Author: Admin

📌Definition Table

Term	Definition
Escape Theory	Hypothesis that viruses evolved from bits of cellular nucleic acids that “escaped” from their host cells.
Regressive Theory	Hypothesis that viruses evolved from more complex free-living organisms that lost cellular structures over time.
Virus-First Theory	Hypothesis that viruses existed before cells and contributed to early molecular evolution.
Endogenous Retrovirus	Viral sequences that have become permanently integrated into the host genome.
Antigenic Drift	Gradual accumulation of mutations in viral genes, leading to minor changes in antigens.
Antigenic Shift	Major genetic changes due to reassortment of genome segments, often producing new viral strains.

📌Introduction

The origin and evolution of viruses remain subjects of scientific debate, with multiple hypotheses proposing different pathways. Viruses evolve rapidly due to high mutation rates, recombination, and reassortment of genetic material, enabling them to adapt to new hosts and evade immune responses. Understanding their evolution is critical for controlling viral diseases and developing effective vaccines.

❤️ CAS Link: Create a public science display showing the different theories of virus origin and how rapid viral evolution impacts human health (e.g., flu vaccine updates).

📌 Theories on the Origin of Viruses

• Escape Theory: Viruses originated from fragments of genetic material that “escaped” from cells.
• Regressive Theory: Viruses evolved from cellular organisms that gradually lost complexity.
• Virus-First Theory: Viruses predate cells and were part of pre-cellular life forms.
• These theories are not mutually exclusive — different viruses may have different origins.
• Evidence is limited due to lack of viral fossils and rapid mutation rates.
• Molecular analysis helps trace evolutionary relationships between viruses and hosts.

📌 Viral Evolution and Mutation

• RNA viruses mutate faster than DNA viruses due to error-prone replication.
• High mutation rates allow viruses to quickly adapt to host immune systems.
• Mutations can alter viral surface proteins, affecting infectivity and host range.
• Recombination occurs when two viruses infect the same cell and exchange genetic material.
• Reassortment in segmented viruses (e.g., influenza) can produce entirely new strains.
• Viral evolution impacts vaccine effectiveness and antiviral drug resistance.

🌍 Real-World Connection: Seasonal influenza vaccines must be updated annually because antigenic drift changes viral surface proteins.

📌 Antigenic Drift vs Antigenic Shift

• Antigenic Drift: Gradual changes due to point mutations in viral genes.
• Antigenic Shift: Abrupt changes due to gene segment reassortment, producing novel antigens.
• Drift leads to reduced vaccine effectiveness over time.
• Shift can cause pandemics when a new strain emerges that the population lacks immunity against.
• Influenza A viruses are well-known for undergoing both drift and shift.
• Shift often occurs when viruses from different species exchange genetic material.

📌Defination Table

Term	Definition
Adaptive Radiation	Rapid diversification of a common ancestor into many species adapted to different niches.
Ecological Niche	The role and position of a species in its environment, including its interactions and resource use.
Polyploidy	Possession of more than two complete sets of chromosomes.
Autopolyploidy	Polyploidy arising from chromosome duplication within a single species.
Allopolyploidy	Polyploidy resulting from hybridisation between different species followed by chromosome doubling.
Instant Speciation	Formation of a new species in one generation, often via polyploidy in plants.
Endemism	Condition of a species being found only in a specific geographic location.

📌Introduction

Adaptive radiation and plant speciation are key drivers of biodiversity. Adaptive radiation produces a burst of new species from a common ancestor, each adapted to a unique niche. In plants, speciation often involves polyploidy, which can cause reproductive isolation almost instantly. These processes help explain the vast diversity in ecosystems, particularly on islands and in environments with varied niches.

❤️ CAS Link: Establish a school garden biodiversity plot, planting species that illustrate adaptive radiation or polyploidy origins.

📌 Adaptive Radiation

• Occurs when a single ancestral species diversifies into many species adapted to different niches.
• Requires available niches and minimal competition or predation.
• Often follows events such as colonisation of a new habitat or mass extinctions.
• Example: Darwin’s finches evolved different beak shapes to exploit different food sources.
• Increases biodiversity rapidly in geological terms.

🧠 Examiner Tip: In adaptive radiation diagrams, label common ancestor and niche specialisation clearly.

