Key Concept: Microscopy, Magnification, Microorganisms

c) Assertion is true, but Reason is false.
[Solution Description]
The assertion is true because the microscope's high magnification enabled scientists to observe microorganisms, which are typically smaller than 0.1 mm and invisible to the naked eye. However, the reason is false because a simple magnifying glass does not provide enough magnification (typically around 5x–10x) to observe tiny unicellular organisms like bacteria (which require at least 400x–1000x magnification). Moreover, Amoeba, though larger than bacteria, still requires a compound microscope for detailed observation. Thus, the correct answer is c).

Key Concept: Observation Techniques

d) It magnifies microorganisms to make them visible for study.
[Solution Description]
Microorganisms are too small to be seen by the naked eye, so microscopes magnify them to a visible scale. Light microscopes use lenses to bend light, while electron microscopes use beams of electrons to achieve even higher magnification. Without magnification, their structures and details remain invisible.

Key Concept: Human eye limitations and magnification

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The unaided human eye has a resolution limit of about 0.1 mm, meaning it cannot distinguish objects smaller than this size. Microorganisms, such as bacteria and Amoeba, are typically much smaller (e.g., bacteria are around 0.001–0.005 mm in size). Therefore, the Assertion is true because microorganisms are indeed invisible to the naked eye. The Reason is also true because their size is below the resolution limit of the human eye, and it correctly explains why the Assertion is valid. Thus, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Function of reading glasses

c) They use convex lenses to focus light properly on the retina.
[Solution Description]
Reading glasses use convex lenses to correct farsightedness by bending light rays to focus properly on the retina.

Key Concept: Role of Lenses and Microscopes

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion is true because Antonie van Leeuwenhoek's improved microscopes allowed him to observe microorganisms like bacteria in the 1660s, which were previously invisible to the naked eye. The reason is also true as microscopes function by using convex lenses to bend light and magnify tiny specimens. However, the reason explains how microscopes work but does not directly justify why their invention led to the discovery of tiny organisms—it was the combination of magnification and scientific curiosity that facilitated these discoveries. Therefore, both statements are true, but the reason is not the correct explanation of the assertion.

Key Concept: Microscope invention

d) Robert Hooke.
[Solution Description]
Robert Hooke, in his book *Micrographia* (1665), was the first scientist to describe and illustrate cells using a microscope. He observed cork tissue and coined the term "cell" due to its resemblance to small rooms (cells) in a monastery.

Key Concept: Historical development of magnification tools

d) Antonie van Leeuwenhoek
[Solution Description]
Antonie van Leeuwenhoek made significant improvements to lenses, allowing him to observe microorganisms like bacteria and blood cells clearly. Due to these contributions, he is credited as the "Father of Microbiology."

Key Concept: Historical development of magnification tools, Robert Hooke's contributions

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that Robert Hooke's Micrographia included detailed drawings of cork cells, leading to the discovery of cells as the basic unit of life. This is true because Hooke coined the term "cell" after observing cork under his microscope. The Reason states that Hooke's microscope could magnify objects 200-300 times, enabling microscopic observations. This is also true as historical records confirm his microscope's magnification capabilities. However, the Reason does not fully explain why Hooke's work was groundbreaking—it was his interpretation and documentation (not just magnification) that contributed to the discovery of cells. Thus, both are true, but the Reason is not the correct explanation of the Assertion.

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: Cell Theory, Historical Context

c) Protozoa
[Solution Description]
The description matches Amoeba, characterized by its single-cell structure, irregular shape, and movement via pseudopodia. As per Table 2.1, Amoeba falls under the Protozoa category.

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: Historical context and contributions

c) Antonie van Leeuwenhoek
[Solution Description]
Antonie van Leeuwenhoek developed superior lenses that allowed him to construct powerful microscopes. His observations of microorganisms like bacteria earned him the title *Father of Microbiology*.

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: Microscope Improvements

c) 17th century
[Solution Description]
Antonie van Leeuwenhoek improved microscope lenses in the 1660s, which corresponds to the 17th century.

