CG Board Class 8 Science Chapter 8 Solutions: Micro-organisms – A Wonderful World (सूक्ष्मजीव – एक अद्भुत संसार) – 2026-27

Write the name of the pigment present in green algae and also the name of the process by which they prepare food.

The pigment present in green algae is chlorophyll. The process by which they prepare their own food in the presence of sunlight is called photosynthesis.

Is the structure of all algae the same?

No, the structure of all algae is not the same. They vary in colour, shape, and structure; some are unicellular (e.g., Chlamydomonas, Chlorella), while others are multicellular and thread-like filaments (e.g., Spirogyra, Ulothrix).

Draw diagrams of the algae you see through the microscope and compare them to the ones given in your book. Point out the features for recognizing them.

• Identification Features of Common Algae:

1. Spirogyra: Long, green thread-like filaments containing ribbon-shaped, spiral (helical) chloroplasts.
2. Chlamydomonas: Microscopic pear-shaped unicellular algae with two flagella at the front end for movement.
3. Chlorella: Single-celled, non-motile spherical green algae.
4. Ulothrix: Unbranched cylindrical filaments made of cells joined end-to-end.

Why is it necessary to add yeast in the batter of idli, dosa and bread?

Yeast is added to the batter because it carries out fermentation. Yeast feeds on sugars present in the dough and releases carbon dioxide (CO₂) gas. This trapped gas causes the batter to swell and rise, making baked items like bread, idli, and dosa soft, porous, and spongy.

Why does the batter rise even when yeast is not added?

Even if commercial yeast is not added, naturally occurring wild yeast cells and bacteria present in the air and environment enter the open batter. Over time, these micro-organisms multiply and ferment the dough, producing carbon dioxide gas that inflates the batter.

In which test tube has the flour risen more? What may be the cause?

The flour ball in test tube ‘A’ (containing flour, yeast, and sugar) rose significantly more than in test tube ‘B’. The reason is that sugar provides readily available nutrients for yeast, accelerating fermentation and carbon dioxide production.

How has adding sugar to the flour affected the rising of the flour?

Adding sugar served as a quick energy source for yeast metabolism, increasing the rate of respiration and causing the dough to inflate much faster and higher.

Is any special food item needed for yeast to multiply?

Yes, yeast requires carbohydrates/sugars (such as glucose or sucrose) along with moisture and mild warmth as energy source for growth and multiplication.

From where did the air come to inflate flour ball?

The gas was carbon dioxide (CO₂), produced internally by living yeast cells during the breakdown (fermentation) of sugar.

Balloon of which test tube expanded the most? What may be the reason?

The balloon attached to the test tube containing water + yeast + sugar/jaggery expanded the most. This is due to rapid fermentation of sugar by yeast, generating a large volume of carbon dioxide gas.

Balloon of which test tube has expanded the least? What is the reason?

The balloon on the test tube with insufficient sugar or minimal yeast expanded the least because the rate of fermentation was very low due to limited nutrients.

Balloon of which test tube has not expanded at all and why is it so?

The test tube containing only sugar solution without any yeast did not expand at all because no micro-organisms were present to carry out fermentation and release gas.

What is the name of this process?

This process is called Fermentation.

Glucose (C_6H₁₂O₆) → [Yeast] 2 Ethanol (C_2H_5OH) + 2 Carbon Dioxide (CO₂↑) + Energy

If the water of the tank is not cleaned for a month, tell what all micro-organisms may be present in it?

Uncleaned tank water can host algae (e.g., Spirogyra, Chlorella), protozoa (e.g., Amoeba, Paramecium, Euglena), various bacteria, and fungal spores.

Why cannot we see the Amoeba and Paramecium with our naked eyes?

Human naked eyes cannot resolve objects smaller than 100 micrometres (μ m). Paramecium is about 100 micrometres and Amoeba is also microscopic and unicellular, making them invisible without a microscope.

Why do the leather shoes get moulds in rainy season?

Rainy weather provides high humidity (moisture) and warm temperatures. Airborne fungal spores land on organic leather shoes and germinate rapidly into visible moulds.

