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

Key Concept: Microscope and Magnification

b) To observe tiny organisms and cells by magnifying them
[Solution Description]
A microscope is used to magnify tiny objects that are not visible to the naked eye. It helps in observing microorganisms, cells, and other small structures by making them appear larger.

Key Concept: Microorganisms

d) They perform all life processes within a single cell.
[Solution Description]
Unicellular microorganisms consist of a single cell that performs all life functions independently, such as reproduction, digestion, and movement. Examples include bacteria and Amoeba. Unlike multicellular organisms, they do not have specialized tissues or organs.

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: Human eye resolution, Microscopy

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The human eye has a resolution limit of approximately 0.1 mm due to the finite size of photoreceptor cells in the retina and optical limitations. This matches the assertion. The resolving power of any optical system, including the eye, is indeed constrained by the wavelength of light ($\lambda$) and the aperture size ($D$), following the Rayleigh criterion $\theta = 1.22\lambda/D$. Thus, both statements are true, and the reason correctly explains the assertion.

Key Concept: Discovery

c) Cork structures
[Solution Description]
Robert Hooke examined cork under his microscope and observed tiny compartments resembling small rooms, which he called “cells.” This became the basis for the cell theory in biology.

Key Concept: Limitations of the Naked Eye, Microscope Magnification

c) 2000 micrometers
[Solution Description]
To find the apparent size under the microscope, multiply the actual size by the magnification factor. The actual size is 5 micrometers and the magnification is 400x. So, the apparent size = $5 \times 400 = 2000$ micrometers or 2 millimeters.

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: Improvement of Microscopes

c) Antonie van Leeuwenhoek
[Solution Description]
Antonie van Leeuwenhoek, a Dutch scientist, developed superior lenses that allowed him to construct more powerful microscopes. His work led to the discovery of bacteria and detailed observations of blood cells, earning him the title *Father of Microbiology*.

Key Concept: Historical Context

c) *Micrographia*
[Solution Description]
Robert Hooke published his detailed drawings and descriptions of cork cells in *Micrographia* in 1665. This book included illustrations of tiny structures he observed using his microscope.

Key Concept: Discovery of Microorganisms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that Antonie van Leeuwenhoek is called the ‘Father of Microbiology’ because he improved microscope lenses, which is true. The Reason explains that his improved lenses allowed clearer observations of microorganisms like bacteria and blood cells, which is also true and correctly justifies why he earned this title. Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Father of Microbiology

c) Antonie van Leeuwenhoek
[Solution Description]
The syllabus mentions that Antonie van Leeuwenhoek improved microscope lenses and discovered bacteria and blood cells, earning him the title of the Father of Microbiology.

Key Concept: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Robert Hooke was the first scientist to observe and name cells while studying cork under a microscope. His microscope had a magnification power of 200 to 300 times, which allowed him to see these tiny structures. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Leeuwenhoek’s discoveries, Microbiology

c) Identification of viruses
[Solution Description]
Leeuwenhoek discovered protozoa (1674), bacteria (1676), spermatozoa (1677), and blood cells (1683). However, the existence of viruses was confirmed much later in the 19th century by Dmitri Ivanovsky and Martinus Beijerinck using filtration experiments, as viruses are too small to be seen with Leeuwenhoek’s light microscopes.

Key Concept: Microscopy

d) He was the first to observe and describe microorganisms such as bacteria and blood cells.
[Solution Description]
Leeuwenhoek improved microscope lenses, allowing higher magnification, and was the first to observe and describe microorganisms like bacteria and blood cells. These contributions established microbiology as a scientific field.

Key Concept: Concept: Significance of Micrographia

c) It was the first book to document microscopic observations and introduce the term “cell”
[Solution Description]
Micrographia was the first book to illustrate microscopic observations, including detailed drawings of cells (from cork), insects, and other minute structures. It introduced the term “cell” to describe biological structures and inspired further microscopy advancements.

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: Levels of Organisation, Cell Function

c) Cell $\rightarrow$ Tissue $\rightarrow$ Organ $\rightarrow$ Organ system $\rightarrow$ Organism
[Solution Description]
The correct hierarchical organisation in living organisms starts with cells forming tissues, which combine to form organs. Organs work together in organ systems, and these systems collectively form an organism.

Key Concept: Types of Cells

c) Amoeba
[Solution Description]
Unicellular organisms consist of a single cell that performs all life functions. Bacteria and protozoa are examples of unicellular organisms.

