The Invisible Living World: Beyond Our Naked Eye Class 8: Cells, Microorganisms and Their Uses

What The Invisible Living World: Beyond Our Naked Eye is about

Key concepts

Lenses, microscopes and the discovery of cells

NCERT section 2.1 What Is a Cell?

Our eyes cannot see objects below a certain size. A curved piece of glass, thick in the middle and thin at the edge (shaped like a lentil, hence lens), makes things look bigger.

Activity 2.1: a round-bottom flask filled with water, placed on a book, makes the letters look larger: the water-filled flask acts like a magnifying glass.

• Robert Hooke (1665) published *Micrographia*. His microscope magnified 200 to 300 times. Looking at a thin slice of cork, he saw small empty compartments like a honeycomb and called each a cell, the first use of the word in science.
• Antonie van Leeuwenhoek (1660s) made better lenses and was the first to clearly see and describe bacteria and blood cells, so he is called the Father of Microbiology.
• School microscopes magnify about 100 to 400 times; low-cost foldable paper microscopes make this world accessible; an electron microscope magnifies about 10,00,000 times, showing structures inside cells.

Magnification: actual size $=$ size of image $÷$ magnification.

Worked example

Under a microscope that magnifies 400 times, the image of a cheek cell measures 24 mm across. What is the actual size of the cell?

Try it first, then show the solutionSolution

1. Actual size $=$ image size $÷$ magnification.
2. $24÷400=0.06$ mm.
3. So the cell is about 0.06 mm across, far too small to see with the naked eye.

Answer: About 0.06 mm.

Parts of a cell: onion peel and cheek cells

NCERT section 2.1 What Is a Cell?

Activity 2.2 (onion peel, teacher demonstration): peel the thin transparent layer from the inner surface of an onion piece with forceps; stain it with safranin (red) for about 30 seconds; rinse in water; place on a slide; add a drop of glycerin (stops drying, improves clarity); lower a coverslip with a needle at an angle so no air bubbles form; wipe extra glycerin with blotting paper; observe. The cells are nearly rectangular and packed closely, like bricks in a wall.

Activity 2.3 (cheek cells): gently scrape the inside of the cheek with the blunt end of a clean toothpick, spread in a drop of water, stain with methylene blue (blue), add glycerin after a minute, cover and observe. The cells are thin, flat and polygon-shaped.

The three basic parts of a cell:

• Cell membrane: the outer boundary; separates one cell from another; porous, letting needed materials in and wastes out.
• Cytoplasm: the jelly-like filling between membrane and nucleus, containing carbohydrates, proteins, fats and mineral salts; most life processes happen here.
• Nucleus: usually round, with its own membrane; controls all the cell's activities and its growth.

Stains increase contrast so the parts are easier to see.

Worked example

Give the reason for each step: (a) adding safranin to onion peel, (b) adding glycerin, (c) lowering the coverslip slowly with a needle.

Try it first, then show the solutionSolution

1. (a) The stain colours the cells, increasing contrast so their parts are visible.
2. (b) Glycerin prevents the cells from drying out and gives a clearer view.
3. (c) Lowering it slowly at an angle stops air bubbles from being trapped, which would block the view.

Answer: Stain for contrast; glycerin against drying; slow coverslip to avoid air bubbles.

Plant cells and animal cells

NCERT section 2.1 What Is a Cell?

Both plant and animal cells have a cell membrane, cytoplasm and nucleus. Plant cells have some extra parts:

• Cell wall: an outer layer around the cell membrane that gives rigidity and strength, so plant cells look firm and closely packed.
• Plastids: small rod-shaped bodies; chloroplasts contain the green pigment chlorophyll and carry out photosynthesis; in non-green parts plastids store substances.
• Large vacuole: a big, empty-looking space that stores substances, helps remove wastes and keeps the cell's shape firm.

