Origins Of Cells
விளையாடிக் கற்றுக்கொள்ளுங்கள்
ஆற்றல் சம்பாதிக்க இந்த கேள்விகளுக்குப் பதிலளியுங்கள், பின்னர் மீன் பிடித்து ஆராயுங்கள். கணக்கு தேவையில்லை.
கல்வியாளர்களுக்கு: Origins Of Cells (Biology, HL)-க்கான தயாரான பாட ஸ்லைடுகள், திருப்புதல் குறிப்புகள் — உங்கள் பாடத்தில் அவற்றைப் பயன்படுத்தவும், அல்லது கற்பவர்கள் நேரலை விளையாட்டாக விளையாடும் ஊடாடும் வகுப்பு செயல்பாடாக தலைப்பை இயக்கவும்.
பாட குறிப்புகள்
Conditions on Early Earth
- Early Earth's atmosphere contained higher levels of carbon dioxide and methane than today, both powerful greenhouse gases.
- These gases trapped infrared radiation, causing the greenhouse effect and much higher surface temperatures.
- The atmosphere lacked free oxygen, so ozone (O₃) could not form; ozone normally absorbs harmful UV radiation.
- Without ozone, UV radiation penetrated to Earth's surface, damaging DNA and increasing mutation rates.
- These conditions may have driven the spontaneous formation of carbon compounds by chemical processes that do not occur today.
- Adding energy (heat or UV radiation) to early atmospheric gases could form amino acids, simple sugars, nucleotides and fatty acids — the building blocks of early cells.
- Oparin and Haldane proposed the 'primordial soup' hypothesis to explain the origin of biological molecules.
- UV radiation may also have catalysed the formation of polymers (proteins, complex sugars, mRNA, phospholipids) from simpler molecules.
The Miller–Urey Experiment
- Miller and Urey recreated early Earth conditions in laboratory apparatus to test the primordial soup hypothesis.
- They boiled water to produce steam, simulating evaporation from high temperatures.
- The steam was mixed with gases including methane, hydrogen and ammonia to recreate the early atmosphere.
- Electrical discharges simulated lightning as an energy source.
- The mixture was cooled, representing condensation of water in the atmosphere.
- After a week, they found traces of simple organic molecules, including amino acids.
- Limitation: they used high levels of methane, but methane may have been in low supply on early Earth.
- Limitation: they used electrical discharge instead of UV light; synthesis of organic molecules from carbon dioxide, nitrogen and water requires nuclear and UV radiation along with electrical discharges.
- Limitation: in water, amino acids tend to remain as monomers rather than joining to form proteins, contradicting the idea that complex molecules formed in the primordial soup.
- Limitation: they were unable to generate nucleotides; nucleotides have since been synthesised by a different approach.
Cells as Units of Life
- Cells are the smallest units of self-sustaining life.
- All cells are enclosed by a plasma membrane that separates cell contents from the outside.
- All cells store genetic information in DNA, which is expressed during protein synthesis.
- Life is defined by features such as metabolic reactions, nutrition, excretion of metabolic waste, reproduction, response to stimuli and growth.
- Reproduction and passing on genetic information allows for evolution by natural selection.
- Viruses are considered non-living: they lack cell structure and organelles, do not carry out metabolic reactions, do not require nutrition, and cannot replicate independently — they rely on host cell components.
Spontaneous Origin of the First Cells
- Cells are complex and normally form only from division of pre-existing cells; the first cells must have originated from non-living components of the primordial atmosphere.
- Step 1: simple organic compounds synthesised from inorganic molecules (as demonstrated by Miller and Urey).
- Step 2: simple organic compounds assembled into polymers.
- Step 3: some polymers developed the ability to self-replicate.
- Step 4: membranes surrounded the polymers, creating compartments with internal chemistry different from the surroundings.
- Protocell-first theory: a cell-like compartment (protocell) capable of basic metabolism arose spontaneously, initially without genetic material, then acquired it (likely RNA).
