Microorganisms Friend & Foe- 3 - Class 8 Science Free MCQ Test with solutions

Correct Answer:
b) Fermentation

Explanation:

What Happens When Yeast is Added to Fruit Juice:

• Yeast, which is a type of fungus (usually called Saccharomyces cerevisiae), feeds on the natural sugars found in fruit juice, like glucose and fructose.

• This process is called fermentation. During fermentation, yeast turns these sugars into alcohol (called ethanol) and carbon dioxide gas.

• Simply put, sugar changes into alcohol and gas through the work of yeast.

Chemical Process (in simple words):

• Sugar → Alcohol + Carbon Dioxide

• So, when fruit juice is left with yeast for a week, it slowly turns into something like wine or cider because alcohol gets formed.

Let’s Look at the Options:

• a) Decomposition:
  This means breaking down of things (like rotting fruit) by bacteria or fungi into simpler stuff. While fruit can rot over time, here the main thing happening is fermentation, not decay.

• b) Fermentation:
  This is the correct answer. Yeast is converting the sugar in fruit juice into alcohol and carbon dioxide — that’s fermentation.

• c) Distillation:
  This is a process where you heat something to separate its parts based on boiling points. It needs special tools and heating, which is not happening here. So, wrong answer.

• d) Oxidation:
  This is when something reacts with oxygen (like when a cut apple turns brown). While some oxidation might happen, the main process going on with yeast is fermentation, which actually happens without oxygen.

Final Answer: b) Fermentation

Viruses are quite different from other microorganisms. They reproduce only inside the host organism: bacterium, plant or animal cell.

The release of carbon dioxide gas causes dough to rise when yeast is added to it. (Fermentation)

- Microorganisms are incredibly versatile and can thrive in diverse environments.
- They are present in:
- Air, water, and soil: Microorganisms like bacteria, fungi, and viruses inhabit these mediums.
- Hot springs and deserts: Extremophiles are adapted to survive in extreme conditions.
- Inside the bodies of plants and animals: Many microorganisms exist symbiotically or as pathogens.
- Therefore, the correct answer is D: All of the above, as microorganisms inhabit all mentioned environments.

The correct answer is:

c) They are small organisms that cannot be seen without a microscope.

Explanation:

• Microbes (microorganisms) are defined by their microscopic size, requiring a microscope to be observed.
• While some microbes can reproduce independently (a), this is not unique to them (e.g., plants and animals also reproduce).
• They are not visible to the unaided eye (b), which is a key characteristic.
• Not all microbes are harmful (d); many are beneficial (e.g., gut bacteria, decomposers, or algae producing oxygen).

Thus, their defining feature is their small size.

• Certain soil bacteria play a crucial role in the nitrogen cycle.
• These bacteria convert nitrogen compounds, such as nitrates, into nitrogen gas (N₂) during the process known as denitrification.
• Nitrogen gas is an inert form and makes up about 78% of the Earth's atmosphere.