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Microorganisms are sorted into five major groups: bacteria, viruses, protozoa, and some fungi and algae.
Bacteria
- Bacteria are tiny single-celled microbes.
- They have a cell wall.
- They have a nucleoid, not a nucleus.
- They are found everywhere.
Shapes of Bacteria
On the basis of shape there are mainly spherical bacteria, comma-shaped bacteria, and rod-shaped or spiral bacteria. Two common examples are Lactobacillus and Rhizobium.
Example 1 — Lactobacillus
- Lactobacillus is a common bacterium.
- It helps turn milk into curd.
- So bacteria can be useful to us.
Example 2 — Rhizobium
- Rhizobium lives in roots of pulse plants.
- It fixes nitrogen from the air.
- So it helps plants grow well.
Important Points
- Having a nucleoid makes bacteria prokaryotic cells.
- Lactobacillus (curd) and Rhizobium (roots) are useful examples.
- Trap: bacterial diseases are cured by antibiotics, not viral ones.
Definition — Bacteria
Very small single-celled microbes with a cell wall and a nucleoid in place of a true nucleus.
Viruses
- Viruses are the smallest microbes.
- They grow only inside a host.
- The host may be animal, plant or bacterium.
- Alone, they seem non-living.
Example 1 — Reproducing in a Host
- A virus cannot grow by itself outside.
- It enters a living cell and multiplies inside it.
- So viruses reproduce only inside a host cell.
Example 2 — Viral Diseases
- Common cold, flu, measles and chickenpox are caused by viruses.
- Polio and smallpox are also caused by viruses.
- So viruses can cause many diseases.
Important Points
- Cold, flu, measles, polio and chickenpox are viral diseases.
- Viruses are much smaller than bacteria.
- Trap: antibiotics work on bacteria, not on viruses.
Definition — Virus
The smallest microbe, which can grow and multiply only inside the cells of a host.
Protozoa
- Protozoa are single-celled microbes.
- They are animal-like.
- They live in ponds and damp soil.
- Examples: Amoeba, Paramecium, Plasmodium.
Example 1 — Amoeba
- Amoeba has an irregular shape.
- It moves with the help of pseudopodia.
- So Amoeba is a moving protozoan.
Example 2 — Paramecium
- Paramecium is found in pond water.
- It moves with the help of cilia.
- So protozoa use special structures to move.
Important Points
- Pseudopodia means "false feet" that Amoeba pushes out.
- Cilia are tiny hairs that beat to move Paramecium.
- Plasmodium, a protozoan, causes malaria.
Definition — Protozoa
Single-celled animal-like microbes such as Amoeba and Paramecium.
Algae
- Algae are simple plant-like organisms.
- They make food by photosynthesis.
- They lack proper roots, stems, leaves.
- Most are multicellular.
Example 1 — Chlamydomonas
- Chlamydomonas is a tiny alga.
- It has chlorophyll and makes its own food.
- So it is plant-like.
Example 2 — Spirogyra
- Spirogyra is a green thread-like alga.
- It floats in ponds.
- So algae are simple green organisms.
Important Points
- Algae release oxygen as they photosynthesise.
- Blue-green algae can also fix nitrogen in soil.
- Trap: algae have chlorophyll, but fungi do not.
Definition — Algae
Simple plant-like organisms that have chlorophyll and make food by photosynthesis.
Fungi
- Fungi are organisms with cell walls.
- They have no chlorophyll.
- They are made of threads called hyphae.
- Yeast and moulds are microscopic fungi.
Example 1 — Yeast
- Yeast is a tiny fungus.
- It is used to make bread rise.
- So some fungi are useful microbes.
Example 2 — Bread Mould
- Bread mould grows on stale bread.
- It is made of fine threads called hyphae.
- So moulds are thread-like fungi.
Important Points
- Fungi feed on dead matter or other living things.
- Yeast is the only single-celled fungus here.
- Penicillin, a medicine, comes from a fungus.
Definition — Fungi
Organisms with cell walls but no chlorophyll, made of fine threads called hyphae.
Objective Type Questions Q1(h) - Page 34 Lakhmir Singh
Question
Name the microorganisms which reproduce only inside the living cells of other organisms.
Show answer
Viruses.
Explanation
- Viruses multiply only inside a living host cell.
- Outside a host they behave as non-living.
Objective Type Questions Q1(i) - Page 34 Lakhmir Singh
Question
Name the simple plant-like microorganisms.
Show answer
Algae.
Explanation
- Algae have chlorophyll and make their own food.
- So they are plant-like microbes.
Objective Type Questions Q1(k) - Page 34 Lakhmir Singh
Question
Name the microorganism used in the baking industry.
Show answer
Yeast.
Explanation
- Yeast gives out carbon dioxide in dough.
- This makes bread soft and fluffy.
Objective Type Questions Q2(e) - Page 34 Lakhmir Singh
Question
Fill in the blank: Amoeba is a ______ organism.
Show answer
Unicellular.
Explanation
- Amoeba is made of just one cell.
- So it is a unicellular organism.
Objective Type Questions Q2(f) - Page 34 Lakhmir Singh
Question
Fill in the blank: ______ are unicellular and can prepare their own food.
Show answer
Algae.
Explanation
- Algae have chlorophyll.
- So they make their own food.
Subjective Type Questions - Short Answer Q9 - Page 35 Lakhmir Singh
Question
Identify the following microorganisms: (a) I live in every kind of environment, and inside your gut. (b) I make bread and cakes soft and fluffy. (c) I live in the roots of pulse crops and provide nutrients for their growth. (NCERT Book Question)
Show answer
(a) Bacteria (b) Yeast (c) Rhizobium.
Explanation
- Bacteria live everywhere, even inside our gut.
- Yeast makes bread fluffy with carbon dioxide.
- Rhizobium lives in pulse roots and fixes nitrogen.
Multiple Choice Questions Q13 - Page 37 Lakhmir Singh
Question
Small unicellular microorganism which has a nucleoid: (a) protozoa (b) bacteria (c) fungi (d) algae
Show answer
(b) bacteria.
Explanation
- Bacteria are single-celled microbes.
- They have a nucleoid instead of a true nucleus.
Multiple Choice Questions Q14 - Page 37 Lakhmir Singh
Question
The microorganism used in the bakery industry: (a) toadstool (b) yeast (c) mould (d) mushroom
Show answer
(b) yeast.
Explanation
- Yeast is used to make bread rise.
Multiple Choice Questions Q15 - Page 37 Lakhmir Singh
Question
A rich source of protein: (a) blue-green algae (b) diatoms (c) spirulina (d) dinoflagellates
Show answer
(c) spirulina.
Explanation
- Spirulina is a microalga rich in protein.
