CBSE Class 8 Science Chapter 02 The Invisible Living World Beyond Our Naked Eye MCQs Set 01

Multiple Choice Questions (MCQs) for Class 8 Science: Chapter 02 The Invisible Living World Beyond Our Naked Eye

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Practice Chapter 02 The Invisible Living World Beyond Our Naked Eye MCQs for Class 8 Science

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Question: What property of a lens made it useful for viewing small objects, leading to the discovery of the microscopic world?
A. Its ability to bend and twist into different shapes
B. Its curved shape—thick in the middle and thin at the edges—similar to a lentil seed
C. Its capacity to generate light when heated
D. Its transparency combined with a perfectly flat surface
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Answer: (B) Its curved shape—thick in the middle and thin at the edges—similar to a lentil seed

Explanation:
The chapter explains that people discovered a curved piece of glass could magnify small things, shaped like a lentil seed with thickness in the middle and thinness at the edges—this design is what gave it the name 'lens' and made it effective for magnification.

Question: Why did Robert Hooke name the tiny structures he observed in cork 'cells'?
A. Because they resembled small prison compartments where organisms were trapped
B. Because their regular arrangement reminded him of the layout of monastery cells
C. Because the empty spaces in the cork resembled the chambers of a honeycomb
D. Because the structures contained liquid that pooled in small spaces
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Answer: (C) Because the empty spaces in the cork resembled the chambers of a honeycomb

Explanation:
The honeycomb comparison is explicit in the text—Hooke saw many small empty spaces in the cork and the compartments reminded him of a honeycomb, leading him to call each space a cell.

Question: What structural difference distinguishes bacteria cells from plant, animal, and fungal cells?
A. Bacteria lack a cell wall entirely
B. Bacteria lack a well-defined nucleus and nuclear membrane, having a nucleoid instead
C. Bacteria contain multiple nuclei arranged in a linear pattern
D. Bacteria have chloroplasts but no cell membrane
Show Answer & Explanation

Answer: (B) Bacteria lack a well-defined nucleus and nuclear membrane, having a nucleoid instead

Explanation:
The text specifically states that bacteria do not have a well-defined nucleus and nuclear membrane, instead possessing a nucleoid—this feature distinguishes them from cells of yeast, protozoa, algae, fungi, plants, and animals.

Question: In Activity 2.2, why was safranin stain applied to the onion peel?
A. To kill any harmful microorganisms on the peel
B. To dissolve the cell wall so cells could be separated
C. To add colour to the cells and improve visibility under the microscope
D. To prevent the peel from drying out during observation
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Answer: (C) To add colour to the cells and improve visibility under the microscope

Explanation:
The activity instructions state that safranin (a red-coloured stain) would give a pinkish colour to the cells and help us see them clearly—this enhanced visibility made the cellular structures easier to observe under magnification.

Question: How do muscle cells' shape and structure relate to their function in the digestive system?
A. Their round shape allows them to rotate food in all directions
B. Their thin, flexible, spindle shape enables them to contract and relax in wave-like motions, pushing food downward
C. Their elongated branches help them sense chemical changes in the food
D. Their flat structure spreads digestive juices evenly across the stomach wall
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Answer: (B) Their thin, flexible, spindle shape enables them to contract and relax in wave-like motions, pushing food downward

Explanation:
The chapter explains that muscle cells in the food pipe are thin, flexible, and spindle-shaped, which allows them to contract and relax in a wave-like manner, pushing food down to the stomach—directly linking their structure to their function.

Question: What is the correct sequence of organisational levels from simplest to most complex?
A. Organism → Organ system → Organ → Tissue → Cell
B. Cell → Tissue → Organ → Organ system → Organism
C. Tissue → Cell → Organ → Organism → Organ system
D. Cell → Organ → Tissue → Organ system → Organism
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Answer: (B) Cell → Tissue → Organ → Organ system → Organism

Explanation:
The chapter explicitly presents the hierarchy: Cell forms Tissue, Tissue forms Organ, Organ combines into Organ system, and all Organ systems together make up an Organism—this is the only sequence that follows the biological organisational structure.

Question: Which statement best explains why the yeast dough in Activity 2.8 rose while the control dough without yeast remained flat?
A. • Yeast cells respired and broke down sugar • Carbon dioxide bubbles were released during respiration • These bubbles made the dough soft and fluffy
B. Warm water causes all flour to expand naturally over time regardless of additives
C. Sugar ferments on its own when exposed to warm conditions without any organisms
D. The dough in bowl A absorbed more water than the dough in bowl B
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Answer: (A) • Yeast cells respired and broke down sugar • Carbon dioxide bubbles were released during respiration • These bubbles made the dough soft and fluffy

Explanation:
The chapter describes yeast as a fungus that respires and breaks down food to release energy; during this respiration process, carbon dioxide is released, which forms bubbles that make the dough soft and fluffy—the control dough without yeast lacks this biological process.

