A2.1 Origin of Cells

pre-biotic atmosphere

• no life, evidence from ancient rocks and gas concentrations

• traces of oxygen because it reacted with other elements

• higher methane concentrations than today due to intense volcanic activity, meteorite bombardment

• higher CO2 emissions from volcano emissions

• higher temperatures - heat trapping greenhouse gases, comet and asteroid impacts raised temperatures

• no stratospheric ozone layer due to lack of oxygen, more UV radiation

• spontaneous reactions that would be impossible today

• organic aerosol haze - carbon compounds formed in specialized environments, may have formed in droplets of water in the atmosphere

effect of evolving organisms on the atmosphere

• increased concentration of O₂ (especially with development of photosynthesis)

• ozone layer formed

• reduced CO₂ concentration

• greenhouse effect reduced

cells as the smallest unit of self-sustaining life

• living things are self-sustaining, individual cells use energy to maintain a highly ordered state

• cells divide to produce more cells

• cells can be taken from body and cultured

Robert Remak

discovered that every cell is formed by division of a pre-existing cell

developments required for origin of cells

• catalysis - control over which chemical reactions occur

• self-assembly - carbon compounds such as amino acids must assemble to form polymers

• compartmentalization - membrane must develop to enclose cell contents

• self-replication of molecules - basis for inheritance and persistence of successful variants

Miller-Urey experiment

attempted to explain origin of carbon compounds

• tested J.B.S. Haldane’s hypothesis that carbon compounds can be made when ultraviolet light acts on a mixture of water, carbon dioxide, and ammonia

• simulated pre-biotic atmosphere: water vapor, methane, hydrogen, ammonia, electrical charges

• results: variety of carbon compounds produced → possible for carbon compounds to form spontaneously on Earth before life evolved

spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers

small droplets of fluid enclosed in a membrane, common in cells

• exosomes - extracellular vesicles

• membrane of vesicles mainly composed of phospholipids

phospholipids

• ampipathic - molecule with both hydrophobic and hydrophilic parts

• naturally assemble into bilayers when mixed with water (hydrophilic heads face outwards, hydrophobic tails face inwards), bilayers that spontaneously form stable spherical structures that are the basis of vesicles

• movement of polar molecules in and out of spherical structures limited by hydrophobic membrane core → vesicles would have developed their own internal chemistry, different from surroundings and cell like