The Carbon Cycle

Overview of the Carbon Cycle

The carbon cycle describes how carbon atoms move continuously between the atmosphere, oceans, land, and living organisms. It is a natural process that balances the amount of carbon dioxide (CO₂) in the atmosphere, helping to maintain Earth's environment.

Carbon exists in different forms such as carbon dioxide gas in the atmosphere, dissolved carbon in oceans, and as organic compounds in plants and animals. Geological processes like the formation of fossil fuels and volcanic activity also play a role in storing and releasing carbon.

This cycle involves both biological processes (like photosynthesis and respiration) and physical processes (like ocean absorption and volcanic eruptions), ensuring carbon is recycled through Earth's systems.

Carbon in the Atmosphere

Carbon dioxide is a key gas in the atmosphere containing carbon. It is produced naturally and by human activities, and it plays an important role in Earth's carbon cycle.

• Sources of carbon dioxide: Respiration by animals and plants releases CO₂ when glucose is broken down for energy.
• Combustion of fossil fuels (like coal, oil, and gas) releases CO₂ when carbon compounds burn in oxygen.
• Volcanic activity releases CO₂ from Earth's interior into the atmosphere.

Carbon dioxide is also removed from the atmosphere by:

• Photosynthesis: Plants absorb CO₂ and use it to make glucose, storing carbon in their tissues.
• Oceans: CO₂ dissolves in seawater where it can be used by marine organisms or stored as carbonate compounds.

Role of Photosynthesis and Respiration

Photosynthesis and respiration are biological processes that keep carbon moving through the cycle.

Photosynthesis occurs in plants and algae. They take in carbon dioxide from the air and water from the soil, using sunlight to produce glucose and oxygen:

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

This process removes CO₂ from the atmosphere and stores carbon in plant biomass.

Respiration happens in all living organisms, including plants and animals. It breaks down glucose to release energy, producing carbon dioxide and water:

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

Respiration returns CO₂ back to the atmosphere, balancing the carbon taken in by photosynthesis.

For instance, when a tree grows, it absorbs CO₂ through photosynthesis, but when it respires, it releases some CO₂. Overall, healthy forests act as carbon sinks because photosynthesis exceeds respiration.

Carbon Storage and Release

Carbon is stored in several natural reservoirs:

• Fossil fuels: Over millions of years, dead plants and animals are compressed underground to form coal, oil, and natural gas, storing carbon.
• Soils: Carbon is stored in organic matter from dead plants and animals decomposing in the soil.
• Oceans: Large amounts of carbon are dissolved in seawater or locked in marine sediments as carbonate rocks.

When fossil fuels are burned, the stored carbon is released rapidly as CO₂ into the atmosphere, increasing atmospheric carbon dioxide levels.

Decomposition of dead organisms by microbes also releases carbon back into the atmosphere as CO₂ or methane. Methane is produced during anaerobic decomposition and is a potent greenhouse gas.

For example, burning 1 kg of coal releases carbon dioxide according to the reaction:

$$\text{C} + \text{O}_2 \rightarrow \text{CO}_2$$

This releases carbon stored for millions of years back into the atmosphere quickly.

Example: Calculating Carbon Dioxide Released from Combustion

If 12 g of carbon burns completely in oxygen, how much carbon dioxide is produced?

The relative atomic mass (Ar) of carbon = 12, oxygen = 16.

Molecular mass of CO₂ = 12 + (16 × 2) = 44.

From the equation $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$, 12 g of carbon produces 44 g of CO₂.

So, 12 g carbon → 44 g CO₂.