and Nutrient Recycling

c) Fungi and bacteria
[Solution Description]
Microorganisms such as fungi and bacteria decompose dead plants and animal waste, converting them into nutrient-rich manure. This process recycles essential nutrients back into nature.

Key Concept: Microorganisms in the Environment

c) Everywhere—water, soil, air, and food
[Solution Description]
Microorganisms are present everywhere, including water, soil, air, and even on food items. They thrive in diverse environments like hot springs, cold regions, and moderate temperatures.

Key Concept: Microorganisms in the Gut

c) Bacteria
[Solution Description]
Microorganisms like bacteria reside in the human gut and play a crucial role in breaking down complex food particles, aiding digestion. They help in nutrient absorption and maintaining gut health.

Key Concept: Microorganisms in Food

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that yeast is used to make bread dough rise, which is true as yeast is a microorganism commonly used in baking. The reason explains that yeast ferments sugar to produce carbon dioxide gas, which causes the dough to expand. This is also true and correctly explains why yeast makes dough rise. Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Manure Formation, Nutrient Recycling

d) The compost pile was too dry and not moist enough
[Solution Description]
For manure formation, microorganisms require optimal temperature and moisture levels to decompose organic waste efficiently. If the pile is too dry or too cold, microbial activity slows down significantly, leading to delayed decomposition.

Key Concept: Microbial Decomposition, Biogas Production

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that methane gas produced during anaerobic decomposition can be directly used as a fuel. This is true as biogas, which contains methane, is indeed used for cooking and electricity generation. The reason explains that anaerobic bacteria decompose organic matter in oxygen-free environments to produce methane, which is also correct. Moreover, the reason correctly explains why methane can be used as fuel (because it is combustible). Thus, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Assertion-Reason Question

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that microorganisms clean the environment by decomposing organic waste, which is true as per the syllabus. The Reason explains that fungi and bacteria break down organic matter into simpler substances, enriching soil fertility, which is also true and directly supports the Assertion. Thus, both statements are correct, and the Reason is the correct explanation for the Assertion.

Key Concept: Fermentation in Food

c) To cause fermentation and help the dough rise.
[Solution Description]
Yeast causes fermentation, leading to the production of gas (carbon dioxide), which helps the dough rise as described in Activity 2.8.

Key Concept: Historical Microbiology

d) Robert Hooke
[Solution Description]
Robert Hooke first used the term “cell” in 1665 after examining cork under a microscope. His observations laid the foundation for cell biology.

Key Concept: Microorganisms and Environmental Cleaning

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Dr. Ananda Mohan Chakrabarty developed a special bacterium in 1971 that could break down oil spills, which helps in cleaning the environment. This demonstrates that certain bacteria indeed have the ability to degrade oil into simpler substances, supporting the assertion. Therefore, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Microorganisms and food

c) It causes the dough to rise by fermentation
[Solution Description]
Yeast is a microorganism that helps in fermentation. When added to dough, it releases carbon dioxide gas, causing the dough to rise. This process is essential for making bread soft and fluffy.

Key Concept: Fermentation, Microbial diversity

b) Lactic acid bacteria producing lactic acid as a byproduct
[Solution Description]
Lactic acid bacteria (LAB) are responsible for fermenting bamboo shoots, converting sugars into lactic acid. This acidification imparts the sour taste and lowers the pH, inhibiting spoilage-causing microorganisms, thereby extending shelf life. Yeast or acetic acid bacteria would produce different byproducts (alcohol/vinegar), while thermophiles are unrelated to low-temperature fermentation.

Key Concept: Microbial Applications in Environmental Solutions

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that microorganisms can be used to produce biogas from organic waste, which is true as certain bacteria break down organic material in the absence of oxygen (anaerobic digestion), producing methane-rich biogas. The reason provided explains the process by which these bacteria decompose organic matter and release methane, which is indeed the main component of biogas. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Bacteria in Curd Formation

d) Lactobacillus
[Solution Description]
The bacteria Lactobacillus feeds on lactose (milk sugar), multiplies, and ferments milk to produce lactic acid, turning it into curd.

Key Concept: Role of Yeast in Dough Preparation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion (A): True. When yeast is added to dough, it ferments the sugar present in the flour, producing carbon dioxide ($CO_2$) gas, which causes the dough to rise. This observation is consistent with Activity 2.8 in the syllabus.
Reason (R): True. The reason correctly explains that fermentation by yeast involves breaking down sugar into ethanol and carbon dioxide ($C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$). The release of $CO_2$ gas is what makes the dough expand.
Since both Assertion and Reason are true, and Reason correctly explains Assertion, the correct answer is option (a).

