Classification of life

What is taxonomy?

The science of classifying organisms. Modern classification groups organisms according to evolutionary relationships and molecular similarities

What is binomial nomenclature?

The two-part naming system devised by Linnaeus. Example: Homo sapiens (genus capitalised, species lowercase, both italicised).

What molecular technique revolutionised classification of life?

Comparison of 16S rRNA sequences, pioneered by Carl Woese, allowing organisms to be classified using molecular differences.

Why are molecular techniques useful for classification?

They reveal evolutionary relationships that may not be obvious from appearance alone. Organisms with similar 16S rRNA sequences are more closely related.

What are the three domains of life?

Bacteria, Archaea, and Eukarya.

Why was the Domain Archaea created?

Molecular evidence showed that prokaryotes were actually two fundamentally different groups: Bacteria and Archaea

What is a prokaryote?

A unicellular organism lacking a membrane-bound nucleus and membrane-bound organelles. Includes members of the domains Bacteria and Archaea.

What characteristics do all prokaryotes share?

They are:

• Unicellular

• Prokaryotic in organisation

• Structurally diverse

• Metabolically diverse

How would you fully describe a prokaryote in an exam answer?

A prokaryote is a unicellular organism lacking a nucleus. Prokaryotes belong to either Bacteria or Archaea and display enormous structural and metabolic diversity despite sharing a simple cellular organisation

Name four differences between Bacteria and Archaea.

• Cell wall structure

• Membrane lipids

• RNA polymerase enzymes

• Structure of other enzymes

What is the main cell wall difference between Bacteria and Archaea?

Bacterial cell walls contain peptidoglycan, whereas archaeal cell walls do not.

Which domain is more similar to Eukarya in some aspects of molecular biology?

Archaea.

What similarities do Bacteria and Archaea share?

• Both are unicellular

• Both are prokaryotic

• Both have diverse surface features

• Both have diverse metabolisms

What is peptidoglycan?

A major component of bacterial cell walls and the target of many antibiotics such as penicillin.

What is a Gram-positive bacterium?

A bacterium with a thick peptidoglycan layer that stains purple during Gram staining

What is a Gram-negative bacterium?

A bacterium with a thinner peptidoglycan layer and an additional outer membrane.

What is a capsule and why is it important?

A polysaccharide layer that protects bacteria and can increase virulence by preventing phagocytosis

What are fimbriae (pili)?

Surface structures involved in attachment and often important in bacterial virulence.

What is a phototroph?

An organism that obtains energy from light. Example: Cyanobacteria.

What is a chemotroph?

An organism that gains energy from chemical compounds. Example: sulphur bacteria.

What is a heterotroph?

An organism that gains nutrients from organic matter produced by other organisms. Many decomposer bacteria are heterotrophs

Why are prokaryotes ecologically important?

They perform nitrogen fixation, nitrification, decomposition, sewage treatment, and waste reclamation.

Give an example of a pathogenic bacterium and explain why it is important.

Vibrio cholerae is pathogenic because it causes cholera, demonstrating how some bacteria can cause disease in humans.

Give an example of a non-pathogenic bacterium and explain its importance.

Rhizobium forms symbioses with plants and performs nitrogen fixation, increasing soil fertility.

Give an example of an archaean

Methanosarcina barkeri is an archaean methanogen that produces methane.

What is an extremophile?

An archaean adapted to extreme conditions such as high temperature or high salinity.

What are the levels of biological organisation?

Cell → Tissue → Organ → Organ System → Organism → Population.

What is a tissue?

A group of cells with similar structure and a common function.

What is an organ?

One or more tissue types organised into a functional unit.

What is an organ system?

A group of organs working together to perform a function.

What are the three main plant organs?

Roots, stems and leaves.

What are the functions of roots?

Resource uptake, anchorage and storage.

What are the functions of stems?

Support, transport, positioning leaves/flowers and storage.

What are the functions of leaves?

Photosynthesis and gas exchange. (Implied from their role in resource acquisition and plant structure.)

Give an example of animal tissue.

Epithelial tissue (skin)

Give an example of an animal organ.

Heart.

Give an example of an animal organ system.

Circulatory system.

Explain how cells become tissues and tissues become organs.

Similar cells group together to form tissues. Different tissues then combine to form organs, and organs work together in organ systems.

Compare Bacteria and Archaea.

Both are unicellular prokaryotes with diverse metabolisms, but they differ in cell wall composition, membrane lipids and molecular machinery. Archaea share some similarities with Eukarya

Why is it not enough to simply memorise examples?

You must place examples into context and explain the biological principles they demonstrate, such as classification, evolution, metabolism, pathogenicity or ecological importance.

What are the levels of biological classification from broadest to most specific?

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

Which taxonomic rank is the broadest?

Domain.

Which taxonomic rank is the most specific?

Species.

What is a phylogenetic tree?

A diagram showing evolutionary relationships between organisms

What does a branch point (node) on a phylogenetic tree represent?

A common ancestor shared by descendant groups.(Added as study support based on the learning outcome about interpreting phylogenies.)

What can phylogenetic trees be used for?

To display evolutionary relationships and patterns of common ancestry between organisms.

Name two bacterial surface structures involved in virulence.

Capsules and fimbriae (pili).

Why can capsules increase bacterial virulence?

They help protect bacteria from phagocytosis

Give an example of a phototroph.

Cyanobacteria.

Give an example of a chemotroph.

Sulphur bacteria.

Compare Bacteria and Archaea.

Both are unicellular prokaryotes with diverse metabolisms, but they differ in cell wall composition, membrane lipids and molecular machinery. Archaea share some similarities with Eukarya.

Why was the Domain Archaea created?

Molecular evidence revealed that prokaryotes consisted of two fundamentally different groups: Bacteria and Archaea.

Give one pathogenic bacterium and its importance.

Vibrio cholerae causes cholera and demonstrates that some bacteria can cause disease in humans.