📌 Ecological Niches and Speciation

• A niche includes all environmental conditions and interactions that allow a species to survive and reproduce.
• Differentiation of niches reduces competition and can drive speciation.
• Closely related species in the same habitat often occupy different niches to avoid direct competition.
• Niche partitioning maintains biodiversity within ecosystems.

🌍 Real-World Connection: Coral reef fish species show niche partitioning by depth, diet, and time of activity.

📌 Polyploidy in Plants

• Polyploidy is common in plants and can cause instant reproductive isolation.
• Autopolyploidy – duplication of chromosome sets within one species; results in fertile polyploids that cannot breed with the original diploid population.
• Allopolyploidy – hybridisation between two species followed by chromosome doubling; produces fertile hybrids reproductively isolated from both parent species.
• Polyploidy can increase vigour, adaptability, and resistance to environmental stress.

📝 Paper 2: Data Response Tip: When interpreting plant speciation data, link chromosome number changes to reproductive isolation.

📌Definition Table

Term	Definition
Speciation	Formation of new species from existing ones.
Population	Group of individuals of the same species living in the same area.
Reproductive Isolation	Mechanisms that prevent gene flow between populations.
Allopatric Speciation	Speciation due to physical/geographic separation of populations.
Sympatric Speciation	Speciation within the same geographic area, often due to ecological or behavioural differences.
Prezygotic Barrier	Isolation mechanisms that occur before fertilisation, preventing mating or fertilisation.
Postzygotic Barrier	Isolation mechanisms after fertilisation that prevent hybrid viability or fertility.
Hybrid	Offspring resulting from mating between two different species or populations.

📌Introduction

Speciation is the evolutionary process by which populations diverge into distinct species. It occurs when gene flow between populations is reduced or eliminated, often through reproductive isolation. Mechanisms of isolation can be geographical, behavioural, temporal, or genetic. Over time, these differences accumulate, leading to the emergence of new species.

❤️ CAS Link: Organise a nature reserve observation project where students record potential cases of speciation in bird populations based on behaviour and habitat differences.

📌 Reproductive Isolation

• Prezygotic barriers prevent fertilisation:
  • Temporal isolation — breeding seasons differ.
  • Behavioural isolation — mating signals or rituals differ.
  • Mechanical isolation — physical incompatibility of reproductive structures.
  • Ecological isolation — species occupy different niches in the same area.
  • Gametic isolation — gametes fail to fuse.
• Postzygotic barriers occur after fertilisation:
  • Hybrid inviability — embryo fails to develop.
  • Hybrid sterility — hybrid grows but is sterile (e.g., mule).
  • Hybrid breakdown — first-generation hybrids are viable, but later generations have reduced fitness.

🧠 Examiner Tip: State whether a barrier is prezygotic or postzygotic and give a clear example.

📌 Allopatric Speciation

• Occurs when a population is split by a physical barrier (mountains, rivers, habitat fragmentation).
• Gene flow stops, and populations evolve independently due to natural selection, genetic drift, and mutations.
• Over time, differences accumulate until the populations can no longer interbreed.
• Example: Squirrel species on opposite sides of the Grand Canyon.

🌍 Real-World Connection: Allopatric speciation is common on islands, where populations are separated by water.

📌 Sympatric Speciation

• Occurs without geographical separation.
• Often due to ecological niche specialisation or behavioural differences within a shared habitat.
• Can result from polyploidy in plants — doubling chromosome numbers creates instant reproductive isolation.
• Example: Cichlid fish species in the same African lake evolved into multiple species with different feeding strategies.

📝 Paper 2: Data Response Tip: When presented with hybridisation data, link it to gene flow and fitness consequences.

📌Definition Table

Term	Definition
Evolution	Changes in the heritable characteristics of organisms over generations.
Natural Selection	Process where advantageous traits become more common because they improve survival and reproduction.
Heritable Characteristics	Traits determined by alleles that can be passed to offspring.
Mutation	Random change in DNA sequence that may produce new alleles.
Homologous Structures	Structures with similar underlying anatomy but different functions, indicating shared ancestry.
Analogous Structures	Structures with similar function but different evolutionary origins, arising from convergent evolution.
Artificial Selection	Human-directed breeding to enhance desirable traits; also called selective breeding.
Molecular Evidence	DNA, RNA, or protein sequence comparisons used to determine evolutionary relationships.

