🧬 Biology · Topic 8 of 10
Start with the study notes on Microorganisms (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
Microorganisms (microbes) are living things too small to see without a microscope. They live almost everywhere: in air, water and soil, inside our bodies, and even in hot springs and polar ice.
Fermentation is the process in which yeast turns sugar into alcohol and carbon dioxide. It was discovered by Louis Pasteur.
Example: A spoonful of curd added to warm milk adds Lactobacillus, which multiplies and sets the milk into curd.
Remember: Antibiotics do not work against viruses, such as colds and flu. Take them only when a doctor prescribes them, and finish the full course.
Microbes that cause disease are called pathogens. A disease that spreads from person to person is communicable.
Example: Plant diseases include citrus canker (bacteria), rust of wheat (fungus) and yellow vein mosaic of bhindi (virus).
Food spoilage happens when microbes grow on food, making it smell and taste bad and sometimes making it poisonous.
Food preservation stops or slows the growth of microbes:
| Misconception | Correct understanding |
|---|---|
| All microbes are harmful. | Many are useful: they make curd, bread and medicines, and enrich the soil. |
| Antibiotics cure colds and flu. | Colds and flu are caused by viruses; antibiotics don't work on them. |
| Ringworm is caused by a worm. | Ringworm is caused by a fungus. |
| Refrigeration kills microbes. | Cold only slows them down; food still spoils eventually. |
| All mosquitoes spread malaria. | Only the female Anopheles spreads malaria; Aedes spreads dengue. |
Which microbe turns milk into curd?
Lactobacillus bacteria.
Why does bread dough rise?
Yeast gives off carbon dioxide, which forms bubbles.
Who discovered penicillin?
Alexander Fleming.
Why are antibiotics useless against the common cold?
The cold is caused by a virus, and antibiotics don't work on viruses.
Which mosquito spreads malaria?
The female Anopheles mosquito.
How does a vaccine protect us?
It trains the body to make antibodies before the real germ attacks.
Why do salt and sugar preserve food?
They draw water out of microbes, so the microbes cannot grow.
Name a microbe that adds nitrogen to soil.
Rhizobium (or blue-green algae).
Key takeaway: Microbes are both friends and foes: they give us curd, bread, medicines and fertile soil, but they also cause disease and spoil food.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
Organisms too small to be seen with the naked eye are called:
Answer: A — Microorganisms (microbes)
Microorganisms can only be seen with a microscope. They live everywhere - in air, water, soil, inside our bodies and even in hot springs and polar ice.
Q2. easy
The main groups of microorganisms are:
Answer: A — Bacteria, fungi, protozoa, algae and viruses
These five groups differ greatly in structure. Viruses are placed with microbes although they reproduce only inside a living host cell.
Q3. medium
Which microorganism reproduces ONLY inside the cells of a host organism?
Answer: A — Virus
Viruses are inert outside a host. Once inside a living cell, they take over its machinery to make copies of themselves, usually destroying the cell.
Q4. medium
Which of these is a single-celled microorganism that moves using pseudopodia?
Answer: A — Amoeba
Amoeba is a protozoan. It changes shape constantly, extending pseudopodia to move and to engulf food particles.
Q6. medium
Which microorganism is used in the baking industry to make bread rise?
Answer: A — Yeast
Yeast ferments the sugars in dough, releasing carbon dioxide. The trapped bubbles make the dough rise and give bread its soft, spongy texture.
Q8. medium
Which scientist discovered the process of fermentation?
Answer: A — Louis Pasteur
Louis Pasteur discovered fermentation in 1857 and also developed pasteurisation, a method of preserving milk by heating and rapid cooling.
Q10. medium
Why is a spoonful of curd added to warm milk to set curd at home?
Answer: A — The curd contains Lactobacillus bacteria that multiply in the milk
The added curd acts as a starter culture, introducing the bacteria. Warmth helps them multiply rapidly and convert the whole quantity of milk into curd.
Q12. medium
Who discovered penicillin, the first antibiotic?
Answer: A — Alexander Fleming
In 1929, Fleming noticed that a mould contaminating a culture plate had killed the surrounding bacteria. That mould, Penicillium, yielded penicillin.
Q14. medium
Why should antibiotics be taken only on a doctor's advice and for the full course?
Answer: A — Misuse kills useful bacteria and makes harmful bacteria resistant
Stopping early leaves the strongest bacteria alive, and they multiply into resistant strains. Antibiotics also destroy the helpful bacteria in our intestines.
