cell structure
Study Guide: Unit 2 Cellular Organisation and Cell Structure
Key Concepts & Main Ideas
Origin of Life and Abiogenesis
Life arose from organic molecules on early Earth (~4.5 billion years ago).
Abiogenesis: life arising from non-living matter.
Emergent properties arise from simple interactions of molecules.
Miller-Urey experiments (1952) simulated early Earth conditions; produced amino acids from simple gases.
Deep-sea hydrothermal vents may have provided energy for primitive life (Michael Russell’s hypothesis).
RNA likely played a crucial role as a self-replicating molecule and catalyst in early life (RNA world hypothesis).
Micelles and vesicles formed from amphipathic molecules (fatty acids/phospholipids) created primitive cell boundaries.
Comets may have delivered organic molecules to Earth during the Late Heavy Bombardment.
Last Universal Common Ancestor (LUCA) links abiotic and biotic phases; evidence from shared genes and fossilized microbes near hydrothermal vents [[3-9]].
Cell Theory
All living organisms are composed of cells.
Cells are the smallest units of life.
All cells arise from pre-existing cells (Pasteur’s experiments disproved spontaneous generation) [[4-5]].
Cell Types and Structure
Unicellular organisms perform all life functions (metabolism, growth, response, homeostasis, nutrition, reproduction, excretion).
Multicellular organisms have specialized cells formed via differentiation (gene expression controls cell fate).
Prokaryotic cells: no nucleus, DNA in nucleoid, smaller ribosomes (70S), simple structure, binary fission reproduction.
Eukaryotic cells: nucleus with nuclear envelope, membrane-bound organelles (mitochondria, ER, Golgi apparatus, lysosomes), larger ribosomes (80S), compartmentalization.
Plant cells have cell walls (cellulose), chloroplasts, large central vacuole; fungal cells have chitin cell walls; animal cells lack cell walls and chloroplasts [[10-16]].
Endosymbiosis Theory
Eukaryotic organelles (mitochondria, chloroplasts) evolved from engulfed free-living prokaryotes.
Evidence: double membrane, own DNA, 70S ribosomes, binary fission capability [[17-18]].
Microscopy
Light microscopes: max magnification ~2000x, resolution ~200 nm, can view living cells.
Electron microscopes (TEM and SEM): max magnification up to 1,000,000x, resolution ~1 nm, require non-living samples, reveal ultrastructure.
Fluorescent and phosphorescent stains improve visualization of cell components.
Cryogenic electron microscopy (cryo-EM) allows molecular-level structural analysis without crystallization [[19-25]].
Viruses
Acellular, no metabolism, require host cells for replication.
Contain DNA or RNA, protein capsid, sometimes viral envelope.
Infect both prokaryotic and eukaryotic cells.
Life cycles: lytic (e.g., bacteriophage lambda) and lysogenic (e.g., HIV).
Origins hypotheses: virus first, progressive (cellular origin), regressive.
Rapid evolution via genetic recombination and high mutation rates.
Example: Influenza virus evolves by RNA segment reassortment.
HIV evolves rapidly due to error-prone RNA polymerase and large population size [[26-33]].
Important Details & Critical Arguments
Miller-Urey Experiment: Demonstrated that organic molecules could form under simulated early Earth conditions (using water, methane, ammonia, hydrogen, and electrical sparks) [[3-5]].
RNA World Hypothesis: RNA’s unique ability to self-replicate and catalyze reactions suggests it was crucial in early life before DNA and proteins evolved [[6-7]].
Endosymbiosis Evidence: Organelles have bacterial-like DNA and ribosomes, can replicate independently, supporting their evolutionary origin from prokaryotes [[17-18]].
Viruses and Living Status: Lack of metabolism and inability to reproduce independently exclude viruses from being considered living; they are obligate intracellular parasites [[26-27]].
Virus Evolution: RNA viruses lack proofreading, leading to rapid mutation rates and evolution of drug resistance (e.g., HIV resistance to nevirapine) [[31-33]].
Study Questions
What conditions on early Earth contributed to the formation of organic molecules?
Explain the significance of the Miller-Urey experiment.
What is abiogenesis and how does it relate to emergent properties?
Describe the key principles of the cell theory.
Compare and contrast prokaryotic and eukaryotic cells.
