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The Invisible Living World: Microorganisms

NCERT Class 8 · Science Based on NCERT Class 8 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter introduces students to the invisible world of microorganisms and the fundamental unit of life—the cell. It explains how the invention of the microscope revealed tiny living beings that were previously unknown, and how these microbes are everywhere—in water, soil, air, and even inside our bodies. The big idea is that all living organisms, from the smallest bacterium to the largest animal, are made up of cells, and that microorganisms play both beneficial and harmful roles in our daily lives, from making curd and bread to cleaning the environment.
Key Concepts (21)
Microorganisms
Living organisms that are so small they cannot be seen with the naked eye and require a microscope to be observed.
e.g. Bacteria in pond water, yeast used in making bread, and Amoeba in stagnant water.
Cell
The basic structural and functional unit of all living organisms.
e.g. Onion peel cells seen under a microscope are rectangular and closely packed like bricks in a wall.
Cell membrane
The thin outer covering of a cell that separates it from other cells and allows entry of essential materials and exit of waste.
e.g. The outer boundary of a cheek cell that lets nutrients in and waste out.
Cytoplasm
The jelly-like substance between the cell membrane and the nucleus where most life processes and cell components are located.
e.g. The space inside an onion cell that contains carbohydrates, proteins, and mineral salts.
Nucleus
The round structure in the middle of a cell that regulates all activities and growth of the cell.
e.g. The dark-stained centre of a cheek cell visible after adding methylene blue.
Cell wall
An extra rigid outer layer present in plant cells, fungal cells, and bacterial cells that provides strength and support.
e.g. The firm outer covering of an onion peel cell that makes it look rectangular and compact.
Unicellular organisms
Living organisms that are made up of only one cell, which carries out all life functions.
e.g. Bacteria, Amoeba, Paramecium, and yeast.
Multicellular organisms
Living organisms that are made up of many cells that work together to perform various functions.
e.g. Humans, plants, animals, and mould (multicellular fungus).
Tissue
A group of similar cells that work together to perform a specific function.
e.g. Muscle cells in the food pipe that contract and relax to push food down.
Organ
A structure made up of different tissues that work together to perform a specific function.
e.g. The stomach, which has muscle cells and cells that produce digestive juices.
Organ system
A group of organs that work together to perform a major function of the body.
e.g. The digestive system, which includes the food pipe, stomach, and intestines.
Decomposition
The process by which microorganisms break down complex substances of dead plants and animals into simpler, nutrient-rich substances.
e.g. Fungi and bacteria turning fallen leaves and vegetable peels into manure in a compost pit.
Fermentation
A process in which microorganisms like yeast and bacteria break down food to release energy, producing carbon dioxide, alcohol, or acids.
e.g. Yeast producing carbon dioxide bubbles that make dough rise and become fluffy for bread.
Lactobacillus
A type of bacterium that converts milk sugar (lactose) into lactic acid, turning milk into curd.
e.g. Adding a spoonful of curd to warm milk to make fresh curd at home.
Rhizobium
A bacterium that lives in root nodules of leguminous plants and converts atmospheric nitrogen into a form usable by plants.
e.g. Bacteria in the roots of pea, bean, or lentil plants that help them grow without nitrogen fertiliser.
Chloroplast
A plastid containing chlorophyll that gives plants their green colour and helps in photosynthesis.
e.g. The green structures in plant cells that make food using sunlight.
Vacuole
A large, empty-looking space in plant cells that stores substances, removes waste, and maintains cell shape.
e.g. The big central space in an onion cell that helps keep the cell firm.
Nucleoid
The region in a bacterial cell that contains genetic material but is not enclosed by a nuclear membrane.
e.g. The area in a bacterial cell where DNA is located, unlike the well-defined nucleus of plant and animal cells.
Viruses
Microscopic, acellular entities that can only multiply inside a living host cell and may cause diseases.
e.g. The virus that causes the common cold or COVID-19.
Biogas
A mixture of gases, mainly methane and carbon dioxide, produced by bacteria decomposing plant and animal waste in an oxygen-free environment.
e.g. The gas used for cooking in some rural households from a biogas plant.
Spirulina