Animal cells usually have no vacuole or only small ones, no cell wall and no chloroplasts, and their shapes vary more (cheek cells are flat and irregular, onion cells nearly rectangular). Both kinds also contain mitochondria and other parts you will study later. A cell is a complex, organised unit, not just a bag of liquid.

Worked example

A slide shows closely packed, nearly rectangular cells with thick outlines, each with a large clear space in the middle. Is it from a plant or an animal? Give two reasons.

Try it first, then show the solutionSolution

1. Thick outlines and regular, closely packed shape suggest a cell wall.
2. A large central vacuole is typical of plant cells.
3. So it is plant tissue.

Answer: Plant: it has a cell wall and a large vacuole.

Shape and function of cells

NCERT section 2.1.1 Variation in shape and structure of cells

Cells differ in shape, size and number, and shape suits function.

• Cheek cells: thin and flat; together they form a protective lining inside the mouth.
• Nerve cells (neurons): very long with branches; they carry messages quickly to distant parts of the body.
• Muscle cells: spindle-shaped, thin and flexible; in the food pipe they contract and relax in waves to push food to the stomach; in the stomach wall they churn food.
• Stomach lining cells: produce digestive juices and acid.
• Plant cells may be rectangular, elongated, oval or tube-like; some form long tubes that carry water through the plant.

So different cells work together in an organ: in the stomach, muscle cells and juice-producing cells together make digestion possible.

Worked example

Explain how the shape of a nerve cell and a muscle cell suits its job.

Try it first, then show the solutionSolution

1. Nerve cell: long and branched, so it can reach far and connect with many cells to pass messages quickly.
2. Muscle cell: spindle-shaped, thin and flexible, so it can contract and relax, for example to push food along the food pipe.

Answer: Long branches let nerve cells carry messages far; flexible spindle shape lets muscle cells contract and relax.

Levels of organisation

NCERT section 2.2 What Are the Levels of Organisation in the Body of a Living Organism?

Just as bricks build a wall, cells build living bodies, in steps:

cell → tissue → organ → organ system → organism

• Tissue: a group of similar cells doing a job (for example muscle tissue).
• Organ: different tissues working together (the stomach).
• Organ system: several organs doing a major function (the digestive system).
• Organism: all organ systems together (a plant, an animal, a human).

Complex organisms begin life as a single cell, the egg, which divides again and again. Organisms made of many cells are multicellular (plants, animals, humans).

Fun fact: the yolk of an ostrich egg is a single cell about 130 to 170 mm across, the largest known cell; the shell and white are non-cellular material that protect and nourish it.

Worked example

Arrange in order from simplest to most complex: digestive system, stomach, human, muscle cell, muscle tissue. Name the level of each.

Try it first, then show the solutionSolution

1. Muscle cell (cell) → muscle tissue (tissue) → stomach (organ).
2. → digestive system (organ system) → human (organism).

Answer: Muscle cell, muscle tissue, stomach, digestive system, human.

Microorganisms and where they live

NCERT section 2.3 What Are Microorganisms?

Microorganisms (microbes) are living things too small to see with the naked eye ('micro' means very small). Some are unicellular (bacteria, Amoeba, Paramecium, yeast); others are multicellular (some fungi such as moulds, and some algae).

Activities 2.4 and 2.5: drops of pond water and of the top layer of a soil suspension viewed under a microscope show tiny moving organisms. Activity 2.6 records them:

• Protozoa: Amoeba (single cell, irregular shape, moves); Paramecium (single cell, moves using special structures).
• Algae: green because of a pigment (chlorophyll); some single-celled and spherical.
• Fungi: bread mould and other moulds, branched threads without chlorophyll.
• Bacteria: spherical, rod, comma or spiral shapes, some with hair-like structures.

Microbes live everywhere: water, soil, air, on leaves, inside our gut, even in hot springs and icy places. Viruses are also microscopic but are not made of cells; they multiply only inside a living cell and may cause disease. Safety: collect pond water with an adult and handle soil with a spoon or gloves.