What evidence supports the endosymbiosis theory?
How do light microscopes and electron microscopes differ in terms of resolution and magnification?
Why are viruses not considered living organisms by most biologists?
Outline the lytic and lysogenic life cycles of viruses.
What are the main hypotheses regarding the origin of viruses?
How do viruses like influenza and HIV evolve rapidly?
Explain the role of micelles and vesicles in the origin of primitive cells.
What is LUCA and what evidence supports its existence?
Key Dates
Earth formation: ~4.5 billion years ago [[3]].
Miller-Urey experiment: 1952 [[3-5]].
Discovery of deep-sea hydrothermal vents: 1977 [[6]].
Discovery of alkaline vent "Lost City": 2000 [[6]].
Rosetta comet probe landing: 2014 [[8]].
Development of electron microscopy: Early 20th century [[22-25]].
HIV discovery and understanding of its life cycle: late 20th century [[28-29]].
COVID-19 pandemic and global research collaboration: 2019-2021 [[31]].
Memory Aids
Cell Theory 3 Key Points:
Living organisms are made of cells
Cells are the smallest units of life
All cells come from pre-existing cells
(Mnemonic: "Life Small Precedes")Functions of Life in Unicellular Organisms:
Metabolism
Growth
Response
Homeostasis
Nutrition
Reproduction
Excretion
(Mnemonic: "My Great Red Hair Needs Really Extra Shampoo")Endosymbiosis Evidence:
DNA, Ribosomes (70S), Membranes, Replication by binary fission
(Mnemonic: "DRMR")Virus Life Cycles:
Lytic = Lysis (cell bursting)
Lysogenic = Latent (DNA incorporated into host genome)
(Mnemonic: "Lytic = Lose cell, Lysogenic = Lie dormant")Microscopy:
Light microscope: Light = Limited resolution (~200 nm)
Electron microscope: Electron = Exceptional resolution (up to 0.2 nm)
(Mnemonic: "Light for Living, Electron for Extreme")
Key Concepts & Main Ideas
1. Origins of Life and Abiogenesis
Life originated from organic molecules on early Earth (~4.5 billion years ago).
Abiogenesis: life arising naturally from non-living matter through stages.
Emergent properties arise from simple molecular interactions (e.g., water).
Miller-Urey experiment (1952): simulated early Earth conditions and produced amino acids from simple molecules (water, methane, ammonia, hydrogen) using electrical sparks to mimic lightning 34.
Deep-sea hydrothermal vents as a source of energy for primitive life (Michael Russell’s theory, 1993).
RNA likely played a crucial role in early life ("RNA world hypothesis") due to its catalytic and self-replicating properties.
Formation of protocells (structures with membranes made of phospholipids/fatty acids) enabled compartmentalization and complex reactions 36.
Comets possibly delivered organic molecules to Earth during the Late Heavy Bombardment (4.1 - 3.8 billion years ago) 7.
Last Universal Common Ancestor (LUCA) likely an anaerobic thermophile living near hydrothermal vents, sharing genetic codes across all life 9.
2. Cell Theory
All living organisms are composed of cells.
Cells are the smallest units of life.
All cells come from pre-existing cells (Pasteur’s experiments disproved spontaneous generation) 45.
3. Cell Structure
Cells can be unicellular or multicellular.
Unicellular organisms perform all functions of life (metabolism, growth, response, homeostasis, nutrition, reproduction, excretion) 10.
Multicellularity enables specialization via cell differentiation (expression of specific genes per cell type).
Prokaryotic cells: no nucleus, simpler structure, 70S ribosomes, binary fission, DNA in nucleoid, plasmids present 1112.
Eukaryotic cells: nucleus with chromosomes, membrane-bound organelles (mitochondria, chloroplasts in plants), 80S ribosomes, cytoskeleton, complex internal compartmentalization 1314.
Differences between animal, plant, and fungal cells summarized (cell walls, vacuoles, chloroplasts, centrioles, food storage) 15.
Xylem and phloem are specialized plant tissues with atypical cells (xylem: dead, lignified vessels; phloem: living sieve tubes with companion cells) 16.
4. Endosymbiosis Theory
Eukaryotic organelles like mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral cells.
Evidence: size and structure of organelle ribosomes (70S), circular DNA, ability to replicate independently, double membranes 1718.