Why is it necessary to dry coriander, cumin seeds and pulses before storing them?

Moisture supports the growth of spoilage bacteria and fungal spores. Drying removes moisture, rendering the food item unfavourable for microbial growth and preventing spoilage during storage.

Usually algae are not parasites. Explain the reasons.

Algae contain chlorophyll pigment and prepare their own food via photosynthesis using sunlight, water, and carbon dioxide. Being autotrophic, they do not rely on host organisms for nourishment and are thus not parasites.

In which root do you find node-like structure?

Node-like structures (root nodules) are found in the roots of leguminous plants like pea, gram, and fenugreek, but not in non-leguminous plants.

In which bowl was the curd formed?

Curd was formed in the first bowl to which a small spoon of curd starter was added.

Why was curd formed?

The added curd starter introduced Lactobacillus bacteria into lukewarm milk. At around 37 deg C, these bacteria multiplied and converted milk sugar into acidic products (lactic acid), thickening the milk into curd.

Herbivorous animals can easily digest grass but humans cannot. Why? Give reasons.

Grass contains a complex carbohydrate called cellulose. Herbivorous animals have specialized symbiotic bacteria in their alimentary canal that secrete cellulose-digesting enzymes. Humans lack these bacteria and enzymes, so they cannot digest cellulose.

Fruits, milk, vegetables, egg, meat kept in the fridge do not get spoilt for a long time. Why?

Refrigerator temperatures are very low, which inhibits the growth, multiplication, and enzyme activity of bacteria and fungi, preventing food spoilage.

If we add a little curd to fresh milk, will it turn to curd?

Yes, provided the milk is lukewarm (around 37 deg C). The Lactobacillus bacteria will multiply and ferment the milk into curd.

Why do we boil milk before using it?

Boiling milk kills disease-causing bacteria and pathogens present in it, ensuring it is safe for consumption and extending its shelf life.

Due to which organism is malaria caused? Write about it.

Malaria is caused by a protozoan parasite called Plasmodium. It is transmitted from a diseased person to a healthy human through the bite of the female Anopheles mosquito vector.

How does leguminous plants make the soil more fertile?

The root nodules of leguminous plants host symbiotic Rhizobium bacteria. These bacteria convert atmospheric nitrogen gas (N₂) into usable nitrogenous compounds (nitrates) in the soil, boosting fertility naturally.

Why is virus known as bridge between living and non-living?

Viruses remain inert like non-living particles outside a living host (no respiration or metabolism). However, upon entering a living host cell, they rapidly reproduce and multiply like living organisms. Thus, scientists call them the bridge between living and non-living.

What is the full form of AIDS and HIV?

• AIDS: Acquired Immuno Deficiency Syndrome.
• HIV: Human Immunodeficiency Virus.

Your friend has cold and fever. On being together, you also get infected with cold. Why? Give reasons.

Common cold is a viral infection spread through air. Coughing or sneezing releases fine fluid droplets containing viruses into the air, which enter a healthy person’s body through inhalation.

You are suffering from cold and even though being together with you, some of your friends do not get infected and some do get infected. Why is it so?

Friends with a strong immune system have active white blood cells (WBCs) that destroy the invading viruses. Friends with weaker disease resistance power fail to kill the viruses and get infected.

Name a disease which has no cure but by taking precautions the disease can be avoided.

AIDS (caused by HIV) has no cure, but it can be prevented completely by adopting proper precautions and awareness.

Why are viruses called strange compared to other living organisms?

Viruses lack cellular organelles and act completely lifeless outside a host; yet, they take control of a host cell’s genetic machinery to replicate once inside.