Worked example

Under a microscope a student sees: (a) a green, round single cell; (b) a single cell with an irregular, changing shape that moves; (c) branched threads without green colour growing on bread. Name the group of each.

Try it first, then show the solutionSolution

1. (a) Green pigment, single cell: an alga.
2. (b) Irregular, moving single cell: a protozoan (like Amoeba).
3. (c) Branched threads without chlorophyll on bread: a fungus (bread mould).

Answer: (a) alga (b) protozoan (c) fungus.

Microbes clean the environment

NCERT section 2.4.1 Key players in cleaning the environment

Activity 2.7: fill a container halfway with garden soil, add fruit and vegetable peels, cover with soil and wait 2 to 3 weeks. The peels turn into dark, nutrient-rich manure.

Soil microbes, mainly bacteria and fungi, break down complex substances in plant and animal waste (and dead bodies) into simpler, nutrient-rich ones: decomposition. The nutrients return to the soil and help plants grow, so microbes recycle materials and keep the surroundings clean. Decomposition works best at a suitable temperature and moisture.

• Biogas: some bacteria living without oxygen break down plant and animal waste or wastewater, releasing biogas, mostly methane with carbon dioxide; it is used for cooking, heating, electricity and even vehicles.
• Oil-spill cleaning: in 1971 Ananda Mohan Chakrabarty developed a bacterium that breaks down oil spills; it was patented in 1980.
• Heritage: Vedic texts, including the Atharvaveda, speak of *krimi*, tiny beings both visible (*drishya*) and invisible (*adrishya*), with good and harmful effects.

Pickles and murabbas resist spoiling because a high concentration of salt or sugar stops microbes from growing.

Worked example

Why do gardeners collect dry leaves and plant waste in pits? What would happen to fallen leaves if there were no microbes?

Try it first, then show the solutionSolution

1. In pits, soil bacteria and fungi decompose the waste into manure, which enriches the soil.
2. Without microbes, dead leaves and waste would pile up, and nutrients locked in them would not return to the soil, so plants would lack nutrients.

Answer: To make manure by decomposition; without microbes, waste would pile up and soil would lose nutrients.

Microbes and food: yeast, Lactobacillus and Rhizobium

NCERT section 2.4.2 Microorganisms and food

Yeast (a unicellular fungus), Activity 2.8: two bowls of 200 g flour with a pinch of sugar; yeast is added only to bowl A (bowl B is the control for comparison); both are kneaded with warm water, covered with a damp cloth and kept warm for 4 to 5 hours. Bowl A rises, becomes fluffy and smells different. Yeast grows well in warmth, feeds on sugar and respires, releasing carbon dioxide, whose bubbles make the dough soft and spongy, and a little alcohol, which gives the smell. It is used for bread, cakes and pastries.

Lactobacillus (a bacterium), Activity 2.9: lukewarm milk with a spoon of curd kept warm turns into curd in a few hours; cold milk kept in a refrigerator does not set. Lactobacillus feeds on milk sugar (lactose), multiplies and makes lactic acid, which makes curd sour. It grows well in warmth. Lactobacillus and other bacteria also ferment idli and dosa batter and bhatura dough.

Rhizobium lives in root nodules of legumes (beans, peas, lentils), trapping nitrogen from the air and making it usable by plants. That is why farmers grow legumes in rotation with other crops: the soil gains nitrogen without chemical fertilisers.

Gas test: carbon dioxide turns lime water milky, a way to confirm the gas from yeast.

Worked example

In the dough activity, why are sugar and warm water added, and why is one bowl kept without yeast?

Try it first, then show the solutionSolution

1. Sugar is food for the yeast; warm water gives the warm conditions in which yeast grows and respires well.
2. The bowl without yeast is the control: any difference between the bowls can then be due only to the yeast.

Answer: Sugar feeds yeast and warmth helps it grow; the bowl without yeast is a control for comparison.

