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[h] IB DP Biology HL A2.1 Origins of cells Flashcards
[q] List reasons why water is a substance on which life depends.
A1.1.1 – Water as medium of life.
[a]
- The first cells originated in water.
- Water is the “universal solvent” allowing it to dissolve and transport molecules around a body.
- Water is a metabolite in condensation and hydrolysis reactions.
- Water is a temperature buffer in bodies and ecosystems.
- Water maintains biological structures (such as phospholipid bilayer, proteins and DNA).
– 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.
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.
2. all cells come from pre-existing cells.
3. cell is the smallest unit of life.
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
Examples: Cells contain catalytic molecules, such as enzymes to speed up chemical reactions within the cell.
Examples: Detecting changes in chemicals in the extracellular environment and moving towards or away from the chemicals.
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.
Examples: Some cells have specialised structures, such as cilia, flagella and pseudopodia to help them move or change position.
Examples: Cells can divide to produce more cells, and they can also increase in size over time.
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.
Examples: Metabolic waste products are transported across the cell membrane, out of the cell into the external environment.
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.
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.
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.
2. The very first protocells did not fossilise.
3. It is thought that cells originated deep in the ocean, making it challenging to reach and collect samples for analysis.
4. There is uncertainty surrounding the exact conditions on pre-biotic Earth, which means scientists cannot replicate the exact conditions that would have existed.
[a] 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.
[q] How did the Urey and Miller experiment show that the origin of life was possible on early Earth.
[a] 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.
[q] Why do some people not agree with Urey and Miller’s findings?
[a] 1. Some people think that the carbon compounds came from comets and meteorites colliding with Earth.
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.
[q] Outline how vesicles may have spontaneously formed by the coalescence of fatty acids into spherical bilayers.
[a] 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.
4. It continued as the cells developed organelles with specialized functions.
[q] Why is RNA presumed to be the first genetic material?
[a] 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.
[q] LUCA
[a] last universal common ancestorIt is thought that LUCA was a simple, single-celledautotrophic 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.
[q] Outline evidences for LUCA.
[a] 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.
[q] Outline approaches to estimate the time over which life has been evolving on Earth.
[a] 1. Fossil Record
2. Microfossils and Stromatolites
4. Human evolution
[q] Outline the evidence of LUCA in the vicinity of hydrothermal vents.
[a] 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. 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.
[q] Endosymbiotic theory
[a] theory that some eukaryotic organelles (mitochondria and chloroplasts) were once free-living prokaryotes that invaded primitive eukaryotic cells, living together beneficially.
[q] universal genetic code
[a] evidence that life only started once on Earth as nearly all organisms use the same sequences of DNA/RNA to determine the same specific amino acids during the protein synthesis.
[q] Louis Pasteur
[a] French scientist who showed bacteria formed due to biogenesis, not spontaneous generation, in experiments using broth and swan neck flasks.
[q] Spontaneous generation
[a] old theory that believed living organisms and matter could form from non-living matter.
[q] Sterilization
[a] techniques used to eliminate micro-organisms to achieve a sterile non-microbial environment.
[q] The cell theory
[a] 1. All organisms are composed of one or more cells
2. All cells come from pre-existing cells
3. Cells are the basis of life
[q] synthesis of simple organic molecules from inorganic materials (step 1)
[a] The first step needed for cells to develop is the production of carbon based organic material such as amino acids and nucleic acids. This could happen as evidenced in the Miller-Urey experiment.
[q] polymerisation of organic monomers (step 2)
[a] Once simple organic molecules were present they needed to form into polymers such as proteins or RNA. This represents the start of metabolism as chemical reactions could happen much quicker, perhaps fueled by the temperature in volcanic vents.
[q] formation of self-replicating polymers (step 3)
[a] DNA and RNA carry the information to produce proteins but are also able to replicate themselves. This is needed to pass on instructions through inheritance. This is the next step after metabolism is established.
[q] Encapsulation of organic molecules into membranes (step 4)
[a] once organic replicating molecules had formed natural amphipathic properties of some may have formed membranes naturally in a watery environment.
[q] Formation of nucleus (first eukaryotes)
[a] Before endosymbiosis occurred the larger prokaryotic cell developed foldings of the plasma membrane which further developed into pores and compartments inside the cell.
[q] Mitochondrial and chloroplast DNA
[a] Evidence for the endosymbiotic theory as this DNA is circular, much the same as bacterial DNA.
[q] division by binary fission (independent of the host cell)
[a] Evidence for the endosymbiotic theory as this is how prokaryotic cells divide as well as mitochondria and chloroplasts.
[q] Own 70s ribosomes for protein synthesis
[a] Evidence for the endosymbiotic theory as these ribosomes are a similar size to those in prokaryotic cells.
[q] Double membranes
[a] Evidence for the endosymbiotic theory as both chloroplasts and mitochondria have these two layers which indicate they were taken in by endocytosis.
[q] Miller-Urey Experiment
[a] The 1953 experiment that simulated the conditions on early Earth and successfully showed organic molecules could be produced from inorganic molecules.
[q] primordial soup
[a] The mix of chemicals believed to be existing on Early Earth from which life arose.
[q] the RNA world hypothesis
[a] The idea that the first replicating molecule was RNA. This theory is supported as RNA has both replicating (can be used as a template) and enzymic properties.
[q] The Great Oxidation Event
[a] the change of Earth’s atmosphere about 2.5bya from reducing to oxidizing, brought about by oxygen-generating photosynthesis that caused a mass extinction event.
[q] LUCA
[a] The theory that there was a Last Universal Common Ancestor from which all currently living organisms are descended from.
[q] Spontaneous formation of vesicles
[a] Fatty acids naturally form bilayers when in a water environment and can coalesce into basic cell shapes – explaining how encapsulation could have occurred.
[q] Hydrothermal vents
[a] Openings in the sea floor out of which heated mineral-rich water flows. A strong candidate for where life could have started.
[q] Conditions of the early Earth
[a] Carbon dioxide and methane giving a high greenhouse effect. No oxygen or ozone so high UV radiation.
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