The majority of carbon dioxide molecules (85 percent) are carried as part of the bicarbonate buffer system. In this system, carbon dioxide diffuses into the red blood cells. Carbonic anhydrase (CA) within the red blood cells quickly converts the carbon dioxide into carbonic acid (H₂CO₃). Carbonic acid is an unstable, intermediate molecule that immediately dissociates into bicarbonate ions (HCO₃ -) and hydrogen (H⁺) ions. Since carbon dioxide is quickly converted into bicarbonate ions, this reaction allows for the continued uptake of carbon dioxide into the blood, down its concentration gradient. It also results in the production of H+ ions. If too much H+ is produced, it can alter blood pH. However, hemoglobin binds to the free H+ ions, limiting shifts in pH. The newly synthesized bicarbonate ion is transported out of the red blood cell into the liquid component of the blood in exchange for a chloride ion (CI_-). This is called the chloride shift. When the blood reaches the lungs, the bicarbonate ion is transported back into the red blood cell in exchange for the chloride ion. The H⁺ ion dissociates from the hemoglobin and binds to the bicarbonate ion. This produces the carbonic acid intermediate, which is converted back into carbon dioxide through the enzymatic action of Ca. The carbon dioxide produced is expelled through the lungs during exhalation.

Overview:
Carbon dioxide (CO2) is a waste product produced by cells during cellular respiration. It needs to be transported out of the body to maintain proper pH levels in the blood. The majority of CO2 is transported in the form of bicarbonates (HCO3-), which is the correct answer.

Explanation:

1. Carbonic Acid (H2CO3):
- Carbon dioxide initially diffuses into the red blood cells (RBCs) and reacts with water to form carbonic acid (H2CO3) through an enzyme called carbonic anhydrase.
- This reaction is reversible, and carbonic acid can dissociate into bicarbonate ions (HCO3-) and hydrogen ions (H+).

2. Bicarbonates (HCO3-):
- The majority of carbon dioxide in the blood is carried in the form of bicarbonates (HCO3-).
- Bicarbonate ions are highly soluble and can easily travel in the plasma without affecting the blood's pH.
- They are transported from the RBCs into the plasma in exchange for chloride ions (Cl-) through a process called the chloride shift.
- The bicarbonate ions can freely move back into the RBCs when they reach the lungs, where they combine with hydrogen ions to form carbonic acid again.
- The carbonic acid is then converted back into carbon dioxide and water through the action of carbonic anhydrase.
- The carbon dioxide is eliminated from the body through exhalation.

3. Carbaminohaemoglobin:
- A small portion of carbon dioxide is also carried by binding to hemoglobin molecules in the blood, forming carbaminohaemoglobin.
- However, this form of CO2 transport is relatively minor compared to bicarbonates.

4. Dissolved CO2:
- A small fraction of carbon dioxide is also dissolved directly in the plasma, without undergoing any chemical reactions.
- However, dissolved CO2 is not the major way in which carbon dioxide is transported in the blood.

Conclusion:
In conclusion, the majority of carbon dioxide in the blood is carried in the form of bicarbonates (HCO3-). This form of transport allows for efficient removal of CO2 from the tissues and its release in the lungs, helping to maintain the body's acid-base balance.