A Virus Cannot Do Anything Until It Gets Inside a Cell

Because it has no cell of its own. A virus has no cell membrane enclosing cytoplasm, no nucleus and no way of feeding, growing or reproducing by itself — it can only multiply inside a living host cell.

Outside a host it does nothing at all, which is why it sits apart from the four groups of microorganisms. This page covers everything in the CBSE Class 8 Science chapter's second part: classifying microorganisms, their role in the environment, fermentation and spoilage, and the cell as the unit of life.

Microorganisms are living things too small to be seen without a microscope. Four groups are cellular organisms, and viruses form a separate case.

- Bacteria — single-celled, with no proper nucleus, in shapes described as rod, spiral, comma and spherical. They live almost everywhere, including soil, water, air and inside our bodies. Examples: Lactobacillus, Rhizobium.
- Fungi — non-green, with no chlorophyll, so they cannot make their own food and absorb it ready-made instead. Some are single-celled and some multicellular. Examples: yeast, bread mould (Rhizopus), mushroom.
- Protozoa — single-celled, animal-like, usually able to move. Examples: amoeba, paramecium, Plasmodium.
- Algae — green, containing chlorophyll, so they photosynthesise like plants. Mostly aquatic. Examples: Spirogyra, Chlamydomonas.

Viruses are different. They are much smaller still, have no cell of their own, and reproduce only inside a host cell. Outside one they show no sign of life, which is why they are described as being on the borderline between living and non-living.

The green film on a damp monsoon wall is algae; bread going mouldy in a humid kitchen is fungi.

The distinction that separates the groups is how they get food. Algae are green and make their own; fungi are non-green and absorb ready-made food. That single test decides which group an unfamiliar microbe belongs to more reliably than its size or shape.

By acting as decomposers — breaking down dead plants, dead animals and waste into simpler substances that return to the soil.

Without them, dead leaves and animal remains would pile up unchanged, and the nutrients locked inside would never be released. Decomposers turn that material into humus, the dark matter that makes soil fertile.

The process runs in any compost pit: kitchen peelings and leaves are broken down by bacteria and fungi over weeks into crumbly manure, which is then dug back into a field or a garden.

Microorganisms also return nitrogen to the soil, and two kinds do it:

- Nitrogen-fixing bacteria, such as Rhizobium, live in the root nodules of leguminous plants such as gram, peas, beans and groundnut. They convert nitrogen from the air into compounds the plant can absorb.
- Decomposing bacteria release nitrogen compounds from dead matter back into the soil.

This is why farmers grow a legume such as gram between two cereal crops — the root nodules leave the field richer in nitrogen than they found it, with no fertiliser added.

The misconception worth clearing is that plants take nitrogen straight from the air. They cannot: nitrogen gas is far too unreactive for a plant to use, so it must first be fixed into compounds absorbed through the roots. That fixing is work only microorganisms can do.

Both use fermentation — the breakdown of sugars by microorganisms without oxygen, releasing products and a little energy.

Curd from milk. A spoon of existing curd added to warm milk introduces the bacterium Lactobacillus. It converts the milk sugar into lactic acid, which thickens the milk and gives curd its sour taste. Warmth matters, which is why curd sets faster in summer than in winter and is often left in a covered vessel overnight.

Bread and bhature. Yeast added to dough ferments the sugars into carbon dioxide and alcohol. The gas collects as thousands of tiny bubbles in the stretchy dough, making it rise and giving baked bread its soft, holey texture. Baking then drives off the alcohol.

Idli and dosa batter works the same way, left to ferment overnight before cooking.

But not all microbial activity is welcome:

- Food spoilage — bacteria and fungi growing on food make it unfit to eat. Bread grows mould, cooked food left out turns sour, and pickles spoil if moisture gets in.
- Disease-causing microbes — bacteria cause typhoid and tuberculosis, protozoa cause malaria and amoebic dysentery, viruses cause the common cold, influenza and measles, and fungi cause ringworm.

Preservation works by denying microbes what they need: salting and sugaring remove water, refrigeration slows them down, boiling and drying kill or starve them.

So the same process is desirable or undesirable depending on what you wanted. Milk turning to curd is welcome; milk turning sour on its own is not — and the chemistry in both cases is bacteria producing acid.

Because every living thing is made of cells, and a cell is the smallest unit that can carry out all the processes of life on its own.

The evidence comes from both ends of the size scale.

From unicellular organisms. An amoeba, a bacterium or a yeast cell is a single cell, and it still performs every life process — it takes in food, respires, grows, removes wastes, responds to its surroundings and reproduces. So one cell is genuinely enough to be alive, which makes it the basic functional unit.

From multicellular organisms. A human being, a mango tree and a mushroom are all built entirely from cells, with no living material outside them. Cells group into tissues, tissues into organs and organs into systems — so the cell is the basic structural unit too.

From growth and repair. Living things grow because their cells divide and increase in number, and a wound heals the same way. Every new cell comes from an existing cell.

From viruses. The contrast completes the argument. A virus has no cell of its own and can do nothing until it enters one — which shows that being alive, in the ordinary sense, means having a cell.

That is the reasoning an answer should reproduce: structural because everything is built from cells, and functional because a single cell can perform all of life's processes by itself.