Key Concept: Microscopic Observation

c) 200–300×
[Solution Description]
Hooke's microscope magnified objects 200 to 300 times their actual size, enabling him to see the tiny structures in cork that he named cells.

Key Concept: Discovery of Microorganisms, Hooke’s Micrographia

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
- Robert Hooke indeed observed cork cells using a microscope with a magnification power of 200 to 300 times, as mentioned in the syllabus.
- The reason states that this magnification enabled him to see and name the tiny compartments ("cells") in cork.
- Both statements are factually correct, and the Reason logically explains why Hooke could observe and describe cells in cork.

Key Concept: Variation in Cell Shape

c) To carry messages quickly and connect with other cells over long distances
[Solution Description]
Nerve cells (neurons) are elongated and branched to efficiently transmit electrical signals across long distances within the body. The long axon helps in transmitting messages quickly, while the branched dendrites help receive signals from multiple sources.

Key Concept: Cell Structure and Function

c) Regulates all cellular activities, including growth and metabolism
[Solution Description]
The nucleus is the control center of the cell. It regulates all cellular activities, including growth, metabolism, and reproduction. It contains genetic material (DNA), which directs protein synthesis and determines the cell's characteristics.

Key Concept: Cell Structure, Plant vs Animal Cells

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the large central vacuole in plant cells helps maintain turgor pressure for structural support, which is true. The reason explains that turgor pressure is generated by osmotic water movement into the vacuole, pushing the cell membrane against the rigid cell wall. Both statements are true, and the reason correctly explains the assertion because the mechanism described in the reason directly causes the effect mentioned in the assertion.

Key Concept: Plant Cell Structures

b) Chloroplast
[Solution Description]
Plant cells have additional structures like the cell wall, plastids, and large vacuoles which are absent or smaller in animal cells.

Key Concept: Chloroplast

c) Help in photosynthesis
[Solution Description]
Chloroplasts contain chlorophyll, which helps plants perform photosynthesis by converting sunlight into energy. Thus, the correct answer is that they help in photosynthesis.

Key Concept: Cell Wall

d) Both b) and c)
[Solution Description]
Plant, fungal, and bacterial cells have a cell wall, while animal cells do not. Therefore, the correct answer is plant cells.

Key Concept: Nucleus Function

c) Regulating all cellular activities and controlling growth
[Solution Description]
The nucleus regulates all cellular activities, including growth, by controlling gene expression and maintaining genetic material.

Key Concept: Cell membrane, Nucleus, Prokaryotic vs Eukaryotic

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the nucleus is absent in bacteria because their genetic material lacks a nuclear membrane. According to the syllabus, bacteria indeed do not have a well-defined nucleus or nuclear membrane; they instead have a nucleoid region where genetic material is present. Thus, the Assertion is true. The Reason states that bacteria are prokaryotes and do not possess membrane-bound organelles. This is also true as bacteria being prokaryotes lack membrane-bound structures like nuclei, mitochondria, etc. Furthermore, the Reason correctly explains the Assertion since the absence of membrane-bound organelles (Reason) directly leads to the absence of a true nucleus (Assertion). Hence, both statements are true, and Reason explains Assertion.

Key Concept: Cell Wall Function

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The cell wall in plant cells is primarily composed of cellulose, a tough and rigid material that provides structural support, rigidity, and strength to the plant. The Assertion correctly states that the cell wall provides rigidity and strength to plants, while the Reason accurately describes that the cell wall is made of cellulose, which is responsible for this function. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Chloroplast Function

c) Chlorophyll
[Solution Description]
Chloroplasts contain chlorophyll, a green pigment that captures light energy for photosynthesis.

Key Concept: Decomposition

c) They break down organic waste into simpler substances
[Solution Description]
Microorganisms decompose organic waste materials such as plant and animal remains, breaking them into simpler substances like humus. This recycling process enriches the soil with nutrients and helps maintain ecological balance by reducing waste accumulation.