Microalgae and the cell as the basic unit of life

NCERT section 2.5 Why Is Cell Considered to Be a Basic Unit of Life?

Microalgae are microscopic plant-like organisms in water, soil, air and on trees. They make food using sunlight, release oxygen (more than half of Earth's oxygen supply), and are food for many water animals. Spirulina, Chlorella and diatoms are used as supplements and medicines; Spirulina is more than 60% protein and provides vitamin $B_{12}$. Microalgae also help clean water and can make biofuel. Pollution, climate change and habitat loss threaten them, so they need conservation.

Why the cell is the basic unit of life: all organisms are made of one or more cells, and each cell has parts that carry out life functions.

• Unicellular organisms (bacteria, protozoa, yeast) do everything in one cell.
• Multicellular organisms (plants, animals, moulds) have specialised cells that cooperate.
• Fungal cells have a cell wall but no chloroplasts, so they cannot make their own food.
• Bacterial cells have a cell wall and membrane but no well-defined nucleus; their genetic material lies in a region called the nucleoid, without a nuclear membrane.

Worked example

List one feature shared by plant, animal and bacterial cells, one feature found only in bacterial cells, and one found in plant but not animal cells.

Try it first, then show the solutionSolution

1. Shared: cell membrane (and cytoplasm).
2. Only bacterial: nucleoid instead of a true nucleus.
3. Plant but not animal: cell wall, chloroplasts or a large vacuole.

Answer: Shared: cell membrane; bacteria only: nucleoid; plant only (vs animal): cell wall or chloroplasts.

Science in real life: The Invisible Living World: Beyond Our Naked Eye

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Cell Structure and Levels of Organisation
• Microorganisms: Types, Habitats and Useful Roles

The Invisible Living World: Beyond Our Naked Eye: formulas and key facts

Actual size from a microscope image

$\text{actual size}=\frac{\text{image size}}{\text{magnification}}$

Levels of organisation

$\text{cell}\to \text{tissue}\to \text{organ}\to \text{organ system}\to \text{organism}$

Basic parts of a cell

$\text{cell membrane}+\text{cytoplasm}+\text{nucleus}$

Yeast respiration (word equation)

$\text{sugar}\stackrel{\text{yeast}}{\to }\text{carbon dioxide}+\text{alcohol}+\text{energy}$

Curd formation (word equation)

$\text{lactose (milk sugar)}\stackrel{\text{Lactobacillus}}{\to }\text{lactic acid}$

Test for carbon dioxide

${\text{CO}}_2\text{ turns lime water milky}$

Magnification scale

$\text{school microscope }100\text{–}400\times ;\text{electron microscope }\approx 10,00,000\times$

The Invisible Living World: Beyond Our Naked Eye: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks The Invisible Living World: Beyond Our Naked Eye

The question patterns to expect, and the method that works for each.

Tricks to remember The Invisible Living World: Beyond Our Naked Eye

Short memory aids. Say each one aloud once, then use it on a question the same day.

• MCN in every cell

  Membrane, Cytoplasm, Nucleus: 'My Cell Notebook'.

  Use it when: The three basic parts.

• Plants add W-V-C

  Plant cells add a Wall, a big Vacuole and Chloroplasts.

  Use it when: Plant versus animal cells.

• Cells Tie Our Organs Systematically

  Cell, Tissue, Organ, Organ system, organiSm.

  Use it when: Levels of organisation.

• Yeast puffs, Lacto sours, Rhizo feeds

  Yeast gives CO₂ (dough rises), Lactobacillus gives lactic acid (curd sours), Rhizobium gives nitrogen (soil fed).

  Use it when: Microbes and food.

• Safranin for shoots, blue for cheeks

  Red safranin for onion (plant), methylene blue for cheek (animal).

  Use it when: Choosing stains.

