WBBSE Solutions For Class 8 School Science Chapter 7 The Microbial World Long Answer Type Questions

Chapter 7 The Microbial World Long Answer Type Questions


Question 1. What are microbes? Where they are found? What are the major classes of microbes?


The term microbe is short for microorganisms, which means small organisms visible only under microscopes. A microbe is any living thing that is too tiny to be seen with the naked eye.

Microbes are the oldest form of life on earth. They may live as individuals or cluster together in communities. Microbes live in the water you drink, the food you eat, and the air you breathe.

Right now, billions of microbes are swimming in your belly and mouth, and crawling on your skin Don’t worry; over 95% of microbes are good for you.

Microbes include bacteria, viruses, fungi, algae, and protozoa. These single-cell organisms are invisible to the eye, but they can be seen with microscopes.

WBBSE Solutions For Class 8 School Science Chapter 7 The Mircobial World Mircobes


Classification of Microbes

The term microbe is short for microorganism, which means small organisms. To help people understand the different types of microbes, they are grouped or classified in various ways.

Microbes are very diverse and represent all the great kingdoms of life. In fact, in terms of numbers, most of the diversity of life on earth is represented by microbes.

Here is an outline of the major groups of microorganisms:

  1. Viruses
  2. Bacteria (Monera)
  3. Algae (Plantae)
  4. Fungi
  5. Protozoa (Protista)

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A virus is acellular and composed basically of a tiny bundle of genetic material (either DNA or RNA) carried in a shell called the viral coat.

They have no boundary wall, cytoplasm, or nucleus. Thousands of different viruses, which come in many shapes, are parasites and disease-causing agents.

Viruses are found on or in just about every material and environment on Earth from soil to water to air. Viruses do not show most of the characteristics of living things outside the living host.

But if they come into contact with a suitable plant, animal, or bacterial cell, they spring into action. They infect and take over the cell like pirates hijacking a ship.

Viruses exist to reproduce only. To do that, they have to take over suitable host cells. The new viral genes then come together and assemble into whole new viruses.

The new viruses are either released from the host cell without destroying the cell or eventually build up to a large enough number that they burst the host cell.


WBBSE Solutions For Class 8 School Science Chapter 7 The Mircobial World virus




Question 2. What are bacteria? Classify them according to the structure.


Bacteria consist of only one cell, but they are a very complex group of living things. Unlike viruses, bacteria feed, move, and respire as well as reproduce on their own.


WBBSE Solutions For Class 8 School Science Chapter 7 The Mircobial World Shape of bacteria


Some bacteria can live in temperatures above the boiling point and in cold below the freezing point. There are thousands of species of bacteria.

Bacteria are classified into 5 groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios), or corkscrew (spirochaetes). They can exist as single cells, in pairs, chains, or clusters.

Some bacteria can make their own food from sunlight, just like plants. Also like plants, they give off oxygen. Other bacteria absorb food from the material they live on or in. They have no membrane-bound cell organelles.

A single teaspoon of soil contains more than a billion (1,000,000,000) bacteria. Bacteria reproduce by binary fission. In this process the bacterium, which is a single cell, divides into two identical daughter cells. Binary fission begins when the DNA of the bacterium divides into two (replicates).

Differences between bacteria and viruses:

Because bacteria and viruses cause many of the diseases we’re familiar with, people often confuse these two microbes. But viruses are entirely different from bacteria.

For one thing, they differ greatly in size. The biggest viruses are only as large as the tiniest bacteria. Another difference is their structure. Bacteria are complex compared to viruses.

A typical bacterium has a rigid cell wall and a thin, rubbery cell membrane surrounding the fluid, or cytoplasm, inside the cell.

A bacterium contains all of the genetic information needed to make copies of itself—its DNA—in a structure called a chromosome. In addition, it may have extra loose bits of DNA called plasmids floating in the cytoplasm.

Bacteria also have ribosomes, necessary for copying DNA so bacteria can reproduce. Some have threadlike structures called flagella that they use to move.


WBBSE Solutions For Class 8 School Science Chapter 7 The Mircobial World bacteriophage


Question 3. What are protozoa? Write about harmful protozoa. What is Achaea?


Protozoa are single-celled organisms having one or more nuclei. They come in many different shapes and sizes ranging from an Amoeba which can change its shape to its fixed shape and complex structure.

They live in a wide variety of moist habitats including freshwater, marine environments, and soil. They can live freely or in a colony.

Protozoa mainly feed on bacteria, but they also eat other protozoa, and sometimes fungi. Some protozoa absorb food through their cell tissues.


WBBSE Solutions For Class 8 School Science Chapter 7 The Mircobial World Protozoa


Others, surround food and engulf it. Others have openings similar to mouth pores into which they sweep food.
Protozoa can be classified into three general groups based on their shape. One group is the Ciliates, which are generally the largest protozoa. The second group is the Amoebae.

The nutrients from living or dead organic matter that they grow on. They absorb simple, easily dissolved nutrients, such as sugars, third group is the Flagellates, which are usually the smallest of the protozoa.

Most protozoa do us no harm. But there are a few that cause diseases. One type of amoeba can live in human intestines. It feeds on red blood cells and causes a disease known as dysentery.

Another species of protozoa can sicken hundreds of thousands of people when it gets into the tap water. Perhaps the best-known deadly protozoa cause malaria, a terrible disease that leads to about 800,000 deaths each year worldwide.


Question 4. What is parasitism? Write about parasitic microbes.


They derive their nutrition from the plants and animals on which they grow. Certain enzymes are produced by them which decompose or kill the protoplasm of the host cells.