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: 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: Assertion and Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that plant cells have chloroplasts while animal cells do not. This is true as chloroplasts are specialized organelles present in plant cells that contain chlorophyll, enabling them to perform photosynthesis. The reason explains that chloroplasts are responsible for photosynthesis, which is indeed a process unique to plant cells. Hence, the reason correctly explains why plant cells have chloroplasts and animal cells do not. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

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) 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: Cytoplasm, Cell membrane, Vacuole

d) Hosts metabolic reactions involving the substances
[Solution Description]
The cytoplasm contains various compounds like carbohydrates, proteins, and fats where most life processes occur. When substances enter through the porous cell membrane, they interact with these components in the cytoplasm for metabolic activities.

Key Concept: Cell Wall

c) Provides structural support and rigidity
[Solution Description]
The cell wall is a rigid outer layer made of cellulose that provides structural support, protection, and maintains the shape of plant cells. It prevents the cell from bursting under osmotic pressure and allows plants to stand upright.

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: Yeast Fermentation

d) It produces carbon dioxide and causes the dough to rise
[Solution Description]
Yeast ferments sugars present in the dough, producing carbon dioxide ($CO_2$) gas and a small amount of alcohol. The $CO_2$ forms bubbles that cause the dough to rise, making it soft and fluffy. This process is essential for bread-making.

Key Concept: Basic cell structure

d) Cell wall
[Solution Description]
The outermost layer of a plant cell is called the cell wall. It provides structural support and protection to the cell. Unlike animal cells, which only have a cell membrane, plant cells have both a cell wall and a cell membrane.

Key Concept: Function of nerve cells

b) Carrying messages
[Solution Description]
Nerve cells (neurons) are responsible for carrying messages throughout the body due to their elongated and branched structure.

Key Concept: Variation in shape and structure of cell

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that the spindle-shaped structure of muscle cells aids in their ability to contract and relax efficiently, which is true as mentioned in the syllabus under Functional Adaptations in Cells. The Reason claims that the unique shape of cells is directly related to their functions, which is also true according to the syllabus (Why Do Cells Look Different?). Furthermore, the Reason correctly explains why muscle cells are spindle-shaped (to facilitate contraction and relaxation). Therefore, both statements are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Levels of Organisation

a) Cells → Tissues → Organs → Organ systems → Organism
[Solution Description]
The levels of organisation in a multicellular organism follow a hierarchical structure starting from the simplest to the most complex. This sequence is:
1. Cell — the basic unit of life.
2. Tissue — a group of similar cells performing a specific function.
3. Organ — made up of different tissues working together.
4. Organ system — several organs performing a major function.
5. Organism — all organ systems combined to form a complete living being.
The correct sequence is: $Cell \rightarrow Tissue \rightarrow Organ \rightarrow Organ system \rightarrow Organism$.

Key Concept: Levels of Organisation, Multicellular Organisms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the hierarchical organisation of cells into tissues, organs, and organ systems is crucial for the survival of multicellular organisms. This is true because multicellular organisms rely on specialised structures to perform complex functions efficiently. For example, the digestive system consists of organs like the stomach and intestines, which are made of tissues that perform specific roles in digestion.
The reason claims that without this hierarchical organisation, complex functions like digestion or respiration cannot be efficiently performed. This is also true because these functions require coordination among different specialised cells and structures. For instance, respiration involves the lungs (organ) made up of lung tissues (tissue), which contain specialised cells (cells) working together to exchange gases.
Moreover, the reason correctly explains why the assertion is true—specialised organisation enables efficient performance of vital functions. Thus, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Levels of Organisation

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that the cell is the basic unit of life, which is true as per the syllabus. The reason states that a group of similar cells forms a tissue, which is also true. However, the reason does not explain why the cell is the basic unit of life; it simply describes the next level of organisation. Therefore, both statements are true, but the reason does not correctly explain the assertion.

Key Concept: Cell, Tissue, Organ

b) Muscle cells represent the tissue level, and pumping blood requires the organ level
[Solution Description]
The muscle cells working together represent the tissue level of organisation because they are similar cells performing a specific function (contraction). However, for complex functions like pumping blood, multiple tissues must combine to form an organ (e.g., the heart), which includes muscle tissue, connective tissue, and nervous tissue.

Key Concept: Multicellular Organisms, Digestive System

c) Muscle cells in the food pipe
[Solution Description]
The wave-like movement in the food pipe (peristalsis) is caused by muscle cells contracting and relaxing. These cells are thin, flexible, and spindle-shaped, allowing them to perform this function efficiently. If this movement stops, the issue lies with the muscle cells in the tissue layer of the food pipe.

