Class 8 Science: What Are Microorganisms? — Practice Questions with Answers

Explanation: Microorganisms are so tiny that our eyes alone cannot see them, so a microscope is used because it magnifies them many times. A telescope is used to view faraway objects such as stars, and a stethoscope is used to listen to heartbeats, so neither can help us see microbes.

Explanation: The main groups of microorganisms are bacteria, fungi, protozoa, algae and viruses. Insects such as ants and houseflies are small animals that can be seen with the naked eye, so they are not counted as microorganisms.

Explanation: Yeast is a type of fungus. It is used in baking because it releases carbon dioxide gas that makes the dough rise and become soft and spongy. It is not a bacterium, protozoan or alga.

Explanation: Curd is formed from milk with the help of bacteria such as Lactobacillus. These bacteria change the sugar in milk into acid, which turns the milk into curd. Algae, viruses and protozoa are not used to make curd.

Explanation: Viruses behave like non-living particles when they are outside a body, but once inside the cells of a host they reproduce like living things. Because of this double behaviour they are placed on the border between living and non-living. Bacteria, fungi and algae are clearly living organisms.

Explanation: Malaria is caused by a protozoan called Plasmodium, which spreads through the bite of an infected female Anopheles mosquito. It is not caused by bacteria, fungi or a virus, so protozoa is the correct group.

Explanation: Converting milk into curd is a helpful job done by microorganisms (bacteria). Causing food to rot, spreading malaria and causing the common cold are all harmful effects of microbes, so they are not useful roles.

Explanation: The fact that microbes can live even in extreme places like hot springs, ice and deserts shows that they are found almost everywhere on Earth. It is wrong to think they live only inside the human body, that they cannot live in water, or that they are all harmful.

Explanation: Algae contain chlorophyll, so like plants they can make their own food by photosynthesis, and large numbers of them can turn pond water green. Protozoa, fungi and viruses do not have chlorophyll and cannot make their own food this way.

Explanation: The fuzzy growth that appears on stale, damp bread is a mould, which is a type of fungus. Fungi grow well in warm, moist conditions and feed on the bread. Algae make their own food and grow in water, protozoa live in water, and viruses cannot grow on their own, so none of these fit.

Explanation: Viruses can multiply only when they are inside the living cells of a host such as a plant, animal or human; on their own they cannot reproduce. This is why they are placed on the border of living and non-living. Viruses are actually the smallest microbes and cannot be seen with a hand lens, and curd is set by bacteria, not viruses.

Explanation: A single-celled organism with no chlorophyll that moves in pond water and engulfs solid food particles is a protozoan, such as Amoeba. Algae are ruled out because they contain chlorophyll and make their own food, while bacteria absorb dissolved food rather than engulfing solid particles, and fungi do not swim about hunting food this way.