IB DP Biology Origins of Cell Study Notes
IB DP Biology Cellular Structure Study Notes
IB DP Biology Cellular Structure Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of
- What plausible hypothesis could account for the origin of life?
- What intermediate stages could there have been between non-living matter and the first living cells?
Standard level and higher level: 4 hours
Additional higher level: 1 hour
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
Additional Higher Level
A2.1.1 – Conditions on Early Earth & Pre-Biotic Formation of Carbon Compounds
🌍 What Was Early Earth Like?
• No oxygen (O₂) in atmosphere
• Intense UV and lightning
• Volcanic activity
• Reducing environment
No free oxygen in the atmosphere meant no ozone layer, allowing intense UV radiation to reach Earth’s surface. The early atmosphere was anoxic (oxygen-free).
Atmosphere Gases:
- High in: CO₂, CH₄ (methane), NH₃ (ammonia), water vapour
- Low or absent: O₂
Frequent Volcanic Eruptions:
- Released gases like CH₄, CO₂, NH₃, H₂O
- Caused extreme heat and unstable surfaces
Intense Energy Sources:
- Lightning storms
- UV radiation from the sun
- Heat from volcanic activity
- Possibly high cosmic radiation (due to weak magnetic field)
💧 Role of Water (Aqueous Environment)
The early oceans acted as a medium for pre-biotic chemistry. Water allowed molecules to mix, react, and remain stable over time, facilitating the origin of life.
🔁 How Did Carbon Compounds Form Pre-Biotically?
Atmospheric gases like CO₂, CH₄, and NH₃ reacted under energy input (lightning, UV radiation), leading to the formation of simple organic compounds:
- Amino acids
- Hydrocarbons
- Fatty acids
Such reactions are no longer common today due to the presence of oxygen, which breaks down these unstable molecules.
🔬 Key Concepts
Enabled formation of complex carbon molecules
Organic molecules could form without life
A2.1.2 – Cells as the Smallest Units of Self-Sustaining Life
🧫 Cell Theory – The Foundation of Biology
• All cells come from pre-existing cells
• The cell is the smallest living unit
The cell theory states that all living organisms are composed of cells. Every new cell comes from the division of an existing cell, and the cell is the smallest structure capable of independent life.
💡 What Makes Something Living?
Living organisms carry out eight essential life functions, remembered by the acronym MR H GREN:
| Function | Description |
|---|---|
| Metabolism | All the chemical reactions in the organism (e.g. respiration, digestion) |
| Reproduction | Ability to produce offspring (asexual or sexual) |
| Homeostasis | Maintaining internal balance (e.g. temperature, pH) |
| Growth | Increase in size or number of cells |
| Response | Reacting to stimuli in the environment |
| Excretion | Removing waste products from metabolism |
| Nutrition | Obtaining and processing food to supply energy |
| Movement | Changing position or moving internal parts (also in unicellular organisms) |
🦠 Why Are Viruses Considered Non-Living?
| Characteristic | Virus | Living Cell |
|---|---|---|
| Made of cells | No | Yes |
| Independent metabolism | No metabolism | Performs all metabolic functions |
| Reproduces independently | Needs host cell | Reproduces on its own |
| Responds to stimuli | Generally not | Yes |
| Uses DNA or RNA | Yes | Yes |
| Growth | No | Yes |
| Homeostasis | No | Yes |
🔍 Conclusion
A2.1.3 – Challenge of Explaining the Spontaneous Origin of Cells
🧠 Why Is the Origin of Cells Difficult to Explain?
Cells are highly organized systems with complex interactions. According to modern cell theory, new cells arise only from existing ones. This makes it difficult to understand how the first living cell could have formed on its own from non-living molecules.
🔑 Key Requirements for the First Cells to Emerge
2. Self-replication: RNA or similar molecules must replicate themselves.
3. Self-assembly: Lipids must spontaneously form membranes or vesicles.
4. Compartmentalization: Membranes must enclose systems to localize reactions and protect fragile molecules.
Obstacles in Testing These Hypotheses
| Scientific Challenge | Explanation |
|---|---|
| No fossil evidence of protocells | Soft, microscopic structures didn’t fossilize |
| Inability to replicate early Earth perfectly | Ancient conditions (gases, temperature, minerals) remain uncertain |
| Limitations in lab simulations | Experiments cannot capture billions of years or all natural environments |
🔍 Nature of Science Insight
Some scientific ideas like the origin of the first cells are difficult to test directly. But if they are consistent with evidence, generate predictions, and can be falsified, they remain scientifically valid.
Miller – Urey and Pre-Cellular Chemistry
The 1953 Miller – Urey experiment simulated early Earth conditions and showed that amino acids and other organic molecules could form abiotically. This supported the idea that prebiotic chemistry could generate the building blocks of life.
