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
A2.1.1—Conditions on early Earth and the pre-biotic formation of carbon compounds
Conditions of early Earth: –
Primitive Atmosphere:
– Composition included water vapor, nitrogen, carbon dioxide, methane, and traces of ammonia.
– Absence of free oxygen in the atmosphere.
– Methane and ammonia gases were in the early earths atmosphere, due to collisions of asteroids and in the many volcanic eruptions that were occurring,
Volcanic Activity:
– Intense volcanic activity characterized the Earth’s surface.
– Molten rock and frequent volcanic eruptions shaped the landscape.
Anoxic Environment:
– no ozone layer present in the atmosphere due to lack of free oxygen in the environment
– Oxygen, a product of photosynthesis, was not yet present in significant amounts.
Energy Sources:
– Frequent lightning storms provide electrical energy.
– Ultraviolet radiation from the Sun contributed to the energy available on Earth.
*This was due to the increased motion within the liquid core resulting in a smaller protective magnetic field than exists today, exposing the planet to much higher levels of cosmic and solar radiation.
Aqueous Environments:
– Oceans and other aqueous settings played a crucial role.
– Chemical reactions, essential for the formation of organic molecules, likely occurred in these environments.
Prebiotic Formation of Carbon Compounds: –
1. The atmosphere was thick with water vapour and other compounds from volcanic eruptions.
2. Lightning was a regular occurrence.
3. The gases, mainly CO2 and CH4, present in high concentration, which allowed UV to penetrate giving a high surface temp.
However, still No free oxygen so no ozone layer to protect from UV radiation.
– This led to a very hot Earth
– High temp and lightning followed by gradual cooling is thought to have led to the spontaneous generation of many carbon compounds.
The reducing gases in the atmosphere would have been able to donate electrons to other molecules, enabling chemical reactions to take place. These reactions resulted in the formation of more complex carbon compounds, including simple amino acids and hydrocarbons.
A2.1.2—Cells as the smallest units of self-sustaining life
Cell Theory: –
1. All organisms are made up of basic living units called cells.
2. All cells come from pre-existing cells
3. Cell is the smallest unit of life
Eight processes can be remembered by the acronym (MR HM GREN)
M – Metabolism
R – Response to stimuli
H – homeostasis
M – Movement
G – Growth
R – Reproduction
E – Excretion
N – Nutrition
Differences between something that is living and something that is non-living: –
Viruses are considered non-living because they are unable to reproduce outside of the host cell. They rely on the host cell for many life processes, including nutrition and growth, and they do not possess a metabolism
A2.1.3—Challenge of explaining the spontaneous origin of cells
Spontaneous origin of cells: –
1. The synthesis of small carbon compounds from abiotic molecules, as demonstrated in Miller-Urey experiments.
2. Small organic molecules need to join to form large-chain polymers. But conditions needs to be correct for this to happen. It is thought that deep sea hydrothermal vents could be perfect environments.
3. Polymers become contained by membranes, giving a protective homeostatic environment around the polymer.
Challenge in explaining the spontaneous origin of cells: –
Cells are highly complex structures that can currently only be produced by division of pre-existing cells.
However, Students should be aware that catalysis, self-replication of molecules, self-assembly and the emergence of compartmentalization were necessary requirements for the evolution of the first cells.
A2.1.4—Evidence for the origin of carbon compounds
Evidence for the origin of carbon compounds
The Miller and Urey Experiment is used to refer to the evidence for the origins of carbon compounds. The experiment stimulated Earth’s pre-biotic atmospheric conditions to show that spontaneous formation of organic molecules could occur
Urey and Miller’s experiment: –
Miller and Urey conducted experiments which showed that the compounds present in the early atmosphere (methane, ammonia and hydrogen (✓✓)) could be used to make the organic carbon compounds necessary for life. They did this by heating up water and passing the steam (✓✓) through ammonia, methane and hydrogen and by using electricity to simulate the lightning (✓✓). They found that these conditions created amino acids (✓✓) and other organic carbon compounds.
Some people not agree with Urey and Miller’s findings because: –
1. Some people think that the carbon compounds came from comets and meteorites colliding with Earth
2. If water was present with proteins then why did the proteins not separate into amino acids
This opposes the idea of a primordial soup leading to complex life.
3. Many scientists are now looking into hydrothermal vents as a more likely alternative to the development of life (A2.1.9)
4. Some scientists think that gases in the experiment were not present on early earth but rather released from volcanoes, however, these gases would have produced a non-reducing environment, due to lack of hydrogen.
