1.2 Five Kingdom classification – A* understanding for iGCSE Biology

The specification has a section called “Variety of Living Organisms”.  In this section, candidates are asked to learn about the features of the Five Kingdoms of living things and certain examples are mentioned.  This model of grouping organisms states that all living things can be allocated to one of these five groups:

  • Bacteria (Monera)
  • Animals
  • Plants
  • Fungi
  • Protoctists (Protista)

It is disappointing that Viruses are added as a sixth group in this section of the syllabus.  Viruses are not classified as a Kingdom of living things as they are not made of cells and have no metabolism.

Bacteria

Bacteria are small, single celled organisms that are made of a fundamentally different kind of cell to all the other Kingdoms.  Bacterial cells are described as being prokaryotic:  they are smaller than other cells, have no nucleus and no membrane-bound organelles (such as mitochondria or chloroplasts).  Bacteria cells have a cell wall containing a cell membrane but their cell wall does not contain any cellulose.  Instead the bacterial cell wall is made mostly of a molecule called proteoglycan a molecule is only found in bacterial cells.

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Bacteria cells contain DNA (all living things use this molecule as their genetic material) but the key idea is that bacterial DNA is not contained inside a nucleus.  Bacterial DNA is in the form of a single circular ring that just floats around in the cytoplasm of the cell.  This circular ring of DNA is sometimes called the bacterial chromosome (but I dislike this term as the DNA molecule in bacteria is not wrapped around a scaffold of protein as in eukaryote cells). Some bacteria contain small additional rings of DNA that are called plasmids.   These plasmids can be transferred from one bacterial cell to another, and can also be used as a vector in genetic engineering.

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Examples of bacteria mentioned in the specification are Lactobacillus bulgaris a rod–shaped bacterium used in the production of yoghurt and Pneumococcus, a spherical bacterium that is the pathogen that causes the infectious disease pneumonia.

Remember that some bacteria are autotrophic and can carry out photosynthesis but most feed by absorbing material through their cell walls.

Animals

Animals by definition are multicellular organisms.  Animal cells do not have a cell wall and do not contain chloroplasts and so cannot photosynthesise.  Animals are often able to move from place to place and have a nervous system.  Animal cells can store carbohydrate in liver and muscle cells in the form of a storage polysaccharide called glycogen.

The examples of animals mentioned in the specification are humans, housefly and mosquito.

Plants

The plant kingdom also contains organisms that are multicellular.  In contrast to animals, plant cells do photosynthesise and do contain chloroplasts.  Plant cells have a cell wall made of the polysaccharide cellulose.  Carbohydrates are stored in plant cells in the form of starch and are transported in the phloem as a sugar called sucrose.

The examples of plants mentioned in the specification are maize, peas and clover.  Maize is a wind-pollinated flowering plant and peas and clover are interesting because they are leguminous plants.  If you remember your work on nitrogen-cycle from E summer, you will know that leguminous plants contain root nodules that contain nitrogen-fixing bacteria.

Fungi

Fungi are a group of organisms that include moulds, mushrooms, toadstools and yeasts.  They are made of cells with a cell wall made of chitin and a nucleus.  Fungi do not photosynthesise and do not contain chloroplasts.  They feed by secreting digestive enzymes onto the food material they are living on and then absorbing the products of digestion:  a process called saprotrophic nutrition.  Fungi store carbohydrate in the form of glycogen.

Multicellular fungi such as Mucor are often organised into a mycelium, a mesh of thread-like structures called hyphae.  Each hypha is a structure containing many nuclei.  Some fungi such as the yeasts used in the brewing and baking industries are single-celled.

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Protoctists

This is the least interesting of the 5 Kingdoms (which is saying something…..)  Protoctists are all single celled organisms but unlike bacteria they are made of eukaryotic cells: cells with a nucleus and organelles like mitochondria and chloroplasts.  Some protoctists like Amoeba share many features with animal cells while others like Chlorella are more plant-like and contain chloroplasts to photosynthesise.  Some protoctists are pathogenic for example Plasmodium, the single celled organism that causes the disease Malaria.

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An Amoeba cell on the left and some Chlorella cells on the right.

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35 comments

  1. williamkidner

    Is there an overlap of species of fungi with plants which maybe photosynthesis and carry out the process saprotrophic nutrition and if so what is the species called?

  2. Robert Winter

    If evolution has proved that eukaryotic cells are more efficient and form the most advanced living organisms such as humans, why are there still prokaryotic cells, and will prokaryotic cells ever not exist

    • Paul Gillam

      This is a good question. Eukaryotic cells are more efficient in some ways, but the reason there are still prokaryotes is that in some environments, they are more suited. Bacteria can reproduce much faster than any eukaryote simply because they are structurally simpler…

  3. Harry Bardrer

    In a typical bacteria cell does the ring of DNA being circular have any advantages, or does each organism just have its own shape for its DNA?