Key Concept: Role of Rhizobium

c) Fixing atmospheric nitrogen
[Solution Description]
$Rhizobium$ bacteria trap nitrogen from the air and convert it into a form usable by plants, thus enriching soil fertility naturally.

Key Concept: Microalgae oxygen production, Spirulina cultivation

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
Microalgae like $Spirulina$ do release oxygen through photosynthesis, which is true (Assertion). However, the Reason states that their high protein content is unrelated to oxygen production, which is also true because protein content pertains to nutritional value, not photosynthetic activity. Thus, both statements are true, but the Reason does not explain the Assertion.

Key Concept: Microalgae oxygen production

d) More than 50%
[Solution Description]
Microalgae release oxygen while making their own food through photosynthesis. According to the syllabus, they produce more than half of Earth’s oxygen supply. So, the correct answer is more than 50%.

Key Concept: Uses of Spirulina

b) Protein
[Solution Description] According to the syllabus, Spirulina is rich in protein, constituting more than 60% of its body weight. It also contains vitamin $B_{12}$, but protein is the most abundant nutrient mentioned.

Key Concept: Microbial Diversity in Nature

c) Certain gut bacteria assist in breaking down food during digestion.
[Solution Description]
Microorganisms demonstrate vast diversity and can survive in extreme conditions, including hot springs or cold snow. Some also live symbiotically in humans, aiding digestion ($\text{Bacteria} \rightarrow \text{Digestive Aid}$).
The correct answer must highlight their adaptability or beneficial roles.

Key Concept: Role in Decomposition and Manure Formation

c) They break down organic matter into manure, enriching nutrients.
[Solution Description]
Microorganisms decompose organic waste into nutrient-rich manure through biochemical processes, enhancing soil fertility. The reaction can be represented as: $\text{Organic Waste} + \text{Microbes} \rightarrow \text{Manure}$.
The correct option should describe this decomposition process.

Key Concept: Microbial Presence in Food Decay

c) Microbial infection by fungi or bacteria
[Solution Description]
The powdery or cotton-like growth on fruits is caused by microorganisms like fungi, which decompose organic matter.

Key Concept: Microorganisms in Digestion

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Lactobacillus converts milk into curd by fermenting lactose into lactic acid, which is true as mentioned in the syllabus under “Microbes in Digestion.” The reason explains that Lactobacillus is a bacteria aiding in digestion and fermentation, which is also correct. Moreover, the reason provides the correct explanation for why Lactobacillus performs the action described in the assertion.
Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Waste decomposition, Biogas production

c) Methane-producing bacteria require anaerobic conditions
[Solution Description]
In a biogas plant, an oxygen-free environment is necessary because biogas is produced by anaerobic bacteria, which work only in the absence of oxygen. These bacteria decompose organic waste such as animal dung, food waste, and plant matter to release $\mathrm{CH}_4$, carbon dioxide, and other gases. If oxygen is present, anaerobic bacteria cannot function properly, and methane production will decrease. Therefore, the digester is kept airtight to allow proper anaerobic decomposition and efficient biogas formation.

Key Concept: Microorganisms in Environmental Cleanup

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that oil-degrading bacteria can help clean up oil spills in oceans, which is true as certain bacteria like Pseudomonas and Alcanivorax are known to degrade hydrocarbons in oil. The reason states that these bacteria break down hydrocarbons into simpler, non-toxic substances, which is also true and directly explains how they help in cleaning oil spills by converting harmful compounds into harmless ones.

Key Concept: Biogas Production

d) Methane ($CH_4$)
[Solution Description]
Bacteria decompose organic waste in anaerobic (oxygen-free) conditions, producing methane ($CH_4$), which can be used as fuel for cooking, heating, and electricity generation.

Key Concept: Nitrogen Fixation

d) *Rhizobium*
[Solution Description]
The bacterium *Rhizobium* forms a symbiotic relationship with leguminous plants (e.g., beans, peas) and fixes atmospheric nitrogen ($N_2$) into a usable form for the plant, enriching soil fertility.

Key Concept: Role of Microbes in Decomposing Waste

b) Fungi and bacteria
[Solution Description]
The syllabus states that fungi and bacteria break down plant waste, such as fruit and vegetable peels, into nutrient-rich manure under optimal conditions.

Key Concept: Steps in Manure Formation

c) By turning waste into nutrient-rich manure
[Solution Description]
Decomposition by microorganisms converts organic waste into nutrient-rich manure, which enhances soil fertility by providing essential nutrients like nitrogen, phosphorus, and potassium. The dark-colored manure improves soil structure and water retention.

Key Concept: Manure Formation

c) Optimal temperature and moisture
[Solution Description]
Manure formation requires optimal temperature and moisture levels for microorganisms like fungi and bacteria to break down plant waste effectively. These conditions ensure efficient decomposition into nutrient-rich manure.