Question: Based on Activity 2.9, what can be inferred about the temperature requirement for curd formation by Lactobacillus?
A. Curd formation occurs equally well at all temperatures
B. Lactobacillus bacteria require cold temperatures below 5°C for fermentation
C. Lactobacillus grows well in warm conditions and forms curd more effectively at higher temperatures
D. Temperature has no effect on the rate of curd formation
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Answer: (C) Lactobacillus grows well in warm conditions and forms curd more effectively at higher temperatures

Explanation:
The text states that Lactobacillus bacteria grow well in warm conditions, and the activity shows that curd forms in bowl A (warm milk) but not in bowl B (cold milk), demonstrating that these bacteria require warmth for efficient fermentation.

Question: What is the primary role of Rhizobium bacteria in legume root nodules?
A. They break down complex proteins into amino acids for plant absorption
B. They trap nitrogen from the air and convert it into a form useful for plant growth
C. They produce vitamins and minerals within the nodule tissue
D. They protect the roots from harmful pathogenic organisms in the soil
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Answer: (B) They trap nitrogen from the air and convert it into a form useful for plant growth

Explanation:
The chapter explicitly states that Rhizobium bacteria trap nitrogen from the air and make it useful for the plants, which is why legume crops like beans, peas, and lentils can grow well without added nitrogen fertilisers.

Question: How do microorganisms contribute to environmental cleanup in the process described in Activity 2.7?
A. They produce chemicals that dissolve waste materials into liquid
B. • They break down complex plant and animal waste into simpler substances • These simpler substances are nutrient-rich • The process is called decomposition • Nutrients return to soil, benefiting plant growth
C. They absorb all waste materials into their cells and store them permanently
D. They convert waste into toxic gases that evaporate into the atmosphere
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Answer: (B) • They break down complex plant and animal waste into simpler substances • These simpler substances are nutrient-rich • The process is called decomposition • Nutrients return to soil, benefiting plant growth

Explanation:
The chapter explains that bacteria and fungi act on plant waste, slowly breaking it down into nutrient-rich manure through decomposition—this recycling process returns nutrients to soil, helping plants grow better while cleaning the environment.

Question: Why would a student expect different observations when comparing bread kept near a sink versus bread kept in a refrigerator after three days?
A. The bread near the sink would absorb moisture and grow larger
B. The bread in the refrigerator would be more mouldy because cold temperatures kill beneficial bacteria
C. The bread near the sink would likely show mould growth due to warmer, more humid conditions favouring microorganism growth, while refrigeration slows microbial activity
D. Both breads would show identical patterns of decay regardless of storage location
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Answer: (C) The bread near the sink would likely show mould growth due to warmer, more humid conditions favouring microorganism growth, while refrigeration slows microbial activity

Explanation:
Microorganisms require optimal temperature and moisture for growth—the warm, humid environment near the sink provides ideal conditions, while the cold, dry refrigerator inhibits microbial reproduction, resulting in different levels of mould formation.

Question: What does the ostrich egg example in the 'Ever heard of' section primarily illustrate about cell size?
A. The largest cells are always found in the largest animals
B. The yolk of an ostrich egg represents the largest known single cell, measuring 130–170 mm in diameter
C. Animal cells are always much larger than plant cells
D. Single-celled organisms can never be visible to the naked eye under any circumstances
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Answer: (B) The yolk of an ostrich egg represents the largest known single cell, measuring 130–170 mm in diameter

Explanation:
The passage directly states that the yolk of an ostrich egg is a single cell—the largest known cell in the living world—measuring about 130 mm to 170 mm in diameter, making it an exceptional example of cell size.

Question: How do plant cells and animal cells differ in their ability to produce food?
A. Animal cells can photosynthesise but plant cells cannot
B. Plant cells contain chloroplasts with chlorophyll for photosynthesis, while animal cells lack chloroplasts and cannot make their own food
C. Both plant and animal cells produce food through identical processes
D. Neither plant nor animal cells are capable of producing their own food
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Answer: (B) Plant cells contain chloroplasts with chlorophyll for photosynthesis, while animal cells lack chloroplasts and cannot make their own food

Explanation:
The 'A step further' section explains that plastids like chloroplasts in plant cells contain chlorophyll, which helps in photosynthesis, whereas animal cells do not have chloroplasts and therefore cannot manufacture their own food.

