Bicarbonates cell membrane transport

Ambient concentrations of COj are very low and usually biolimiting. Hence phytoplankton generally rely on bicarbonate as their carbon source. Phytoplankton must convert this bicarbonate to COj to enable production of organic matter. This conversion is facilitated by the Zn-containing enzyme, carbonic anhydrase (Table 11.4). Some phytoplankton release carbonic anhydrase into seawater with the resulting COj then transported across their cell membrane. 

Remember that biological membranes contain a great number of different types of proteins, which typically account for about 60% of their total mass. There are about 20 prominent proteins found in erythrocyte membranes. The chloride-bicarbonate anion exchanger accounts for about 30% of the total protein content of the red blood cell membrane. As carbonic anhydrase converts C02 to the more water-soluble form of HCOj", this membrane protein will transfer the ion across the erythrocyte membrane. At the same time, chloride ions are transported in the opposite direction to prevent the... 

Intracellular fluids (also called the cytosol) are quite different compositionally from plasma and interstitial fluids (Table 4, Figures 4 and 5). The internal pH of many cells is maintained near 6.9-7.0. via various membrane transport mechanisms such as Na" /H and CP/HCO exchangers, and various phosphate and protein buffers. In contrast to the plasma, the intracellular fluids have substantially lower concentrations of sodium, calcium, chloride, and bicarbonate and higher to substantially higher concentrations of potassium, magnesium,... 

Two of the prominent integral proteins in the red blood cell membrane are glycophorin, which provides an external negative charge that repels other cells, and band 3, which is a channel for bicarbonate and chloride exchange. The transport of bicarbonate into the red blood cell in exchange for chloride helps to carry the bicarbonate to the lungs, where it is expired as COj. 

Fig. 10.6. Proteins in the red blood cell membrane. The proteins named Band 3 (the bicarbonate-chloride exchange transporter) and glycophorin contain nonpolar a-helical segments spanning the lipid bilayer. These proteins contain a large number of polar and charged hydrophilic amino acids in the intracellular and extracellular domains. On the inside of the cell, they are attached to peripheral proteins constituting the inner membrane skeleton. Band 3 is connected to spectrin filaments via the protein ankyrin. Glycophorin is connected to short actin filaments and spectrin via protein 4.1.

Protein-mediated transport systems, whether facilitative or active, are classified as antiports if they specifically exchange compounds of similar charge across a membrane they are called symports or cotransporters if they simultaneously transport two molecules across the membrane in the same direction. Band 3 in the red blood cell membrane, which exchanges chloride ion for bicarbonate, provides an example of an antiport. 

Sodium bicarbonate reabsorption by the proximal tubule is initiated by the action of a Na+/H+ exchanger located in the luminal membrane of the proximal tubule epithelial cell (Figure 15-3). This transport system allows sodium to enter the cell from the tubular lumen in exchange for a proton from inside the cell. As in all portions of the nephron, Na+/K+ ATPase in the basolateral... 

FIGURE 2.27 The parietal cell in the acid-producing slate. The large circles represent membrane-bound proteins. The apical membrane contains the H,K-ATPase and the KCI transporter shown in Figme 2.26- Also shown is cytoplasmic carbonic anhydrase and the chloride-bicarbonate exchanger in the basal membrane of the cell. The terms "apical" and "basal" refer to opposite sides of the plasma membrane of epithelial cells. 

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