📌Definition Table

📌Introduction

Evidence for the evolution of life combines molecular biology, geology, and paleontology. DNA and protein comparisons reveal shared ancestry, while fossils and dating techniques establish timelines for life’s emergence. Geological evidence suggests life may have originated in extreme environments, such as hydrothermal vents, where chemical energy supported early organisms.

❤️ CAS Link: Create an interactive school exhibit showing models of hydrothermal vents and extremophile habitats.

📌 Molecular Evidence for Common Ancestry

• All living organisms share the same genetic code, supporting a common origin.
• DNA sequences and protein structures show strong similarities across species.
• Universality of basic biochemical pathways (glycolysis, ATP synthesis) points to a shared ancestor.
• Molecular clock analysis uses mutation rates to estimate divergence times.
• Similarity in ribosomal RNA sequences is a strong evolutionary marker.
• LUCA likely had a simple but fully functioning genetic and metabolic system.

🧠 Examiner Tip: In “evidence” questions, include molecular, structural, and biochemical similarities — not just DNA sequences.

📌 Geological and Fossil Evidence

• The oldest known microfossils date to around 3.5 billion years ago.
• Stromatolites, layered structures from microbial mats, provide early evidence of life.
• Fossils are dated using radiometric methods such as uranium–lead or potassium–argon dating.
• Sedimentary rock layers help reconstruct the sequence of evolutionary events.
• Fossil evidence is limited due to the rarity of preservation and erosion over time.
• Isotopic signatures in ancient rocks suggest biological carbon fixation occurred early in Earth’s history.

🌍 Real-World Connection: Fossil dating techniques are also applied in archaeology to date ancient human remains and artefacts.

📌 Hydrothermal Vent Hypothesis

• Proposes that LUCA evolved in deep-sea hydrothermal vents.
• Vents provide heat, minerals, and chemical gradients — all potential energy sources.
• Extremophile microbes found in vents today may resemble early life forms.
• Vents’ stable conditions could protect early life from surface hazards like UV radiation.
• Chemosynthetic bacteria use vent chemicals to produce food without sunlight.
• Fossil and chemical evidence suggests vent ecosystems existed over 3 billion years ago.

🔍 TOK Perspective: How do scientists decide between competing origin hypotheses when direct observation is impossible?

🌐 EE Focus: An EE could analyse genomic evidence of extremophile adaptations to assess whether vent environments are plausible origins of life

📝 Paper 2: Data Response Tip: When explaining the hydrothermal vent hypothesis, always include energy source, protection from UV, and chemical gradients for full marks

📌Definition Table

📌Introduction

The evolution of cells marks the transition from non-living chemistry to living systems capable of growth, reproduction, and metabolism. This process involved the development of self-replicating molecules, the formation of protective membranes, and the emergence of the first prokaryotic cells. While several hypotheses exist, evidence points to RNA as playing a key early role in life’s origin.

📌 Stages in the Origin of Cells

• Abiotic synthesis of organic molecules (amino acids, nucleotides) in early Earth conditions.
• Polymerisation into macromolecules such as proteins and nucleic acids.
• Self-replication: first genetic material likely RNA due to its dual role as information carrier and catalyst.
• Compartmentalisation: formation of membranes/vesicles to protect and concentrate reactions.
• Development of metabolic pathways to harness energy and resources.
• Transition from protocells to the Last Universal Common Ancestor (LUCA).

🧠 Examiner Tip: Always mention compartmentalisation as a key step — many students forget it and lose marks in origin-of-life questions.

📌 RNA World Hypothesis

• RNA can store genetic information and act as an enzyme (ribozyme).
• This dual role supports the idea that RNA preceded DNA and proteins.
• RNA ribozymes could catalyse their own replication in early life.
• DNA likely evolved later for greater stability in information storage.
• Proteins took over most catalytic functions due to greater versatility.
• Lab experiments have shown that short RNA sequences can self-replicate under certain conditions.

📌 Compartmentalisation and Protocells

• Fatty acids can spontaneously form vesicles in water.
• Vesicles can encapsulate RNA, proteins, and other molecules.
• This separation from the environment allows controlled internal chemistry.
• Vesicles can grow and divide without complex machinery.
• Early membranes were likely more permeable than modern phospholipid bilayers.
• Protocells could have formed naturally in volcanic pools or oceanic hydrothermal vents.

