Domains, Kingdoms or Both

Classification

What types of evidence are now used to classify organisms?

Morphological observations together with biochemical and molecular evidence, including DNA, RNA and proteins.

Why is biochemical evidence useful in classification?

Similarities and differences in biological molecules can reveal evolutionary relationships between organisms.

What is biochemical analysis?

The analysis of biological molecules, such as proteins and nucleic acids, to identify relationships between organisms.

How can biochemical analysis support morphological classification?

Organisms that look similar may also have similar biochemical pathways, supporting the idea that they share a common ancestor.

How can biochemical analysis conflict with morphological classification?

Organisms may look similar but have very different biochemical characteristics, suggesting they are not closely related.

What is gel electrophoresis?

A method of separating fragments of proteins or nucleic acids according to their electrical charge and size.

What substances can be separated using gel electrophoresis?

DNA fragments, RNA fragments, proteins and amino acids.

How can gel electrophoresis help classify organisms?

It produces patterns that can be compared between organisms to identify similarities and evolutionary relationships.

What does greater similarity in DNA or protein patterns suggest?

The organisms are likely to be more closely related and share a more recent common ancestor.

Why are amino acid sequences useful in classification?

Similarities in the amino acid sequences of proteins can indicate evolutionary relationships.

How can blood pigments provide evidence for classification?

Different animal groups contain characteristic respiratory pigments, which can be compared to determine relationships.

What respiratory pigment is found in vertebrates and many invertebrates?

Haemoglobin.

What respiratory pigment is found in polychaete worms?

Chlorocruorin.

What respiratory pigments are found in many molluscs and crustaceans?

Haemocyanin.

How can fibrinogen be used to investigate evolutionary relationships?

Differences in its amino acid sequence can be compared between species, with fewer differences indicating a closer relationship.

Why is combining DNA, protein and morphological evidence useful?

It provides a more reliable picture of evolutionary relationships than relying on one type of evidence alone.

What is a phylogenetic tree?

A model used to show the evolutionary relationships between different groups of organisms.

What does a branch point on a phylogenetic tree represent?

A common ancestor from which different groups evolved.

What does it mean if two groups share a recent branch point on a phylogenetic tree?

They share a relatively recent common ancestor and are more closely related.

What is molecular phylogeny?

The use of molecular evidence, such as DNA and protein sequences, to determine evolutionary relationships.

Why have phylogenetic trees changed as scientific techniques have developed?

DNA and biochemical evidence can reveal relationships that were not apparent from morphology alone.

What was the traditional two-domain view of life?

Organisms were divided into prokaryotes and eukaryotes.

What are prokaryotes?

Organisms whose cells lack a membrane-bound nucleus and membrane-bound organelles.

What are eukaryotes?

Organisms whose cells contain a membrane-bound nucleus and membrane-bound organelles.

What does the endosymbiotic theory explain?

How some organelles in eukaryotic cells may have evolved from free-living prokaryotic organisms.

How may mitochondria have evolved according to endosymbiotic theory?

An ancestral eukaryotic cell engulfed an aerobic prokaryote which was not digested and eventually became a permanent mitochondrion.

What advantage did engulfing an aerobic prokaryote give an ancestral eukaryotic cell?

It allowed more efficient aerobic respiration, providing more energy for growth and reproduction.

How may chloroplasts have evolved according to endosymbiotic theory?

An ancestral eukaryotic cell engulfed a photosynthetic prokaryote which was not digested and eventually became a permanent chloroplast.

What advantage did engulfing a photosynthetic prokaryote provide?

It allowed the host cell to photosynthesise and produce its own organic nutrients.

What is endosymbiosis?

A relationship in which one organism lives inside the cells or body of another organism.

What evidence is used to investigate the evolutionary origins of prokaryotes and eukaryotes?

Molecular phylogeny, including comparisons of ribosomal proteins, enzymes and genetic material.

What are the three domains of life?

Bacteria, Archaea and Eukaryota.

Which two domains contain prokaryotic organisms?

Bacteria and Archaea.

Which domain contains eukaryotic organisms?

Eukaryota.

Why were prokaryotes divided into Bacteria and Archaea?

Molecular evidence revealed major differences between the two groups despite both having prokaryotic cells.

Which two domains appear more closely related according to molecular evidence?

Archaea and Eukaryota.

What does the three-domain model suggest about Archaea and Eukaryota?

They share a more recent common ancestor with each other than either does with Bacteria.

Do Bacteria have a membrane-bound nucleus?

No.

Do Archaea have a membrane-bound nucleus?

No.

Do Eukaryota have a membrane-bound nucleus?