Q16. medium
Who discovered the vaccine for smallpox?
Answer: A — Edward Jenner
Edward Jenner developed the smallpox vaccine in 1798 after noticing that milkmaids who had caught cowpox did not get smallpox. Smallpox was declared eradicated in 1980.
Q18. medium
Children are vaccinated against which of these diseases?
Answer: A — Polio, measles, tuberculosis, tetanus and hepatitis
India's immunisation programme protects children from these serious diseases. Polio in particular has been almost eliminated in India through mass vaccination.
Q20. medium
An animal that carries disease-causing microbes from one person to another is called a:
Answer: A — Carrier (vector)
The female Anopheles mosquito carries the malaria parasite, the Aedes mosquito carries dengue, and houseflies carry germs from waste to our food.
Q22. medium
Malaria is spread by the bite of the:
Answer: A — Female Anopheles mosquito
Only the female mosquito bites, as she needs blood to develop her eggs. In doing so she injects the malarial parasite into the bloodstream.
Q24. medium
The best way to prevent mosquito-borne diseases is to:
Answer: A — Prevent water from collecting anywhere so mosquitoes cannot breed
Mosquitoes lay eggs in stagnant water. Emptying coolers, covering tanks, clearing puddles and using nets and repellents break the breeding cycle.
Q26. medium
Which of these diseases is caused by a VIRUS?
Answer: A — Measles, polio and chicken pox
Viral diseases spread easily and cannot be treated with antibiotics. Vaccination is the main defence against measles, polio and chicken pox.
Q28. medium
Which of these plant diseases is caused by microorganisms?
Answer: A — Citrus canker, rust of wheat and yellow vein mosaic of bhindi
Citrus canker is bacterial, wheat rust is fungal and yellow vein mosaic is viral. They reduce crop yields and spread through air, water, seeds and insects.
Q30. medium
Microorganisms that break down dead plants and animals are called:
Answer: A — Decomposers
Bacteria and fungi decompose dead matter, clearing waste and returning nutrients to the soil. Without them, dead material would pile up endlessly.
Q32. medium
Which microorganism is used to produce alcohol on a large scale?
Answer: A — Yeast
Yeast ferments the sugars in grains and fruit juices into alcohol and carbon dioxide. This is the basis of the brewing and wine industries.
Q34. medium
The process of heating milk to a high temperature and cooling it rapidly to kill microbes is called:
Answer: A — Pasteurisation
Milk is heated to about 70 °C for 15-30 seconds and chilled suddenly. This kills harmful microbes without spoiling the taste, so the milk keeps longer.
Q36. medium
Adding salt or sugar preserves food because:
Answer: A — It draws water out of microbes, so they cannot grow
High concentrations of salt or sugar remove water from microbial cells by osmosis. This is why pickles, jams and salted fish keep for months.
Q38. medium
Keeping food in a refrigerator preserves it because low temperature:
Answer: A — Slows down the growth and multiplication of microbes
Cold does not kill microbes but greatly slows their activity. That is why refrigerated food lasts days rather than hours, and frozen food lasts months.
Q40. medium
Nitrogen fixation by microorganisms is important because:
Answer: A — Plants cannot use atmospheric nitrogen directly and need it in a fixed form
Although 78% of air is nitrogen, plants can absorb it only as nitrates or ammonium compounds. Bacteria and lightning convert nitrogen gas into these usable forms.
Q42. medium
Which of these microorganisms can make its own food by photosynthesis?
Answer: A — Algae
Algae such as Chlamydomonas and Spirogyra contain chlorophyll and photosynthesise. They are important producers of oxygen and food in water bodies.
Q44. medium
Which of these is a harmful effect of microorganisms?
Answer: A — They cause diseases in humans, animals and plants, and spoil food
The same group that gives us antibiotics and curd also causes cholera, tuberculosis, crop diseases and food poisoning. Microbes are both helpful and harmful.
Q46. medium
Bread left in a warm, moist place grows a fuzzy growth. This is:
Answer: A — Fungus (bread mould)
Spores of Rhizopus present in air settle on the bread and, given warmth and moisture, germinate into a cottony mass with black spore-containing heads.
Q48. medium
Antibiotics are also mixed into animal feed and used on crops. A concern about this is that:
Answer: A — It can lead to antibiotic-resistant bacteria that are hard to treat
Widespread low-dose use gives bacteria constant exposure, letting resistant strains survive and spread. This is a serious worldwide public-health problem.
