Bicarbonate ions transport

Over the whole range of variation of the Cl- HC03— ratio in the cathode compartment, the transport number of chloride was somewhat higher than the molar ratio of chloride in the solution. In other words, in all the three membranes tested chloride ion transport is favored over bicarbonate ion transport. This fact is not unrelated to the data on water transport. The electro-osmotic water transport with chloride ions is smaller than with bicarbonate ions. It is known that, other factors being equal, low water transport and high mobility of ions in a membrane-as measured, for instance, by electrical conductivity-are correlated (13,16). 

The enzyme carbonic anhydrase promotes the hydration of COg. Many of the protons formed upon ionization of carbonic acid are picked up by Hb as Og dissociates. The bicarbonate ions are transported with the blood back to the lungs. When Hb becomes oxygenated again in the lungs, H is released and reacts with HCO3 to re-form HgCOj, from which COg is liberated. The COg is then exhaled as a gas. 

Step 1 of Figure 29.13 Carboxylation Gluconeogenesis begins with the carboxyl-afion of pyruvate to yield oxaloacetate. The reaction is catalyzed by pyruvate carboxylase and requires ATP, bicarbonate ion, and the coenzyme biotin, which acts as a carrier to transport CO2 to the enzyme active site. The mechanism is analogous to that of step 3 in fatty-acid biosynthesis (Figure 29.6), in which acetyl CoA is carboxylated to yield malonyl CoA. 

The remaining 60% of carbon dioxide is transported in the blood in the form of bicarbonate ions. This mechanism is made possible by the following reaction ... 

This entire reaction is reversed when the blood reaches the lungs. Because carbon dioxide is eliminated by ventilation, the reaction is pulled to the left. Bicarbonate ions diffuse back into the red blood cells. The hemoglobin releases the hydrogen ions and is now available to load up with oxygen. The bicarbonate ions combine with the hydrogen ions to form carbonic acid, which then dissociates into carbon dioxide and water. The carbon dioxide diffuses down its concentration gradient from the blood into the alveoli and is exhaled. A summary of the three mechanisms by which carbon dioxide is transported in the blood is illustrated in Figure 17.8. 

Exocrine glands within the pancreas secrete an aqueous fluid referred to as pancreatic juice. This fluid is alkaline and contains a high concentration of bicarbonate ion it is transported to the duodenum by the pancreatic duct. 

Some transporters such as Na+-dependent dicarboxylate transporter (NADC1), Na+-dependent bicarbonate transporter 2 (SBC2), Na+-dependent bicarbonate transporter HNBC1, several ion transporters, and channels are also expressed in the intestinal tissues [4]. 

Figure 2.3 Absorption of bile acids by the cholangiocyte in the cholehepatic shunt. Bile acids are absorbed at the apical membrane of the cholangioc5de by the apical sodium-dependent bile-acid transporter (ASBT) that causes cholehepatic shunting of bile acids back to the hepatocyte. Absorbed bile adds are exported across the basolateral membrane by multi-drug-resistance-associated protein 3 (MRP3), a truncated form of ASBT or by the het-eromeric organic solute (OST) a and p forms. Bile adds cause choleresis that is rich in bicarbonate ions secreted by the chloride/bicarbonate ion exchanger.

Human carbonic anhydrase II, found primarily in the erythrocyte, is the prototypical member of the family of carbonic anhydrases and has been extensively reviewed (Pocker and Sarkanen, 1978 Lindskog, 1983, 1986 Silverman and Lindskog, 1988). Within the erythrocyte carbonic anhydrase II hydrates CO2 to form bicarbonate ion plus a proton via tandem chemical processes (Silverman and Lindskog, 1988) (Scheme 2). Most of the carbon dioxide generated during the process of respiration requires this carbonic anhydrase Il-catalyzed event for transport out of the cell. The resultant protons of CO2 hydration are taken up by His-146)8, His-122a, and the amino terminus of the a subunits of the hemoglobin tetramer. As a reference. Scheme 3 outlines the interconversions... 