Key Concept: Cell Division, Multicellular Organisms

a) Cell
[Solution Description]
Multicellular organisms begin as a single cell (egg) that divides repeatedly to form different tissues, organs, and systems. If the organism fails to form multiple tissues, it means it couldn’t progress beyond the cell level to the tissue level of organization. Hence, development stops at the cell level.

Key Concept: Microorganisms Types

d) Virus
[Solution Description]
The question refers to an acellular microorganism that replicates only within living cells. From the “Ever heard of…” section, viruses are described as acellular and require entry into a living cell to multiply.

Key Concept: Microorganisms – Unicellular vs Multicellular

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that Amoeba is a unicellular microorganism, which is true as per the syllabus (Table 2.1). The Reason states that Amoeba cannot be seen with the naked eye because it is microscopic in size, which is also true and correctly explains why Amoeba is not visible without magnification. Thus, both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

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: Protozoa, Algae

b) Paramecium
[Solution Description]
Protozoa are single-celled microorganisms that move using specialized structures like cilia or flagella. They are commonly found in aquatic environments. Examples include Paramecium, which uses cilia for movement.

Key Concept: Types of Microorganisms

c) Rhizobium
[Solution Description]
Rhizobium is a type of bacteria that forms symbiotic associations with the roots of leguminous plants. It helps in fixing atmospheric nitrogen into a form that plants can absorb, thereby aiding in plant growth.

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: 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: Microbial decomposition, Nutrient recycling

c) Microorganisms convert waste into nutrients that enrich the soil
[Solution Description]
Microorganisms like fungi and bacteria decompose plant and animal waste, breaking down complex organic matter into simpler nutrients. These nutrients are then recycled back into the soil, enhancing its fertility. Without microbial decomposition, organic waste would accumulate, and essential nutrients would remain locked in the waste, unavailable to plants.

Key Concept: Gut microbiota, Microbial diversity

c) Our relationship with microbes depends on multiple factors
[Solution Description]
This scenario highlights the complex nature of our symbiotic relationship with gut microbes. As mentioned in the syllabus, some microorganisms live inside our bodies beneficially (like aiding digestion), but can become harmful under certain conditions. The same microbe can be beneficial in one context (gut) but harmful in another (bloodstream), showing that our connection with microbes depends on multiple factors like location and quantity.

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: Role of Microorganisms

c) They break down organic waste into manure
[Solution Description]
Microorganisms decompose organic waste like fruit and vegetable peels into nutrient-rich manure, which enhances soil fertility.

Key Concept: Microbes in Waste Decomposition

c) Fungi and Bacteria
[Solution Description]
The decomposition of organic waste like fruit and vegetable peels into manure is primarily carried out by certain microorganisms such as fungi and bacteria. These microbes break down complex organic matter into simpler substances, enriching the soil with nutrients. Hence, fungi and bacteria are the key players.

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: Nutrient Recycling, Scientific Heritage

b) It shows ancient people understood microbial decomposition and nutrient cycling
[Solution Description]
Modern science confirms that microorganisms decompose organic matter, recycling nutrients back into the soil, which aligns with ancient descriptions of ‘Krimi’ breaking down waste. This process maintains ecological balance and supports plant growth.

Key Concept: Biogas Production

c) Methane and carbon dioxide.
[Solution Description]
Microorganisms decompose organic matter in oxygen-free environments to produce biogas, mainly consisting of methane and carbon dioxide.

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: 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: 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: Environmental applications of microbes

c) He developed a bacterium to clean oil spills
[Solution Description]
Dr. Ananda Mohan Chakrabarty developed a special bacterium that could break down oil spills, helping to clean up environmental pollution. His work demonstrated how microorganisms can be used to solve real-world problems.

Key Concept: Environmental Application

c) Breaking down plastic waste
[Solution Description]
Microorganisms have various applications in solving environmental issues. They can break down pollutants, decompose organic waste, and even remove toxins from contaminated sites. Some examples include cleaning oil spills (as demonstrated by Dr. Chakrabarty), treating wastewater, and biodegrading plastic waste. Among the options, plastic waste accumulation is a major problem that certain microbes can degrade.