⚗️ IA Tips & Guidance: Simple lipid vesicle experiments can be modelled in the lab using micelle and emulsion formation — ideal for chemistry–biology crossover IAs.

📌 Competing Hypotheses: Gene-First vs Metabolism-First

• Gene-First Model: Self-replicating molecules (RNA/DNA) appeared first, later supported by metabolism.
• Metabolism-First Model: Self-sustaining chemical cycles evolved first, creating a framework for genetic material to develop.
• Hydrothermal vents could have supported metabolism-first life by providing continuous chemical gradients.
• Both models may have operated together — early metabolic cycles could stabilise and support genetic molecules.
• Debate continues due to limited fossil and experimental evidence.

🌐 EE Focus: An EE could explore metabolism-first vs gene-first origins using modern synthetic biology studies as evidence.

📝 Paper 2: Data Response Tip: In origin-of-life questions, clearly link abiotic synthesis → polymerisation → replication → membranes in the correct sequence.

📌Definition Table

📌Introduction

The formation of carbon compounds was a key step in the origin of life. Before cells existed, simple molecules like water, methane, ammonia, and hydrogen reacted to produce the organic building blocks of life — amino acids, sugars, lipids, and nucleotides. These compounds then assembled into polymers and eventually into membrane-bound structures, paving the way for the first living cells.

❤️ CAS Link: Organize a science outreach activity where students build models of early Earth and simulate vesicle formation using soap bubbles or oil-in-water emulsions.

📌 Conditions on Early Earth

• Early Earth’s atmosphere likely contained methane, ammonia, hydrogen, and water vapour, but little to no oxygen.
• High energy sources — UV radiation, lightning, volcanic heat — drove chemical reactions.
• Oceans acted as a solvent where compounds could accumulate.
• No ozone layer meant high UV penetration, which could both drive synthesis and degrade molecules.
• These conditions favoured the formation of organic molecules from inorganic gases.

🧠 Examiner Tip: In essays, link atmospheric composition and energy sources to the formation of organic molecules — omitting one loses marks.

📌 Primordial Soup Hypothesis

• Proposed independently by Oparin and Haldane in the 1920s.
• Suggests early oceans formed a “soup” of organic molecules from atmospheric gases.
• Energy from lightning and UV drove the formation of simple organic compounds.
• Molecules accumulated in oceans, providing building blocks for life.
• This set the stage for polymerisation and protocell formation.
• Hypothesis has been tested experimentally with partial success.

🌍 Real-World Connection: Understanding abiogenesis informs astrobiology, guiding the search for life on planets like Mars and moons like Europa.

📌 Miller–Urey Experiment

• Conducted in 1953 to test the primordial soup hypothesis.
• Simulated early Earth atmosphere with methane, ammonia, hydrogen, and water vapour.
• Passed electric sparks to mimic lightning.
• After a week, found amino acids and other simple organic molecules in the apparatus.
• Proved organic compounds can form under abiotic conditions.
• Later research showed early Earth atmosphere may have been less reducing, raising questions about the experiment’s exact relevance.

📌 Assembly into Polymers

• Organic monomers can link to form polymers (proteins, nucleic acids, polysaccharides).
• Polymerisation can occur on mineral surfaces like clay, which act as catalysts.
• Lipids can spontaneously form micelles and vesicles in water.
• Vesicles provide compartmentalisation, protecting reactions from the external environment.
• These membrane-bound structures are possible precursors to the first protocells.
• Fatty acids, more permeable than modern phospholipids, may have formed primitive membranes.

⚗️ IA Tips & Guidance: For a practical link, consider IAs on self-assembly of lipids into micelles/vesicles or polymerization under simulated conditions — easy to model with safe lab analogues.

🌐 EE Focus: A possible EE topic could be comparing pathways for abiotic synthesis of amino acids in different atmospheric models, linking to planetary habitability.

📝 Paper 2: Data Response Tip: If shown the Miller–Urey setup, label the gas mixture, condenser, spark chamber, and collection trap. Always mention the type of molecules produced.

📌 Definition Table

📌 Introduction

The history of nucleic acids is shaped by key experiments and technological advances that proved DNA, not protein, is the genetic material, and revealed its structure and function. These discoveries illustrate how scientific knowledge evolves through falsification, technological innovation, and collaboration.