Key Concept: Formation of manure and biogas

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that aerobic decomposition is faster due to higher efficiency of aerobic microorganisms, which is true as aerobic conditions allow for quicker breakdown of organic matter compared to anaerobic conditions. The Reason correctly identifies the different byproducts of aerobic (CO$_2$ and H$_2$O) and anaerobic (CH$_4$ and CO$_2$) decomposition processes. However, the Reason does not explain why aerobic decomposition is faster—it merely describes the byproducts. Thus, both statements are true, but the Reason does not correctly explain the Assertion.

Key Concept: Microorganisms and Food

b) Milk turns into yogurt by forming lactic acid
[Solution Description]
The bacterium *Lactobacillus* converts lactose in milk into lactic acid through fermentation. This process sours the milk and forms curd. The chemical reaction can be represented as $ \text{milk} + \text{Lactobacillus} \rightarrow \text{curd} $.

Key Concept: Historical Contribution: Dr. Ananda Mohan Chakrabarty

c) Developing an oil-spill-cleaning bacterium
[Solution Description]
Dr. Chakrabarty developed a genetically modified bacterium in 1971 capable of breaking down oil spills. This innovation helped in environmental cleanup and earned him a patent in 1980.

Key Concept: Microorganisms in Food, Agriculture

b) The fixed nitrogen is being consumed by other soil microorganisms before the wheat plants can absorb it.
[Solution Description]
The $\text{Rhizobium}$ bacteria present in the root nodules of legumes like peas fix atmospheric nitrogen ($\text{N}_2$) into a form usable by plants. This benefits the subsequent crop, usually a non-legume like wheat, by providing additional nitrogen. However, if the wheat yield does not improve, it suggests that either:
1) The $\text{Rhizobium}$ population is insufficient or inactive,
2) The fixed nitrogen is being depleted by other factors (e.g., leaching, competing weeds), or
3) The wheat crop has other limiting factors (e.g., phosphorus deficiency, water stress).
The most plausible scientific reason among the options is that the fixed nitrogen is being utilized by other soil microbes before the wheat can absorb it, as microbial activity can rapidly consume available nitrogen.

Key Concept: Role of yeast

c) To make the dough rise and become fluffy
[Solution Description]
Yeast ferments the sugar in the dough, releasing carbon dioxide gas which causes the dough to rise and become fluffy. It does not primarily add flavor, color, or act as a preservative.

Key Concept: Role of sugar and temperature, Multi-concept

b) No dough rise as yeast dies, but honey caramelizes.
[Solution Description]
Honey contains fermentable sugars, so yeast can still metabolize it. However, boiling water kills yeast cells, halting fermentation entirely. Thus, despite honey’s usability, no $CO_2$ production or dough rising occurs.

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 primarily due to carbon dioxide produced by yeast, which is true as per the syllabus. The Reason explains that yeast converts glucose into ethanol and carbon dioxide ($C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$), which is also correct and directly supports the Assertion. Therefore, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Lactobacillus activity, temperature effect

d) Curd forms much slower at 10°C than at 40°C
[Solution Description]
Lactobacillus bacteria thrive in warm conditions (around 37-45°C). At 10°C, their metabolic activity slows down significantly, reducing lactic acid production and delaying curd formation. At 40°C, they actively ferment lactose to lactic acid, causing faster curdling.
Step-by-step explanation:
1. Lactobacillus bacteria convert lactose in milk to lactic acid through fermentation.
2. The enzyme activity of Lactobacillus is optimal around 37-45°C.
3. At 10°C, the bacterial metabolism slows down, reducing lactic acid production.
4. Less lactic acid means slower coagulation of milk proteins (casein), delaying curd formation.
5. At 40°C, bacteria are highly active, producing more lactic acid rapidly, leading to quicker curdling.
Therefore, at 10°C, curd formation will be much slower compared to 40°C.

Key Concept: Bacterial growth, starter culture

c) Bacterial growth is limited by environmental factors
[Solution Description]
Bacterial growth follows logistic growth patterns with limiting factors.
Step-by-step explanation:
1. Initially, more starter culture means more Lactobacillus bacteria to begin fermentation.
2. However, bacterial population growth is limited by:
– Available nutrients (lactose)
– Accumulation of waste products (lactic acid)
– Physical space constraints
3. Beyond the optimal amount, additional bacteria compete for limited resources.
4. The environment becomes increasingly acidic, inhibiting further bacterial growth.
5. The maximum growth rate is determined by these limiting factors, not just initial quantity.
Therefore, after reaching the optimal starter amount, adding more won’t speed up curd formation.