Key Concept: Cell Structure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the nucleus is the control center of the cell, which is true. The reason given is that it contains genetic material (DNA) that regulates cell activities, which is also true and correctly explains why the nucleus is considered the control center. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Cell shape and function, Plant vs Animal cells

d) Assertion is false, but Reason is true.
[Solution Description]
Muscle cells are spindle-shaped to allow contraction and relaxation for movement, but chloroplasts are absent in animal cells (including muscle cells). Chloroplasts are exclusive to plant cells and are involved in photosynthesis, not muscle function. Thus, Assertion is false because muscle cells do not contain chloroplasts, and Reason is also false since chloroplasts are not related to muscle cell flexibility or contractility.

Key Concept: Levels of organization, cell specialization

d) Specialized cells form tissues, which form organs, which then collaborate in organ systems.
[Solution Description]
Cells specialize to perform specific functions (e.g., muscle cells for contraction). Tissues form when similar cells work together (e.g., muscle tissue). Organs consist of multiple tissues (e.g., stomach), and organ systems combine organs (e.g., digestive system). This hierarchy ensures efficient functioning.

Key Concept: Multicellular Organisms

a) Tissue
[Solution Description]
In multicellular organisms, cells with similar structures and functions come together to form tissues. These tissues work collectively to perform specific tasks, such as muscle contraction or nutrient transport. Therefore, the correct answer is 'Tissue.'

Key Concept: Organ Systems

a) Digestive system
[Solution Description]
An organ system consists of multiple organs that work together to perform a major function in the body. For example, the digestive system includes the stomach, intestines, liver, and other organs involved in breaking down food and absorbing nutrients. Hence, the correct answer is 'Digestive system.'

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: Cell as Basic Unit of Life

c) Cell
[Solution Description]
The basic structural and functional unit of life is the cell. Cells are the smallest entities that can carry out all life processes independently or as part of a larger organism.

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: Levels of Organisation, Organ Systems in Humans and Plants

d) Assertion is false, but Reason is true.
[Solution Description]
To analyze this assertion-reason pair:
- Assertion claims the vacuole is an organ system in plants responsible for water transport. However, according to the syllabus, the vacuole is part of a cell's structure, not an organ system. Organ systems are higher levels of organization formed by multiple organs working together (e.g., xylem and phloem form the plant's vascular system for water transport). Hence, the Assertion is false.
- Reason correctly describes the function of vacuoles as per the syllabus text, stating they store substances, remove waste, and maintain cell shape. Therefore, the Reason is true.
Since Assertion is false but Reason is true, option d) is correct.

Key Concept: Observation Tools for Microorganisms

c) Microscope
[Solution Description]
A microscope is used to magnify microorganisms so they become visible to the human eye.

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: Cell Structure

c) Genetic material floating freely in the nucleoid region
[Solution Description]
Bacterial cells lack a well-defined nucleus and membrane-bound organelles. Instead, their genetic material is found in a nucleoid region. In contrast, fungi and protozoa have a proper nucleus enclosed within a nuclear membrane.

Key Concept: Fungi, Algae

d) Fungi
[Solution Description]
Fungi are characterized by their branched filamentous structures and the presence of a cell wall. Unlike algae, they lack chloroplasts and cannot perform photosynthesis. Examples include bread mould and other moulds.

Key Concept: Bacteria, Fungi, Protozoa, Algae

c) Rhizobium
[Solution Description]
Rhizobium bacteria live in the root nodules of leguminous plants like beans, peas, and lentils. They have a symbiotic relationship with these plants by fixing atmospheric nitrogen into a form that plants can use, thereby enriching the soil with nitrogen.

Key Concept: Types of Microorganisms: Protozoa

c) Amoeba
[Solution Description]
$Amoeba$ is a protozoan that has an irregular shape and moves using temporary projections called pseudopodia.

