How oxygen increased
The rise of atmospheric oxygen over time
Earth’s atmosphere did not always contain as much oxygen as it does today. Oxygen built up gradually over a very long time as photosynthetic organisms released it into their surroundings.
About 2.7 billion years ago, photosynthetic algae began producing oxygen. Soon afterwards, oxygen began to appear in the atmosphere. Over the following billion years, its percentage gradually increased. Later, plants evolved and also released oxygen through photosynthesis.
The slow build-up of oxygen changed Earth’s environment. Eventually, atmospheric oxygen reached a level that allowed animals to evolve.
Notes for this section are being prepared.
Balanced equation for photosynthesis
Photosynthesis uses light energy to form glucose and oxygen from carbon dioxide and water. The balanced symbolic equation is:
The numbers in front of the formulae are coefficients. They show that six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen. No coefficient is written before glucose because its coefficient is one.
A coefficient multiplies the whole formula that follows it; it does not change the formula of an individual molecule. For example, $$\ce{6H2O}$$ means six water molecules, each containing two hydrogen atoms and one oxygen atom. Together, they contain twelve hydrogen atoms and six oxygen atoms.
Why the equation is balanced
The equation is balanced because both sides contain the same number of each type of atom:
- Carbon: six on each side.
- Hydrogen: twelve on each side.
- Oxygen: eighteen on each side. On the left, twelve come from carbon dioxide and six from water. On the right, six are in glucose and twelve are in oxygen molecules.