📌 Hershey – Chase Experiment

• Used bacteriophages (viruses that infect bacteria) to test whether protein or DNA carried genetic information.
• DNA was labeled with radioactive phosphorus (³²P), protein coat with radioactive sulfur (³⁵S).
• After infection, ³²P was found inside bacteria, ³⁵S remained outside.
• Conclusion: DNA is the genetic material passed to offspring, not protein.
• Provided final confirmation after earlier evidence from other studies.

🧠 Examiner Tip: State both radioisotopes and which macromolecule each labels — examiners award marks for detail.

🌍 Real-World Connection: Radioisotope tracing is now used in medicine (PET scans) and forensic science.

📌 Chargaff’s Rules

• Erwin Chargaff discovered that in any DNA sample: %A ≈ %T and %G ≈ %C.
• Ratios vary between species, but A always pairs with T and G with C.
• This disproved the tetranucleotide hypothesis (equal quantities of all bases).
• Provided key evidence for complementary base pairing in the double helix.
• Also explained why purines pair with pyrimidines for uniform helix width.

🔍 TOK Perspective: Chargaff’s work shows how unexpected data can challenge accepted theories, raising the question — how open should science be to overturning long-held beliefs?

📌 Falsification of the Tetranucleotide Hypothesis

• Early scientists believed DNA was too simple to carry genetic information.
• Proposed repeating units of four bases in equal amounts.
• Chargaff’s data disproved this — base composition is species-specific.
• This paved the way for the Watson–Crick model of DNA.
• Shows how scientific theories are discarded when evidence contradicts them.

🌐 EE Focus: Explore the process of falsification in biology by comparing historical theories about DNA with modern understanding.

📌 Technological Advances in Molecular Biology

• X-ray diffraction (Franklin & Wilkins) revealed DNA’s helical structure.
• Molecular visualization software now models DNA and proteins in 3D.
• Computer modelling helps predict protein folding and genetic interactions.
• Databases store genome sequences for research and biotechnology.
• These tools improve our understanding of structure–function relationships in biology.

⚗️ IA Tips & Guidance: Consider using online DNA modeling tools or protein databases in IAs to investigate structural properties.

📝 Paper 2: Data Response Tip: When asked about experiments like Hershey–Chase, always include the method, results, and conclusion. Missing any one of these will lose marks.

📌 Definition Table

📌 Introduction

The genetic code is the link between the information stored in nucleic acids and the proteins that carry out cellular functions. Its structure, universality, and redundancy allow life to exist and evolve while maintaining accuracy in protein synthesis. The way DNA is transcribed and translated is fundamental to understanding heredity, gene expression, and biotechnology.

📌 Nature of the Genetic Code

• Made of triplet codons — each codon specifies one amino acid or a stop signal.
• Code is universal — the same in nearly all organisms, evidence for common ancestry.
• Code is redundant — multiple codons can code for the same amino acid.
• Code is non-overlapping — codons are read sequentially in sets of three.
• Start codon AUG codes for methionine and begins translation.
• Stop codons UAA, UAG, UGA terminate protein synthesis.

🧠 Examiner Tip: Always include “triplet, non-overlapping, universal, redundant” when defining the genetic code in exams.

📌 Storage and Transmission of Information

• DNA stores instructions for building proteins in the sequence of its bases.
• Complementary base pairing ensures faithful replication during cell division.
• mRNA carries a temporary copy of a gene from DNA to ribosomes.
• The sequence of bases in DNA directly determines protein structure.
• Changes in base sequence can alter amino acid sequence, potentially affecting function.
• Genome size and gene number vary widely across organisms.

🌍 Real-World Connection: Genome sequencing projects help identify genes linked to diseases, enabling targeted therapies and personalized medicine.

📌 Protein Synthesis Overview

• Transcription: DNA sequence is copied into mRNA by RNA polymerase.
• mRNA processing in eukaryotes includes splicing, capping, and poly-A tail addition.
• Translation: Ribosomes read codons on mRNA to assemble amino acids in sequence.
• tRNA molecules match their anticodons to mRNA codons, delivering amino acids.
• Ribosomes catalyse peptide bond formation to create a polypeptide chain.
• Folding and modifications create a functional protein.

⚗️ IA Tips & Guidance: In protein synthesis modelling or bioinformatics IAs, link codon sequences to protein structure and function.

📌 Universality and Biotechnology

• The universality of the genetic code allows gene transfer between species.
• Bacteria can produce human proteins like insulin when given the human gene.
• Genetically modified crops use foreign genes to gain desired traits.
• The redundancy of the code reduces harmful effects of some mutations.
• Ethical debates arise over the use of genetic engineering in food and medicine.
• The universality also supports the theory of evolution.