The interrelationship between the parasitic microbe and the host is called parasitism. Such effects of the parasites on the host become visible to the naked eye as disease symptoms.

Many well-known diseases of human beings like typhoid, tetanus, tuberculosis, pneumonia, and many others are due to parasitic bacteria.

Microbes are also known as pathogenic bacteria. Some bacteria grow well only in the presence of oxygen, while others grow well in absence of oxygen. The former is known as aerobes and the latter are anaerobes.

Question 5. What are saprophytes? Write a note about saprophytic microbes.


They grow on dead and decaying plants and animals, dung,’ rotten wood, stagnant water, and many other decaying substances rich in organic matter.

Certain enzymes secreted by the bacteria decompose the complex organic substances of the substrate, converting them into simpler ammonium compounds.

They cause decay and therefore are also known as putrefying bacteria. The souring of milk, the manufacture of cheese, the preparation of butter from milk, and vinegar from sugarcane juice, are various processes completed by the action of certain specific saprophytic bacteria.

Zygomonas ferments glucose-producing alcohol, lactic acid, and carbon dioxide, and plays a significant role in the wine industry.

Acetobacter oxidizes organic compounds to organic acids such as lactic acid thus having a significant role in the vinegar industry. Lactobacillus converts sugars into lactic acid. Canned food is spoiled by some Bacillus and Clostridium bacteria.

Question 6. What is symbiosis? Explain the term with examples from microbes.


Symbiosis is the phenomenon by which two organisms maintain a relationship with each other to be mutually benefitted. In symbiotic mode, organisms develop a special relationship with certain other organisms to obtain nourishment.

Organisms involved in this type of relationship are called symbionts. Rhizobium bacteria is a striking example of this type.

They occur in root nodules of leguminous plants and help in fixing the free nitrogen of the atmosphere in the soil for the plants which in return provides carbohydrates and protection to the bacteria.

They are also called nitrogen-fixing bacteria and add to the fertility of the soil. Azotobacter and Clostridium are other examples of nitrogen-fixing bacteria.

They are found in chalky soil and obtain energy from the carbohydrates present in the soil. The energy so obtained is used in fixing atmospheric nitrogen into amino acids in the soil which react with the calcium salts, forming nitrites and thereafter nitrates.

Lichens form a very good example of a symbiotic relationship. Lichens are formed by the symbiotic relationship between algae and fungi sharing a single colony.

Fungus provides water, nutrients, and shelter in the form of the network formed by mycelium. Algae synthesize the food for the entire colony through the process of photosynthesis.

Question 7. Write about various microbial habitats with suitable examples.

Microbial Habitat:

Microbes live in almost every nook and corner you can think of, from 20 miles beneath the Earth’s surface to 20 miles overhead. They live at temperatures less than -20 degrees Celsius to temperatures hotter than the boiling point.

Microbes thrive on a huge range of food including oil and toxic wastes. Every time you walk on the ground you step on billions of microbes.

Microbes live in the soil, on rocks, inside roots, buried under miles of earth, in compost piles, and toxic waste all over the earth’s surface.

Microbes are found in boiling hot springs and on frozen snow fields. Most animals interact with microbes in important ways, and all animals, as well as all plants and fungi, depend on microbes for their survival.

Microbes live in their digestive systems, in their mouths, and on their skin. Microbes are important for the good health of animals.

Animals provide microbes with food and shelter. That is why animals are called “hosts” to microbes. For example, cows depend on microbes for their food.

Cows and other animals called ruminants have special stomachs called rumens which are host to billions of microbes that help these animals survive.

Also, we depend on microbes to clean up our environment. Without microbes to get rid of all the dung from animals, the world would be a really smelly place. Microbes make this world a cleaner place.

Question 8. What is lichen? Mention some economic importance of lichens.


A lichen is a composite organism that emerges from algae or cyanobacteria (or both) living among filaments of a fungus in a mutually beneficial (symbiotic) relationship.

The fungus benefits from the symbiotic relationship because algae or cyanobacteria produce food by photosynthesis. The algae or cyanobacteria benefit by being protected from the environment by the filaments of the fungus,

  1. which also gathers moisture and nutrients from the environment, and (usually) provides an anchor to it.
  2. Economic importance of Lichens:
  3. Lichens are used in the tanning of leather.
  4. Important dyes are prepared from lichens for use in woolen and silken clothes.
  5. Lichens like Evernia is used in cosmetics and perfumes.
  6. Lichens are indicators of pollution
  7. Lichens play important role in soil formation.

Question 9. Write the full form of DOTS. What is food poisoning? Mention the names of two bacteria that cause food poisoning. What are the symptoms of food poisoning?


DOTS stands for Directly Observed Treatment, short course.

The disease caused due to the presence of a large number of microorganisms (like bacteria and fungi) in the food or due to the presence of toxic substances in food formed by the action of microorganisms, is called food poisoning.

Bacteria Food poisoning
Clostridium botulinum Botulism
Salmonella Typhimurium Salmonellosis


Question 10. Mention two types of yeast that are used as food. Name two microbial vitamins along with their sources. Name a disease-causing microorganism in animals.

Two types of yeast that are used as food:

Yeast type Organism used Purpose
Baker’s Yeast Saccharomyces cerevisiae Used to leaven bread
Food Yeast Torulopsis utilize Used as a dietary supplement in deficient diets


Microbial vitamin Source
Vit. B12 Bacillus coagulans
Riboflavin Ashbya gossypii

Anthrax un animals is caused by bacillus anthracis

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