    • Paul Gillam

      I’m not sure what advantages being circular might have for DNA… Perhaps it helps in replicating the molecule as there is no beginning or end of the molecule? Good question!

  4. Cary Godsal

    Can some single celled fungi such as yeasts also be protoctists, as they are both single celled eukaryotic cells?

    • Paul Gillam

      Good question – no single celled fungi are not classified as protoctists, simply because they have a cell wall made of chitin and feed saprotrophically…..

  5. Morrison Cleaver

    Since viruses are not living organisms and are not made of cells as specified above, how can their shape change making new diseases such as ebola?

    • Paul Gillam

      Well their genetic material can mutate (change) when they are inside another cell. This allows the proteins that make up the outside of the virion change shape. Viruses are products of natural selection even though they are not alive….. Good question!

  6. Sebastian Stanga

    How did Eukaryotic cells evolve to have organelles in their cell? Is it to do with mutations in the DNA which caused the organelles to form?

    • Paul Gillam

      Excellent question – this is a big question in the history of life. How did the first eukaryote cell evolve? We know that it happened by what is called the Endosymbiont theory – this means that one bacterium engulfed one or two other bacteria to form the first cell with organelles. Organelles such as mitochondria, chloroplasts and the nucleus are descended from free-living bacteria.

  7. Arnav Jain

    What do fungi feed on and how if they have to first secrete their enzymes onto it and then absorb it, which takes time?

    • Paul Gillam

      They feed on whatever dead matter they happen to land on… The fungi grow through the dead matter releasing enzymes and absorbing the products of digestion. This is quite a slow process but they are not going anywhere……

  8. Tatham

    Howcome bacteria have remained simple whereas animals for instance have decided to become much more complex, even though we have had the same time to evolve. What triggers the change?

    • Paul Gillam

      Good question – the answer is that bacteria are still bacteria because there is a perfectly good way of existing as a bacteria. Animals didn’t decide to evolve, it is just that in a world without multicellular, motile heterotrophs, there is a way of living that works that can be filled…. Hence animals!

    • Paul Gillam

      Yes I guess that is true except that protoctists is only a miscellaneous kingdom for unicellular eukaryote organisms. Hyphae are structural components of fungi involved in feeding. Their long thin filament-like shape is an adaptation to increase the sa/vol ratio.

    • Paul Gillam

      Well they are important in the Nitrogen cycle that you will learn about in E block. They help leguminous plants by giving them a ready supply of nitrate ions which the plant can use to make amino acids and proteins. This will make more sense after you have studied Nitrogen cycle and Biological molecules in E block. Good question though!

  9. victor hope

    sir, if plants are classified as autotrophic organisms, would something like a venus fly trap be counted as heterotrophic as it too is a plant, but it is not entirely slef-sufficient?

  10. Tom Dale

    What kingdom do cancer cells/stem cells fall into? They are living organisms but do they fit into one of the categories above?

  11. Henry Pearson

    Certain Bacteria form colonies and produce gametes like a multi-cellular organism. Could these colonies be counted as animals?

  12. James Xu

    If a eukaryote cell multiplies slower than a prokaryote cell, surely that means bacterial diseases are more efficient than protistan diseases? If not, what makes a protistan disease more ‘lethal’?

  13. Matt

    How did prokaryotes like bacterium evolve to form the eukaryotes that make up most of the world today. Was there something that triggered the evolution or not?

  14. Vin

    As they seem to be structurally similar, did prot(oct)ists and then fungi evolve from bacteria/monera, and if so, is there any chance that the kingdom ‘bacteria’ will cease to exist by mutating/evolving into prot(oct)ists? How come prot(oct)ists and bacteria seem quite similar yet are also different?

  15. Cameron Thurlow

    What is so special about proteoglycan that means that it was not carried on through evolution to eukaryotic cells/beyond? What purpose does it play in particular for the bacterium?

  16. Harry Lyons

    Do all fungi reproduce asexually and do the fungus’s spores just die if they don’t land on favourable conditions for growth and germination

  17. Armel

    With so many undiscovered species, how can scientists be sure that there are only five main groups in the animal kingdom?

  18. Aera

    I love your site! This helped me so much when it comes to understanding the topics. Thank you so much!

  19. Aera

    I searched the internet about the difference of saprophytic means and parasitic means but I still don’t understand D: Is it okay with you to clarify the difference of the two for me? Again, thank you for your wonderful webite, sir!

    • Paul Gillam

      A saprophyte (better term now is saprotroph) is an organism that gets its energy from breaking down dead or decaying matter. All fungi feed as saprotrophs by secreting digestive enzymes into the material they are growing through, and then absorbing the products of digestion. Some bacteria (called decomposers) are also saprotrophs.

      A parasite gets its energy not from dead or decaying material but directly from another living organism. Parasites often live inside another organism (e.g. Plasmodium parasite that causes malaria) or on the surface of an organism (e.g.a tick that feeds on blood).

      Hope this helps!

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