🌐 EE Focus: An EE could investigate mutation effects on protein function using in-silico translation tools.

📌 Errors and Mutations in the Code

• Mutations are changes in DNA base sequence — may be silent, missense, or nonsense.
• Silent mutations do not change the amino acid sequence due to redundancy.
• Missense mutations replace one amino acid with another, possibly altering function.
• Nonsense mutations create premature stop codons, shortening proteins.
• Frameshift mutations (insertion/deletion) alter the reading frame, often severely damaging function.
• Mutation effects depend on location, type, and protein function.

🔍 TOK Perspective: Are “errors” in the genetic code always harmful, or can they be a source of creativity in evolution?

📝 Paper 2: Data Response Tip: If given a DNA or mRNA sequence, be ready to transcribe and translate it, then determine amino acids. Show all working to get method marks.

📌 Definition Table

📌 Introduction

Nucleic acids are the molecules of heredity — storing, transmitting, and expressing genetic information in all living organisms. Their structure, composed of nucleotides arranged in specific sequences, determines how genetic instructions are preserved and used. The structure of DNA and RNA underpins every process in molecular biology, from replication to protein synthesis.

📌 DNA as Genetic Material

• DNA is found in the nucleus of eukaryotes and the cytoplasm of prokaryotes.
• It stores hereditary information in the form of base sequences.
• Viruses may have DNA or RNA as their genetic material.
• The double helix structure allows stability and long-term information storage.
• DNA’s complementary base pairing supports accurate replication.
• Its stability allows it to persist for generations without rapid degradation.

🧠 Examiner Tip: In “structure–function” questions, link double helix stability to information storage and complementary base pairing to accurate replication.

📌 Components of Nucleotides

• Each nucleotide contains a pentose sugar (deoxyribose in DNA, ribose in RNA).
• A phosphate group forms the sugar–phosphate backbone.
• Nitrogenous bases are either purines (A, G) or pyrimidines (C, T, U).
• Bases attach to the 1′ carbon of the sugar; phosphate attaches to the 5′ carbon.
• The sugar–phosphate backbone is connected by phosphodiester bonds.
• Nitrogenous bases form the genetic code through specific sequences.

⚗️ IA Tips & Guidance: In gel electrophoresis or molecular modeling IAs, discuss how nucleotide structure (size, charge) influences DNA movement or stability.

📌 Structure of DNA

• DNA consists of two antiparallel strands forming a double helix.
• Strands are held together by hydrogen bonds between complementary bases.
• Adenine pairs with thymine (A–T) via two hydrogen bonds.
• Guanine pairs with cytosine (G–C) via three hydrogen bonds.
• The helical twist creates major and minor grooves where proteins can bind.
• DNA is always read and replicated in the 5′→3′ direction.

🌐 EE Focus: Explore how GC content affects DNA melting temperature in organisms adapted to extreme environments.

📝 Paper 2: Data Response Tip: If a diagram shows DNA, label the 5′ and 3′ ends, indicate hydrogen bonds, and state the number of bonds in each base pair for full marks.

📌 Structure of RNA

• RNA is usually single-stranded and shorter than DNA.
• Contains ribose sugar and uracil instead of thymine.
• Three main types: mRNA (codes for proteins), tRNA (transfers amino acids), and rRNA (structural component of ribosomes).
• Can fold into secondary structures due to internal base pairing.
• Less stable than DNA — suited for short-term genetic instructions.
• RNA can act as an enzyme (ribozymes) as well as an information carrier.

❤️ CAS Link: Create a school science workshop using models to show differences between DNA and RNA, highlighting their roles in cells.

📌 Nucleosomes and DNA Packaging

• In eukaryotes, DNA wraps around histone proteins to form nucleosomes.
• Each nucleosome contains eight histone subunits.
• Nucleosomes compact DNA so it fits inside the nucleus.
• They help regulate gene expression by controlling accessibility to DNA.
• DNA between nucleosomes is called linker DNA.
• Histone modifications can activate or silence genes.

🌍 Real-World Connection: Epigenetic research studies how histone modifications change gene expression, which is important in cancer and aging.

🔍 TOK Perspective: Models of DNA simplify its structure for understanding. To what extent does this simplification distort the reality of how DNA functions in the cell?