Q50. medium
Ringworm, a common skin infection, is caused by:
Answer: A — A fungus
Despite its name, ringworm is fungal, not a worm. It spreads by contact with infected people, animals or shared clothing and towels.
Q52. medium
Yeast is used in the preparation of idli and dosa batter along with bacteria because they:
Answer: A — Ferment the batter, producing gas that makes it soft and spongy
Microbes present naturally in the rice and dal ferment the batter overnight. Carbon dioxide bubbles make it rise, and the fermentation also improves the nutrition.
Q54. medium
Which disease spreads through the AIR when an infected person coughs or sneezes?
Answer: A — Tuberculosis, influenza and the common cold
Tiny droplets carrying germs are released into the air and inhaled by others nearby. Covering the mouth while coughing and wearing masks limits this spread.
Q56. medium
Which of these correctly pairs a microorganism with its use?
Answer: A — Yeast - baking and brewing; Lactobacillus - curd; Rhizobium - nitrogen fixation
Each useful microbe has a specific role. Notice that viruses have no such beneficial everyday use - they are almost entirely disease-causing.
Q58. medium
Which instrument is needed to see microorganisms?
Answer: A — A microscope
Microorganisms are far too small to see without help. A microscope magnifies them hundreds or thousands of times, which is how they were first discovered.
Q60. medium
Which of these is the smallest of all microorganisms and is not considered fully living outside a host?
Answer: A — A virus
Viruses are much smaller than bacteria and show no signs of life on their own. They reproduce only inside a living host cell, which is why they sit on the border of living and non-living.
Q5. hard
Which of these microorganisms is a FUNGUS?
Answer: A — Bread mould (Rhizopus)
Bread mould, yeast and mushrooms are fungi. They have no chlorophyll and feed on dead or living organic matter.
Q7. hard
The process by which yeast converts sugar into alcohol and carbon dioxide is called:
Answer: A — Fermentation
Fermentation is anaerobic respiration in yeast. It is used to make bread, idli and dosa batter, and alcoholic drinks. Louis Pasteur discovered it in 1857.
Q9. hard
Curd is formed from milk by the action of:
Answer: A — Lactobacillus bacteria, which convert lactose into lactic acid
Lactobacillus multiplies in warm milk and produces lactic acid, which curdles the milk protein and sets it into curd, giving the characteristic sour taste.
Q11. hard
Medicines that kill or stop the growth of disease-causing microorganisms are called:
Answer: A — Antibiotics
Antibiotics such as penicillin, streptomycin and tetracycline are made from bacteria and fungi. They work against bacterial infections, not against viruses.
Q13. hard
Antibiotics are NOT effective against:
Answer: A — Viral diseases like the common cold and flu
Viruses reproduce inside our own cells and lack the structures antibiotics target. Taking antibiotics for a cold is useless and encourages resistant bacteria.
Q15. hard
A preparation of dead or weakened microbes given to build immunity is called a:
Answer: A — Vaccine
The vaccine trains the body to produce antibodies without causing the disease. Those antibodies remain ready to fight the real germ in future.
Q17. hard
Which of these diseases has been completely ERADICATED worldwide through vaccination?
Answer: A — Smallpox
A worldwide vaccination drive wiped out smallpox, declared eradicated by the WHO in 1980 - the first disease ever eliminated by human effort.
Q19. hard
Diseases that spread from an infected person to a healthy person are called:
Answer: A — Communicable diseases
Communicable diseases spread through air, water, food, contact or carriers. Cholera, tuberculosis, measles and influenza are examples.
Q21. hard
Malaria is caused by a protozoan called:
Answer: A — Plasmodium
Plasmodium is carried by the female Anopheles mosquito. It multiplies in the liver and red blood cells, causing repeated cycles of fever and chills.
Q23. hard
Dengue is caused by a virus carried by the:
Answer: A — Aedes mosquito, which breeds in clean stagnant water
Aedes aegypti breeds in clean water collected in coolers, flowerpots and discarded tyres. Emptying such containers weekly is the best preventive measure.
Q25. hard
Which of these diseases is caused by BACTERIA?
Answer: A — Tuberculosis and typhoid
Tuberculosis, typhoid, cholera and tetanus are bacterial. Colds, flu, measles and polio are viral, while malaria and amoebic dysentery are caused by protozoa.
Q27. hard
Cholera and typhoid spread mainly through:
Answer: A — Contaminated water and food
These water-borne diseases spread where sewage contaminates drinking water. Boiling water, proper sanitation and hand washing prevent them.