📌Introduction

Evolution explains the diversity of life on Earth as species change over time through genetic variation and natural selection. Evidence from molecular biology, comparative anatomy, selective breeding, and fossil records supports the theory. While Darwin’s natural selection remains the central explanation, modern molecular data have refined our understanding of how species evolve and adapt.

❤️ CAS Link: Collaborate with a local science museum to create an educational exhibit explaining how molecular evidence and anatomy support evolution.

📌 Darwinian vs. Lamarckian Evolution

• Darwin’s Theory: Variation exists within populations due to random mutations. Individuals with advantageous traits survive and reproduce more successfully, passing those traits on. Over time, these traits become more common.
• Lamarck’s Theory: Suggested traits acquired during an organism’s lifetime (e.g., stretched giraffe neck) could be inherited. This has been disproven except for limited epigenetic influences.
• Darwin’s theory requires heritability; Lamarck’s does not.
• Modern evidence strongly supports Darwin’s model, with minor updates such as recognising that evolution can sometimes occur rapidly (e.g., antibiotic resistance).

🧠 Examiner Tip: In Paper 2, contrast mechanism and heritability when comparing Darwin and Lamarck.

📌 Molecular Evidence for Evolution

• Sequence data from DNA, RNA, and proteins allow scientists to compare species.
• More similarities in sequences indicate closer evolutionary relationships.
• Conserved sequences (e.g., haemoglobin genes) are used because they change slowly and are found across many species.
• Example: Humans and chimpanzees share ~99% DNA sequence identity, indicating recent common ancestry.
• Data from multiple genes increases certainty and can be used to construct evolutionary trees.

🌍 Real-World Connection: Genomic comparisons are used to track zoonotic disease origins, like tracing SARS-CoV-2 back to coronaviruses in bats.

📌 Artificial Selection (Selective Breeding)

• Humans breed organisms with desirable traits over many generations.
• Works faster than natural selection because humans control which individuals reproduce.
• Examples:
• Cows bred for higher milk yield.
• Crops bred for disease resistance.
• Demonstrates evolution in action by showing how heritable changes accumulate over time.

📌 Homologous Structures

• Structures with similar anatomy but adapted for different functions.
• Example: The pentadactyl limb in humans, whales, birds, frogs, and alligators has the same bone layout but is adapted for walking, swimming, flying, or jumping.
• Supports adaptive radiation — species evolved from a common ancestor but adapted to different environments.

📝 Paper 2 Tip: Use function + structure when explaining homologous structures.

📌Definition Table

Term	Definition
Reclassification	Changing the classification of an organism based on new evidence.
Paraphyletic Group	A group containing a common ancestor but not all of its descendants.
Domain	Highest taxonomic rank, above kingdoms, based on fundamental genetic differences.
Archaea	Domain of single-celled prokaryotes distinct from bacteria and eukaryotes.
Eubacteria	Domain of true bacteria, prokaryotic organisms with peptidoglycan in their cell walls.
Eukaryote	Domain of organisms with membrane-bound organelles and a nucleus.
Convergent Evolution	Evolution of similar traits in unrelated species due to similar environments.

📌Introduction

Reclassification occurs when new evidence, particularly from molecular data, reveals that the current taxonomy does not accurately reflect evolutionary relationships. DNA sequencing has shown that many traditional groupings, based on morphology alone, do not form true clades. This has led to splitting, merging, and reorganising taxa. One major outcome has been the development of the three-domain system, recognising fundamental differences between Archaea, Eubacteria, and Eukaryotes.

❤️ CAS Link: Host a student science fair where groups research and present a reclassification case study, such as the figwort family.

📌 Why Reclassification Happens

• New molecular evidence (DNA, mRNA, protein sequences) can contradict morphology-based groupings.
• Traditional taxonomy sometimes placed unrelated organisms together due to convergent evolution.
• Correct clade-based classification ensures groups contain only close evolutionary relatives.
• Reclassification can involve moving species between families, splitting large groups, or merging smaller ones.

🧠 Examiner Tip: When explaining reclassification, always reference new evidence and clade accuracy.

