The Invisible Living World: Beyond Our Naked Eye — Class 8 MCQs with Answers

The Invisible Living World: Beyond Our Naked Eye — important questions & MCQs with answers (Class 8 Science)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Living things too small to be seen with the naked eye are called:

   AinsectsBlarge plantsCmicroorganismsDonly animals
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   Answer: microorganisms

   Explanation: Microorganisms are living things too small to see unaided — insects, large plants and animals are all visible without help, so tiny-yet-alive is the defining trait, not being non-living.

2. Q2 of 6Easy

   Which of these is a group of microorganisms?

   AmammalsBreptilesCbacteriaDbirds
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   Answer: bacteria

   Explanation: Bacteria form one of the main microorganism groups, alongside fungi, protozoa and algae — mammals, reptiles and birds are visible multicellular animals, not microbes.

3. Q3 of 6Medium

   In the yeast-and-sugar balloon experiment, the balloon inflates because yeast:

   Aheats the airBmakes water vapourCproduces a gas (carbon dioxide)Dreacts with the balloon
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   Answer: produces a gas (carbon dioxide)

   Explanation: Yeast ferments the sugar and releases carbon dioxide gas, which fills and inflates the balloon — it is not from heating the air or forming water vapour.

4. Q4 of 6Medium

   Food spoils faster in warm, moist conditions because microbes:

   Agrow and multiply quickly thereBdie immediatelyCstop movingDturn into manure
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   Answer: grow and multiply quickly there

   Explanation: Warmth and moisture are ideal growth conditions for microbes, so they multiply rapidly and spoil food faster — they do not die immediately or stop moving in such conditions.

5. Q5 of 6Hard

   Ranking a virus, a bacterium and a human by cell number, the human is:

   Asingle-celledBnon-livingCmulticellular, made of many cellsDa fungus
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   Answer: multicellular, made of many cells

   Explanation: A human body is built from trillions of cells and is multicellular, a bacterium is a single cell, and a virus is not even made of cells at all.

6. Q6 of 6Hard

   A pond turns green in summer. This is most likely due to a rapid growth of:

   AvirusesBcowsCsaltDalgae
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   Answer: algae

   Explanation: A green bloom on a pond's surface signals rapid growth of algae, which multiply quickly in warm, nutrient-rich water — not viruses, cows or salt.

The Invisible Living World: Beyond Our Naked Eye notes: key concepts (Class 8 Science)

This chapter opens the hidden world of microorganisms — living things too small to see without a microscope. You meet the main groups (bacteria, fungi, protozoa, algae and viruses) and learn which are single-celled and which are many-celled. Microbes are both friends and foes: they decompose waste into manure, fix nitrogen in legume roots (Rhizobium), and ferment food (yeast makes dough rise with carbon dioxide, Lactobacillus makes curd and idli batter), yet some spoil food or cause disease — which is why we preserve food with salt, sugar, drying and cold.

Microorganism

A living thing too small to be seen with the naked eye; a microscope is needed to view it. Microbes live almost everywhere — in soil, water, air and inside our bodies.

Unicellular vs multicellular

Unicellular organisms are made of a single cell (bacteria, protozoa, yeast); multicellular ones have many cells (moulds, algae, plants, animals).

Bacteria

Single-celled microbes that keep their genetic material in a nucleoid rather than a membrane-bound nucleus; some are helpful (curd, manure) and some are harmful.

Fungi

Microbes that can be single-celled, like yeast, or many-celled, like the moulds that grow on stale bread.

Protozoa and algae

Protozoa such as Amoeba and Paramecium are single-celled; algae are simple, often green microbes of ponds and water that can clean water and make biofuel.

Viruses

On the borderline of living and non-living: a virus can only multiply inside a living host cell and is inert outside it.

Decomposers and manure

Bacteria and fungi break down dead plants and animals into simple, nutrient-rich manure, returning nutrients to the soil and cleaning the environment.

Nitrogen fixation

Rhizobium bacteria live in the root nodules of legumes (peas, beans, lentils) and trap nitrogen from the air, increasing soil fertility.

Fermentation

Yeast ferments dough and releases carbon dioxide, making bread rise; Lactobacillus ferments milk into curd and batter for idli and dosa.

Biogas

When microbes decompose waste without air, they produce biogas, a mixture mainly of methane and carbon dioxide that can be used as a fuel.

Harmful microbes

Some microorganisms spoil food or cause diseases and infections, which spread through contaminated food, water and air.

Food preservation

Salting, adding sugar, drying and refrigeration all preserve food by making conditions too harsh for microbes to grow.