The Link Reaction, also known as pyruvate decarboxylation, occurs after glycolysis and immediately before the Krebs cycle in animal cells. This transformation prepares the product of glycolysis for entry into the main energy-generating pathway. It generates a carbon dioxide molecule considered a metabolic waste product, and its efficient removal is fundamental to the animal’s survival. This article traces the journey of that carbon dioxide molecule from its creation deep within the cell to its ultimate expulsion from the body.
Where CO2 Is First Produced
The origin of this carbon dioxide is the mitochondrial matrix. Pyruvate, a three-carbon molecule derived from glycolysis, must be transported into this matrix to continue aerobic respiration. Once inside, it undergoes a transformation catalyzed by the large enzyme complex called pyruvate dehydrogenase.
This complex removes one carboxyl group from the three-carbon pyruvate molecule. The removed carboxyl group is released as carbon dioxide. The remaining two-carbon fragment is then attached to coenzyme A, forming Acetyl-CoA, which proceeds into the Krebs cycle. Since one glucose molecule yields two pyruvate molecules, this decarboxylation event occurs twice for every molecule of glucose metabolized.
Moving CO2 Out of the Cell
Carbon dioxide is a small, nonpolar gas that moves easily through biological membranes. A high concentration of CO2 in the mitochondrial matrix creates a steep concentration gradient. This gradient immediately drives the molecule out of the reaction site.
The CO2 diffuses across the mitochondrial membranes into the cell’s cytoplasm. From the cytoplasm, it crosses the cell membrane to enter the interstitial fluid. The molecule then reaches the wall of a local capillary, where it diffuses into the bloodstream down its concentration gradient.
How the Blood Carries CO2
Once in the blood, carbon dioxide is transported to the lungs using three distinct mechanisms. The majority is chemically modified for better solubility. Only 5 to 10 percent of the total remains dissolved directly in the blood plasma, contributing to the partial pressure of CO2.
Another 10 to 20 percent of the CO2 binds directly to the amino groups of proteins, primarily the globin chains of hemoglobin, to form a compound known as carbaminohemoglobin. This binding is reversible and occurs more readily when hemoglobin has released its oxygen load, a phenomenon called the Haldane effect. This increases the blood’s capacity to carry CO2 away from the tissues.
The most significant portion, accounting for 70 to 90 percent of the transported CO2, is carried in the form of bicarbonate ions (\(\text{HCO}_3^-\)). The \(\text{CO}_2\) first diffuses into red blood cells, where the enzyme carbonic anhydrase rapidly catalyzes the reaction between \(\text{CO}_2\) and water to form carbonic acid (\(\text{H}_2\text{CO}_3\)).
Carbonic acid is unstable and quickly dissociates into a hydrogen ion (\(\text{H}^+\)) and a bicarbonate ion (\(\text{HCO}_3^-\)). The hydrogen ions are buffered by binding to hemoglobin, which prevents a drastic drop in blood \(\text{pH}\). The bicarbonate ions then move out of the red blood cell into the plasma in exchange for a chloride ion (\(\text{Cl}^-\)), a process known as the chloride shift, which maintains the electrical neutrality of the cell.
The Body’s Final Exit Strategy
The blood travels through the veins back to the heart and is pumped to the lungs. Upon reaching the pulmonary capillaries, the concentration gradient is reversed. The partial pressure of \(\text{CO}_2\) is high in the blood and low in the alveolar air.
This gradient drives the entire transport process into reverse. Bicarbonate ions re-enter the red blood cells in exchange for chloride ions. Inside the red blood cell, bicarbonate recombines with hydrogen ions released from hemoglobin, re-forming carbonic acid.
Carbonic anhydrase then converts the carbonic acid back into water and gaseous \(\text{CO}_2\). This \(\text{CO}_2\), along with the amount released from carbaminohemoglobin, diffuses out of the blood into the air space of the lungs. This metabolic waste product is expelled during exhalation, which also regulates the blood’s \(\text{pH}\).