Why Can’t Cells Form Spontaneously Today?
• Existing organisms rapidly consume or destroy organic precursors
• Conditions today are very different from the early Earth, making spontaneous cell formation virtually impossible
A2.1.4 – Evidence for the Origin of Carbon Compounds
🌍 Early Earth Conditions
- No oxygen or ozone layer
- Atmosphere rich in methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water vapour (H₂O)
- Frequent lightning, volcanic eruptions, and intense UV radiation
- Hot, unstable, and chemically reactive environment
The Miller – Urey Experiment (1953)
Stanley Miller and Harold Urey tested whether life’s building blocks could form naturally under simulated early Earth conditions.
Mixed gases (CH₄, NH₃, H₂, H₂O) in a closed system
Applied electrical sparks to simulate lightning
Allowed the gases to circulate for a week
🔍 What they found:
Amino acids (protein building blocks) formed spontaneously
Other small organic molecules appeared
Proved organic compounds could form under early Earth-like conditions
Why This Matters
It gave the first experimental evidence that life’s essential molecules can arise without life itself. This supported the theory of abiogenesis the chemical origin of life.
Limitations & Alternative Views
| Concern | Explanation |
|---|---|
| Gases used | Assumed a reducing atmosphere; real early Earth may have had more CO₂, less CH₄/NH₃ |
| No real life formed | Formed amino acids, not full cells or genetic molecules |
| Other theories | Organic compounds may have come from space or deep-sea hydrothermal vents |
🧠 Final Conclusion
A2.1.5 – Spontaneous Formation of Vesicles by Coalescence of Fatty Acids
🧠 Why Is This Important?
Early life needed some way to organize and protect its molecules. The spontaneous formation of vesicles made this possible, even before enzymes or genes existed.
🧪 What Happened on Early Earth?
- Fatty acids could have naturally formed from simple carbon compounds
- These molecules have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails
- In water, they spontaneously arrange into bilayers and fold into vesicles – spherical compartments that enclose fluid
These vesicles resembled early cell membranes and didn’t require enzymes, proteins, or any life to form – just water and fatty acids.
💡 Why Is This Crucial for Early Life?
| Feature | Explanation |
|---|---|
| Compartmentalization | Created internal environments where reactions could happen safely and separately |
| Controlled chemistry | Vesicles could hold different pH or molecule concentrations than their surroundings |
| Step toward cells | Provided early structure to trap RNA, proteins, or other essential molecules |
| Experimental evidence | Lab experiments show vesicles form spontaneously from fatty acids under prebiotic conditions |
🧬 From Vesicles to Cells…
These simple vesicles could grow, merge, divide, and trap molecules behaviors similar to primitive cells. Over time, membranes evolved to include proteins and complex structures like the nucleus and organelles.
Spontaneous formation of vesicles from fatty acids provided early life with separation, protection, and control — making complex chemistry and cell-like functions possible even before true cells existed.
A2.1.6 – RNA as the Presumed First Genetic Material
🧠 Why Do Scientists Think RNA Came Before DNA?
RNA has two crucial properties that make it a candidate for the first biomolecule:
- Replication – it can copy itself under certain conditions.
- Catalysis – it can act like an enzyme, speeding up reactions.
🧪 The “RNA World” Hypothesis
This hypothesis suggests life began in a world where RNA molecules carried genetic information and performed enzymatic functions:
- RNA formed from simple molecules in early Earth conditions
- Some RNA could self-replicate using catalytic RNA (ribozymes)
- RNA catalyzed basic chemical reactions, including protein formation
- Vesicles formed, enclosing RNA to create primitive protocells
- DNA evolved later for stability; proteins evolved for better catalysis
🧬 Modern Evidence Supporting RNA’s Role
| Feature | Explanation |
|---|---|
| Self-replication | RNA can pair with complementary bases and copy itself (in lab settings) |
| Catalysis | Ribozymes in ribosomes still catalyze peptide bond formation today |
| Versatility | One molecule can both store instructions and catalyze reactions |
| Still used in cells | Ribosomal RNA is essential in all cells, reflecting its ancient role |
RNA may have been the first molecule of life, acting as both information carrier and catalyst. Over time, DNA replaced it for stability and proteins for more efficient catalysis but RNA’s role is still visible in modern cells.
A2.1.7 – Evidence for a Last Universal Common Ancestor (LUCA)
🌍 What is LUCA?
LUCA = Last Universal Common Ancestor
A primitive cell-like organism that lived ~3.5 billion years ago, likely in hydrothermal vents.