A microalga rich in protein and vitamin B12, used as a health supplement and superfood.
e.g. A green powder added to smoothies or sold as tablets for nutrition.
How It Works
Observation of Onion Peel Cells (Activity 2.2)
  1. Cut an onion bulb vertically and remove a thin transparent layer (onion peel) from the inner surface using forceps.
  2. Place the peel in a petri dish with a few drops of safranin stain for 30 seconds to colour the cells pink.
  3. Transfer the peel to water to rinse off extra stain, then place it on a glass slide.
  4. Add a drop of glycerin to prevent drying and improve clarity.
  5. Carefully place a coverslip over the peel using a needle, avoiding air bubbles.
  6. Wipe off excess glycerin with blotting paper and observe under a microscope.
💡 Students often forget to add glycerin or trap air bubbles under the coverslip, which ruins the slide. Also, the stain is safranin (red), not methylene blue (blue) — that is for cheek cells.
Observation of Human Cheek Cells (Activity 2.3)
  1. Rinse mouth with clean water.
  2. Gently scrape the inside of the cheek with the blunt end of a clean toothpick.
  3. Place the scraped material in a drop of water on a clean glass slide and spread evenly.
  4. Add a drop of methylene blue stain to improve visibility.
  5. After one minute, add a drop of glycerin to prevent drying.
  6. Place a clean coverslip carefully and remove excess glycerin with blotting paper.
  7. Observe under a microscope.
💡 The stain used here is methylene blue (blue), not safranin. Also, the scraped material must be spread evenly, or cells will clump and be hard to see.
Curd Formation by Lactobacillus (Activity 2.9)
  1. Take two bowls — A (lukewarm milk) and B (cold milk).
  2. Add a small spoonful of curd to each bowl and mix well.
  3. Cover both bowls. Keep bowl A in a warm place and bowl B in a cool place (refrigerator).
  4. Observe after a few hours or overnight.
💡 Curd forms only in warm conditions because Lactobacillus bacteria grow and multiply best at warm temperatures. Cold milk does not curdle properly. Students often think curd forms faster in cold — it does not.
Yeast Fermentation in Dough (Activity 2.8)
  1. Take two bowls — A (flour + sugar + yeast + warm water) and B (flour + sugar + warm water, no yeast).
  2. Knead both to make soft dough, cover with a damp cloth, and keep in a warm place.
  3. Observe after 4–5 hours.
💡 Yeast respires and releases carbon dioxide gas, which forms bubbles that make the dough rise and become fluffy. The dough without yeast remains flat. Students often forget that sugar is added as food for yeast, and warm water provides optimal temperature for yeast activity.
Decomposition and Manure Formation (Activity 2.7)
  1. Fill a container halfway with garden soil.
  2. Add fruit and vegetable peels, then cover with a layer of soil.
  3. Leave for 2–3 weeks.
  4. Observe that the peels have turned into dark, nutrient-rich manure.
💡 Microorganisms like fungi and bacteria in the soil break down the complex substances in the peels into simpler, nutrient-rich manure. This process requires optimal temperature and moisture. Students often think the peels just rot — they are actually being decomposed by microbes.
Levels of Organisation in Living Organisms
  1. Cells are the basic unit of life, like bricks in a wall.
  2. Similar cells group together to form a tissue.
  3. Different tissues organise to form an organ.
  4. Several organs work together to form an organ system.
  5. All organ systems together make up a complete organism.
💡 The sequence is: Cell → Tissue → Organ → Organ System → Organism. Students often confuse the order or skip 'tissue' or 'organ system'. Remember the analogy: bricks (cells) → wall (tissue) → room (organ) → house (organ system) → colony (organism).
Exam Traps — Don't Make These Mistakes
⚠ Confusing cell wall and cell membrane
✗ The cell wall is present in all cells, including animal cells.
→ The cell wall is an extra layer found only in plant cells, fungal cells, and bacterial cells. Animal cells (like human cheek cells) have only a cell membrane, not a cell wall.
✓ The cell wall is present in plant cells, fungal cells, and bacterial cells, but absent in animal cells. Animal cells have only a cell membrane.
⚠ Mixing up stains for different cells
✗ We used methylene blue to stain onion peel cells.
→ Onion peel cells are stained with safranin (red-coloured stain), not methylene blue. Methylene blue is used for human cheek cells.
✓ Onion peel cells are stained with safranin to give them a pinkish colour, while human cheek cells are stained with methylene blue.
⚠ Thinking all microorganisms are harmful
✗ Microorganisms are harmful because they cause diseases and spoil food.
→ While some microorganisms cause diseases, many are beneficial. They help in decomposition, curd formation, fermentation, nitrogen fixation, biogas production, and cleaning the environment.
