Absorption of glucose in the ileum
Na ions diffuse down their
concentration gradient into the cell
from the lumen through a transport
protein. This is a co-transport
protein (symport) meaning that as
Na ions are moving into the cell,
glucose also moves into the cell,
against its concentration gradient.
The concentration of Na ions is kept
low as they are actively transported
out of the cell. The concentration of
Na ions is much lower inside the cell
than in the lumen of the ileum.
Glucose moves by
facilitated diffusion through
a different carrier protein
from the cell into the
capillary.
Sodium ions are actively
transported out of epithelial cells by
the sodium potassium pump, into
the blood. This carrier protein
(antiport) is on the membrane
adjacent to the capillary. K ions
move from the blood to the
epithelial cell and Na ions in the
opposite direction.