How is carbon cycled at a global scale?

The carbon atoms undergo a complicated chemistry forming what is known as the global carbon cycle, as do oxygen, nitrogen, and other elements; but the carbon cycle is the most widely recognized. An animal produces carbon dioxide and consumes oxygen in its metabolism of food. Glucose is a typical food and a metabolic reaction can be represented by:

A plant and green bacteria, on the other hand, produces oxygen and consumes carbon dioxide in its photo synthesis. Energy in the form of electromagnetic radiation (or photons) is supplied so that the low-energy-content carbon dioxide can be converted to high-energy-content glucose. An overall reaction for the complicated multi-step photosynthesis reaction can be represented by:

At a glance, animals and plants make food for each other. The plants convert solar energy into high-energy food for the animals. Water is a reactant and a product in the photosynthesis. Radioactive labeled studies showed that the oxygen in the water produced comes from those in carbon dioxide. You may be thinking about plants with leaves that give beautiful flowers. In fact, primitive plants in the ocean play a more important role in in the photosynthesis process, because of the large number of them.

The solubility of carbon dioxide depends on its partial pressure. As we know, carbon dioxide dissolves in water to form carbonic acid:

H₂CO₃→H++HCO−3Ka₁=4.2×10⁻⁷

The dissolved carbon dioxide further reacts with metal ions in the water forming calcium and magnesium carbonates. The K_sp values for CaCO₃ and MgCO₃ are 5×10−9 and 3×10−3 respectively. Extensive limestone (CaCO₃) and dolomite (mixture of CaCO₃ and MgCO₃) have been formed this way.

Some believed that this is how lime stone produced. Lime stone is soluble in acidic solutions, which may be formed by dissolving large amount of carbon dioxide.

or,

When the concentration of carbon dioxide is reduced, the acidity decreases and the reverse reaction takes place forming a solid, CaCO₃(s). Thus, metabolism, photosynthesis, mineralization and geological process are the major chemical processes in the global carbon cycle.