Microorganisms and Viruses

The main groups of microorganisms

• Bacteria, single-celled organisms without a true nucleus, found almost everywhere.
• Fungi, organisms such as moulds and yeasts that absorb nutrients from organic matter.
• Protozoa, single-celled organisms with a true nucleus, such as Amoeba and Plasmodium.
• Algae, mostly aquatic organisms that photosynthesise, ranging from single cells to seaweeds.

Beneficial and harmful roles

Many microorganisms are beneficial. Decomposer bacteria and fungi break down dead organisms and waste, recycling nutrients back into the ecosystem.

Yeast is used in baking and brewing, and certain bacteria and fungi are used to produce antibiotics and fermented foods such as tempeh and soy sauce.

Other microorganisms are harmful. Pathogenic bacteria, fungi and protozoa can cause disease in humans, animals and plants, for example Salmonella (food poisoning), ringworm fungus, and Plasmodium (malaria).

Viruses: not quite a cell

A virus is a very small, non-cellular particle made up of genetic material (DNA or RNA) enclosed in a protein coat called a capsid. Unlike bacteria, fungi and protozoa, a virus has no cytoplasm, organelles, or cell membrane, and it cannot carry out any life process on its own.

A virus can only reproduce by infecting a living host cell and using the host's machinery to make copies of itself. This dependence on a host is why viruses are usually considered to be on the boundary between living and non-living things, and why viral diseases such as influenza, dengue and chickenpox cannot be treated with antibiotics, which act only on bacteria.

How this is examined

SPM questions commonly ask you to classify a described organism as a bacterium, fungus, protozoan, alga or virus, to give an example of a beneficial and a harmful microorganism, or to explain why a virus is not considered a complete cell.

Comparing a bacterium and a virus

Common misconceptions

Worked exam-style question

Question. Organism X is described as follows: it consists only of genetic material enclosed in a protein coat, it has no cytoplasm or organelles, and it can only increase in number after entering a living cell. (a) Identify the group to which organism X belongs.

(b) Give one reason for your answer. (c) Explain why an antibiotic would not be an effective treatment for an infection caused by organism X.

Model answer. (a) Organism X is a virus. (b) It has no cytoplasm, organelles or cell membrane, so it is not a complete cell, and it can only reproduce inside a living host cell.

(c) Antibiotics work by disrupting the life processes of bacterial cells, such as cell wall formation; since a virus has no cell structure or life processes of its own for the antibiotic to act on, antibiotics cannot destroy it.

Practice question

A student observes a single-celled organism under a microscope. It has a distinct nucleus, moves using extensions of its cytoplasm, and engulfs smaller food particles directly.

Identify the group this organism most likely belongs to, and justify your answer.

Exam tip

Key terms

• Pathogen, a microorganism that causes disease in its host.
• Decomposer, a bacterium or fungus that breaks down dead matter and recycles nutrients.
• Fermentation, the anaerobic breakdown of sugars by microorganisms such as yeast, used in baking and brewing.
• Classification, the arrangement of organisms, including microorganisms, into groups based on shared characteristics.

Source:SRC-DSKP-EN

Frequently asked questions

More for Biodiversity