- Gene-first theory: a self-replicating nucleic acid (likely RNA) arose first; natural selection led to variants that developed a membrane and metabolism.
- Metabolism-first theory: life began as a self-sustaining system of chemical reactions that later evolved cells and genetic material; favoured by many scientists because most life processes require energy from metabolic reactions.
Testing Theories on the Origin of Cells
- Hypotheses and theories in science should be testable.
- Testing origin-of-cells theories requires replicating early Earth conditions in the laboratory.
- It is not possible to replicate early Earth conditions exactly as they might have been.
- The exact nature of the first cells is unknown because none of these early cells fossilised.
- This makes it difficult to test the hypotheses underpinning theories about the origin of life.
Formation of Vesicles
- Membranes separate genetic material and biochemical processes from the outside environment; this is compartmentalisation.
- Membrane formation was a crucial step in the origin of cells.
- First cell membranes were likely composed of fatty acids because of their amphipathic nature (polar and non-polar parts).
- A few lipid molecules in water form a monolayer; more lipids form bilayers with polar parts facing water and non-polar parts pointing inward.
- Bilayers spontaneously form microspheres (small vesicles), which could have formed early cell membranes.
- These early membranes separated internal chemistry from the outside environment.
- Fatty acids could have combined with glycerol via condensation reactions to form triglycerides, which could then undergo phosphorylation to form simple phospholipids — the main component of modern cell membranes.
- Eukaryotic cells later evolved multiple internal compartments, allowing further division of activity within cells.
RNA as the First Genetic Material
- Early life required a self-replicating system and the ability to catalyse chemical reactions.
- In modern cells these roles are carried out by DNA and enzymes, but neither would have been present in the pre-biotic world.
- RNA can store genetic information and has enzymatic properties, so it may have performed both functions — the RNA world hypothesis.
- As life evolved, DNA took over genetic storage and proteins (enzymes) became biological catalysts.
- Properties of RNA supporting this: it can assemble spontaneously from nucleotides, replicate itself, and control the rate of chemical reactions.
- Modern cells contain ribozymes that catalyse the formation of peptide bonds.
- Evidence RNA came before DNA: ribose can be formed from methanal, a main product of the Miller–Urey experiment; deoxyribose in DNA is produced from ribose in an enzyme-catalysed reaction; ribozymes can join amino acids to form proteins from an RNA template.
The Last Universal Common Ancestor (LUCA)
- All life on Earth is thought to have evolved from an ancient common ancestor, LUCA, believed to have existed about 4 billion years ago.
- In a phylogenetic tree of life, LUCA is at the very base.
- Evidence for common ancestry: same biochemistry in all organisms, same DNA bases and genetic code, and same shared amino acids forming proteins.
- Shared genes in eubacteria and archaea indicate inheritance from LUCA.
- Other organisms may have evolved alongside LUCA but became extinct due to competition for shared resources; LUCA's descendants outcompeted them.
- Similar bone structure of the vertebrate forelimb across species indicates inheritance from a common ancestor.
- DNA sequence similarity is used to determine evolutionary relationships: more similar sequences mean closer relatedness.
Timescale and Evidence for Evolution of Life
- Fossils provide evidence of the history of life and are used to determine the timescale of evolutionary events.
- Carbon dating of carbon-14 is used for samples up to approximately 60,000 years old.
- Radiometric dating measures relative proportions of radioactive substances (e.g. carbon-13 to carbon-12) in a sample.
- Older rocks are expected to contain evidence of more ancient life, so dating them indicates when life may have originated.
- Genome analysis can determine age: DNA mutations accumulate over time, and the number of mutations between species indicates when they branched from a common ancestor.
- Changes in amino acid composition of proteins also reflect DNA changes; assuming a constant rate of change forms the basis of a molecular clock.
- The molecular clock can be used to estimate when life on Earth originated.
- LUCA may have evolved in hydrothermal vents deep in the ocean, where organisms can generate energy by chemosynthesis.