Yes.

Do Bacteria have membrane-bound organelles?

No.

Do Archaea have membrane-bound organelles?

No.

Do Eukaryota have membrane-bound organelles?

Yes.

Which domain has peptidoglycan in its cell walls?

Bacteria.

Do Archaea have peptidoglycan in their cell walls?

No.

Do Eukaryota have peptidoglycan in their cell walls?

No.

What type of membrane lipids are found in Bacteria?

Ester-linked, unbranched lipids.

What type of membrane lipids are found in Archaea?

Ether-linked, branched lipids.

What type of membrane lipids are found in Eukaryota?

Ester-linked, unbranched lipids.

What size ribosomes are found in Bacteria?

70S ribosomes.

What size ribosomes are found in Archaea?

70S ribosomes.

What size ribosomes are found in Eukaryota?

80S ribosomes in the cytoplasm.

What initiator amino acid is used in protein synthesis by Bacteria?

Formylmethionine.

What initiator amino acid is used by Archaea?

Methionine.

What initiator amino acid is used by Eukaryota?

Methionine.

How do Bacteria normally reproduce?

By binary fission.

How do Archaea normally reproduce?

By binary fission.

What are extremophiles?

Organisms, particularly some Archaea, that can survive extreme conditions of heat, cold, pH, salinity or pressure.

Why are Archaea biochemically distinct from Bacteria?

They differ in features such as membrane lipids, cell-wall composition and aspects of their molecular biology.

Why is the three-domain system widely accepted?

Molecular and biochemical evidence shows major differences between Bacteria and Archaea and similarities between Archaea and Eukaryota.

Why can classification systems change?

New scientific evidence and improved techniques can reveal previously unknown evolutionary relationships.

What was Linnaeus's original kingdom classification?

He classified living organisms into two kingdoms: Plantae and Animalia.

Why did scientists move away from a two-kingdom system?

Improved knowledge of microorganisms showed that many organisms did not fit properly into either plants or animals.

What are the six kingdoms used in the six-kingdom classification system?

Archaebacteria, Eubacteria, Protista, Fungi, Plantae and Animalia.

Which kingdom contains Archaea?

Archaebacteria.

Which kingdom contains Bacteria?

Eubacteria.

What are Archaebacteria?

Prokaryotic organisms belonging to the Archaea domain, including many organisms adapted to extreme environments.

What are Eubacteria?

The true bacteria, which are prokaryotic organisms in the Bacteria domain.

What is the Protista kingdom?

A diverse group containing mainly single-celled eukaryotic organisms, including protoctists, algae and slime moulds.

What is the Fungi kingdom?

A eukaryotic kingdom of heterotrophic organisms including yeasts, moulds and mushrooms.

Are fungi autotrophic or heterotrophic?

Heterotrophic.

Can fungi be unicellular or multicellular?

Yes. For example, yeasts are unicellular while moulds and mushrooms are multicellular.

What is the Plantae kingdom?

A eukaryotic kingdom containing mainly multicellular autotrophs that photosynthesise using chlorophyll.

What are examples of organisms in Plantae?

Mosses, liverworts, ferns, gymnosperms and flowering plants.

Are plants autotrophic or heterotrophic?

Mainly autotrophic.

What is the Animalia kingdom?

A eukaryotic kingdom containing multicellular heterotrophic organisms.

Are animals autotrophic or heterotrophic?

Heterotrophic.

How do animals obtain nutrients?

By consuming other organisms or organic material.

What is the five-kingdom classification system?

A system containing Monera, Protista, Fungi, Plantae and Animalia.

What is Monera?

A kingdom in the five-kingdom system containing prokaryotic organisms.

Why is the five-kingdom system increasingly replaced by a six-kingdom system?

Molecular evidence shows that Archaea and Bacteria are sufficiently different to be placed into separate kingdoms.

What is the main difference between the five-kingdom and six-kingdom systems?

The five-kingdom system groups all prokaryotes into Monera, while the six-kingdom system separates them into Archaebacteria and Eubacteria.

How does the three-domain system classify organisms at the highest level?

It divides all organisms into Bacteria, Archaea and Eukaryota.

How are the kingdoms related to the three-domain system?

Bacteria contains Eubacteria, Archaea contains Archaebacteria, and Eukaryota contains Protista, Fungi, Plantae and Animalia.

Why is critical evaluation important when developing classification systems?

Scientists must compare new evidence with existing models and revise classifications when better evidence becomes available.

Why is molecular evidence often more reliable than appearance alone?

Physical similarities can evolve independently, whereas DNA and protein similarities can provide stronger evidence of shared ancestry.