📌 Case Study: The Figwort Family

• Originally classified in the late 1700s based on shared morphological traits (e.g., tube-shaped flowers).
• Grew to over 275 genera in the Scrophulariaceae family.
• DNA analysis of three chloroplast genes revealed the group was paraphyletic.
• Resulted in:
  • Creation of new families.
  • Movement of genera into existing families.
  • Reduction of the figwort family to less than half its original size.
• Demonstrates how morphological similarity can be due to convergent evolution, not shared ancestry.

🌍 Real-World Connection: Archaeal enzymes from thermophiles are used in high-temperature industrial processes like PCR.

📌 The Three-Domain System

• Developed after rRNA analysis revealed two distinct groups of prokaryotes.
• Domains are the largest taxonomic rank:
• Archaea – Prokaryotes living in extreme environments; unique cell wall and membrane composition; ribosomes more similar to eukaryotes.
• Eubacteria – True bacteria with peptidoglycan cell walls.
• Eukaryotes – Organisms with a nucleus and membrane-bound organelles.

Feature	Archaea	Eubacteria	Eukaryotes
Cell Type	Prokaryotic	Prokaryotic	Eukaryotic
Chromosome	Circular	Circular	Linear (plus circular in mitochondria/chloroplasts)
Cell Wall	Without peptidoglycan	With peptidoglycan	Sometimes present, never with peptidoglycan
Membrane Lipids	Glycerol-ether	Glycerol-ester	Glycerol-ester
Ribosomes	70S (subunit similar to eukaryotes)	70S	80S (cytoplasm) & 70S (organelles)
Histones	Present	Absent	Present
Introns	Sometimes	Rare	Present

📝 Paper 2: Data Response Tip: When given a classification table, identify domain-level differences in cell wall composition, ribosomes, and genetic features.

📌Definition Table

Term	Definition
Lytic Cycle	Viral replication process resulting in destruction of the host cell and release of new viruses.
Lysogenic Cycle	Viral replication strategy where viral DNA integrates into the host genome and remains dormant before activation.
Provirus	Viral genome integrated into the host DNA.
Latency	Period where a virus remains dormant in host cells without producing new viruses.
Reverse Transcriptase	Enzyme that synthesises DNA from an RNA template (in retroviruses).
Lysozyme	Enzyme used by bacteriophages to break bacterial cell walls during infection.

📌Introduction

Viral replication requires host cell machinery because viruses lack the enzymes and structures necessary for independent reproduction. Depending on the virus, replication may follow a lytic pathway, where new viruses are made immediately, or a lysogenic pathway, where the viral genome integrates into the host cell and remains inactive until triggered.

❤️ CAS Link: Develop a school science outreach activity where students model viral replication using simple props to show lytic vs lysogenic cycles.

📌 General Process of Viral Replication

• Attachment: Virus recognises and binds to specific host cell receptors.
• Entry: Virus injects its genome or enters the host via endocytosis/membrane fusion.
• Replication: Host cell machinery is hijacked to replicate viral genetic material.
• Assembly: Viral components self-assemble into new virions.
• Release: Viruses exit the cell via lysis (bursting) or budding (enveloped viruses).
• Cycle repeats upon infection of new host cells.

🧠 Examiner Tip: Always include attachment → entry → replication → assembly → release in sequence when describing viral replication for full marks.

📌 Lytic Cycle

• Immediate replication upon infection.
• Host cell is destroyed at the end of the cycle.
• Steps:
  • Attachment to host cell surface.
  • Injection or entry of viral genome.
  • Replication of genome and synthesis of viral proteins.
  • Assembly of new viral particles.
  • Host cell lysis via enzymes like lysozyme, releasing new viruses.
• Example: T4 bacteriophage.
• Rapidly produces many new viruses, often causing acute infections.

🌍 Real-World Connection: The lytic cycle explains why some viral diseases (e.g., influenza) cause sudden, severe symptoms.

📌 Lysogenic Cycle

• Viral DNA integrates into the host genome as a provirus (in eukaryotes) or prophage (in bacteria).
• Host cell continues normal functions while replicating viral DNA along with its own.
• The virus can remain dormant for years (latency).
• Environmental triggers (e.g., UV light, stress) can activate the lytic cycle.
• Lysogeny allows viruses to persist without killing the host.
• Example: Lambda bacteriophage, herpesviruses.

📝 Paper 2: Data Response Tip: In questions with viral life cycle diagrams, always identify key triggers for switching from lysogenic to lytic — these are often part of the mark scheme.