Carbonic anhydrase plays an important role in the secretion of aqueous humor [1,2]. This enzyme was first demonstrated to be present in the ciliary processes of the rabbit, and its presence was later confirmed in human ciliary processes [3,4]. Carbonic anhydrase is responsible for the generation of bicarbonate anions which are secreted from the ciliary process into the posterior chamber, with sodium being the counter ion. Inhibition of carbonic anhydrase in the ciliaiy processes of the eye decreases aqueous humor secretion, presumably by slowing the formation of bicarbonate ions with subsequent reduction in sodium and fluid transport. The role of the enzyme in aqueous humor secretion has been reviewed in detail by Maren [1]. 

Binding of C02 Most of the carbon dioxide produced in metabolism is hydrated and transported as bicarbonate ion (see p. 9). However, some CO2 is carried as carbamate bound to the uncharged a-amino groups of hemoglobin (carbamino-hemo-globin see Figure 3.7), which can be represented schematically as follows ... 

Bicarbonate buffer system, acidosis, and alkalosis Radioisotopes, nuclear medicine Cell crenation/rupture Ion transport... 

As the estimations above display, the net flows of chloride and bicarbonate ions are negligible, and the transport of ions is passive. 

Carbon dioxide is a major end product of aerobic metabolism. In complex organisms, this carbon dioxide is released into the blood and transported to the lungs for exhalation. While in the blood, carbon dioxide reacts with water. The product of this reaction is a moderately strong acid, carbonic acid (pAT = 3.5), which becomes bicarbonate ion on the loss of a proton. 

This is an important reaction in your life. This reaction is occurring in the blood vessels of your lungs as you read these words. The carbon dioxide gas produced in your cells is transported in your blood in the form of the bicarbonate ion (HC03 ). In the blood vessels of your lungs, the HC03 ions combine with H+ ions to produce CO2, which you exhale. This reaction also occurs in sodium bicarbonate products, such as those shown in Figure 10-14, that are made with baking soda. 

The morphological classification adopted in an earlier section (see pp. 89— 91) emphasizes the important role of membrane systems in calcification. Membranes, both in intracellular and extracellular calcification, are thought to be involved in an active transport of calcium to the site of calcification. They may also be involved in facilitating the availability of bicarbonate ions and in removing protons released during calcification. Thus, all the main ion species involved in biological calcification may be controlled by membrane processes. The ions are related according to the empirical equation... 

For efficient transport of relatively insoluble CO2 from the tissues where it is formed to the lungs where it must be exhaled, the buffers of the blood convert CO2 to the very soluble anionic form HCOJ (bicarbonate ion). The principal buffers in blood are bicarbonate-carbonic acid in plasma, hemoglobin in red blood cells, and protein functional groups in both. The normal balance between rates of elimination and production of CO2 yields a steady-state concentration CO2 in the body fluids and a relatively constant pH. 

As the concentration of HCO3 (i.e., of metabolic CO2) in red blood cells increases, an imbalance occurs between the bicarbonate ion concentrations in the red blood cell and plasma. This osmotic imbalance causes a marked efflux of HC03 to plasma and consequent influx of Cl from plasma in order to maintain the balance of electrostatic charges. The latter osmotic influx, known as the chloride shift, is accompanied by migration of water to red blood cells. Thus, transport of metabolic CO2 in the blood occurs primarily in the form of plasma bicarbonate formed after CO2 diffuses into red blood cells. 

Hydrogen ions are transported by an indirect mechanism in the upper small intestine. As sodium is absorbed, hydrogen ions are secreted into the gut. Hydrogen ions then combine with bicarbonate ions to form carbonic acid, which then dissociates into carbon dioxide and water. Carbon dioxide readily diffuses into the blood for expiration through the lung. The water remains in the chyme. 

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