Q29. hard
Which microorganisms fix atmospheric nitrogen in the root nodules of leguminous plants?
Answer: A — Rhizobium bacteria
Rhizobium lives symbiotically in the nodules, converting nitrogen gas into compounds the plant can use, and receiving food and shelter in return.
Q31. hard
The conversion of kitchen and farm waste into manure by microorganisms is called:
Answer: A — Composting
Microbes break down vegetable peels, leaves and dung into dark, nutrient-rich compost, an excellent natural fertiliser that also improves soil texture.
Q33. hard
Which microorganism is used to clean up oil spills and sewage?
Answer: A — Certain bacteria that break down oil and organic waste
Specific bacteria digest hydrocarbons and organic pollutants. They are used in sewage treatment plants and to help clean up oil spills - a technique called bioremediation.
Q35. hard
Food spoilage is caused by:
Answer: A — Microorganisms growing on the food and producing harmful substances
Bacteria and fungi grow on food in warm, moist conditions, changing its taste and smell and sometimes producing toxins that cause food poisoning.
Q37. hard
Which chemicals are commonly used as food preservatives?
Answer: A — Sodium benzoate and sodium metabisulphite
These approved preservatives stop microbial growth in jams, squashes and sauces. Salt, sugar, oil and vinegar are traditional preservatives.
Q39. hard
Oil and vinegar are added to pickles because they:
Answer: A — Prevent bacterial growth by keeping out air and creating acidic conditions
A layer of oil seals the pickle from air, and the acid in vinegar creates conditions in which most spoilage microbes cannot survive.
Q41. hard
In the nitrogen cycle, which bacteria convert nitrogen compounds back into nitrogen gas?
Answer: A — Denitrifying bacteria
Denitrifying bacteria in the soil return nitrogen to the atmosphere, completing the cycle and keeping the level of atmospheric nitrogen roughly constant.
Q43. hard
The green slimy growth seen in stagnant water is usually:
Answer: A — Algae
Algae multiply rapidly in still water rich in nutrients and sunlight, forming a green film or mat. Excessive growth can starve the water of oxygen.
Q45. hard
Which practice helps prevent the spread of communicable diseases?
Answer: A — Washing hands with soap, covering the mouth while coughing, and drinking clean water
Simple hygiene interrupts the route by which germs travel from person to person, and is often more effective than any medicine.
Q47. hard
Some microorganisms live in our intestines and are useful because they:
Answer: A — Help digestion and produce vitamin B and vitamin K
The gut flora aids digestion, makes certain vitamins and crowds out harmful bacteria. Overuse of antibiotics can disturb this useful population.
Q49. hard
Which disease is caused by a protozoan and spreads through contaminated water?
Answer: A — Amoebic dysentery
Entamoeba histolytica infects the intestines and causes dysentery with loose, blood-stained stools. Clean drinking water and sanitation prevent it.
Q51. hard
Which microorganism is used to produce citric acid and other chemicals industrially?
Answer: A — Certain fungi and bacteria
Industrial fermentation using selected microbes produces citric acid, acetic acid, vitamins, enzymes and many medicines cheaply and in large quantities.
Q53. hard
Which of these is an example of a microorganism that lives in extreme conditions?
Answer: A — Bacteria found in hot springs and in polar ice
Some bacteria survive in boiling hot springs, deep sea vents, salt lakes and polar ice - conditions in which no other life can exist.
Q55. hard
Boiling drinking water helps prevent disease because it:
Answer: A — Kills the bacteria, viruses and protozoa present in the water
Bringing water to a rolling boil for a few minutes destroys the microbes that cause cholera, typhoid, jaundice and dysentery - a simple, reliable safeguard.
Q57. hard
Which overall statement about microorganisms is CORRECT?
Answer: A — Microorganisms are both useful and harmful - they make food and medicines and enrich soil, but also cause disease and spoil food
This sums up the topic: microbes give us curd, bread, antibiotics, compost and soil nitrogen, yet the same group causes cholera, malaria, crop diseases and food spoilage.
Q59. hard
Blue-green algae (cyanobacteria) that grow in fields are useful because they:
Answer: A — Fix atmospheric nitrogen and act as a natural fertiliser, especially in paddy fields
Cyanobacteria such as Anabaena and Nostoc fix nitrogen directly. They enrich the soil of rice fields, reducing the need for chemical fertilisers.