📌 Definition Table

📌 Introduction

Water has a set of unique physical and chemical properties that make it ideal for supporting life. These include its cohesive and adhesive behavior, thermal stability, solvent capabilities, and low density of ice. Most of these properties stem from water’s polarity and hydrogen bonding, making it a critical component in temperature regulation, transport, metabolism, and survival of organisms in various habitats.

📌 Cohesion and Adhesion

• Cohesion results from hydrogen bonds between water molecules, allowing them to stick together.
• This creates surface tension, allowing some organisms (like pond skaters) to walk on water.
• Cohesion enables the formation of continuous water columns in plant xylem.
• Adhesion allows water to bind to polar surfaces like cellulose in plant cell walls.
• The combination of cohesion and adhesion drives capillary action, helping water move through narrow tubes.
• These forces are vital for water transport in plants during transpiration.

🧠 Examiner Tip: Use “hydrogen bonding” to explain both cohesion and adhesion, and relate them to plant transport or surface tension in short-answer questions.

📌 Water as a Solvent

• Water’s polarity allows it to dissolve ionic and polar compounds such as salts, sugars, and amino acids.
• Substances that dissolve in water are called hydrophilic, while non-polar substances are hydrophobic.
• Water forms hydration shells around ions, separating and stabilizing them in solution.
• It enables transport of dissolved substances in blood, lymph, and plant sap.
• Most biochemical reactions occur in aqueous solutions due to water’s solvent nature.
• Water facilitates interactions between molecules, making it a medium for metabolism.

⚗️ IA Tips & Guidance: When investigating solubility or reaction rates, discuss how water’s polarity and hydrogen bonding influence molecular interactions and transport mechanisms.

📌 Specific Heat Capacity

• Water has a high specific heat capacity of 4200 J/kg/°C, which means it resists rapid temperature changes.
• This stability is due to hydrogen bonds absorbing heat before increasing molecular motion.
• It helps maintain stable aquatic environments despite fluctuating air temperatures.
• Organisms can maintain homeostasis as water buffers against heat gain/loss.
• Aquatic animals benefit from minimized temperature extremes in water bodies.
• This is especially important for enzyme function, which depends on optimal temperatures.

🌐 EE Focus: An EE could investigate how water’s thermal properties influence organism survival or biochemical reaction rates in different temperature conditions.

📌 Thermal Conductivity and Latent Heat

• Water has relatively high thermal conductivity, helping distribute heat efficiently in organisms and environments.
• This property supports internal temperature regulation and even heat distribution in multicellular organisms.
• Water has a high latent heat of vaporization, requiring significant energy to evaporate.
• This allows for effective cooling mechanisms such as sweating or transpiration.
• Ice, due to stable hydrogen bonds, forms a lattice structure that insulates water beneath.
• These features make water excellent for thermoregulation and habitat stability.

🔍 TOK Perspective: How do we “know” water is essential for life? Are our definitions of life biased by Earth-based systems and water-dependent models?

📌 Density of Ice and Buoyancy

• Ice is less dense than liquid water due to hydrogen bonding creating a structured crystal lattice.
• This makes ice float, insulating the water below and protecting aquatic life in winter.
• Floating ice forms stable platforms for animals like seals and polar bears.
• This property contributes to the thermal stability of oceans and lakes.
• Aquatic organisms survive harsh winters because water beneath the ice remains liquid.
• Buoyancy is also affected by body composition; animals like seals use blubber for buoyancy and insulation.

❤️ CAS Link: Develop a CAS project involving ecosystem protection of aquatic life or education about climate change’s impact on ice habitats.

📌 Viscosity and Movement

• Water has a low viscosity compared to other fluids, enabling efficient flow through vessels.
• This allows blood, lymph, and xylem sap to move with minimal resistance.
• Organisms like fish and aquatic birds are adapted to water’s viscosity for smooth movement.
• Water’s viscosity supports buoyant force, helping organisms float or swim.
• Streamlined body shapes evolve to reduce drag and move efficiently through water.
• Lower viscosity compared to oil or honey makes water ideal for internal circulation systems.

🌍 Real-World Connection: Water’s physical properties are crucial in engineering, medical fields (IV fluids), and climate science. Its viscosity and heat properties are considered in designing life support and artificial habitats.

📝 Paper 2: Data Response Tips: Use terms like “high specific heat capacity” or “high latent heat of vaporization” to explain temperature or evaporation trends. Always connect the data back to hydrogen bonding for full marks.