Question: What scientific principle does Ananda Mohan Chakrabarty's work with oil-spill-decomposing bacteria demonstrate?
A. Bacteria are always harmful and should be eliminated from the environment
B. Microorganisms with naturally occurring abilities can be applied to solve real-world environmental problems like pollution
C. Oil spills cannot be cleaned because bacteria are too small to have any effect
D. Patent law prevents the development of new microorganism-based solutions
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Answer: (B) Microorganisms with naturally occurring abilities can be applied to solve real-world environmental problems like pollution

Explanation:
The 'Be a scientist' section describes how Chakrabarty developed a bacterium that could break down oil spills, showing that microorganisms can be harnessed to address environmental challenges—his work exemplifies the practical application of microbiology to solve pollution problems.

Question: Why are microalgae considered critical organisms despite their microscopic size?
A. They are the primary source of nitrogen for all land plants
B. • They produce more than half of Earth's oxygen supply • They serve as food for many aquatic animals • Some provide health supplements and medicines for humans • They help clean water and are used to make biofuel
C. They are the only organisms capable of breaking down plastic waste
D. They provide the majority of food directly consumed by humans
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Answer: (B) • They produce more than half of Earth's oxygen supply • They serve as food for many aquatic animals • Some provide health supplements and medicines for humans • They help clean water and are used to make biofuel

Explanation:
The section on microalgae explains multiple crucial roles: they produce over half of Earth's oxygen, serve as nutritional sources for aquatic life, provide human supplements and medicines, and assist in water purification and biofuel production—collectively making them vital to ecosystems and human welfare.

Question: In Activity 2.1, when a water-filled flask is placed over printed letters on a page, the letters appear larger. Which explanation best describes why this magnification occurs?
A. The glass of the flask bends the light rays, making objects appear magnified
B. The curved surface of the flask acts as a lens that refracts light passing through the water
C. Water particles scatter light in a way that increases the size of the image
D. The thickness of the glass creates a prism effect that enlarges all objects
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Answer: (B) The curved surface of the flask acts as a lens that refracts light passing through the water

Explanation:
A curved container of liquid functions as a simple lens by bending (refracting) light rays. The chapter describes how a piece of glass shaped like a lentil seed—thick in the middle and thin at the edges—could make small things look bigger, which is precisely how the flask-and-water setup works.

Question: Which of the following best describes what all living organisms have in common based on the chapter's discussion of organisational levels?
A. They are all multicellular with many different tissue types
B. They are all composed of cells as their basic unit of structure
C. They all have the same number and arrangement of cells
D. They all contain visible microorganisms within their bodies
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Answer: (B) They are all composed of cells as their basic unit of structure

Explanation:
The chapter explicitly states that all living beings are made up of cells, and that the cell is the basic unit of life. This applies to both multicellular organisms like plants and animals, and unicellular ones like bacteria and protozoa.

Question: When observing cheek cells in Activity 2.3, they appear as polygon-shaped structures rather than rectangular ones like onion peel cells. What does this difference primarily reflect?
A. Cheek cells are younger and have not yet developed a defined shape
B. Animal cells lack a cell wall, so they adopt whatever shape results from their arrangement and packing
C. The stain used (methylene blue) causes animal cells to become polygonal
D. Cheek cells belong to a different kingdom of organisms and therefore have fundamentally different geometry
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Answer: (B) Animal cells lack a cell wall, so they adopt whatever shape results from their arrangement and packing

Explanation:
Animal cells lack a cell wall, which means they can pack together in various ways depending on how they are pressed against neighbouring cells, resulting in polygon shapes. Plant cells have rigid cell walls that keep them more rectangular. The chapter notes that onion peel cells are closely arranged without spaces and appear rectangular because of this structure.

Question: In the context of Activity 2.8 with yeast fermentation, why were students instructed to add sugar to the flour and use warm water?
A. Sugar and warm water prevent the growth of unwanted bacteria
B. Warm water dissolves the sugar, which yeast needs as food, and warmth accelerates yeast respiration and growth
C. Sugar and warm water change the chemical properties of flour to activate yeast enzymes
D. These additions eliminate the need for oxygen during fermentation
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Answer: (B) Warm water dissolves the sugar, which yeast needs as food, and warmth accelerates yeast respiration and growth

Explanation:
The chapter explains that yeast breaks down food (in this case, sugar) to release energy for growth and life processes, and that yeast grows well in warm conditions. The combination of sugar as a food source and warmth creates optimal conditions for yeast activity, producing the carbon dioxide that makes the dough rise.