✓ Microorganisms can be both beneficial and harmful. Beneficial microbes help in making curd, bread, manure, and biogas, while harmful ones cause diseases and food spoilage.
⚠ Confusing bacteria and viruses
✗ Viruses are unicellular microorganisms like bacteria.
→ Bacteria are cellular (have a cell membrane, cytoplasm, and nucleoid) and can live independently. Viruses are acellular (not made of cells) and can only multiply inside a living host cell.
✓ Bacteria are unicellular microorganisms with a cell membrane, cytoplasm, and nucleoid. Viruses are acellular and can only reproduce inside a host cell.
⚠ Forgetting the role of sugar and warm water in yeast activity
✗ Yeast alone makes the dough rise without any other ingredient.
→ Yeast needs sugar as food to respire and produce carbon dioxide. Warm water provides the optimal temperature for yeast growth. Without sugar or warm water, yeast activity is very slow or absent.
✓ Yeast requires sugar as a food source and warm water for optimal growth. During respiration, yeast breaks down sugar and releases carbon dioxide, which makes the dough rise.
⚠ Thinking curd forms equally well in cold and warm conditions
✗ Curd forms in both warm and cold milk if left overnight.
→ Lactobacillus bacteria grow best in warm conditions. In cold milk (refrigerator temperature), bacterial growth is very slow, so curd does not form properly or takes much longer.
✓ Curd forms best in warm conditions because Lactobacillus bacteria multiply rapidly at warm temperatures. Cold temperatures slow down bacterial growth, preventing proper curd formation.
Compare & Contrast
Presence of cell wall
Plant cell
Animal cell
Plant cells have an extra rigid cell wall outside the cell membrane, while animal cells have only a cell membrane.
Presence of chloroplasts and vacuoles
Plant cell
Animal cell
Plant cells have chloroplasts (for photosynthesis) and a large central vacuole, while animal cells lack chloroplasts and have small or no vacuoles.
Cellular structure and reproduction
Bacteria
Viruses
Bacteria are unicellular organisms with a cell membrane, cytoplasm, and nucleoid, and can reproduce independently. Viruses are acellular (not made of cells) and can only multiply inside a living host cell.
Product of fermentation
Yeast (fungus)
Lactobacillus (bacterium)
Yeast produces carbon dioxide and alcohol during fermentation (used in bread and cake making), while Lactobacillus produces lactic acid (used in curd formation).
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (3 marks), 1 Long Answer (5 marks). Total approximately 10-12 marks.
Most asked: Cell structure and its parts (cell membrane, cytoplasm, nucleus, cell wall) · Differences between plant and animal cells · Types of microorganisms (bacteria, fungi, protozoa, algae, viruses) · Beneficial uses of microorganisms (curd formation, fermentation, decomposition, nitrogen fixation) · Levels of organisation (cell → tissue → organ → organ system → organism)
Don't skip: Draw and label a plant cell and an animal cell, and write three differences between them.; Explain the role of microorganisms in making curd and bread, mentioning the specific microbes involved.; Describe the levels of organisation in a living organism with examples.; How do microorganisms help in cleaning the environment? Explain with the process of decomposition.
One-Line Revision (read aloud before the exam)
  1. Microorganisms are tiny living beings that cannot be seen with the naked eye and require a microscope.
  2. The cell is the basic structural and functional unit of all living organisms.
  3. All cells have three main parts: cell membrane, cytoplasm, and nucleus.
  4. Plant cells have an extra cell wall, chloroplasts, and a large vacuole, which animal cells lack.
  5. Bacteria are unicellular and have a nucleoid instead of a well-defined nucleus.
  6. Viruses are acellular and can only multiply inside a living host cell.
  7. Yeast is a unicellular fungus used in making bread and cakes by releasing carbon dioxide.
  8. Lactobacillus is a bacterium that converts milk into curd by producing lactic acid.
  9. Rhizobium bacteria live in root nodules of leguminous plants and fix atmospheric nitrogen.
  10. Decomposition is the process by which microbes break down dead plants and animals into nutrient-rich manure.
  11. The levels of organisation in a living organism are: cell → tissue → organ → organ system → organism.
  12. Microorganisms can be both beneficial (making curd, bread, manure, biogas) and harmful (causing diseases).
  13. Robert Hooke first observed cells in cork and named them 'cells' in 1665.
  14. Antonie van Leeuwenhoek is known as the Father of Microbiology.
  15. Spirulina is a microalga rich in protein and vitamin B12, used as a health supplement.

