The Invisible Living World: Beyond Our Naked Eye · Lesson 4 of 9
Discovering Microorganisms Around Us
“Use water and soil observations to recognise the diversity of tiny living organisms.”
• Define microorganism and name habitats in which microbes are found. • Describe what pond-water and soil-suspension observations investigate. • Compare the forms of protozoa, algae, fungi and bacteria in the chapter tables. • Explain why viruses are distinct from cellular microorganisms.
Living things in ordinary places
A pond drop or a small sample of garden soil can look unremarkable. Under a microscope, each may reveal many tiny organisms. A microorganism, or microbe, is a living organism too small to be seen clearly with unaided eyes. Some, such as bacteria and Amoeba, consist of a single cell; some algae and fungi may consist of many cells. A low-cost foldable paper microscope can make this world more accessible, though it may show less detail than a powerful laboratory instrument.
An organism so small that a microscope is generally needed to see it.
Microbes occur in water, soil and air, on surfaces and in or on living bodies. Some live in hot springs or very cold regions; helpful bacteria also live in our gut. An apparently empty space does not prove an absence of microbes.
Investigating water and soil
For the pond-water observation, a teacher or elder helps collect water; a drop is placed on a slide beneath a coverslip. For the soil observation, use a spoon or gloves to collect moist soil, stir it into water, allow particles to settle, and place a drop from the upper layer on a slide. These are the chapter’s two investigations. The follow-up study compares observations and information in two tables. Record what you actually see before trying to give each form a name.
The textbook suggests magnification around 100–400 times for these observations. An individual slide may show only some of the listed organisms, and an image in a table is a comparison guide rather than proof that a particular sample contains it.
| Source | Illustrated organism | Features to notice |
|---|---|---|
| Pond water | Amoeba, protozoan | One cell; irregular, changing outline and movement |
| Pond water | Paramecium, protozoan | One cell; movement aided by specialised structures |
| Pond water | Green alga | Chlorophyll; movement with specialised structures in the illustrated example |
| Soil suspension | Bread mould and other moulds, fungi | Branched filaments; sac-like or brush-like features; no chlorophyll |
| Soil suspension | Alga | Spherical form and green chlorophyll in the example |
| Soil suspension | Bacteria | Spherical, comma-shaped, spiral or rod-like forms; some projections |
Problem
A student sees one irregular moving shape and another moving shape with a more consistent outline. Can both be microbes?
- 1.Small size and movement do not require the same outline.
- 2.The examples could resemble Amoeba and Paramecium; careful observation and further evidence are needed for identification.
Problem
Both samples are small. Why should they not be placed in the same category just by size?
- 1.The illustrated mould has branching filaments without chlorophyll.
- 2.The illustrated alga is green with chlorophyll. Structure and pigment provide more evidence than size alone.
The limits of a category
Bacteria, protozoa, fungi and some algae are groups represented by microscopic living organisms. Viruses are microscopic too, but they are acellular: they are not made of cells. They multiply only inside a living host cell and may infect plants, animals or bacteria. Calling every tiny thing a one-celled organism would therefore be incorrect. The chapter explores useful microbes in detail; disease-causing microbes are developed in the following chapter.
Not composed of cells.
Viruses are microscopic but acellular. Fungal mould can be multicellular, while yeast is unicellular. Do not equate “microbe” with one identical cell type.
Check your understanding
Classify organisms by evidence such as cell number, form and chlorophyll, while keeping observations separate from guesses about identity.
Quiz
Why is the upper layer of a settled soil suspension used for a slide?
Which listed pond organism has an irregular moving outline?
What characterises the mould drawings in the soil table?
What distinguishes viruses from the cellular microbes studied here?
What conclusion follows from finding different shapes in a soil drop?
Problem
A soil slide reveals several moving specks after larger particles settle. What can you conclude?
- 1.The specks are visible only under magnification and may be microorganisms.
- 2.Describe their shapes and movements first. Avoid claiming a species name without enough evidence.
Practice Problems
- Describe how you would prepare a slide from pond water and what you would record before naming an organism.
- Explain the purpose of allowing a soil suspension to settle.
- Compare Amoeba, Paramecium and an illustrated green alga using two observable features.
- Describe two differences between the mould and alga entries in the soil table.
- Why is it inaccurate to say every microscopic entity is a unicellular organism?
- Give three places besides a pond where microbes may occur.
Key Takeaways
• Microorganisms occur in water, soil, air and living bodies. • Microscope observations reveal differences in form, movement and pigment. • Bacteria, protozoa, some algae and fungi include microscopic organisms. • Viruses are acellular and multiply inside living cells.