- Fossilised structures in sedimentary rocks near deep-sea hydrothermal vents in Quebec, Canada, are similar to those produced by modern prokaryotes near hydrothermal vents.
- These fossils are at least 3.77 billion years old, possibly more than 4 billion years old — among the oldest forms of life found.
- The fossil structures are small tubes of haematite (iron(III) oxide); carbonate and other carbonaceous material indicate oxidation and biological activity.
- This suggests ancient bacteria had a similar biochemistry to modern iron-oxidising bacterial communities near hydrothermal vents.
- Sequence data from modern hydrothermal vent species indicate they share a common ancestor.
- Based on amino acid sequence data, LUCA may have been anaerobic, converted carbon dioxide into glucose, used hydrogen as an energy source instead of sunlight, converted nitrogen into ammonia for amino acid synthesis, and survived at very high temperatures (thermophilic).
- Fossil and genetic evidence suggests LUCA may have been an autotrophic extremophile living in hydrothermal vents with abundant hydrogen, carbon dioxide and iron.
- This is not the only hypothesis for the origin of life; scientists continue to gather and analyse data that may support or refute existing theories.
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பயிற்சி கேள்விகள்
இலவச முன்னோட்டம் — 58-இல் 8 கேள்விகள். அனைத்தையும் பார்க்க பதிவு செய்யவும்.
1.Which of the following is not a piece of evidence that suggests that all life descended from a common ancestor called LUCA?
Easy- AAll living organisms have the same biochemistry, e.g. lipids and carbohydrates
- BAll living organisms have the same DNA bases and genetic code
- CAll living organisms have shared amino acids forming protein molecules
- DAll living organisms can be infected by viruses
2.What was a key limitation of the Miller-Urey experiment according to current understanding?
Easy- AThey used too much UV radiation in their experiment
- BThe experiment did not run for long enough
- CThe high levels of methane they used did not accurately reflect early Earth conditions
- DThey only included methane, ammonia, hydrogen and water in the experiment
3.What property of fatty acids made them likely candidates for early cell membranes?
Easy- ATheir amphipathic nature
- BTheir ability to store genetic information
- CTheir catalytic properties
- DTheir ability to self-replicate
4.Which of the following is a challenge of explaining the spontaneous origin of cells?
Medium- AThere are so many different theories of how the first cells originated that it is impossible to make sense of all the conflicting information.
- BIt is impossible to know the conditions on early Earth and so it is impossible to replicate any of those conditions experimentally.
- CThe current scientific equipment that exists is not able to withstand the temperature and pressure conditions of early Earth and so the correct conditions cannot be replicated experimentally.
- DEarly cells did not fossilise well, making it difficult to know their exact nature or confirm hypotheses about their formation.
5.All living organisms share the same core characteristics in order to be classified as living. Relating to this, which of the following statements about viruses is correct?
Medium- AViruses are non-living because they are obligate parasites and rely on other organisms to survive.
- BViruses are living because they are able to reproduce themselves using their own metabolic pathways inside the host cell.
- CViruses are non-living because they cannot reproduce without infecting a host cell.
- DViruses are living because they contain genetic information in the form of DNA or RNA.
6.Why was UV radiation significant for the origin of carbon compounds on early Earth?
Medium- AIt interacted with ozone to form free oxygen in the atmosphere.
- BIt provided energy for the formation of larger polymers from simpler molecules.
- CIt allowed plants to carry out photosynthesis using the energy from the radiation which allows them to create carbon compounds.
- DIt caused mutations in DNA which led to the diversification of early living organisms.
7.What is the main observation from the Miller-Urey experiment?
Medium- AThe formation of simple organic molecules, including amino acids, from inorganic gases and water.
- BThe spontaneous formation of vesicles from fatty acids.
- CThe replication of RNA molecules without enzymes.
- DThe synthesis of nucleotides from methane and ammonia.
8.Which of the following is a feature of all cells?
Easy- AA nucleus
- BA plasma membrane
- CMitochondria
- DCell wall
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