Maintaining the sodium ion gradient
Co-transporter proteins in the cell-surface membranes of epithelial cells allow amino acids and glucose to be absorbed from the ileum.
The mechanism depends on a sodium ion concentration gradient. A sodium-potassium pump actively moves sodium ions from the epithelial cell into the blood, lowering the sodium ion concentration inside the epithelial cell.
Sodium ions pumped out
Sodium ions are actively transported from the epithelial cell into the blood by a sodium-potassium pump.
Co-transport into the cell
Sodium ions then move from the lumen into the epithelial cell down their concentration gradient. Glucose or an amino acid enters through the same co-transporter at the same time.
Movement into blood
The concentration of glucose or amino acids inside the epithelial cell rises, so these molecules can pass down their concentration gradients into the blood by facilitated diffusion.
How the mechanism is powered
The movement through the sodium-dependent co-transporter is passive because sodium ions are moving down their concentration gradient.
However, ATP is needed for the sodium-potassium pump that creates and maintains this gradient. For this reason, co-transport is considered overall to be an active transport process.
Glucose and amino acids follow the same overall mechanism, using the appropriate sodium-dependent co-transporter.
Exam Tip: In a co-transport explanation, link the stages: the sodium-potassium pump maintains a low sodium ion concentration inside the epithelial cell, sodium ions return down their gradient through a co-transporter, and glucose or amino acids subsequently enter the blood by facilitated diffusion.