Question: Based on the chapter's description of microorganism diversity, which pairing of a microorganism with its characteristic is correct?
A. Bacteria have a well-defined nucleus enclosed by a nuclear membrane
B. Paramecium moves using specialised structures visible under magnification
C. Algae lack chlorophyll and cannot make their own food
D. Fungi contain chloroplasts that produce chlorophyll for photosynthesis
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Answer: (B) Paramecium moves using specialised structures visible under magnification

Explanation:
• Table 2.1 shows that Paramecium is a protozoa that moves from one place to another with the help of specialised structures
• Bacteria lack a well-defined nucleus and have a nucleoid instead
• Algae contain chlorophyll (green pigment) and make their own food
• Fungi lack chloroplasts and cannot perform photosynthesis

Question: What inference can be drawn from the observation that decomposition in Activity 2.7 occurs 'at optimal temperature and appropriate moisture level'?
A. Microorganisms are unable to function in very cold or dry conditions
B. The rate of decomposition by microbes depends on environmental conditions, and farmers can accelerate manure formation by controlling these factors
C. Only certain species of bacteria can survive in non-optimal temperatures and moisture
D. Temperature and moisture are the only factors that determine whether decomposition will occur at all
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Answer: (B) The rate of decomposition by microbes depends on environmental conditions, and farmers can accelerate manure formation by controlling these factors

Explanation:
The mention of optimal conditions implies that while decomposition happens naturally, it can be faster or slower depending on the environment. Farmers who collect leaves and waste in pits are likely using their understanding of these conditions to speed up the natural decomposition process and create manure more efficiently.

Question: Which statement about plant and animal cells is supported by evidence presented in section 2.5 of the chapter?
A. Plant cells always have more mitochondria than animal cells
B. Both plant and animal cells are enclosed by a cell membrane, but plant cells have an additional cell wall
C. Animal cells contain chloroplasts while plant cells do not
D. Plant cells are always larger than animal cells because they have a cell wall
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Answer: (B) Both plant and animal cells are enclosed by a cell membrane, but plant cells have an additional cell wall

Explanation:
The chapter clearly distinguishes that both cell types have a cell membrane, but plant cells have an additional outer layer called a cell wall. This is shown in the schematic diagrams (Fig. 2.5) and discussed in the text describing the structural differences between the two cell types.

Question: The chapter states that Spirulina 'is being used by humans as health supplements.' What does this suggest about how scientific understanding of microorganisms has evolved?
A. Ancient scientists knew about Spirulina but rejected it as a food source
B. Modern science has enabled humans to identify and utilise microorganisms for human benefit rather than viewing all as harmful
C. Spirulina is a recent discovery that did not exist before the invention of microscopes
D. Microorganisms are becoming dangerous only in recent times, so supplements are now needed
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Answer: (B) Modern science has enabled humans to identify and utilise microorganisms for human benefit rather than viewing all as harmful

Explanation:
Spirulina's use as a health supplement demonstrates that scientists now understand microorganisms well enough not only to identify them, but to recognise their beneficial properties and deliberately cultivate them for human nutrition. This marks a shift from microscopy simply revealing the existence of tiny life to applying that knowledge practically.

Question: How does the chapter's discussion of Lactobacillus in curd formation (Activity 2.9) illustrate the relationship between temperature and microorganism function?
A. Bacteria are killed if the temperature is not warm enough to activate their genes
B. The same microorganism behaves differently depending on environmental conditions—Lactobacillus produces curd rapidly in warm milk but grows very slowly in cold milk
C. Cold temperatures reverse the fermentation process, turning curd back into milk
D. Lactobacillus cannot survive in cold conditions, so it must be added fresh to warm milk every time
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Answer: (B) The same microorganism behaves differently depending on environmental conditions—Lactobacillus produces curd rapidly in warm milk but grows very slowly in cold milk

Explanation:
Activity 2.9 directly compares curd formation in warm milk (Bowl A) versus cold milk (Bowl B), showing that the same bacterium (Lactobacillus) functions more effectively at higher temperatures. The chapter notes these bacteria 'grow well in warm conditions,' explaining the observed difference in curd formation rate.

Question: According to the chapter, viruses are distinguished from other microorganisms primarily by which characteristic?
A. They are larger than bacteria and can be seen with a light microscope
B. They are acellular and reproduce only inside a living host cell
C. They have a cell wall but no cell membrane like bacteria
D. They contain chlorophyll and can produce their own food
Show Answer & Explanation

Answer: (B) They are acellular and reproduce only inside a living host cell

Explanation:
The 'Ever heard of' box clearly states that viruses are microscopic and acellular, and that they multiply when they enter a living cell. This reproduction requirement—that they can only replicate inside a host organism—is what fundamentally distinguishes them from other microorganisms like bacteria and fungi, which can reproduce independently.

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