MCQs — 10 Questions with Answers

Q1. Which scientist first observed and drew cells from a thin slice of cork?

  • A. Robert Hooke ✓
  • B. Antonie van Leeuwenhoek
  • C. Charles Darwin
  • D. Louis Pasteur

Answer: A — Robert Hooke published Micrographia in 1665 with detailed drawings of cork cells, which he called 'cells' after the honeycomb-like structure he observed.

Q2. What is the basic unit of life in all living organisms?

  • A. Tissue
  • B. Organ
  • C. Cell ✓
  • D. Organism

Answer: C — A cell is the smallest unit of life and the basic structural and functional unit of all living organisms.

Q3. Which of the following is NOT a main part of a cell?

  • A. Cell membrane
  • B. Cytoplasm
  • C. Chlorophyll ✓
  • D. Nucleus

Answer: C — Chlorophyll is a pigment found inside chloroplasts (which are in the cytoplasm), but it is not itself a main part of a cell.

Q4. Why do we stain cells with safranin or methylene blue before observing them under a microscope?

  • A. To make cells larger
  • B. To give colour and increase contrast for clear visibility ✓
  • C. To protect cells from drying
  • D. To kill harmful bacteria in the cells

Answer: B — Stains give colour to cells and increase contrast with the background, making it easier to see cell structures clearly under the microscope.

Q5. A student observes an onion peel cell and a cheek cell under the microscope. Which statement best explains their shape difference?

  • A. Onion peel cells are always larger than cheek cells
  • B. Onion peel cells have a cell wall so they are rectangular; cheek cells lack a cell wall so they are polygon-shaped ✓
  • C. Cheek cells contain more cytoplasm than onion cells
  • D. Onion cells are older and thus more deformed than cheek cells

Answer: B — The presence of a rigid cell wall in plant cells (like onion) forces them into rectangular shapes, while animal cells (like cheek cells) without cell walls adopt round or polygon shapes.

Q6. A nerve cell in the human body has a long, branched structure. How does this structure help the nerve cell perform its function?

  • A. It stores more cytoplasm for energy production
  • B. It allows the cell to reach different parts of the body and pass messages quickly ✓
  • C. It prevents the cell from being destroyed by disease
  • D. It increases the number of vacuoles for better storage

Answer: B — Nerve cells (neurons) are elongated with branches specifically to extend throughout the body and rapidly transmit messages between different regions.

Q7. In a plant cell, large vacuoles help in storing water and nutrients. In contrast, animal cells have very small vacuoles or none at all. What does this difference suggest?

  • A. Animal cells do not need to store any substances
  • B. Plant cells must store water to maintain turgor pressure and shape, while animals regulate water differently through their circulatory system ✓
  • C. Vacuoles are more important than the nucleus in plant cells
  • D. Animal cells are more primitive than plant cells

Answer: B — Plants use large vacuoles to maintain water pressure (turgor) for structural support, while animals have specialized systems for water regulation and shape maintenance.

