The system of glucose transport in the small intestine is very efficient at maximizing the absorption of glucose from the lumen into the blood.
The direct source of energy used to move glucose into the epithelial cell from the intestinal lumen is the sodium gradient.
The sodium-glucose cotransporter (SGLT1) is a transport protein that couples the movement of sodium and glucose across the apical membrane of the epithelial cell.
SGLT1 uses the energy stored in the sodium gradient to transport glucose against its concentration gradient.
The sodium gradient is established by the sodium-potassium pump (Na+/K+-ATPase), which uses ATP to pump sodium out of the epithelial cell and potassium into the cell.
This creates a higher concentration of sodium outside the cell than inside.
When glucose and sodium bind to SGLT1, the transporter changes its shape and transports both molecules into the epithelial cell.
The sodium gradient drives the movement of glucose into the cell, even though the concentration of glucose is higher in the lumen than in the cell.
How this system maximizes glucose absorption from the intestinal lumen into the epithelial cells and from the epithelial cells into the blood:
The sodium gradient provides a continuous source of energy for glucose transport.
The sodium-potassium pump is constantly pumping sodium out of the cell, maintaining a high sodium gradient.
This means that SGLT1 can always transport glucose into the cell, even if the concentration of glucose in the lumen is low.
SGLT1 transports glucose against its concentration gradient.
This means that glucose can be absorbed into the epithelial cell even when the concentration of glucose inside the cell is higher than in the lumen.
Glucose is transported across the basolateral membrane of the epithelial cell into the blood by facilitated diffusion.
This means that glucose moves from the epithelial cell to the blood down its concentration gradient.
This ensures that glucose is constantly removed from the epithelial cell, which keeps the sodium gradient high and maximizes glucose absorption.