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Strand 1: Organisation of Life

Characteristics of Life

The defining features of living things, viruses, and the domains and classification of life.

1.1 Characteristics of Living Things

In brief:All living organisms share certain characteristics that distinguish them from non-living things. These characteristics form the basis for defining life.

All living organisms share 8 characteristics that distinguish them from non-living things. The mnemonic MRS NERG can help you remember them:

Movement - response to stimuli by changing position
Respiration - releasing energy from food molecules
Sensitivity - detecting and responding to changes in the environment
Nutrition - obtaining food for energy, growth, and repair
Excretion - removal of metabolic waste products
Reproduction - producing new individuals of the same species
Growth - permanent increase in size and complexity

Plus Organisation: all living things are made of cells arranged in levels of increasing complexity.

Viruses challenge the definition of life: they have DNA or RNA and can replicate, but they cannot reproduce independently, carry out metabolism, or respond to stimuli. They need a host cell to reproduce.

Animal cell

All living things are made of cells - the basic unit of life

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diagram

Levels of Organisation

Cell
→
Tissue
→
Organ
→
Organ System
→
Organism
table

MRS NERG - Characteristics of Life

LetterCharacteristicMeaningExample
MMovementChanging position in response to stimuliPlant growing towards light
RRespirationReleasing energy from foodGlucose → CO₂ + H₂O + ATP
SSensitivityResponding to environmentPupil dilating in dark
NNutritionObtaining foodPhotosynthesis / eating
EExcretionRemoving metabolic wasteCO₂ from lungs, urea from kidneys
RReproductionProducing offspringSexual or asexual
GGrowthIncrease in size/complexityCell division

Key Points

  • 1The characteristics of life: Organisation, Growth, Response, Metabolism, Respiration, Nutrition, Excretion, Reproduction.
  • 2Organisation: living things are made of cells arranged in increasing complexity.
  • 3Metabolism: all the chemical reactions occurring in an organism (anabolism + catabolism).
  • 4Viruses have DNA or RNA and a protein coat but cannot reproduce independently.
  • 5Viruses are difficult to classify as living or non-living - they need a host cell to replicate.

Learning Outcomes

  • Outline the characteristics of living things
  • Discuss the difficulty of defining viruses, their economic and medical importance

Domains & Classification

In brief:Classification is the organisation of living things into groups based on shared characteristics. Modern classification uses three domains based on genetic evidence.

Classification is the organisation of living things into groups based on shared characteristics. Modern classification uses three domains based on genetic (DNA) evidence.

The Three Domains (proposed by Carl Woese in 1990):

1. Bacteria - prokaryotic, most diverse and abundant. E.g. E. coli, Streptococcus
2. Archaea - prokaryotic, often found in extreme environments (hot springs, salt lakes)
3. Eukaryota - all organisms with membrane-bound nuclei: Protista, Fungi, Plantae, Animalia

Binomial Nomenclature (devised by Linnaeus): every species has a two-part Latin name - Genus species. E.g. Homo sapiens, Escherichia coli. The genus is capitalised, both italicised.

Taxonomic hierarchy: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species

table

The Three Domains of Life

FeatureBacteriaArchaeaEukaryota
Cell typeProkaryoticProkaryoticEukaryotic
NucleusNoNoYes (membrane-bound)
Cell wallPeptidoglycanPseudopeptidoglycanVaries (cellulose/chitin/none)
HabitatsEverywhereExtreme environmentsEverywhere
ExamplesE. coliMethanogensHumans, fungi, plants
process

Taxonomic Hierarchy (King Philip Came Over For Good Spaghetti)

Domain
→
Kingdom
→
Phylum
→
Class
→
Order
→
Family
→
Genus
→
Species

Key Points

  • 1Three Domains: Archaea, Bacteria, Eukaryota - based on genetic characteristics.
  • 2Six Kingdoms: Bacteria, Archaea, Protista, Fungi, Plantae, Animalia.
  • 3Prokaryotic cells (Bacteria, Archaea): no membrane-bound nucleus.
  • 4Eukaryotic cells (Protista, Fungi, Plantae, Animalia): membrane-bound nucleus and organelles.
  • 5Phylogeny: classification based on evolutionary development - genus and species.
  • 6Classification systems evolve as new evidence (e.g. DNA analysis) emerges.

Learning Outcomes

  • Outline the domains of life system based on genetic characteristics
  • Use classification principles to identify and classify living things in known and unknown contexts; outline the importance of classification systems in biology