Key Concept: Lactobacillus in curd formation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that curd forms faster in bowl A due to its warmer temperature, which aligns with the observation from Activity 2.9. The reason explains that this occurs because Lactobacillus bacteria thrive and ferment lactose efficiently in warmth, producing lactic acid. Both statements are true, and the reason correctly explains the assertion.

Key Concept: Fermentation in Milk

b) Bacteria produce lactic acid by fermenting lactose
[Solution Description]
The bacteria Lactobacillus present in the curd ferments lactose (milk sugar) and produces lactic acid. In warm conditions, these bacteria multiply rapidly, converting milk into sour-tasting curd.

Key Concept: Traditional Fermented Foods

b) Lactobacillus bacteria
[Solution Description]
The fermentation process in idli and dosa batter involves bacteria such as Lactobacillus, which ferments the carbohydrates in the batter, producing gases that make the batter fluffy.

Key Concept: Traditional Fermented Foods

c) Lactobacillus (bacteria)
[Solution Description]
Lactobacillus is a bacterium that ferments the batter for idli and dosa by breaking down sugars into lactic acid. This gives the batter a slightly sour taste and makes it fluffy due to gas production.

Key Concept: Unicellular vs Multicellular Organisms

c) Unicellular organisms perform all functions in one cell, while multicellular organisms have specialized cells
[Solution Description]
Unicellular organisms consist of a single cell, whereas multicellular organisms are made up of many cells working together. Unicellular organisms perform all life functions within one cell, while multicellular organisms have specialized cells for different functions.

Key Concept: Cell Functionality, Specialization in Multicellular Organisms

c) To increase efficiency by dividing labor among cells
[Solution Description]
Multicellular organisms consist of many cells that perform specialized functions. According to the syllabus, such specialization allows cells to work more efficiently while also cooperating to enhance survival. None of the other options correctly explain the reason for specialization in multicellular organisms.

Key Concept: Microscopy and Cell Observation

c) It magnifies cells up to 10,00,000 times, revealing tiny structures.
[Solution Description]
An electron microscope is required because it provides much higher magnification (up to 10,00,000 times) compared to light microscopes. Subcellular structures are too small to be resolved under a light microscope, so the superior resolution of an electron microscope is needed to visualize them clearly.

Key Concept: Plant Cell Structure

c) Cell wall
[Solution Description]
The cell wall is a rigid outer layer found in plant cells that provides structural support and protection, giving plants their firmness.

Key Concept: Structure of a Cell

c) Acts as a medium for cellular activities and houses organelles
[Solution Description] The cytoplasm is a jelly-like substance inside the cell that contains all other cell components, such as organelles. It is where most life processes like metabolism occur, providing a medium for chemical reactions.

Key Concept: Levels of Organisation in Living Organisms

c) Cell $\rightarrow$ Tissue $\rightarrow$ Organ $\rightarrow$ Organ System $\rightarrow$ Organism
[Solution Description] The correct sequence starts with the cell, which is the basic unit of life. Cells form tissues, tissues form organs, organs work together in organ systems, and organ systems make up an organism.

Key Concept: Specialization in Multicellular Organisms

c) To increase efficiency by dividing labor among cells
[Solution Description]
Specialization allows cells in multicellular organisms to perform specific tasks efficiently, increasing the overall survival chances of the organism. For instance, muscle cells specialize in movement, while nerve cells specialize in transmitting signals.

Key Concept: Examples of Multicellular Organisms

c) Human
[Solution Description]
Multicellular organisms consist of multiple cells and include plants and animals. Yeast is unicellular, while humans are multicellular, making them the correct choice in this context.

Key Concept: Unicellular vs Multicellular

c) Bacteria
[Solution Description]
Unicellular organisms are those that consist of a single cell and perform all life functions within it. Bacteria and protozoans are examples of unicellular organisms. Among the given options, bacteria fit this description.

Key Concept: Nucleus, Unicellular Organisms

c) The nucleus regulates essential cellular activities and growth.
[Solution Description]
The nucleus is essential for regulating all cellular activities, including growth and metabolism. In unicellular organisms, all life processes occur within a single cell, so the absence of the nucleus disrupts vital functions, leading to cell death.

Key Concept: Cell Functionality

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that the nucleus regulates all cellular activities, which is true as per the syllabus. The Reason mentions that the cytoplasm contains essential compounds and hosts most life processes, which is also true. However, while both statements are correct, the Reason does not explain why the nucleus regulates activities; rather, it describes the role of cytoplasm. Thus, the Reason is not the correct explanation of the Assertion.

Key Concept: Plant Cell Structures

c) Helps in photosynthesis
[Solution Description]
Chloroplasts contain chlorophyll, which helps in photosynthesis by capturing sunlight to produce food for the plant.