- 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
Describe the 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.
Describe the 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
Explain 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
Explain why cells are the smallest units of self-sustaining life?
Because they contain all the components necessary to carry out all eight processes of life at some point in their life cycle.
These 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
What is the differences between something that is living and something that is non-living. Include reasons that viruses are considered to be 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
Metabolism
Define: Chemical reactions that take place within the cell(s) of an organism
Examples: Cells contain catalytic molecules, such as enzymes to speed up chemical reactions within the cell
Response to Stimuli
Define: Responding to changes in the external environment
Examples: Detecting changes in chemicals in the extracellular environment and moving towards or away from the chemicals
Homeostasis
Define: The maintenance of constant internal conditions, despite changes in their external environments
Examples: Moving ions or other molecules into or out of the cell across the cell membrane to control the concentration of certain substances in the cell
Movement
Define: Having some control over their place and position
Examples: Some cells have specialised structures, such as cilia, flagella and pseudopodia to help them move or change position
Growth
Define: Increasing in size over a period of time. In multicellular organisms, growth can also refer to an increase in the number of cells that make up an organism
Examples: Cells can divide to produce more cells, and they can also increase in size over time
Reproduction
Define: The production of offspring
Examples: Cells contain genetic material which contains the instructions for the cell to function and reproduce. During the reproduction of a cell, this genetic material will be copied so it can be passed on to the offspring
Excretion
Define: The removal of metabolic waste
Examples: Metabolic waste products are transported across the cell membrane, out of the cell into the external environment
Nutrition
Define: The intake or production of nutrients. Heterotrophic organisms obtain their nutrients from the external environment, whereas autotrophic organisms can produce nutrients from inorganic material
Examples: Some cells can produce their own nutrients through processes such as photosynthesis; other cells obtain their nutrients by consuming other organisms or organic molecules. Cells can also obtain nutrients by diffusion of the molecules across the membrane into the cell and by endocytosis
A2.1.3—Challenge of explaining the spontaneous origin of cells
Explain the 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.
What are the 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
How did the Urey and Miller experiment show that the origin of life was possible on early Earth.
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.
Why do some people not agree with Urey and Miller’s findings?
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
Outline how vesicles may have spontaneously formed by the 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
Why is RNA presumed to be the 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
Outline approaches to estimate the time over which life has been evolving on Earth.
1. Fossil Record
2. Microfossils and Stromatolites
3. Stratigraphy
4. Human evolution
From this fossil evidence and genetic analysis, scientists have inferred features and characteristics of LUCA:
Phylogenetic analysis and fossilised evidence suggest that LUCA evolved in hydrothermal vents – fissures in the ocean floor through which mineral rich water escapes (Figure 2). Hydrothermal vents are thought to have provided LUCA physical protection from the external ocean.
Hydrothermal vents have high temperatures, which could have provided the energy necessary for the formation of complex organic molecules required for cellular formation