Key Concept: Microbes in Waste Decomposition

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that microorganisms such as bacteria and fungi are essential for decomposing organic waste, which is true as they break down materials like leaves and food scraps. The reason explains that this decomposition process recycles nutrients into the soil, aiding plant growth, which is also true. Furthermore, the reason correctly explains why the assertion is true because nutrient recycling is a direct result of microbial decomposition. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Microbial Decomposition, Nitrogen Fixation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that microorganisms such as *Rhizobium* and decomposer bacteria play a key role in breaking down organic waste and enriching soil fertility. This is true because *Rhizobium* converts atmospheric nitrogen into nitrates (a usable form for plants), and decomposer bacteria break down dead organic matter into humus, releasing nutrients back into the soil. The reason correctly explains the assertion by detailing how these two types of microorganisms contribute to soil fertility through different but complementary mechanisms. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Fermentation, Food production

c) Fermentation releasing carbon dioxide
[Solution Description]
Yeast ferments sugars present in the dough, producing carbon dioxide ($CO_2$) and ethanol as byproducts. The $CO_2$ gas gets trapped in the dough, causing it to expand and rise. This process, called fermentation, is essential for giving bread its light and airy texture.

Key Concept: Microbial fermentation, Food preservation

d) Jar A (no treatment)
[Solution Description]
According to the syllabus, microorganisms don't grow on pickles/murabbas due to high salt or sugar concentrations which act as preservatives by creating hypertonic conditions that draw water out of microbial cells. Oil immersion also creates an anaerobic environment unsuitable for many microbes. Untreated Jar A would allow natural microbial growth since it has no preservation method applied. Therefore, only Jar A would show spoilage.

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 Food

d) It produces gas to make the dough rise
[Solution Description]
Yeast ferments sugars in the dough, producing carbon dioxide gas that causes the dough to rise, making bread fluffy.

Key Concept: Environmental Role of Microbes

c) They decompose dead organisms and recycle nutrients.
[Solution Description]
Microorganisms decompose dead plants, animals, and other organic waste, recycling nutrients back into the soil. This process helps maintain environmental balance and reduces pollution.

Key Concept: Biogas Production

c) Methane
[Solution Description]
Microorganisms such as bacteria decompose plant and animal waste in oxygen-free environments, releasing methane gas along with carbon dioxide. Methane is a major component of biogas used as fuel.

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: Scientific heritage, Microbial applications

c) It reduces pollution by decomposing oil spills.
[Solution Description]
Dr. Chakrabarty's bacterium was engineered to degrade hydrocarbons found in oil spills, which helps in cleaning polluted water bodies. This reduces environmental damage caused by oil spills and aids in ecosystem restoration.

Key Concept: Microorganisms in the Environment

b) They decompose organic waste into simpler substances and biogas.
[Solution Description]
Microorganisms decompose organic waste into simpler substances and biogas, contributing to environmental cleanliness as mentioned in the syllabus.

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: Environmental Applications of Microorganisms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that genetically modified microorganisms can degrade plastic waste, which is true as researchers have engineered bacteria (e.g., Ideonella sakaiensis) to produce PETase, an enzyme that breaks down PET plastics. The reason correctly explains this by mentioning the role of bacterial enzymes like PETase in degrading PET into non-toxic components. Thus, both the assertion and reason are true, and the reason logically 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: 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: Yeast Fermentation

c) Carbon dioxide ($CO_2$)
[Solution Description]
Yeast (Saccharomyces cerevisiae) ferments sugar in the dough to produce carbon dioxide ($CO_2$) gas. This gas gets trapped in the dough, making it rise and become fluffy.

Key Concept: Microbial Fermentation

c) Lactobacillus
[Solution Description]
Lactobacillus is the bacterium that ferments lactose in milk, producing lactic acid, which causes the milk to turn into sour curd. This process occurs under warm conditions and distinguishes bowl A (with yeast) from bowl B (without yeast).

Key Concept: Microalgae nutrient content, Environmental threats

c) Pollution and habitat destruction
[Solution Description]
Spirulina requires clean water, moderate temperature, and sunlight for optimal growth. Pollution can contaminate water, climate change can alter temperature conditions, and habitat destruction reduces available growing areas. All these factors can negatively impact Spirulina's growth and oxygen production.

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: Microalgae uses

b) Health supplements
[Solution Description]
Spirulina is rich in nutrients and serves as a health supplement due to its high protein content (more than 60% of its body weight) and vitamin B12. It is also used in medicines. The correct answer is health supplements.

