How Life Changed the Atmosphere

Early Life and Atmosphere

Life on Earth began around 3.5 billion years ago with simple, single-celled organisms. These early life forms were anaerobic, meaning they did not require oxygen to survive because the atmosphere had very little oxygen at that time.

Among these early organisms were photosynthetic bacteria, such as cyanobacteria. These organisms could use sunlight to convert carbon dioxide and water into glucose and oxygen through photosynthesis:

$$6 \text{CO}_2 + 6 \text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6 \text{O}_2$$

Photosynthesis by these organisms was the first significant source of oxygen on Earth, gradually increasing the oxygen levels in the atmosphere over millions of years. This increase in oxygen is known as the Great Oxygenation Event, which dramatically changed Earth's atmosphere and life.

For instance, cyanobacteria in ancient oceans released oxygen as a waste product, which slowly accumulated in the atmosphere.

Impact of Photosynthesis

As photosynthetic organisms produced oxygen, the amount of oxygen in the atmosphere increased. This had several important effects:

• Increase in atmospheric oxygen: Oxygen levels rose from almost zero to about 21%, the level we have today.
• Formation of the ozone layer: Oxygen molecules (O₂) in the upper atmosphere were converted into ozone (O₃) by ultraviolet (UV) radiation from the Sun. The ozone layer absorbs harmful UV rays, protecting life on Earth.
• Reduction of carbon dioxide: Photosynthesis uses carbon dioxide, so as photosynthetic organisms flourished, atmospheric CO₂ levels decreased.

The formation of the ozone layer was crucial because it allowed life to move from water to land by shielding organisms from damaging UV radiation.

Example: If photosynthesis uses 6 molecules of CO₂ to produce 1 molecule of glucose and 6 molecules of O₂, how many molecules of oxygen are produced if 12 molecules of CO₂ are used?

Since 6 CO₂ molecules produce 6 O₂ molecules, 12 CO₂ molecules will produce:

$$\frac{6 \text{ O}_2}{6 \text{ CO}_2} \times 12 \text{ CO}_2 = 12 \text{ O}_2$$

So, 12 molecules of oxygen are produced.

Consequences for Earth

The increase in oxygen had major consequences for life and the atmosphere:

• Development of aerobic respiration: Organisms evolved to use oxygen for respiration, a more efficient way to release energy from food compared to anaerobic respiration.
• Changes in atmospheric composition: The atmosphere shifted from being mostly carbon dioxide and nitrogen to one rich in oxygen and nitrogen, similar to today’s atmosphere.
• Support for diverse life forms: Higher oxygen levels allowed more complex multicellular organisms to evolve, leading to the biodiversity we see today.

Aerobic respiration uses oxygen to break down glucose, releasing energy, carbon dioxide, and water:

$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$$

Example: Calculate the volume of oxygen used when 10 dm³ of glucose reacts completely with oxygen in aerobic respiration. The balanced equation is:

$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$$

From the equation, 1 volume of glucose reacts with 6 volumes of oxygen.

So, 10 dm³ of glucose will react with:

$$10 \times 6 = 60 \text{ dm}^3 \text{ of oxygen}$$