📌Definition Table

Term	Definition
Virus	An infectious particle made of nucleic acid enclosed in a protein coat, sometimes with an envelope.
Capsid	Protein shell that encloses the viral genome.
Envelope	Membrane derived from the host cell, containing viral proteins and glycoproteins.
Bacteriophage	Virus that infects bacteria.
Retrovirus	Virus with RNA genome that uses reverse transcriptase to integrate into host DNA.
Host Range	The spectrum of host species or cells a virus can infect.

📌Introduction

Viruses are non-cellular infectious agents that rely entirely on a host cell to reproduce. They have no metabolism or organelles and cannot carry out life processes independently. Structurally, viruses are made up of nucleic acids (DNA or RNA) surrounded by a capsid, and in some cases, an additional lipid envelope.

❤️ CAS Link: Develop an educational infographic explaining how viruses differ from living cells, for use in a school health awareness campaign.

📌 Basic Structure of Viruses

• All viruses contain genetic material — either DNA or RNA, but never both.
• The genome can be single-stranded (ss) or double-stranded (ds), and linear or circular.
• A capsid made of protein subunits (capsomeres) encloses and protects the genome.
• Some viruses have an envelope derived from the host cell membrane, containing viral glycoproteins.
• Enveloped viruses are generally more sensitive to heat, detergents, and desiccation.
• Non-enveloped viruses rely on their stable capsid for protection and tend to be more resistant to environmental changes.

🧠 Examiner Tip: Always state whether a virus is enveloped or non-enveloped when describing its structure — this is often linked to exam questions about viral survival outside a host.

📌 Structural Diversity in Viruses

• Helical viruses: Capsid proteins arranged in a spiral (e.g., tobacco mosaic virus).
• Icosahedral viruses: Symmetrical 20-sided capsid (e.g., adenoviruses).
• Complex viruses: More elaborate structures (e.g., bacteriophages with head-tail arrangement).
• Enveloped viruses: Have a lipid layer containing viral glycoproteins (e.g., influenza virus, HIV).
• Non-enveloped viruses: Lack lipid envelope (e.g., poliovirus).
• Capsid shape is determined by the arrangement of capsomeres and can influence host interactions.

🌍 Real-World Connection: The structural stability of non-enveloped viruses makes them persistent on surfaces, explaining why norovirus outbreaks spread rapidly in schools and cruise ships.

📌 Special Adaptations for Host Infection

• Viral surface proteins (ligands) bind to specific host cell receptors, determining host range.
• Bacteriophages have tail fibres for attachment to bacterial surfaces.
• Enveloped viruses enter cells via membrane fusion or endocytosis.
• Non-enveloped viruses often inject genetic material directly into the host cytoplasm.
• Some viruses encode proteins that suppress host immune responses.
• Structural adaptations allow evasion of immune detection and efficient cell entry.

📝 Paper 2: Data Response Tip: When interpreting virus diagrams, always label genome type, capsid, envelope, and any specialised attachment structures — missing one loses easy marks.

📌Definition Table

Term	Definition
Magnification	The number of times an image is enlarged compared to its actual size.
Resolution	The ability of a microscope to distinguish two points as separate.
Light Microscope	Microscope using visible light and lenses to view specimens.
Electron Microscope	Microscope using beams of electrons to achieve higher resolution.
Scanning Electron Microscope (SEM)	Electron microscope producing 3D surface images.
Transmission Electron Microscope (TEM)	Electron microscope producing 2D images of thin specimens.

📌Introduction

Comparing cell types and understanding microscopy techniques are essential for studying cell ultrastructure. Key differences between prokaryotic and eukaryotic cells, and between plant and animal cells, help explain how structural features relate to function. Microscopy allows these features to be visualised in detail, with advances in technology significantly improving our ability to study cells.

❤️ CAS Link: Organise a microscopy workshop for younger students, where they can observe plant and animal cells and learn how to calculate magnification from scale bars.