LUCA is the proposed ancestor of all modern life. It was probably:
- A simple unicellular organism
- Existed in high-temperature environments (e.g., deep-sea vents)
- Possibly used RNA first, but later DNA for genetic storage
🔬 Evidence Supporting LUCA’s Existence
| Evidence | Explanation |
|---|---|
| Universal Genetic Code | All living organisms use the same 64 codons to code for amino acids. |
| Shared Genes | Core genes (e.g., ribosomes, ATP synthase) are conserved across all domains. |
| Cellular Structure | All cells have phospholipid membranes, nucleic acids, and ribosomes. |
What Happened to Other Life Forms?
Many primitive life forms may have existed. LUCA’s descendants outcompeted or survived better, while others went extinct.
🌿 Lineage After LUCA
| Domain | Origin |
|---|---|
| Bacteria | Early divergence from LUCA |
| Archaea | Also diverged early from LUCA |
| Eukarya | Evolved later through endosymbiosis |
Endosymbiosis Theory (Linked to LUCA)
Eukaryotes evolved when a LUCA-descendant cell engulfed smaller prokaryotes, which became mitochondria or chloroplasts.
This is supported by the fact that these organelles contain their own DNA and ribosomes.
LUCA is the most recent common ancestor of all life. Shared molecular traits — like the genetic code and ribosomes — support its existence. While other early life forms may have existed, LUCA’s descendants gave rise to all modern life.
A2.1.8 – Approaches Used to Estimate Dates of the First Living Cells and LUCA
Why Do We Date Early Life?
Dating early life helps scientists determine when the first living cells appeared and estimate the timeline for LUCA (Last Universal Common Ancestor).
🧬 Method 1: Fossil Evidence – Stromatolites
Stromatolites are layered structures formed by ancient microbial mats.
The oldest known stromatolites are about 3.42 billion years old.
Conclusion: Life must have existed at least 3.42 billion years ago.
🧬 Method 2: Carbon Isotope Analysis
Living organisms preferentially use lighter carbon isotopes (¹²C over ¹³C).
Some ancient rocks contain carbon isotope ratios that suggest biological origin – as early as 4.1–4.2 billion years.
Conclusion: Life may have started even earlier than fossil records indicate.
⚠️ Challenges in Estimating Dates
| Challenge | Why it matters |
|---|---|
| Lack of old rocks | Earth’s crust is recycled by plate tectonics – rocks older than 4 billion years are rare. |
| Ambiguous evidence | Some chemical signatures may have formed through non-biological (e.g., volcanic) processes. |
| LUCA didn’t fossilize | LUCA likely lived in deep-sea hydrothermal vents, leaving no fossil evidence behind. |
🧩 Putting It Together
Scientists synthesize fossil and chemical data to estimate:
- First cells: at least 3.4–3.5 billion years ago
- LUCA: possibly older, but date remains uncertain
Multiple methods – fossil stromatolites and carbon isotope data – suggest that life has existed for over 3.4 billion years. The LUCA likely predates these fossils, but precise dating remains difficult due to limited ancient evidence.
A2.1.9 – Evidence for the Evolution of the Last Universal Common Ancestor (LUCA) in the Vicinity of Hydrothermal Vents
What Are Hydrothermal Vents?
Hydrothermal vents are deep-sea openings in the ocean floor where superheated, mineral-rich water escapes. They occur near mid-ocean ridges and provide unique chemical and thermal environments.
🧬 Why Are Hydrothermal Vents Important to the Origin of Life?
These vents supply the critical components early life may have needed:
- Heat and chemical energy to drive reactions
- Reduced compounds like H2, CH4, H2S
- Mineral-rich surfaces that may act as catalysts
Fossil Evidence: Life at Ancient Vents
Fossilized microbial structures have been found in ancient mineral deposits from vent environments, dating back over 3.7 billion years.
These fossils formed in metal-rich conditions – consistent with hydrothermal vent activity – and suggest early microbial life adapted to vent settings.
🧪 Genomic Evidence: LUCA’s Biochemical Traits
Comparative studies of genes across all life domains suggest LUCA had:
- Genes for hydrogen metabolism
- Genes for carbon dioxide fixation
- Genes for nitrogen processing
These traits are common among microbes currently found in hydrothermal vents, supporting the idea that LUCA evolved in a similar habitat.
Conclusion: LUCA likely evolved in a hot, anaerobic, mineral-rich environment – similar to modern hydrothermal vents.
🔬 Summary Table
| Evidence | Key Findings |
|---|---|
| Fossils | Microbial fossils in ancient vent rocks suggest early life adapted to these settings. |
| Genomic Analysis | LUCA’s genes resemble those of vent-dwelling microbes. |
| Chemistry | Vents provide energy, minerals, and reduced chemicals for prebiotic reactions. |
Hydrothermal vents are one of the strongest candidates for the cradle of life. LUCA’s traits and fossil evidence strongly support a vent-based origin – and research continues to uncover more clues.