The Invisible Living World: Beyond Our Naked Eye: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in The Invisible Living World: Beyond Our Naked Eye

The slips examiners see most often, and what to write instead.

• ✗Mistake: Drawing a cell wall in animal cells

  Plant cell features are copied onto animal cells.

  Wrong working

  Cheek cell drawn with a thick wall and a large vacuole.

  Right working

  Cheek cells have only a cell membrane, cytoplasm and a nucleus; no wall and no large vacuole.

  ✓Do this: Check the list of plant-only parts before labelling.

  Usually costs: 1-2 marks in diagram questions.

• ✗Mistake: Saying bacteria have a nucleus

  All cells are assumed to be alike.

  Wrong working

  A bacterial cell has a nucleus surrounded by a nuclear membrane.

  Right working

  Bacteria have a nucleoid region with no nuclear membrane.

  ✓Do this: Remember: no true nucleus in bacteria.

  Usually costs: 1 mark.

• ✗Mistake: Calling viruses microorganisms made of cells

  They are microscopic, so they are grouped with bacteria.

  Wrong working

  Viruses are unicellular microbes.

  Right working

  Viruses are acellular; they multiply only inside living cells.

  ✓Do this: Note the word 'acellular'.

  Usually costs: 1 mark.

• ✗Mistake: Explaining dough rising as 'yeast swells'

  The gas is forgotten.

  Wrong working

  Yeast absorbs water and swells, so the dough rises.

  Right working

  Yeast respires, releasing carbon dioxide bubbles that make the dough rise and become spongy.

  ✓Do this: Always mention carbon dioxide from respiration.

  Usually costs: 1-2 marks.

• ✗Mistake: No control in an experiment

  Only the treated set-up is described.

  Wrong working

  Put yeast in sugar solution and see if a balloon inflates.

  Right working

  Also set up an identical flask without yeast; only the yeast flask's balloon inflates.

  ✓Do this: Pair every test with a control that differs in one factor.

  Usually costs: 1 mark in design questions.

NCERT exercise guide: The Invisible Living World: Beyond Our Naked Eye

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on The Invisible Living World: Beyond Our Naked Eye

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Who is known as the Father of Microbiology?
   (a) Robert Hooke
   (b) Antonie van Leeuwenhoek
   (c) Ananda Mohan Chakrabarty
   (d) Louis Pasteur

   Hint

   He first described bacteria.

   Answer

   (b) Antonie van Leeuwenhoek

2. Q2 · 1 mark · MCQExam level

   Which of these is a unicellular fungus?
   (a) Bread mould
   (b) Yeast
   (c) Spirulina
   (d) Rhizobium

   Hint

   Used in baking.

   Answer

   (b) Yeast

3. Q3 · 1 mark · Very short answerExam level

   Name the gas that makes dough rise when yeast is added.

   Hint

   It turns lime water milky.

   Answer

   Carbon dioxide

4. Q4 · 1 mark · Very short answerExam level

   Why is a drop of glycerin added to a slide before placing the coverslip?

   Hint

   Think of drying.

   Answer

   It prevents the cells from drying out and makes them clearer to see.

5. Q5 · 2 marks · Very short answerHarder exam level

   How does the shape of a cheek cell suit its function?

   Hint

   Lining.

   Answer

   Cheek cells are thin and flat, so they fit closely together to form a protective lining inside the mouth.

6. Q6 · 2 marks · Very short answerHarder exam level

   An electron microscope magnifies about 10,00,000 times. If a structure appears 5 mm long, what is its actual length?

   Hint

   Divide.

   Answer

   $5÷1000000=0.000005$ mm

7. Q7 · 2 marks · Very short answerHarder exam level

   An ostrich egg yolk is about 150 mm across and a cheek cell about 0.06 mm. How many times wider is the yolk?

   Hint

   Divide the sizes.

   Answer

   $150÷0.06=2500$ times

8. Q8 · 3 marks · Short answerHarder exam level

   Give three reasons why microalgae are important.

