CIE AS/A Level Biology -18.1 Classification- Study Notes- New Syllabus

Ace A level Biology Exam with CIE AS/A Level Biology -18.1 Classification- Study Notes- New Syllabus

Key Concepts:

discuss the meaning of the term species, limited to the biological species concept, morphological species concept and ecological species concept
describe the classification of organisms into three domains: Archaea, Bacteria and Eukarya
state that Archaea and Bacteria are prokaryotes and that there are differences between them, limited to differences in membrane lipids, ribosomal RNA and composition of cell walls
describe the classification of organisms in the Eukarya domain into the taxonomic hierarchy of kingdom, phylum, class, order, family, genus and species
outline the characteristic features of the kingdoms Protoctista, Fungi, Plantae and Animalia
outline how viruses are classified, limited to the type of nucleic acid (RNA or DNA) and whether this is single stranded or double stranded

CIE AS/A Level Biology 9700-Study Notes- All Topics

Meaning of the Term “Species”

🧬 1. Biological Species Concept

Definition: A species is a group of organisms that can interbreed and produce fertile offspring in nature.
Focus: Reproductive compatibility.
Example: Horses and donkeys can mate, but their offspring (mules) are infertile, so they are different species.

🌿 2. Morphological Species Concept

Definition: A species is a group of organisms that share common physical features.
Focus: Observable structural characteristics.
Example: Different species of oak trees are distinguished by leaf shape, size, and bark texture.

🌳 3. Ecological Species Concept

Definition: A species is a group of organisms that occupy the same ecological niche and have similar interactions with the environment.
Focus: Role in the ecosystem and adaptation.
Example: Two species of warblers may live in the same forest but feed in different parts of trees, occupying distinct niches.

📌 Key Points

Biological: Based on reproduction.
Morphological: Based on physical traits.
Ecological: Based on ecological role/niche.

Classification of Organisms: Three Domains

🧬 Key Concept

All living organisms are classified into three domains based on cell type, genetics, and biochemistry.
The three domains are: Archaea, Bacteria, and Eukarya.

🌋 1. Archaea

Cell type: Prokaryotic (no nucleus)
Cell wall: Lacks peptidoglycan (different from bacteria)
Environment: Often extremophiles (e.g., hot springs, salty lakes, acidic environments)
Genetics: Genes and metabolic pathways more similar to eukaryotes than bacteria
Example organisms: Methanogens, Halophiles, Thermophiles

🦠 2. Bacteria

Cell type: Prokaryotic
Cell wall: Contains peptidoglycan
Environment: Found in most habitats, including soil, water, and human body
Metabolism: Diverse (photosynthesis, nitrogen fixation, decomposition)
Example organisms: Escherichia coli, Streptococcus, Cyanobacteria

🌿 3. Eukarya

Cell type: Eukaryotic (with nucleus and membrane-bound organelles)
Cell wall: Present in plants and fungi (cellulose or chitin), absent in animals
Organisms included: Plants, animals, fungi, and protists
Example organisms: Humans (Homo sapiens), Oak tree (Quercus), Yeast (Saccharomyces)

📌 Key Points

Domain	Cell Type	Cell Wall	Habitat/Characteristics	Example
Archaea	Prokaryote	No peptidoglycan	Extremophiles	Methanogens
Bacteria	Prokaryote	Peptidoglycan	Widespread, diverse metabolism	E. coli
Eukarya	Eukaryote	Cellulose/chitin in some	Plants, animals, fungi, protists	Humans, Yeast

Prokaryotes: Archaea vs Bacteria

🌱 Key Concept

Both Archaea and Bacteria are prokaryotic organisms (no nucleus, no membrane-bound organelles).
Despite similarities, they differ in membrane lipids, ribosomal RNA, and cell wall composition.

🔹 Differences Between Archaea and Bacteria

Feature	Archaea	Bacteria
Membrane lipids	Ether-linked lipids (more stable, often branched)	Ester-linked lipids (unbranched)
Ribosomal RNA	Unique sequences, more similar to eukaryotes	Distinct bacterial sequences
Cell wall composition	Lacks peptidoglycan (may have pseudopeptidoglycan)	Contains peptidoglycan

📌 Key Points

Both are prokaryotic but genetically and biochemically distinct.
Archaea often live in extreme environments due to their unique cell structures.

