Characteristics of Living Organisms Notes

Edexcel IGCSE Biology: Revision notes

Key facts

  • Living organisms show movement, respiration, sensitivity, growth, reproduction, excretion and nutrition (MRS GREN).
  • Eukaryotes (plants, animals, fungi, protoctists) have a nucleus; prokaryotes (bacteria) do not.
  • A pathogen is a micro-organism that causes disease.
  • Viruses are not living: they can only reproduce inside a host cell.

MRS GREN

All living organisms show seven characteristics, remembered as MRS GREN.

Learn each in full biological terms. Movement: an action by an organism or part of it causing a change of position or place. Respiration: chemical reactions that release energy from food. Sensitivity: detecting and responding to stimuli. Growth: a permanent increase in size. Reproduction: producing offspring. Excretion: removing waste products of metabolism. Nutrition: taking in food for energy and materials.

  1. 1

    Movement

    change of position

  2. 2

    Respiration

    releases energy from food

  3. 3

    Sensitivity

    responds to stimuli

  4. 4

    Growth

    permanent increase in size

  5. 5

    Reproduction

    produces offspring

  6. 6

    Excretion

    removes metabolic waste

  7. 7

    Nutrition

    takes in food

MRS GREN: the characteristics of living organisms

Removing the waste products of metabolism is called...

Eukaryotes

Eukaryotes have cells with a nucleus and membrane-bound organelles. There are four kingdoms.

Plants make food by photosynthesis. Animals feed on other organisms. Fungi feed by extracellular digestion (saprotrophic nutrition). Protoctists are mostly single-celled with varied features.

MitochondrionCell wallVacuoleCell membraneBudNucleusCytoplasm
Yeast is a fungus: its nucleus and membrane-bound organelles make it a eukaryote

Plants

Example:
A rose
Key feature:
Photosynthesis

Animals

Example:
A human or dog
Key feature:
Feed on other organisms

Fungi

Example:
Mushrooms, yeast
Key feature:
Extracellular digestion

Protoctists

Example:
Amoeba, algae
Key feature:
Mostly single-celled

Which kingdom feeds by extracellular digestion?

Prokaryotes

Prokaryotes such as bacteria have simpler cells with no true nucleus.

A bacterium has a cell wall, cell membrane, cytoplasm, plasmids (small circles of extra DNA) and a single circular chromosome of DNA that is free in the cytoplasm.

Cell wallCell membranePlasmidCytoplasmChromosomal DNAFlagellumRibosome
A prokaryotic cell (bacterium): no nucleus, DNA free in the cytoplasm.

Where is the DNA in a bacterium?

Pathogens and viruses

Pathogens cause disease. Viruses are extremely small and are not classed as living because they only reproduce inside host cells.

A pathogen can be a fungus, bacterium, protoctist or virus. A virus is genetic material (DNA or RNA) in a protein coat, much smaller than a bacterium. It cannot carry out most life processes independently.

Surface proteinsProtein coatGenetic materialEnvelope
A virus: genetic material inside a protein coat, with no cell structure.
  1. 1

    Virus enters

    genetic material is injected into a host cell

  2. 2

    Host machinery hijacked

    the cell is made to copy the viral genetic material and proteins

  3. 3

    New viruses released

    copies leave to infect other cells

How a virus reproduces

Why are viruses not classed as living?

Try an exam question

Give three reasons why a virus is not considered to be a living organism.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.