CIE Categories Archives: 1. Characteristics and Classification of Living Organisms

1.4) Dichotomous keys

1.4) Dichotomous keys

  • Used to identify unfamiliar organisms.
  • They simplify the process of identification.
  • Each key is made up of pairs of contrasting features. (dichotomous means two branches)
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1.3) Features of organisms

1.3) Features of organisms


All living organisms have certain features in common, including the presence of cytoplasm, cell membranes, DNA as genetic material. Also contain ribosomes (in the cytoplasm), floating freely or attached to membranes called rough endoplasmic reticulum. Ribosomes are responsible for protein synthesis and enzymes involved in respiration

The Whittaker five kingdom scheme: Animal, Plant, Fungus, Prokaryote and Protoctist.


The plant kingdom:

  • Multicellular
  • Cell wall made up of cellulose
  • Contains chloroplasts with photosynthetic pigments
  • Make their own food by photosynthesis



  • Produces gametes but no seeds


Flowering plants

  • Divided into two subclasses: Monocotyledon and Dicotyledon

The animal kingdom:

  • Multicellular organisms
  • No cell wall or chloroplasts
  • Coelenterates, Flatworms, Nematode worms, Annelids, Arthropods, Molluscs, Echinoderms, Vertebrates.



  • Vertebrates are animals which have a vertebral column called the spinal column or just the spine and consists of a chain of cylindrical bones joined end to end.
  • Poikilothermic (variable temperature) cold blooded.
  • Homoiothermic (constant temperature) warm blooded.


The fungi kingdom:

  • Made up of thread-like hyphae, rather than cells.
  • Many nuclei distributed throughout the cytoplasm in their hyphae.
  • Mushrooms, toadstools, puffballs, bracket fungi that grow on tree trunks.
  • Mould fungi which grow on stale bread, cheese, fruit or other food.
  • The yeasts are single-celled fungi.

The prokaryote kingdom:

  • Bacteria and blue-green algae.
  • Consist of single cells.
  • Different to other single-cell organisms because their chromosomes are not organised into a nucleus.
  • Bacteria are very small organisms.
  • Cell walls are made, of cellulose, but of a complex mixture of proteins, sugars and lipids.
  • Some bacteria have a slime capsule outside their cell wall.
  • Cytoplasm may contain granules of glycogen, lipid and other food reserves .
  • Each bacterial cell contains a single chromosome, consisting of a circular strand of DNA.
  • The chromosome is not enclosed in a nuclear membrane but is coiled up to occupy part of the cell.
  • Flagella can flick and move the cell about.


The protoctist kingdom:

  • Single-celled (unicellular) organisms
  • Their chromosomes enclosed in a nuclear membrane to form a nucleus.
  • Euglena, possess chloroplasts and make their food by photosynthesis. Often referred to as unicellular ‘plants’ or
  • Amoeba and Paramecium, take in and digest solid food. May be called unicellular ‘animals’ or



  • Have a central core of RNA or DNA surrounded by a protein coat.
  • No nucleus, cytoplasm, cell organelles or cell membrane.
  • Virus particles therefore are not cells.
  • Do not feed, respire, excrete or grow.
  • Do reproduce, but only inside the cells of living organisms, using materials provided by the host cell.
  • The nucleic acid core is a coiled single strand of RNA.
  • The coat is made up of regularly packed protein units called capsomeres each containing many protein molecules.
  • The protein coat is called a capsid.


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1.2) Concept and use of classification system

1.2) Concept and use of classification system


  • Species: is a group of organisms that can reproduce to produce fertile offspring.
  • Binomial system: is an internationally agreed system in which the scientific name of an organism is made up of two parts showing the genus and the species.
  • It is important to classify organisms:
  • Possible to identify those most at risk of extinction.
  • Understand evolutionary relationships. (by studying the anatomy of different groups of vertebrates)


  • Morphology: the study of the form, or outward appearance, of organisms.
  • Anatomy: the study of their internal structure, as revealed by dissection.


  • The use of DNA has revolutionised the process of classification.
  • Eukaryotic organisms contain chromosomes made up of strings of genes.
  • DNA is made up of a sequence of bases, coding for amino acids and proteins.
  • Each species has a distinct number of chromosomes and a unique sequence of bases in it DNA, making it identifiable and distinguishable from other species.
  • This helps when different species are very similar morphologically and anatomically.
  • Cladistics: the process of biological classification.
  • Organisms which share a more recent ancestor (more closely related) have DNA base sequences that are more similar than those that share only a distant ancestor.
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1.1) Characteristics of living organisms

1.1) Characteristics of living organisms


Movement: is an action by an organism or part of an organism causing a change of position or place.

Respiration: describes the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

Sensitivity: is the ability to detect or sense stimuli in the internal or external environment and to make appropriate responses.

Growth: is a permanent increase in size and dry mass by an increase in cell number or cell size or both.

Reproduction: is the processes that make more of the same kind of organism.

Excretion: is the removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials and substances in excess of requirements.

Nutrition: is the taking in of materials for energy, growth and development. Plant require light, CO2, H2O and ions. Animals need organic compounds and ions and H2O.

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