Glucose Transport

Transport of monosaccharides from small intestine to tissues

• Stomach: minimal carbohydrate digestion/absorption

• Small intestine (Jejunum)

  • Glucose and galactose are absorbed via the SGLT1 transporter (Na⁺-dependent) 

  • Fructose is absorbed via GLUT5 (facilitated diffusion) 

  • All monosaccharides exit the enterocyte into the blood via GLUT2 

• Portal Circulation To liver

  • Transport monosaccharides to liver by hepatic portal vein

  • Liver stores glucose as glycogen or releases to blood

  • Fructose/galactose is converted to glucose or glycolytic intermediates

• Systemic distribution: glucose transported in blood to tissue is mediated by GLUT transporters

  • GLUT1: most tissues (basal uptake, brain, RBCs) s .

  • GLUT2: liver, pancreas (bidirectional) s .

  • GLUT3: neurons (high-affinity) s .

  • GLUT4: muscle, adipose (insulin-dependent

Cori Cycle

• Metabolic pathway where lactate produced by anerobic glycolysis In muscle is transported to the liver, converted back to glucose by gluconeogenesis and returned to muscle during blood

Steps

1. Intensive Anaerobic Activity

   1. Muscle import Glucose from blood

2. Export lactate waste

3. Recycling

   1. Liver imports lactate from blood and converts it to glucose by gluconeogenesis

4. Glucose exported into blood

5. Recovery (once activity is done)

   1. Liver and muscle store glucose and glycogen for further needs

GLUT transporters

• Facilitative uniporters, mediate glucose move down it's concentration gradient as sole driving force

• Not coupled to any energy

  • In the cori cycle, liver must generate a high intracellular glucose to release it into te blood while muscle consumes glucose rapidly to keep uptake favourable

Glucose transport into cell follow kinetics similar to enzyme-catalyzed reacitons

• Substrate is glucose outside cell, product is glucose inside cell

• High extracellular concentrations of glucose (if intracellular concentration is low), the rate of glucose uptake approaches mac value, V max

  • Kt is like Km, [S] to reach 50% of Vmax

  • Channels is always plugged with Glucose, V max is the channel is always transporting a glucoset35qttvfrccd

Liver

•  bidirectional glucose transport

  • Import (after a meal, glycogen synthase)

  • Export (cori cycle, lactate to glucose)

• Low Blood Glucose, time of need

  • [Glc] liver is higher than blood, drive export of glucose into blood during gluconeogenesis (Glc synthesis from precursors) or glycogenolysis (breakdown of glycogen to Glc)

• High Blood Glc, time of plent

  • [Glc] liver is lower than blood, drives import of glucose from blood into liver after a meal to be stored as glycogen

• Bidirectional flux is mediated by GLUT uniporter and concentration gradients

SGLT transporters

• Sodium-coupled glucose transporters, imports glucose against it's concentration gradient

• Glc is taken from digestive tract, into intestinal epithelial cells unidirectionally against its concentration gradient by coupled transport

• Sodium is high in the digestive tract and low in epithelial cell, Glc is low in digestive tract and high in epithelial cell

  • SGLT symport moves 2 Na+  down its concentration gradient, the energy released is used to power Glc to transport it against it's concentration gradient

  • GLUT uniport exports Glc down it's conc. Grad from cell cytosol to blood

• Kidney sues SFLT to uptake Glc form urine

• Na+ gradient is maintained by Na+/K+ ATPase pump

  • 3 Na+ is pumped out from the cell cytosol into the blood

  • 2 K+ is pumped from the blood into the cell cytosol

  • Keeps [Na+] cytosol low to allow for SGLT to continue working

Glc transport summary

• Liver hepatocytes can import Glc from blood for long-time storage as glycogen

• In response to epinephrine liver will synthesize Glc (gluconeogenesis) and release Glc from glycogen (glycogenolysis) to export to tissue in need

• Hepatocytes have bidirectional Glc transport drive by Glc concentration gradient using GLUT

• Intestinal Enterocytes performs unidirectional Glc transport (gut to blood) using active transport in SGLT

• Tissue controls transport direction to match physiological roles, it does this by

  • Regulating pathways: turn on/off specific metabolic routes as needed

  • Controlling concentration: adjust substrate and product levels to drive reaction direction

  • Coupling reaction: linking transport to exergonic process to achieve flow