Key Concept: Microbial Presence in Food Decay

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that high salt or sugar concentrations prevent microbial growth, which is true as these conditions make the environment unsuitable for most microbes. The reason explains that this happens due to plasmolysis caused by the hypertonic environment, which correctly justifies the assertion. Hence, both are true, and the reason is the correct explanation of the assertion.

Key Concept: Microbial presence in food decay

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that microorganisms cause food items like lemons and tomatoes to rot. This is true as per the syllabus, which mentions microbial presence leads to food decay. The Reason states that microorganisms break down organic matter into simpler substances, enriching the soil with nutrients. This is also true and directly explains why the Assertion occurs. The decomposition of food by microbes results in nutrient recycling, which supports the rotting process mentioned in the Assertion.

Key Concept: Extremophiles, gut microbiota

d) Thermally stable proteins and membranes
[Solution Description]
Thermophilic bacteria have specially adapted proteins and cell membranes. Their proteins have more stable structures due to increased ionic bonds and hydrophobic interactions. Membrane lipids contain more saturated fatty acids which remain stable at high temperatures. This contrasts with mesophilic human gut bacteria whose proteins and membranes would denature at such high temperatures.

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: Microalgae: Tiny Helpers in Water

c) Contribution to over 50% of Earth’s oxygen
[Solution Description]
Microalgae contribute over 50% of Earth’s oxygen through photosynthesis. The process can be represented by the equation $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$. They also serve as a nutrient-rich food source for aquatic life and aid in water purification.

Key Concept: Microorganisms in Environmental Cleanup

c) *Rhizobium*
[Solution Description]
*Rhizobium* bacteria form a symbiotic relationship with legumes like beans and peas. They convert atmospheric nitrogen ($N_2$) into ammonia or other usable forms, thereby enhancing soil fertility naturally without chemical fertilizers.

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: Microbial Byproduct in Oxygen-Free Environments

d) Methane ($CH_4$)
[Solution Description]
The syllabus mentions that some bacteria decompose waste in oxygen-free environments, releasing methane ($CH_4$) and carbon dioxide ($CO_2$). Methane is used as fuel for cooking, heating, and electricity.

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: Decomposition, Biogas Production

c) 20%
[Solution Description]
Let's consider 100 g of organic waste with 60 g of carbon. During anaerobic decomposition:
1. The reaction produces equal moles of $CH_4$ and $CO_2$, so for every 1 mole of $CH_4$ (16 g), there is 1 mole of $CO_2$ (44 g).
2. Total carbon in products: $CH_4$ has 12 g carbon (molecular weight 16 g/mol, carbon 12 g/mol) and $CO_2$ has 12 g carbon (molecular weight 44 g/mol, carbon 12 g/mol).
3. Out of total 24 g carbon in products, 12 g is in $CH_4$.
4. Percentage of carbon converted to methane = $(12/60) \times 100 = 20\%$.

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: 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: 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: Microorganisms in environmental cleanup, microbial fermentation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that genetically modified bacteria are used for oil spill cleanup, which aligns with Dr. Chakrabarty's work on a patented bacterium capable of degrading oil spills. The reason provides the historical context and scientific basis for this application, explaining that microbes can indeed break down complex pollutants. Therefore, both statements are true, and the reason correctly explains the assertion.

Key Concept: Microbes as a Source of Biogas

d) Methane
[Solution Description]
Bacteria decompose organic matter in oxygen-free environments, releasing biogas, which primarily contains methane $\mathrm{CH}_4$ along with other gases like carbon dioxide. Methane is the main component used as fuel.

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: Fermentation process

c) Carbon dioxide ($CO_2$)
[Solution Description]
During fermentation, yeast breaks down sugars and releases carbon dioxide ($CO_2$). This gas forms bubbles in the dough, making it rise and become fluffy. The correct answer is $CO_2$.

