Learn · Science · Class 7

Microorganisms & Decomposers

The smallest living things, and the essential job they do for every ecosystem.

10 parts
Scroll to begin ↓
📘 DefinitionWhat a microorganism isPart 1 of 10
🧩 The partsAre viruses alive?Part 2 of 10
🔍 In detailDecomposersPart 3 of 10
✏️ Worked exampleWhy decomposers matterPart 4 of 10
🌍 In the real worldMicroorganisms in a food webPart 5 of 10
🔍 In detailUseful microorganismsPart 6 of 10
🌍 In the real worldWhat conditions do they need?Part 7 of 10
⚠️ Common mistakesWhere marks goPart 8 of 10
✅ SummaryWhat to rememberPart 9 of 10
✏️ Worked exampleWatch one worked throughPart 10 of 10
1📘 DefinitionPart 1 / 10

What a microorganism is

A microorganism is a living thing too small to see without a microscope.

The main groups are bacteria, fungi (such as yeast and moulds), and protists. Viruses are usually studied alongside them, though whether they are truly alive is debated.

Check yourself: Which of these is a fungus?
2🧩 The partsPart 2 / 10

Are viruses alive?

Viruses are not made of cells and cannot reproduce on their own: they must take over a host cell.

They do not feed, grow or respire. That is why many scientists say they are not truly living, and it is a good example of a scientific definition being genuinely uncertain.

Check yourself: Why do many scientists say viruses are not alive?
3🔍 In detailPart 3 / 10

Decomposers

Decomposers (mainly bacteria and fungi) break down dead plants and animals and their waste.

They feed on the dead material, and in doing so release nutrients back into the soil for plants to absorb.

Check yourself: Which two groups are the main decomposers?
4✏️ Worked examplePart 4 / 10

Why decomposers matter

Without decomposers, dead material would pile up and the nutrients locked inside it would never return to the soil.

Plants would run out of the minerals they need, and every food chain would eventually collapse. Decomposers are the recycling system of the whole planet.

Check yourself: Without decomposers, plants would eventually…
5🌍 In the real worldPart 5 / 10

Microorganisms in a food web

Decomposers can be drawn into a food web with arrows pointing into them from dead material at every level.

Some microorganisms are also producers: algae in a pond photosynthesise and start their own food chains.

Check yourself: Algae in a pond act as…
6🔍 In detailPart 6 / 10

Useful microorganisms

Yeast makes bread rise and is used in brewing. Bacteria turn milk into yoghurt and cheese.

Some produce medicines, penicillin comes from a mould. Others are used to treat sewage. Only a small minority cause disease.

Check yourself: Penicillin was originally obtained from…
7🌍 In the real worldPart 7 / 10

What conditions do they need?

Microorganisms grow fastest in warmth, moisture and with a food supply.

That is exactly why we refrigerate food (cold slows growth), dry it, or salt it, all three remove one of the conditions they need.

Check yourself: Why does refrigeration preserve food?
8⚠️ Common mistakesPart 8 / 10

Where marks go

  • "All microorganisms are harmful." Most are harmless or essential.
  • "Decomposers are just insects." Insects help break material up, but the decomposers proper are bacteria and fungi.
  • "Refrigeration kills germs." It slows their growth; freezing usually only pauses it.
9✅ SummaryPart 9 / 10

What to remember

  • Bacteria, fungi and protists; viruses are a debated case.
  • Decomposers return nutrients to the soil.
  • Without them, nutrient cycles and food chains collapse.
  • They need warmth, moisture and food, which is how we preserve food.
10✏️ Worked examplePart 10 / 10

Watch one worked through

Here is one done slowly, with the reason for every move. Then one for you.

Watch one first

Bread kept in a sealed bag in a warm kitchen grows mould in three days. The same bread in the fridge does not. Explain.

  1. List what is the same in both: same bread, same bag, same mould spores in the air.
    bread same ✓ bag same ✓
    Anything identical in both cannot be the cause. Crossing those off is how any fair test is read.
  2. Find the one thing that differs.
    kitchen warm fridge cold
    One difference and one different result means that difference is the cause.
  3. Mould is a fungus, and like all microorganisms it grows faster when warm.
    warm → fast growth cold → slow growth
    Cold does not kill the mould, it only slows it down. That is why fridge food still goes off eventually.
  4. The sealed bag kept moisture in, which mould also needs.
    needs: warmth + moisture + food
    Removing any one of the three stops growth. That is the whole principle behind drying, salting and refrigerating food.

Answer: The warm kitchen gave the mould the warmth it needs to grow quickly; the fridge slowed it down

Now you try

Name one condition besides warmth that mould needs. One word.

Show the answer

moisture

Practise Microorganisms & Decomposers

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