📌 Prokaryotic vs Eukaryotic Cells

• Prokaryotic cells are smaller (0.1–5 μm) and lack membrane-bound organelles; eukaryotic cells are larger (10–100 μm) and have organelles.
• DNA in prokaryotes is circular and located in the nucleoid; in eukaryotes it is linear and enclosed in a nucleus.
• Prokaryotes have 70S ribosomes, eukaryotes have 80S ribosomes (and 70S in mitochondria/chloroplasts).
• Prokaryotes reproduce by binary fission; eukaryotes by mitosis and meiosis.
• Prokaryotic cell walls contain peptidoglycan; eukaryotic plant cell walls contain cellulose.
• Prokaryotes generally lack cytoskeletal structures, while eukaryotes have a complex cytoskeleton.

🧠 Examiner Tip: In “compare and contrast” questions, always balance similarities and differences — unbalanced answers can lose marks.

📌 Plant vs Animal Cells

• Plant cells have a rigid cell wall, large central vacuole, and chloroplasts; animal cells do not.
• Animal cells have centrioles for cell division; plant cells generally do not.
• Plant cells store starch; animal cells store glycogen.
• Lysosomes are more common in animal cells.
• Plasmodesmata allow communication between plant cells; gap junctions serve this role in animal cells.
• Both plant and animal cells have a plasma membrane, mitochondria, ER, Golgi apparatus, and ribosomes.

🌍 Real-World Connection: Understanding plant–animal cell differences is essential in biotechnology, such as tissue culture and crop genetic engineering.

📌 Light Microscopy vs Electron Microscopy

• Light microscopes use visible light and glass lenses; resolution ~200 nm.
• Electron microscopes use electron beams; resolution ~0.1 nm.
• TEM produces 2D images of thin specimens, showing internal structures.
• SEM produces 3D images of surfaces.
• Electron microscopy requires specimens to be in a vacuum and coated in metal.
• Light microscopy can be used for living cells; electron microscopy cannot.

📝 Paper 2: Data Response Tip: When given a micrograph with a scale bar, measure with a ruler in mm, convert to μm or nm, then apply the magnification formula — don’t forget units.

📌Definition Table

Term	Definition
Eukaryote	A cell containing a true nucleus and membrane-bound organelles.
Organelle	A specialised structure within a cell with a specific function.
Cytoskeleton	A network of protein filaments providing structural support and movement.
Endoplasmic Reticulum (ER)	Network of membranes for protein and lipid synthesis (rough ER) or lipid metabolism (smooth ER).
Golgi Apparatus	Organelle that modifies, packages, and distributes proteins and lipids.
Mitochondrion	Site of aerobic respiration, producing ATP.

📌Introduction

Eukaryotic cells are larger and more complex than prokaryotic cells, found in animals, plants, fungi, and protists. They contain membrane-bound organelles that compartmentalise cellular functions, allowing greater efficiency and specialisation. Their internal complexity enables them to perform more advanced metabolic processes and support multicellular life.

📌 General Characteristics of Eukaryotes

• Possess a nucleus containing DNA wrapped around histones.
• Contain 80S ribosomes in the cytoplasm and 70S ribosomes in mitochondria and chloroplasts.
• Exhibit compartmentalisation — separation of functions into organelles.
• Larger cell size (10–100 μm) compared to prokaryotes.
• Have a complex cytoskeleton for movement, shape, and intracellular transport.
• Reproduce through mitosis (asexual) and meiosis (sexual).

🧠 Examiner Tip: Always include the presence of membrane-bound organelles when defining eukaryotic cells — many students miss this and lose marks.

📌 Nucleus and Genetic Control

• Enclosed by a double membrane (nuclear envelope) with pores for molecule exchange.
• Contains chromatin (DNA + proteins) and the nucleolus, where ribosomes are assembled.
• DNA is linear and associated with histones.
• Controls gene expression and coordinates cell activities.
• Nuclear pores allow mRNA and ribosomal subunits to exit to the cytoplasm.
• Replication and transcription occur inside the nucleus.

🌍 Real-World Connection: Knowledge of plant cell structure supports crop improvement strategies in agriculture.

📌 Endomembrane System

• Rough ER has ribosomes and synthesises proteins for secretion or membranes.
• Smooth ER synthesises lipids, detoxifies drugs, and stores calcium.
• Golgi apparatus modifies, sorts, and packages proteins and lipids into vesicles.
• Lysosomes contain hydrolytic enzymes for digesting macromolecules and old organelles.
• Secretory vesicles transport molecules to the plasma membrane for exocytosis.
• Endocytosis brings materials into the cell via vesicles.