Q8. Which structure in a cell is responsible for regulating growth and controlling all activities occurring within the cell?

  • A. Cell membrane
  • B. Cytoplasm
  • C. Nucleus ✓
  • D. Cell wall

Answer: C — The nucleus acts as the control centre of the cell, regulating all activities and controlling growth through the genetic material it contains.

Q9. A student prepares a microscope slide of an onion peel cell. Why is glycerin added to the slide before placing the coverslip?

  • A. Glycerin is a stain that colours the cells
  • B. Glycerin prevents cells from drying and improves clarity for better visualization ✓
  • C. Glycerin is necessary for the cell wall to remain rigid
  • D. Glycerin kills bacteria that might contaminate the sample

Answer: B — Glycerin prevents the cells from drying out under the heat of the microscope light and maintains clarity by keeping the cell structures intact.

Q10. Two cells—a muscle cell and a cheek cell—are observed under a microscope. The muscle cell appears spindle-shaped while the cheek cell appears polygon-shaped. What is the most likely reason for this difference?

  • A. The muscle cell is older and has deformed over time
  • B. The cheek cell is surrounded by a cell wall that constrains its shape
  • C. Each cell's shape is adapted to its specific function in the body; muscle cells are elongated for contraction, while cheek cells form a flat protective lining ✓
  • D. The muscle cell contains more nucleus material than the cheek cell

Answer: C — Cell shape is directly related to function; spindle-shaped muscle cells allow contraction and movement, while flat polygon cheek cells form protective linings on tissue surfaces.

Flashcards

What is the smallest unit of life called?

The cell is the smallest unit of life and the basic building block of all living organisms.

Who first saw and named cells?

Robert Hooke first observed cells in cork tissue in 1665 and named them after the small empty spaces he saw.

What are the three main parts of a cell?

The three main parts are cell membrane (outer boundary), cytoplasm (material inside), and nucleus (control centre).

Name one structure found in plant cells but not in animal cells.

Plant cells have a cell wall outside the cell membrane, which provides rigidity and strength to the plant.

What is the function of the cell membrane?

The cell membrane is porous and controls the entry of materials needed for life and the exit of waste products.

What does the nucleus do in a cell?

The nucleus regulates all activities inside the cell and controls growth and reproduction.

Why do we use stains like safranin when observing cells?

Stains like safranin give colour to cells and increase contrast, making them clearly visible under the microscope.

How does cell shape relate to its function?

The unique shape and structure of a cell help it perform its specific function, such as nerve cells having branches to carry messages.

What is cytoplasm and where do life processes occur?

Cytoplasm is the material between the cell membrane and nucleus containing carbohydrates, proteins, fats, and minerals where most life processes happen.

What is the difference between a plant cell and an animal cell in terms of shape?

Plant cells are typically rectangular with a cell wall, while animal cells are usually round or polygon-shaped without a cell wall.

Important Board Questions

Define a cell. [1 mark]

A cell is the smallest unit of life and the basic structural and functional unit that makes up all living organisms.

Name any two structures found in plant cells but not in animal cells, and state their functions. [2 marks]

Cell wall (provides rigidity and strength) and large vacuoles (store water, nutrients, and maintain cell shape). Chloroplasts (contain chlorophyll for photosynthesis) is also acceptable.

Describe the structure and function of the three main parts of a cell: cell membrane, cytoplasm, and nucleus. Explain how their functions are essential for the cell to survive. [3 marks]

Cell membrane (boundary, porous, controls entry/exit); Cytoplasm (contains compounds, site of life processes); Nucleus (control centre, regulates growth). Each is essential for cell survival and function.

Why do cells of different organisms show variation in shape and structure? Explain with two examples (one plant, one animal) how cell shape relates to cell function. Also, describe the procedure to observe onion peel cells under a microscope, including the role of staining and glycerin. [5 marks]

Variation due to specialized functions. Examples: long nerve cells for message transmission; tube-like plant cells for water transport. Procedure: wash onion → remove peel → stain with safranin (increases visibility) → rinse → mount on slide with glycerin (prevents drying) → cover with coverslip → observe under microscope. Draw and label a diagram showing onion peel cells with cell wall, cell membrane, cytoplasm, and nucleus.

