1.5 - Origin of cells

• Pasteur's Experiment:

  • Louis Pasteur designed an experiment to test whether sterile nutrient broth could spontaneously generate microbial life.
• Method of Pasteur's experiment:

  1. Two experiments were setup
  2. In both, Pasteur added nutrient broth to flasks and bent the necks of the flasks into S shapes
  3. Each flask was then heated to boil the broth in order than all existing microbes were killed.
  4. After the broth had been sterilized, Pasteur broke off the swan necks from the flasks in Experiment 1, exposing the nutrient broth within them to air from above.
  5. The flasks in Experiment 2 were left alone.
• Results and conclusion of Pasteur's experiment:

  • The broth in experiment 1 turned cloudy whilst the broth in experiment 2 remained clear.
  • This indicates that mircobe growth only occurred in experiment 1.
  • Conclusion: Pasteur rejected the hypothesis of spontaneous generation as for growth of microbes to occur a source of contamination was needed.
• Evidence to support cell theory:

  • Cells are highly complex structures and no mechanism has been found for producing cells from simpler subunits.
  • All known examples of growth be it of a tissue, an organism or a population, are all a result of cell division.
  • Viruses are produced from simpler subunits, but they do not consist of cells, and they can only be produced inside the host cells that they have infected.
  • Genetic code is universal: each of the 64 codons produces the same amino acid in translation, regardless of the organism
• Key problems with arrival of first cells:

  • Non-living synthesis of simple organic molecules, e.g. sugars and amino acids
  • Assembly of these organic molecules into polymers
  • Formation of polymers that can self-replicate (enabling inheritance)
  • Formation of membranes to package the organic molecules
• Earth’s atmosphere was ‘reducing’ in the early days. It did not contain oxygen gas until after plants started photosynthesising

  The atmosphere contained:

  • Hydrogen
  • Nitrogen
  • Water vapour
  • Methane
  • Ammonia
  • Hydrogen sulfide
• • The monomers mixed in the ‘primeval soup’, shallow oceans laden with chemicals where it is thought that they reacted to form biological molecules
  • Miller and Urey tried to recreate these conditions in the lab
  • They were trying to demonstrate ‘chemical evolution’, the formation of more complex molecules from simpler stock in the primeval soup
• Miller and Urey recreated the conditions of pre-biotic Earth in a closed system:

  1. conditions included a reducing atmosphere (low oxygen), high radiation levels, high temperatures and electrical storms
  2. Water was boiled to form vapour and then was mixed with methane, ammonia and hydrogen
  3. The mixture of gases was exposed to an electrical discharge (sparks) to simulate lightning
• After a week Miller and Urey found:

  • Thirteen of the twenty naturally occurring amino acids
  • Around 15% of the carbon was now in organic compounds
• Conditions used in Miller and Urey experiment formed amino acids but also tended to hydrolyse bonds preventing polymers forming.

  Deep-sea thermal vents:

  • Fissures in a planet's surface from which geothermally heated water issues. Vents are commonly found near in volcanically active areas)
  • Along with heat energy the Vents issue a ready supply of reduced inorganic chemicals
  • Vents provide the right conditions and chemicals to allow organic polymers to arise.
• Formation of polymers that can self replicate:

  • DNA though very stable and effective at storing information is not able to self-replicate – enzymes are required
  • However RNA can both store information and self-replicate - it can catalyse the formation of copies of itself.
  • In ribosomes RNA is found in the catalytic site and plays a role in peptide bond formation
• Formation of membranes to package organic molecules:

  • Experiments have shown that phospholipids natural assemble into bilayers, if conditions are correct.
  • Formation of the bilayer creates an isolated internal environment.
  • The formation of an internal environment means that optimal conditions, e.g. for replication or catalysis can be maintained.
• Endosymbiotic theory explains the existence of several organelles of eukaryotes.

  The theory states that the organelles (e.g. mitochondria and chloroplasts) originated as symbioses between separate single-celled organisms,
• Development of the Nucleus

  • A prokaryote grows in size and develops folds in it’s membrane to maintain an efficient SA:Vol
  • The infoldings are pinched off forming an internal membrane
  • The nucleoid region is enclosed in the internal membrane and hence becomes the nucelus
• Development of Mitochondria

  • An aerobic proteobacterium enters a larger anaerobic prokaryote (possibly as prey or a parasite)
  • It survives digestion to become a valuable endosymbiont
  • The aerobic proteobacterium provides a rich source of ATP to it’s host enabling it to out-compete other anaerobic prokaryotes
  • As the host cell grows and divides so does the aerobic proteobacterium therefore subsequent generations automatically contain aerobic proteobacterium.
  • The aerobic proteobacterium evolves and is assimilated and to become a mitochondrion.
• Evidence for endosymbiotic theory for mitochodria and chloroplasts:

  • They have their own DNA (naked and circular)
  • They have ribosomes that are similar to prokaryotes (70S)
  • They have a double membrane and the inner membrane has proteins similar to prokaryotes
  • They are roughly the same size as bacteria and are susceptible to the antibiotic chloramphenicol
  • They transcribe their DNA and use the mRNA to synthesize some of their own proteins.
  • They can only be produced by division of pre-existing mitochondria and chloroplasts.