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 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: Fermentation, Role of Yeast

b) 2.49 L
[Solution Description]
The balanced chemical equation for fermentation is:
$C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$
Molar mass of glucose ($C_6H_{12}O_6$) = 180 g/mol.
10 g of sugar corresponds to $\frac{10}{180} = 0.0556$ moles of glucose.
From the equation, 1 mole glucose produces 2 moles $CO_2$.
So, 0.0556 moles glucose will produce $0.0556 \times 2 = 0.111$ moles $CO_2$.
At STP, 1 mole of gas occupies 22.4 L.
Volume of $CO_2$ produced = $0.111 \times 22.4 = 2.49$ L.

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 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: Conservation of Microalgae

b) They help maintain oxygen balance and support aquatic life
[Solution Description] The syllabus highlights that conserving microalgae is crucial for maintaining oxygen balance on Earth, ensuring food security, and supporting livelihood development. Pollution, climate change, and habitat destruction threaten their diversity and abundance.

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: 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: Microbial diversity, Environmental cleaning

d) Assertion is false, but Reason is true.
[Solution Description]
The assertion claims that microorganisms cannot survive in extreme conditions due to their cellular structure. However, this is false because microorganisms exhibit high adaptability with specialized enzymes and cell membranes that function effectively in extreme temperatures, as studied in the syllabus. On the other hand, the reason correctly explains microbial survival by highlighting their metabolic and structural diversity, which is true as per the syllabus (e.g., extremophiles in hot springs and polar regions).

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: Role of Lactobacillus

c) *Lactobacillus*
[Solution Description]
*Lactobacillus* bacteria ferment lactose in milk, converting it into lactic acid. This process thickens the milk and turns it into curd. Bowl A in Activity 2.9 demonstrates this transformation when a spoon of curd (containing *Lactobacillus*) is added to boiled milk.

Key Concept: Microbial decomposition, biogas production

a) 0.75
[Solution Description]
Given:
– 1 kg waste $\rightarrow$ 0.25 m$^3$ methane under optimal conditions.
– Only 60% of 5 kg waste is decomposed.
Step 1: Calculate effective waste decomposed = 5 kg $\times$ 0.60 = 3 kg.
Step 2: Methane produced = 3 kg $\times$ 0.25 m$^3$/kg = 0.75 m$^3$.

Key Concept: Decomposition Process

b) Optimal temperature and moisture
[Solution Description]
Microorganisms require optimal temperature and moisture levels to break down plant waste into nutrient-rich manure efficiently. These conditions help maintain their metabolic activity.

Key Concept: Microbial decomposition, environmental impact

c) 30
[Solution Description]
Given:
– Decomposition time at 30°C = 15 days.
– Rate halves at 20°C $\Rightarrow$ time doubles.
Step 1: New decomposition time = 15 days $\times$ 2 = 30 days.

Key Concept: Manure Formation, Microorganisms

b) High temperature and turning waste regularly
[Solution Description]
For faster decomposition:
1. Microorganisms like Lactobacillus require optimal moisture and temperature.
2. Turning the waste increases aeration, aiding aerobic bacteria.
3. Smaller particle size increases surface area for microbial action.
4. Adding nitrogen-rich material provides nutrients for microbes.

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: Formation of manure and biogas

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that microorganisms decompose organic waste into manure, which is true as fungi and bacteria break down plant waste into nutrient-rich manure. The Reason explains that these microorganisms convert complex organic compounds into simpler substances, enhancing soil fertility. Since the Reason correctly explains why the Assertion occurs (microbial action leads to manure formation), both are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Historical and cultural significance of microorganisms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Saccharomyces cerevisiae has been used in fermentation for thousands of years, which is historically accurate as evidence shows its use in ancient brewing and baking. The reason explains the role of Saccharomyces cerevisiae in producing alcohol and carbon dioxide during fermentation, which directly supports why this microorganism was significant in food and beverage production. Thus, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Microorganisms in Food Preservation

c) It dehydrates microbial cells, inhibiting growth
[Solution Description]
High salt or sugar concentrations create a hypertonic environment, causing water to move out of microbial cells via osmosis, preventing their growth and spoilage of food.

