“Beginnings of Cellular Life” Harold Horowitz (1992)• All life is cellular.• All living things are from 50 to over 90% water, the source of protons, hydrogen and oxygen in photosynthesis and the solvent of biomolecules.• The major elements of covalently bound biomolecules are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.• There is a universal set of small molecules: (i.e. sugars, amino acids, nucleotides, fatty acids, phospholipids, vitamins and coenzymes.)* The principle macromolecules are proteins,lipids, carbohydrates and nucleic acids.
“Beginnings of Cellular Life” Harold Horowitz (1992)• There is a universal type of membrane structure (lipid bilayer).• The flow of energy in living things involves formation and hydrolysis of phosphate bonds, usually ATP.• The metabolic reactions of any living species is a subset of a universal network of intermediary metabolism (glycolysis, the Krebs cycle, the electron transport chain)• Every replicating cell has a genome made of DNA that stores the genetic information of the cell which is read out in sequences of RNA and translated into protein.
“Beginnings of Cellular Life” Harold Horowitz (1992)• All growing cells have ribosomes, which are the sites of protein synthesis.• All living things translate information from nucleotide language through specific activating enzymes and transfer RNAs.• All replicating biological systems give rise to altered phenotype due to mutated genotypes.• Reactions that proceed at appreciable rates in all living cells are catalyzed by enzymes.
Life from inanimate matter?*Spontaneous interaction of simple molecules(ammonia, phosphates, methane, UV light, heat,electricity, lightning) = primordial organic soup(Haldane & Oparin)Primary QUESTIONS:(1) Where did the raw materials for life come from?(2) How did monomers develop?(3) How did polymers develop?(4) How did an isolated cell form?
Where did the raw materials for life come from?Electric sparks can generate amino acidsand sugars from an atmosphere loaded withwater, methane, ammonia and hydrogen, aswas shown in the famous Miller-Ureyexperiment (1953) = LIGHTNING createdbuilding blocks of life on Earth.- similar amino acids from Murchisonmeteorite
Community Clay (Alexander Graham Cairns-Smith)• Polymerization on clays or the evaporation of amino acids containing water near volcanic vents- Sidney Fox experiment: heated amino acids = proteinoids• The first molecules of life might have met on clay• Mineral crystals in clay could have arranged organic molecules into organized patterns. After a while, organic molecules took over this job and organized themselves.
Deep Sea Vent Theory- Suggests that life may have begun at submarine hydrothermal vents, spewing key hydrogen-rich molecules. Their rocky nooks could have concentrated these molecules & provided mineral catalysts for critical reactions.
Chilly Start- Ice might have covered the oceans 3 billionyears ago, as the sun was about a third lessluminous than it is now.- This layer of ice, possibly hundreds of feetthick, might have protected fragile organiccompounds in the water below fromultraviolet light and destruction from cosmicimpacts.- The cold might have also helped thesemolecules to survive longer, allowing keyreactions to happen.
RNA World• DNA needs proteins in order to form, and proteins require DNA to form, so how could these have formed without each other? RNA = store info like DNA, serve as an enzyme, & help create both DNA & proteins. Question remains: how was RNA created in the 1st place?
Self-Replicating RNA and the Dawn of Natural SelectionRNA molecules called ribozymes have been found tocatalyze many different reactions – For example, ribozymes can make complementary copies of short stretches of their own sequence or other short pieces of RNA• Early protobionts with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection• The early genetic material might have formed an “RNA world”
Protobionts• Replication and metabolism are key properties of life• Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure• Protobionts exhibit simple reproduction and metabolism and maintain an internal chemical environment
Endosymbiosis Hypothesis• Proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells• An endosymbiont is a cell that lives within a host cell
Panspermia• Svante Arrhenius in 1908• Perhaps life did not begin on Earth at all but was brought here from elsewhere in space• Rocks regularly get blasted off Mars by cosmic impacts reaching Earth.• Martian meteorites brought microbes on Earth thereby evolving into complex systems.• Other scientists have suggested that life might have hitchhiked on comets/ portals from other star systems
Fig. 25-7 Humans Colonization of land Animals Origin of solar system and Earth 1 4 Proterozoic Archaean Prokaryotes 2 3 Multicellular eukaryotes Single-celled eukaryotes Atmospheric oxygen
Fig. 25-13 Present Cenozoic Eurasia Africa 65.5 India South America Madagascar Antarctica Millions of years ago 135 Mesozoic 251 Paleozoic
Cell TheoryTheodore Schwann, Matthias Schleiden & Rudolf Virchow
Schwann & Schleiden 18391. The cell is the basic unit structure, physiology, and organization in living things.2. The cells retains a dual existence as a distinct identity and a building block in the construction of organism. Virchow 1858: All cells came from pre-2. Cells form (Omnis cellula e cellula). existing cells by free-cell formation, similar to the formation of crystal (spontaneous generation)
Modern Cell Theory1. All known living things are made up of cells2. The cell is structural and functional unit of all living things3. All cells comes from pre-existing cells by division.4. Cells contains hereditary information which is passed from cell to cell by division5. All cells are basically the same in chemical composition6. All energy flow (metabolism andbiochemistry) of life occurs within the cells
Primordial Soup Theory- Russian Chemist A.I. Oparin and English Geneticist J.B.S. Haldane in 1920.- basic building blocks of life came from simple molecule then energized by lightning and the rain from the atmosphere created the "organic soup”.- The first organisms would have to be simple heterotrophs. In order to survive, they consume other organisms for energy. They would become autotrophs by mutation. Evidence now suggest the first organisms were autotrophs.
Urey-Miller Experiment (1950)- They mixed gases thought to be present on primitive earth: ammonia, methane, water, hydrogen- then electrically sparked the mixture to signify lightning = amino acids- electricity, UV light, heat, & shock- Sidney Fox experiment: heated amino acids = proteinoids
Simple Beginnings• Instead of developing from complex molecules such as RNA, life might have begun with smaller molecules interacting with each other in cycles of reactions.• These might have been contained in simple capsules akin to cell membranes, and over time more complex molecules that performed these reactions better than the smaller ones could have evolved• "metabolism-first" models, as opposed to the "gene-first" model of the "RNA world" hypothesis.