Characteristics of Living OrganismsEdexcel IGCSE Biology: Revision notes
Section 1
What are the eight characteristics of living organisms?
All living organisms share eight characteristics. A useful mnemonic is MRS GREN:
- Movement — an action by an organism causing a change of position
- Respiration — chemical reactions that release energy from food
- Sensitivity (response to surroundings) — detecting and responding to stimuli
- Growth — a permanent increase in size
- Reproduction — producing offspring
- Excretion — removal of waste products of metabolism
- Nutrition — obtaining food to provide energy and materials for growth
- Control of internal conditions (homeostasis)
MRS GREN is a well-known mnemonic, but make sure you can also state each characteristic in full biological terms in an exam answer.
Section 2
What are eukaryotic organisms?
Eukaryotic organisms have cells containing a nucleus and membrane-bound organelles. The four eukaryotic kingdoms are:
- Plants — e.g. a flowering plant such as a rose; make food by photosynthesis
- Animals — e.g. a human or a dog; obtain food by feeding on other organisms
- Fungi — e.g. mushrooms or yeast; feed by extracellular digestion (saprotrophic nutrition)
- Protoctists — e.g. Amoeba or algae; mostly single-celled, with varied features
Section 3
What are prokaryotic organisms?
Prokaryotic organisms, such as bacteria, have simpler cells with no true nucleus. They contain:
- Cell wall
- Cell membrane
- Cytoplasm
- Plasmids (small circles of extra DNA)
- A single circular chromosome of DNA, which is not enclosed in a nucleus
Bacteria do have a cell wall — but it is chemically different from a plant cell wall (bacterial walls are not made of cellulose).
Section 4
What are pathogens?
A pathogen is a micro-organism that causes disease. Pathogens may be fungi, bacteria, protoctists or viruses.
Section 5
What are viruses, and why are they not classed as living?
Viruses are extremely small infectious particles, much smaller than bacteria. A virus consists of genetic material (DNA or RNA) surrounded by a protein coat. Viruses are not considered living organisms because they cannot carry out most life processes independently — in particular, they can only reproduce inside a living host cell, using the host's machinery.
The influenza virus injects its genetic material into a host cell, hijacking the cell's machinery to make copies of itself.
Must Know
- The eight characteristics of living organisms: nutrition, respiration, excretion, response to surroundings (sensitivity), movement, control of internal conditions, reproduction, growth and development (MRS GREN)
- Eukaryotic organisms (nucleus present) include plants, animals, fungi and protoctists
- Prokaryotic organisms (e.g. bacteria) have a cell wall, cell membrane, cytoplasm, plasmids and a circular chromosome, but no nucleus
- A pathogen is a disease-causing micro-organism: fungus, bacterium, protoctist or virus
- Viruses are not living organisms — they can only reproduce inside a host cell
That's the notes covered.
Carry on to the next subtopic.