(1) Things which cannot be seen by naked eyes are of the size:**

• Options: (a) Smaller than 1 micron (b) Smaller than 10 micron (c) Smaller than 100 micron (d) Smaller than 1000 micron

(c) Smaller than 100 micron

• (2) The first person to see micro-organisms by observing bacteria through the self made microscope was:
• Options: (a) Robert Hooke (b) Antonia Leewan Hawk (c) Edward Jenner (d) Louis Pasteur

(b) Antonia Leewan Hawk (Antonie van Leeuwenhoek)

• (3) The germs causing malaria is:
• Options: (a) Bacteria (b) Virus (c) Protozoa (d) Fungi

(c) Protozoa

• (4) The micro-organisms present in the roots of bengal gram is:
• Options: (a) Bacteria (b) Virus (c) Protozoa (d) Fungi

(a) Bacteria

• (5) Mushroom is a:
• Options: (a) Protozoa (b) Algae (c) Flower (d) Fungi

(d) Fungi

• (6) HIV/AIDS is caused due to:
• Options: (a) Bacteria (b) Virus (c) Protozoa (d) Fungi

(b) Virus

(1) How does boot polish protect leather shoes?**

Boot polish creates a smooth, wax-coated water-resistant layer over leather shoes. This layer keeps moisture out, preventing fungal spores from germinating and spoiling the leather.

• (2) Why does curd sometimes turn bitter?

Leaving curd at warm temperatures for too long causes excessive multiplication of Lactobacillus and wild bacteria, producing excess lactic acid and protein decomposition products that make it bitter.

• (3) Why must the milk be lukewarm when curd is to be made from it?

Lukewarm milk (around 37 deg C) provides the optimum temperature required for Lactobacillus bacteria to grow fast and set curd. Extremely cold or boiling milk prevents bacterial growth.

• (4) How does the leguminous plants increase the fertility of the soil?

Symbiotic Rhizobium bacteria residing in the root nodules of leguminous plants fix atmospheric nitrogen gas into soluble nitrates in the soil, enhancing soil fertility.

• (5) Why must we safeguard food items from micro-organisms?

Micro-organisms spoil food by decomposing it and producing toxic by-products, which cause food poisoning, vomiting, diarrhea, and diseases.

• (6) What are the mediums through which micro-organisms enter our body?

Pathogens enter our body through

1. Contaminated food and drinking water
2. Inhaled air containing cough droplets
3. Vector bites like mosquitoes and flies
4. Direct physical contact, infected blood, or skin wounds

• (7) Must we use the handkerchief used by a person suffering from cold and fever or not? Explain with reasons.

No, we must not use it because respiratory viruses and bacteria contaminate the handkerchief. Using it transfers viruses directly to our hands and respiratory passages, leading to infection.

• (8) What happens, when our body loses its resistance power?

When disease resistance drops, white blood cells fail to destroy pathogens. Micro-organisms multiply rapidly, causing disease symptoms like fever, weakness, and recurring infections.

• (9) Why is cleanliness necessary for being healthy?

Hygiene prevents the growth and spread of disease-causing bacteria, viruses, fungi, and insect vectors, keeping our body and surroundings free of infections.

• (10) What will happen if a small amount of curd is added to milk to prepare curd and then heated?

High heat kills the beneficial Lactobacillus bacteria present in the curd starter, preventing fermentation and failing to set the curd.

• (11) Why do the doctors advise us to drink boiled water?

Boiling kills all harmful disease-causing micro-organisms (bacteria, protozoa, viruses) present in water, protecting us from waterborne diseases like cholera and dysentery.

(1) With the help of your teachers, relatives, newspapers and magazines try to make a list of beneficial and harmful processes caused by micro-organisms.**

• Beneficial Processes:

1. Formation of curd, butter, and cheese from milk using Lactobacillus bacteria.
2. Fermentation of dough for bread, idli, dosa, and jalebi using Yeast.
3. Atmospheric nitrogen fixation by Rhizobium in legume roots.
4. Decomposition of dead organic matter into fertile manure.
5. Industrial manufacture of antibiotics, vaccines, alcohol, and vinegar.

• Harmful Processes:

1. Causing diseases in humans (e.g., cholera, tuberculosis, polio, malaria, AIDS).
2. Food spoilage and rotting of fruits, vegetables, and milk.
3. Destruction of leather, wood, and clothes by fungal decay.
4. Crop diseases in agriculture (e.g., Irish potato blight).