A2.1.5—Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers
A necessary step in the evolution of the first cells was the spontaneous formation of vesicles by the coalescence (merge) of fatty acids into spherical bilayers. Phospholipids naturally assemble into bilayers, if conditions are correct
1. Formation of bilayer creates an isolated internal environment
2. The formation of an internal environment means that optimal conditions, e.g. for replication or catalysis can be maintained.
3. This sectioning is known as compartmentalization
4. It continued as the cells developed organelles with specialized functions
A2.1.6—RNA as a presumed first genetic material
The development of self-replicating molecules so that inheritance of characteristics can occur is hypothesised to be RNA. It may have acted initially as both the genetic material and the enzymes of the earliest cells.
1. RNA was formed from inorganic sources.
2. RNA was able to replicate using ribozymes.
3. RNA was able to catalyse protein synthesis.
4. Membrane compartmentalisation occurred.
5. Inside the cell, RNA was able to produce both protein and DNA.
6. DNA took over as the main genetic material because it is more stable.
7. Proteins took over as the catalytic form (enzymes) because they are more capable of variability.
A2.1.7—Evidence for a last universal common ancestor
LUCA (Last Universal Common Ancestor)
It is thought that LUCA was a simple, single-celled autotrophic microbe with probably an RNA genome that existed between 2.5 and 3.5 billion years ago. Although there are arguments that LUCA had a DNA genome. The final decision has yet to be made.
Scientists think that LUCA, or descendants of LUCA, outcompeted the other life forms existing on early Earth, leading to the extinction of the other life forms.
Outline evidences for LUCA
Scientists now think that two domains, bacteria and archaea, arose from LUCA, with the third domain, eukaryotes, evolving much more recently by the process of endosymbiosis
Endosymbiosis: A theory that eukaryotic cells evolved by engulfing and incorporating prokaryotic cells, which then became organelles within the eukaryotic cell.
A2.1.8—Approaches used to estimate dates of the first living cells and the last universal common ancestor
Dating the First Cells and LUCA: A Fossil and Isotope Odyssey
- The Stromatolite Trail: Stromatolites, layered structures formed by microbial mats, provide some of the oldest evidence of life on Earth. Their presence in rocks dating back to 3.42 billion years ago sets a minimum age for the earliest cells and the last universal common ancestor (LUCA).
- Carbon Isotope Clues: Rocks older than 4.0 billion years are rare due to Earth’s active geology. However, analysis of ancient zircons (tiny crystals) from Western Australia has revealed carbon isotope ratios consistent with biological activity, suggesting that life may have existed even further back in time.
- The Challenge of Interpretation: While these findings provide valuable clues, it’s important to remember that interpreting ancient geological records can be complex. Alternative explanations for some observations are possible, and further research is needed to refine our understanding of the earliest stages of life on Earth.
In essence, scientists are piecing together a timeline of early life using a combination of fossil evidence and geochemical analyses. While challenges remain, these approaches are slowly unveiling the fascinating history of life on our planet.
A2.1.9—Evidence for the evolution of the last universal common ancestor in the vicinity of hydrothermal vents
LUCA: Life at the Vents
- Hydrothermal Vents: A Cradle for Life: Evidence suggests that LUCA, the last universal common ancestor, likely evolved in the vicinity of deep-sea hydrothermal vents. These vents release hot, mineral-rich fluids into the cold ocean water, creating unique and energy-rich environments.
- Energy-Rich Conditions: Hydrothermal vents provide a rich source of energy, including hydrogen, methane, and sulfide, which early life forms could have used to assemble carbon compounds into polymers. The high concentrations of these chemicals would have been readily available for early life to utilize.
- LUCA’s Metabolic Capabilities: Analysis of genes shared by bacteria, archaea, and eukaryotes suggests that LUCA likely had a metabolism capable of utilizing hydrogen, carbon dioxide, and nitrogen. This suggests that LUCA was adapted to the environment found around hydrothermal vents.
- Continued Research: While the “hydrothermal vent hypothesis” is gaining traction, research is ongoing. Understanding the exact nature of LUCA and the early stages of life on Earth remains a subject of ongoing scientific investigation.
In essence, the evidence points towards hydrothermal vents as a plausible cradle for life, providing the energy and chemical building blocks necessary for the emergence of LUCA and the subsequent diversification of life on Earth.