Key Concept: Microbes in Food Preservation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that yeast makes dough rise by fermenting sugar and producing carbon dioxide, which is true as yeast converts sugars into $\text{CO}_2$ and ethanol during fermentation. The reason states that fermentation increases dough volume by releasing gases, which is also true and directly explains how yeast causes dough to rise. Thus, both statements are correct, and the reason correctly explains the 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 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: Yeast fermentation process

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the dough rises when yeast is added due to the production of carbon dioxide during respiration. This is true because yeast breaks down sugar in anaerobic conditions (fermentation), producing $CO_2$ gas bubbles that make the dough fluffy.
The reason explains that yeast respires anaerobically to break down sugar, releasing $CO_2$ and alcohol. This is also true and correctly explains why the dough rises. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Fermentation byproducts, Chemical equation

c) $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$; $CO_2$ creates air pockets.
[Solution Description]
The fermentation equation is:
$C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + \text{energy}$
Here, $CO_2$ gas forms bubbles in dough, making it fluffy. Ethanol evaporates during baking, leaving no residue.

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 Lactobacillus is used to convert milk into curd, which is true as per the syllabus. The reason states that Lactobacillus ferments lactose in milk to produce lactic acid, which is also true and correctly explains why Lactobacillus converts milk into curd. Therefore, both the assertion and reason are true, and the reason is the correct explanation of 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: Role of Lactobacillus in curd formation

b) Converts lactose to lactic acid
[Solution Description]
Lactobacillus bacteria convert lactose in milk into lactic acid ($C_3H_6O_3$) through fermentation, which sours the milk and forms curd. Warm conditions favor this process.

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

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: Cultural significance of fermented foods, Conditions required for fermentation

c) Lactic acid bacteria are inactive in cold conditions
[Solution Description]
Lactobacillus bacteria require warm temperatures (30-40°C) to ferment lactose into lactic acid ($C_3H_6O_3$), which thickens the milk and gives curd its sour taste. Cold temperatures slow down bacterial metabolism, delaying or preventing fermentation. The reaction is:
$C_{12}H_{22}O_{11} + H_2O \rightarrow 4C_3H_6O_3$

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: Cell Structure, Unicellular vs Multicellular Organisms

b) A bacterial cell
[Solution Description]
The question describes an organism lacking a well-defined nucleus and having a nucleoid. According to the syllabus, bacteria have a nucleoid instead of a proper nucleus. Therefore, this organism must be a bacterium. Fungi (yeast), protozoa, and algae are eukaryotic and have a well-defined nucleus.

Key Concept: Basic Unit of Life

c) It is the smallest unit capable of performing all life functions
[Solution Description]
The cell is the smallest unit that can perform all life processes independently. All living organisms are composed of cells, and cells carry out essential functions necessary for survival, such as metabolism, growth, and reproduction.

Key Concept: Cell as the Basic Unit of Life

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion (A) is true because the cell is indeed the fundamental structural and functional unit of all living organisms, from microorganisms to complex multicellular organisms. The reason (R) is also true because cells carry out vital functions like growth, reproduction, and metabolism on their own, which supports life. Furthermore, the reason correctly explains why the cell is the basic unit of life since it highlights the independent functioning of cells in sustaining life processes.

Key Concept: Variation in Cells, Multicellular Organisms

d) Differentiation leads to specialized structures suited to their specific roles.
[Solution Description]
Muscle cells are spindle-shaped for contraction, while nerve cells are elongated with branches for signal transmission. This structural variation arises due to differentiation, where cells specialize during development to perform specific functions, ensuring efficient operation of tissues and organs.

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: 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: Specialized Cells, Survival Advantage

c) Specialized cells enhance functional efficiency, enabling adaptation to diverse environments.
[Solution Description]
Specialized cells in multicellular organisms perform specific functions efficiently, improving overall survival. For example, muscle cells enable movement, nerve cells transmit signals, and blood cells transport oxygen. This division of labor allows the organism to exploit diverse resources and environments. Unicellular organisms must perform all functions alone, limiting their efficiency in complex tasks.

Key Concept: Assertion and Reason

c) Assertion is true, but Reason is false.
[Solution Description]
The Assertion states that unicellular organisms perform all life processes within a single cell, which is true as per the syllabus (e.g., bacteria, protozoa). The Reason claims that in unicellular organisms, specialized cells work together to carry out various functions, which is false because unicellular organisms consist of only one cell and do not have specialized cells working together. Therefore, the Assertion is true, but the Reason is false.

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: 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.

Key Concept: Unicellular vs Multicellular organisms

b) Unicellular organisms perform all vital functions in a single cell
[Solution Description]
Unicellular organisms consist of only one cell which performs all life processes necessary for survival. Multicellular organisms are composed of many cells that work together and specialize in different functions. The key distinction is that unicellular organisms carry out all essential functions in a single cell, whereas multicellular organisms have differentiated cells specialized for various tasks.