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 yeast is added to dough to make it soft and fluffy, which is true as yeast causes fermentation in baking. The reason explains that yeast produces carbon dioxide during respiration, forming bubbles in the dough. This is also true and correctly explains why the dough becomes soft and fluffy. Therefore, both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Effect of lactic acid on curd

b) Causes the milk to solidify and turn sour
[Solution Description]
Lactic acid ($C_3H_6O_3$) produced by Lactobacillus lowers the pH of milk, making it sour and causing the proteins to coagulate, forming curd with a thick texture.

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: Lactobacillus fermentation, optimal conditions for curd formation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that curd does not form in cold milk due to the inability of Lactobacillus to ferment lactose into lactic acid at low temperatures. This is true because Lactobacillus requires warm conditions for optimal growth and metabolic activity. The reason explains that the metabolic activity of Lactobacillus is inhibited in cold conditions, preventing the fermentation process necessary for curd formation. Both statements are true, and the reason correctly explains the assertion as it directly links temperature to bacterial metabolism and fermentation.

Key Concept: Role of microorganisms in fermentation, Chemical reactions involved in fermentation

d) Optimal temperature increases yeast metabolic rate
[Solution Description]
The rate of fermentation increases with temperature because microbial activity is temperature-dependent. At 35°C, yeast (Saccharomyces cerevisiae) metabolizes sugars more efficiently, producing $CO_2$ gas faster, which makes the dough rise quicker. The reaction is:
$C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + \text{Energy}$
Higher temperatures (within 30-40°C) optimize enzyme activity in yeast, accelerating fermentation.

Key Concept: Microbial Role in Fermentation

c) Produces carbon dioxide and alcohol, making the dough rise and fluffy
[Solution Description]
Yeast is a microorganism that ferments sugars in the dough, producing carbon dioxide and alcohol as byproducts. The carbon dioxide forms bubbles, making the dough rise and become fluffy, while the alcohol evaporates during baking. This process is called fermentation.

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: Microscopic Observation, Cell Components

b) Because subcellular components are too small for light microscopes to resolve
[Solution Description]
The syllabus states that some subcellular structures require high magnification (such as 10,00,000x provided by an electron microscope) to be observed. Light microscopes cannot provide sufficient resolution, and staining or fixing does not compensate for inadequate magnification.

Key Concept: Unicellular vs. Multicellular Organisms

c) Unicellular organisms perform all life functions in a single cell while multicellular organisms use specialized cells.
[Solution Description]
Unicellular organisms, such as bacteria and protozoa, are made up of a single cell that performs all necessary life functions independently. Multicellular organisms, like plants and animals, consist of multiple cells that specialize in different functions. Fungi can be either unicellular (e.g., yeast) or multicellular (e.g., mould). Therefore, the correct answer must reflect that only unicellular organisms carry out all life processes in a single cell.

Key Concept: Function of Nucleus

c) Nucleus
[Solution Description]
The nucleus is the control center of the cell. It regulates all cellular activities, including growth, by controlling gene expression and DNA replication.

Key Concept: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the cell is the basic unit of life, which is true because all living organisms are composed of cells, whether unicellular or multicellular. The reason given is also true as it correctly explains why the cell is considered the basic unit of life—because all living beings are made up of one or more cells. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

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: 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: Examples of Unicellular and Multicellular Organisms

c) Mould
[Solution Description]
Multicellular organisms consist of multiple cells working together, such as plants, animals, and some fungi like mould. Unicellular organisms, like yeast or bacteria, consist of a single cell performing all functions.

Key Concept: Specialization in Multicellular Organisms

c) Cells carry out specialized functions and cooperate
[Solution Description]
Multicellular organisms are made up of many cells that specialize in different functions. These specialized cells work together to ensure the survival of the organism. This cooperation is a defining feature of multicellularity.

Key Concept: Nucleoid, Prokaryotic Cells

c) The nucleoid reduces the complexity of gene regulation mechanisms.
[Solution Description]
The nucleoid in bacteria contains genetic material but lacks a nuclear membrane, meaning DNA regulation is less compartmentalized compared to eukaryotes. This affects processes like transcription and translation efficiency.

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: 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.