Classification of Eukarya: Taxonomic Hierarchy

🌱 Key Concept

Organisms in the Eukarya domain are classified into a hierarchical system from broad to specific groups.
The hierarchy helps organize biodiversity and shows relationships between organisms.

🔹 Taxonomic Hierarchy

Kingdom

The broadest group of related organisms.
Example: Animalia, Plantae, Fungi, Protista.

Phylum

Groups organisms within a kingdom that share major body plans or structures.
Example: Chordata (animals with a notochord).

Class

Divides phyla into groups with similar characteristics.
Example: Mammalia (all mammals).

Order

Further divides classes into groups sharing more specific traits.
Example: Primates (monkeys, apes, humans).

Family

Groups genera that are closely related.
Example: Hominidae (great apes, humans).

Genus

Groups species that are very similar and closely related.
Example: Homo (humans and closely related extinct species).

Species

Most specific group; organisms that can interbreed and produce fertile offspring.
Example: Homo sapiens (modern humans).

📌 Key Points

Hierarchy goes from broad → specific: Kingdom → Phylum → Class → Order → Family → Genus → Species.
Each level shows increasing similarity and relatedness among organisms.
Scientific names use binomial nomenclature: Genus species (e.g., Homo sapiens).

Characteristic Features of Eukaryotic Kingdoms

🦠 1. Kingdom Protoctista

Cell type: Eukaryotic (with nucleus)
Organisation: Mostly unicellular, some multicellular or colonial
Nutrition: Autotrophic (photosynthetic) or heterotrophic
Cell wall: Present in some, made of cellulose
Movement: Some have cilia, flagella, or pseudopodia
Example organisms: Amoeba, Chlamydomonas, Paramecium

🍄 2. Kingdom Fungi

Cell type: Eukaryotic
Organisation: Mostly multicellular (except yeasts, which are unicellular)
Nutrition: Heterotrophic (absorptive nutrition; saprophytic or parasitic)
Cell wall: Made of chitin
Reproduction: Spores, both sexual and asexual
Example organisms: Saccharomyces (yeast), Mucor, Agaricus

🌱 3. Kingdom Plantae

Cell type: Eukaryotic
Organisation: Multicellular
Nutrition: Autotrophic (photosynthesis)
Cell wall: Made of cellulose
Reproduction: Sexual (alternation of generations) and asexual
Example organisms: Mosses, Ferns, Flowering plants

🐾 4. Kingdom Animalia

Cell type: Eukaryotic
Organisation: Multicellular
Nutrition: Heterotrophic (ingestion of food)
Cell wall: Absent
Movement: Usually capable of locomotion at some stage of life
Reproduction: Mostly sexual, some asexual in lower animals
Example organisms: Humans, Sponges, Insects, Birds

📌 Key Points

Kingdom	Cell Type	Organisation	Nutrition	Cell Wall	Example
Protoctista	Eukaryotic	Mostly unicellular	Autotrophic/heterotrophic	Sometimes cellulose	Amoeba, Chlamydomonas
Fungi	Eukaryotic	Mostly multicellular	Heterotrophic (absorptive)	Chitin	Yeast, Mucor
Plantae	Eukaryotic	Multicellular	Autotrophic	Cellulose	Ferns, Flowering plants
Animalia	Eukaryotic	Multicellular	Heterotrophic (ingestion)	Absent	Humans, Insects

Classification of Viruses

🌱 Key Concept

Viruses are non-cellular infectious agents.
Classified based on their type of nucleic acid and strand structure.

🔹 Classification by Nucleic Acid

Nucleic Acid Type	Strand Structure	Example Viruses
DNA	Double-stranded (dsDNA)	Adenovirus, Herpesvirus
DNA	Single-stranded (ssDNA)	Parvovirus
RNA	Double-stranded (dsRNA)	Rotavirus
RNA	Single-stranded (ssRNA)	Influenza virus, HIV, Poliovirus

📌 Key Points

Viruses lack cellular structure and cannot reproduce independently.
Classification is based on nucleic acid type because it determines replication strategy.
Single-stranded RNA viruses can be further classified into positive-sense or negative-sense RNA, but basic classification is by ss or ds.

CIE AS/A Level Biology -18.1 Classification- Study Notes

CIE AS/A Level Biology -18.1 Classification- Study Notes- New Syllabus