Table of Contents
The Great Oxygenation Event
The primordial Earth atmosphere was reducing and contained very little oxygen gas. Fossil evidence illustrates the emergence of cyanobacteria about 3.5 billion years ago (3.5 Ga). Dissolved iron in the primordial oceans served as a sink to hold onto atmospheric oxygen. As photosynthesis increased, the amount of atmospheric oxygen remained low because dissolved oxygen began to oxidize into iron oxides which precipitated into sediment. The banded iron stone illustrates the deposition of iron oxides over geologic time. These “rust” bands progressively increased between 2.5-1.8 Ga. About 1.85 Ga, the iron sink effectively ran out and the oxygen began to accumulate in the atmosphere. This switch to an oxidizing environment caused mass extinctions of organisms incapable of detoxifying the oxygen. It also removed greenhouse gasses from the atmosphere to cause a severe cooling of the planet called the Huronian glaciation leading to a long “snowball Earth”.
Chloroplasts
Chloroplasts arose through a second endosymbiotic event in plants and various protists. These light harvesting organelles share similarity in structure and genome to photoautotrophic cyanobacteria.
Chloroplasts contain the light-harvesting pigment chlorophyll. The chlorophyll molecules are found on the internal thylakoid membranes. These membranes are stacked in columns called grana.
Summary of Photosynthesis
In photosynthesis, photoautotrophic organisms use light energy to convert carbon dioxide and water into carbohydrates. The reaction of photosynthesis can be written as follows:
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
Like cellular respiration, photosynthesis is a redox reaction. Carbon dioxide is reduced to glucose and water is oxidized to oxygen.
Photosynthesis occurs in two stages: the light dependent reactions and the light independent reactions (Calvin cycle).
