Increase of Atmospheric Oxygen

• Explain how algae and plants produced the oxygen currently present in the atmosphere through the process of photosynthesis.
• Recall and apply the word and symbol equations for photosynthesis:

$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

• State that algae first produced oxygen approximately 2.7 billion years ago, leading to the initial appearance of oxygen in the atmosphere.
• Describe how the evolution of plants over the subsequent billion years caused a gradual increase in the percentage of atmospheric oxygen, eventually reaching levels that enabled the evolution of animals.