5. Microscopy Developments
Light microscopes: magnify up to 2000x, limited resolution (~200 nm), can see living or dead specimens.
Electron microscopes (TEM and SEM): much higher resolution (~1 nm), reveal detailed cell ultrastructure but only view dead specimens prepared with heavy metals 1922.
Fluorescence microscopy: uses fluorescent and phosphorescent stains to highlight specific cellular components 21.
Cryogenic electron microscopy (cryo-EM): new technology to resolve biomolecules at molecular level without crystallization 23.
6. Viruses
Viruses are acellular, lack metabolism, require a host cell to replicate.
Composed of DNA or RNA inside a capsid; some have an envelope derived from host membranes with glycoproteins aiding infection 26.
Life cycles: lytic (e.g., bacteriophage lambda) and lysogenic (e.g., HIV) 2728.
Diverse genetic material types (RNA/DNA, single/double-stranded).
Hypotheses for viral origins:
Virus-first hypothesis: viruses predate cells.
Progressive (cellular origin) hypothesis: viruses evolved from mobile genetic elements.
Regressive hypothesis: viruses evolved by reduction from parasitic cells 30.
Viruses evolve rapidly by genetic recombination and high mutation rates (especially RNA viruses like HIV and influenza) 32.
Reassortment in segmented RNA viruses leads to new strains (e.g., influenza) 32.
HIV’s rapid mutation leads to drug resistance; HAART (multiple drug therapy) slows resistance development 3233.
Important Details & Critical Arguments
RNA's dual role as genetic material and catalyst supports the RNA world hypothesis 67.
Pasteur’s experiment disproved spontaneous generation, reinforcing cell theory 4.
The endosymbiotic theory is supported by multiple lines of molecular and structural evidence 18.
Microscopy advancements have been fundamental to cell biology understanding, with electron microscopy vastly extending resolution limits 2324.
Virus evolution challenges vaccine development due to high mutation rates and genetic reassortment 32.
International collaboration has been crucial in recent viral research, exemplified by COVID-19 response 31.
Study Questions
What molecules and conditions on early Earth likely contributed to the origin of life?
Define abiogenesis and emergent properties with examples.
What were the key findings of the Miller-Urey experiment?
How do the deep-sea vent hypothesis and the RNA world hypothesis complement each other in explaining the origin of life?
What are the key principles of the cell theory?
Compare and contrast prokaryotic and eukaryotic cells.
Describe the process and significance of cell differentiation in multicellular organisms.
Summarize the evidence supporting the endosymbiotic theory.
Explain the differences between light, transmission electron, and scanning electron microscopes.
Why are viruses not considered living organisms by most biologists?
Outline the differences between the lytic and lysogenic viral life cycles.
Discuss the three hypotheses proposed for the origin of viruses.
What mechanisms contribute to the rapid evolution of viruses such as influenza and HIV?
How do the structures of xylem and phloem cells differ from typical plant cells, and why?
What roles do micelles and vesicles play in the origin of early cells?
Key Dates
Earth formation: ~4.5 billion years ago 3
Miller-Urey experiment: 1952 3
Discovery of deep-sea hydrothermal vents: 1977 6
Michael Russell's alkaline vent hypothesis: 1993 6
Rosetta mission comet probe landing: 2014 7
COVID-19 pandemic onset and global research effort: 2019-2021 31
Memory Aids
"ABC of Cell Theory":
All living organisms are made of cells
Basic unit of life is the cell
Cells come from pre-existing cells
"Functions of Life" (7 Rs):
Respiration (metabolism)
Reproduction
Response to environment
Regulation (homeostasis)
Require nutrients (nutrition)
Removal of waste (excretion)
Reproduction (growth)
"Prokaryote vs Eukaryote":
Pro=no nucleus, simple, 70S ribosomes
Eu=true nucleus, complex, 80S ribosomes
"Virus Lifecycle":
Lytic: virus replicates and bursts cell
Lysogenic: viral DNA integrates into host genome
"RNA World": RNA = Replicate + Act as enzyme
This study guide covers foundational concepts on cell biology, the origin of life, microscopy, and viruses based on the provided document. For detailed explanations and diagrams, refer to the specified pages [[3]-[33]].
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