   Hint

   Oxygen, food, uses.

   Answer

   They release oxygen (more than half of Earth's supply); they are food for many aquatic animals; some (Spirulina, Chlorella) are used as supplements and medicines (also: clean water, biofuel).

9. Q9 · 2 marks · Very short answerHarder exam level

   A student pulls out a gram (chana) plant and finds small round swellings on its roots. What are they, and why does a crop grown in that field next season need less nitrogen fertiliser?

   Hint

   Root nodules.

   Answer

   They are root nodules containing Rhizobium bacteria. Rhizobium traps nitrogen from the air and makes it available to the plant, leaving the soil richer in nitrogen for the next crop.

10. Q10 · 5 marks · Long answerHarder exam level

    Make a table comparing plant, animal and bacterial cells for: cell wall, cell membrane, nucleus, chloroplasts, large vacuole.

    Hint

    Five rows, three columns.

    Answer

    Cell wall: plant yes, animal no, bacterium yes. Cell membrane: all three yes. Nucleus: plant and animal yes; bacterium no (nucleoid). Chloroplasts: plant yes (green parts), animal no, bacterium no. Large vacuole: plant yes, animal no (small or none), bacterium no.

11. Q11 · 3 marks · Short answerHOTS / competency

    Why are stains used when observing cells, and why must air bubbles be avoided under the coverslip?

    Hint

    Contrast and clear view.

    Answer

    Most cells are nearly transparent; stains colour parts like the nucleus, increasing contrast so they can be seen. Air bubbles look like dark circles and hide the cells, so the coverslip is lowered slowly at an angle.

12. Q12 · 3 marks · Short answerHOTS / competency

    In a biogas plant, cow dung and kitchen waste are fed into a closed tank. Explain why the tank is closed, what gas is made, and what is done with the leftover slurry.

    Hint

    Oxygen-free microbes.

    Answer

    The bacteria that make biogas live without oxygen, so the tank is closed. They release biogas, mostly methane with some carbon dioxide, used as fuel. The leftover slurry is rich in nutrients and is used as manure.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Who first used the word 'cell' and in which year?Show answer

   Robert Hooke, in 1665 (Micrographia).

2. 2.Who is called the Father of Microbiology?Show answer

   Antonie van Leeuwenhoek.

3. 3.Name the three basic parts of a cell.Show answer

   Cell membrane, cytoplasm and nucleus.

4. 4.Which stain is used for onion peel and which for cheek cells?Show answer

   Safranin for onion peel; methylene blue for cheek cells.

5. 5.Give two parts found in plant cells but not animal cells.Show answer

   Cell wall and chloroplasts (also a large vacuole).

6. 6.State the levels of organisation.Show answer

   Cell, tissue, organ, organ system, organism.

7. 7.Name four groups of microorganisms.Show answer

   Bacteria, fungi, protozoa and algae.

8. 8.What gas makes dough rise?Show answer

   Carbon dioxide, released by yeast.

9. 9.Which bacterium turns milk into curd?Show answer

   Lactobacillus.

10. 10.What does Rhizobium do?Show answer

    It lives in root nodules of legumes and traps nitrogen from the air for the plant.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you explain why a water-filled flask magnifies?
• ☐Can you describe Hooke's and Leeuwenhoek's contributions?
• ☐Can you list the steps of making an onion peel slide with reasons?
• ☐Can you draw and label onion and cheek cells?
• ☐Can you give three differences between plant and animal cells?
• ☐Can you relate the shape of three cells to their function?
• ☐Can you arrange the levels of organisation with examples?
• ☐Can you name and describe protozoa, algae, fungi and bacteria seen under a microscope?
• ☐Can you explain decomposition, manure and biogas?
• ☐Can you explain the roles of yeast, Lactobacillus and Rhizobium?

The Invisible Living World: Beyond Our Naked Eye: frequently asked questions