Assertion-Reason Questions for The Invisible Living World: Microorganisms

Q1. Assertion (A):

All organisms, including both plants and animals, are made up of cells.

Reason (R):

Cells are the basic structural and functional units of all living organisms, as discovered by Robert Hooke and confirmed through observations of plant and animal tissues.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why all organisms are made of cells. Hooke's observation of cork cells and subsequent studies of onion and cheek cells confirmed that cells are the universal building blocks of life. This aligns with modern cell theory.

Q2. Assertion (A):

Plant cells from onion peel appear rectangular in shape when observed under a microscope.

Reason (R):

Plant cells are rectangular because they need to be tightly packed together to form strong tissues that provide structural support to the plant.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — The assertion is true—onion peel cells are indeed rectangular. However, the reason, while partially logical, is not the complete scientific explanation. Cells are rectangular in onion peel because they fit together efficiently in a tissue, but the rectangular shape itself is not determined by the packing requirement; rather, it results from cell division patterns and the structure of plant tissues. The reason is not the correct explanation for the shape observed.

Q3. Assertion (A):

Methylene blue stain makes cheek cells visible under a microscope.

Reason (R):

Methylene blue is a blue-coloured dye that increases contrast between cells and the background, allowing structures to be distinguished clearly.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains how methylene blue functions in microscopy. The stain works by adding colour and creating contrast, which is essential for visibility. This is explicitly stated in Activity 2.3 of the chapter.

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Case-Based Questions (CBQ) for The Invisible Living World: Microorganisms

Case Study:

A school science lab received three different sets of slides prepared by students for observing plant cells from onion peel. Slide Set 1 was prepared using safranin stain and glycerin. Slide Set 2 was prepared using only water (no stain or glycerin). Slide Set 3 was prepared with safranin stain but without glycerin. When all three sets were observed under the same microscope with identical magnification, different results were noted. Set 1 showed clear, well-defined rectangular cells with pink colour. Set 2 showed very faint, barely visible cell outlines. Set 3 showed cells with colour but the structures appeared distorted and fuzzy after 10 minutes of observation.

Q1. Why did Slide Set 2 (water only, no stain) show barely visible cell outlines?

  • A. Water is toxic to onion cells and causes them to become transparent
  • B. The cells lack colour and contrast against the background, making them difficult to see ✓
  • C. Water evaporates quickly and removes the cells from the slide
  • D. Onion cells cannot be observed in water-only preparations

Answer: B

Q2. Slide Set 3 showed distorted and fuzzy structures after 10 minutes despite having colour from safranin. What was most likely the cause?

  • A. The safranin stain was faulty and began to fade quickly
  • B. The absence of glycerin allowed water to evaporate, causing cells to dehydrate and lose clarity ✓
  • C. The microscope's magnification lens became dirty during observation
  • D. Safranin reacts with oxygen in the air, causing the cells to distort

Answer: B

Q3. Based on the three slide sets, which statement correctly explains why Slide Set 1 remained clear throughout the observation period?

  • A. Safranin provides colour while glycerin prevents drying and maintains cell clarity ✓
  • B. Glycerin increases the magnification power of the microscope
  • C. Safranin kills bacteria that would otherwise cloud the view
  • D. Pink colour naturally reflects light better, making observations clearer

Answer: A

Q4. If a fourth slide set (Slide Set 4) were prepared with safranin stain and a different oil-based mounting medium instead of glycerin, what would likely be observed?

  • A. The cells would be clearly visible with colour, remaining stable for extended observation because the oil-based medium also prevents drying ✓
  • B. The cells would appear colourless because oil prevents stain from entering cells
  • C. The cells would dissolve in the oil-based medium, making observation impossible
  • D. The safranin stain would turn yellow in the presence of oils

Answer: A

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