3709 3710 3711 3712 *Table 8.1 Tissue Distribution of Selected GLUTs* 3713 3714 3715

	Location	Function	K_m (mM)
GLUT-1	Most tissues	Basal glucose uptake	1
GLUT-2	Liver, kidneys, pancreas	Removes excess glucose from blood	15-20
GLUT-3	Most tissues	Basal glucose uptake	1
GLUT-4	Muscle and fat	Removes excess glucose from blood	5
GLUT-5	Small intestine, testes	Transport of fructose	10

3716 3717 3718 ## B. Sodium- and ATP-dependent cotransport of glucose 3719 3720 This type of glucose cotransport with sodium occurs in the epithelial cells of the intestine, the renal tubules, and the choroid plexus. This is an energy-requiring process that transports glucose against (up) its concentration gradient, from low extracellular concentrations to higher intracellular concentrations while Na+ is transported down its electrochemical gradient. There is a much higher extracellular than intracellular concentration of Na+, which is the result of the Na+-K++ATPase. The Na+ concentration gradient powers the transport of glucose against its concentration gradient; ATP hydrolysis is an indirect energy source because it is necessary to establish the Na+ gradient. (see also Fig. 7.10). Because this secondary active transport of glucose requires the concurrent uptake (symport) of Na+, the transporter is a sodium-dependent glucose cotransporter (SGLT). (Note: The choroid plexus, part of the blood–brain barrier, also contains GLUT-1.)c 3721 3722 3723 3724 3725 ![The text describes the function of sodium-dependent glucose cotransporter protein 2 (SGLT2) in the kidneys and its role in glucose reabsorption.](https://cdn.orris.care/cdss_images/5daaaeb73c9e7aa954ef463fbd8c92d4ff7cfbabea9b71a750d81153eb8c3741.png) 3726 3727 3728 ![A diagram illustrates a process with arrows showing the conversion of glucose into pyruvate, alongside ADP, ATP, and NAD+ molecules.](https://cdn.orris.care/cdss_images/62cd341cc0632f076f19d3c61ca62a477d60c0388911fd5bd4487a416a397d7e.png) 3729 3730 *Figure 8.11* 3731 3732 Two phases of aerobic glycolysis. NAD(H) = nicotinamide adenine dinucleotide; ADP = adenosine diphosphate. 3733 3734 3735 3736 3737 ## V. GLYCOLYSIS REACTIONS 3738 3739 The conversion of glucose to pyruvate occurs in two stages (Fig. 8.11). The first five reactions of glycolysis correspond to an energy-investment phase in which the phosphorylated forms of intermediates are synthesized at the expense of ATP. The subsequent reactions of glycolysis constitute an energy-generation phase in which a net of two molecules of ATP are formed by substrate-level phosphorylation per glucose molecule metabolized. 3740 3741 3742 ## A. Glucose phosphorylation 3743 3744 Phosphorylated sugar molecules do not readily penetrate cell membranes because there are no specific transmembrane carriers for these compounds and because they are too polar to diffuse [... Output truncated due to length. 2646 tokens/9361 characters removed ...]