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The Nature and Variety of Living Organisms

Characteristics of living organisms — Edexcel Linear - OL Revision Notes & Topical Questions

5 min read 14 parts Lesson 3 of 4 Unit quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Lesson summary

Features of Eukaryotes and Prokaryotes — EDEXCEL LINEAR - OL notes: Compare animals, plants, fungi, protoctists and prokaryotes by their cell structure, nutrition and carbohydrate storage, and identify pathogens and viruses.Living organisms are grouped by the features they share:…

  • saprobionts
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Worked example 5

Part 1 of 7Eukaryotes and prokaryotes

Living organisms are grouped by the features they share: plants, animals, fungi, protoctists and prokaryotes. The first four are eukaryotes: their cells contain a nucleus with a distinct membrane, and they may be single-celled or multicellular. Both animal and plant cells seen under a light microscope show a membrane-bound nucleus.

Prokaryotes (for example bacteria) are always single-celled, their cells do not contain a nucleus, and their genetic material lies free in the cytoplasm as a single, circular chromosome of DNA. Prokaryotic cells are also substantially smaller than eukaryotic cells.

Part 2 of 7Comparing animals, plants and fungi

Structures of animal and plant cells compared
Structures of animal and plant cells compared
FeatureAnimalsPlantsFungi
CellsMulticellularMulticellularUsually multicellular (Mucor); some single-celled (yeast)
NucleusYes, with membraneYes, with membraneYes, with membrane
Cell wallNoneCelluloseChitin
ChloroplastsNoYesNo
NutritionFeed on organic substances made by othersPhotosynthesisFeed on dead or living organic matter (saprotrophic)
Carbohydrate storeGlycogenStarch or sucroseGlycogen
Nervous coordinationUsually yesNoNo

Examples of fungi include moulds, mushrooms and yeasts. Multicellular fungi are made of thread-like hyphae that contain many nuclei and form a network called a mycelium.

Part 3 of 7Protoctists

The protoctists are a very diverse group with features that differ from the other three eukaryotic groups. Examples: Amoeba, Paramecium, Plasmodium and Chlorella.

Part 4 of 7Prokaryotes and their cell components

Bacterial cells showing cell wall and cytoplasm
Bacterial cells showing cell wall and cytoplasm

Bacteria have a cell wall, cell membrane, cytoplasm, plasmids and circular DNA. Most feed on other living or dead organisms. If they feed on dead organic matter they are known as saprobionts or decomposers. Compared with eukaryotic cells, a bacterium lacks a nucleus and membrane-bound organelles and has plasmids as well as its main chromosome.

Part 5 of 7Pathogens and viruses

A pathogen is an organism that causes disease in another organism. Groups that include pathogens are bacteria, fungi, protoctists and viruses. Pathogenic bacteria do not always infect the cells of hosts; they can remain within body cavities or spaces. Example: Pneumococcus, a spherical bacterium, causes pneumonia.

Viruses are small particles, always smaller than bacteria; they are parasitic and can only reproduce inside living cells; they infect every type of living organism; they have no cellular structure but a protein coat and contain one type of nucleic acid, either DNA or RNA.

Part 6 of 7Using the comparison in exam questions

Exam questions often give a list of features and ask you to identify the group, or give a table with blanks to complete.

1
Work through the features in a fixed order: (1) Is there a nucleus with a membrane?
2
If not, the organism is a prokaryote.
3
(2) Is the organism multicellular?
4
Animals and plants always are; fungi usually are; protoctists mostly are not.
5
(3) What is the cell wall made of: cellulose (plant), chitin (fungus), none (animal)?
6
(4) Does it have chloroplasts and photosynthesise (plant, and some protoctists such as Chlorella)?
7
(5) How are carbohydrates stored: glycogen (animals, fungi) or starch/sucrose (plants)?

For example, an organism with thread-like hyphae, a chitin wall and no chloroplasts is a fungus; a single-celled organism with a circular chromosome and no nucleus is a bacterium. Remember too that the size order from largest to smallest is: eukaryotic cells, then bacteria, then viruses, so a virus is never a cell and is always smaller than a bacterium.

Part 7 of 7Worked examples

  1. State two ways in which a plant cell differs from an animal cell. [2]2 marks

  2. Give two features that fungi and animals have in common and one that distinguishes them. [3]3 marks

  3. Describe the genetic material of a bacterial cell. [2]2 marks

  4. Explain why viruses are not classed as living cells. [2]2 marks

  5. Name the structure formed by the hyphae of a multicellular fungus. [1]1 mark

Key terms

TermMeaning
EukaryoteAn organism whose cells have a nucleus surrounded by a membrane.
ProkaryoteA single-celled organism whose cells have no nucleus; DNA is a circular chromosome in the cytoplasm.
HyphaeThread-like structures that make up the body of a multicellular fungus.
MyceliumThe network of hyphae of a fungus.
SaprobiontAn organism that feeds on dead organic matter; a decomposer.
PathogenAn organism that causes disease in another organism.
VirusA tiny non-cellular particle with a protein coat and DNA or RNA that reproduces only inside living cells.

Avoid these mistakes

1
Saying bacteria have a nucleus — their DNA lies in the cytoplasm.
2
Saying fungal cell walls are made of cellulose — they are made of chitin.
3
Saying all fungi are multicellular — yeast is single-celled.
4
Thinking all bacteria are pathogens — only some cause disease; many are saprobionts.
5
Calling viruses cells — they are non-cellular particles.

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