🔍 TOK Perspective: The endosymbiotic theory shows how indirect molecular evidence can strongly support historical biological events.

📌 Specialisation: Plant vs Animal Cells

• Plant cells have a rigid cell wall, chloroplasts, and a large central vacuole.
• Animal cells have centrioles and more lysosomes.
• Plant vacuole maintains turgor pressure and stores materials.
• Cell wall provides structure and protection.
• Some plant cells have plasmodesmata for cell-to-cell communication.
• Animal cells often have specialised shapes for specific functions.

📝 Paper 2: Data Response Tip: When comparing plant and animal cells, use a table format for clarity and always include at least three similarities and three differences.

📌Definition Table

Term	Definition
Prokaryote	A unicellular organism lacking a true nucleus and membrane-bound organelles.
Nucleoid	Region in a prokaryotic cell containing the circular DNA molecule.
Plasmid	Small, circular DNA molecule in prokaryotes, often carrying extra genes.
Binary Fission	A method of asexual reproduction in prokaryotes where the cell divides into two identical cells.
Pili	Hair-like structures on bacteria used for attachment or DNA transfer.
Flagella	Long, whip-like structures used for movement in some prokaryotes.

📌Introduction

Prokaryotic cells are the simplest and oldest forms of life, found in domains Bacteria and Archaea. They are small, typically 0.1–5.0 μm, and lack a nucleus and membrane-bound organelles. Despite their simplicity, prokaryotes display a diverse range of adaptations, enabling them to survive in almost every environment on Earth.

❤️ CAS Link: Create a public awareness campaign about biofilms and their role in medical device infections.

📌 General Characteristics of Prokaryotes

• Unicellular with simple internal organisation.
• DNA is located in the nucleoid region, not enclosed by a membrane.
• Contain 70S ribosomes for protein synthesis.
• Reproduce mainly through binary fission.
• Some possess additional DNA in plasmids, which can be transferred between cells.
• Cell wall composition varies — Gram-positive vs Gram-negative bacteria.

🧠 Examiner Tip: Always state “70S ribosomes” for prokaryotes and “80S ribosomes” for eukaryotes — mixing them up is a common error in exams.

📌 Cell Wall and Plasma Membrane

• Cell wall maintains shape, protects against osmotic pressure, and provides structural support.
• Peptidoglycan is the main component in bacterial cell walls.
• Gram-positive bacteria have thick peptidoglycan layers; Gram-negative have thinner layers and an outer membrane.
• Plasma membrane controls substance exchange with the environment.
• Some bacteria have a capsule for protection and attachment to surfaces.
• Archaea have cell walls without peptidoglycan (pseudopeptidoglycan instead).

🌍 Real-World Connection: Antibiotics like penicillin target bacterial cell wall synthesis, making them ineffective against Archaea.

📌 Genetic Material: Nucleoid and Plasmids

• Nucleoid contains a single circular chromosome made of DNA.
• DNA is supercoiled to fit into the cell.
• Plasmids carry extra genes, often for antibiotic resistance.
• Plasmids can be transferred between bacteria via conjugation (pili-mediated).
• Chromosome replication is linked to binary fission.
• No histones in bacteria, but Archaea have histone-like proteins.

📌 Movement and Surface Structures

• Flagella provide motility, enabling movement towards nutrients or away from toxins.
• Flagellar movement is driven by a rotary motor powered by proton gradients.
• Pili aid in surface attachment and genetic exchange.
• Fimbriae are shorter and more numerous, mainly for adhesion.
• Some prokaryotes secrete a slime layer to aid movement on surfaces.
• Motility and adhesion are important for colonisation in both pathogenic and symbiotic bacteria.

📌 Binary Fission

• DNA is replicated beginning at the origin of replication.
• Chromosomes attach to the cell membrane and are pulled apart as the cell elongates.
• The plasma membrane pinches inwards to divide the cell.
• Produces two genetically identical daughter cells.
• Occurs rapidly under favourable conditions — some bacteria divide every 20 minutes.
• Allows for fast population growth but limits genetic diversity unless horizontal gene transfer occurs.

📝 Paper 2: Data Response Tip: If asked to draw or label a prokaryotic cell, always include: cell wall, plasma membrane, cytoplasm, nucleoid, plasmid, 70S ribosomes, pili, and flagella — missing any of these may cost marks.