Final answer:
The movement of sodium ions out of epithelial cells is active transport, as is the entry of glucose molecules into these cells. Sodium ions into the cell can be involved in secondary active transport. Both processes rely on energy, either directly from ATP or indirectly through an established sodium gradient.
Step-by-step explanation:
The transport processes for sodium ions and glucose molecules in epithelial cells can either be active or passive, depending on the direction relative to their concentration gradient and whether energy is expended.
The movement of sodium ions out of epithelial cells is typically an active transport process, especially when considering the sodium-potassium pump. This pump actively exchanges sodium (Na+) for potassium (K+), using ATP as energy, and contributes to the net negative charge inside the cell.
When considering the entry of sodium ions into epithelial cells, this can be a part of a secondary active transport process known as co-transport or symport, where the movement of sodium down its electrochemical gradient is coupled with the transport of another substance, such as glucose.
In the intestines, glucose is actively transported into epithelial cells alongside sodium ions because this allows for glucose absorption even when its concentration inside the cell is higher than in the digestive tract. This process also relies indirectly on the energy provided by the sodium-potassium ATPase pump.
Lastly, the ingestion of a glucose molecule into an epithelial cell via active transport in the small intestine is necessary when the concentration of glucose inside the cell already exceeds that in the lumen, which is often the case after a meal when glucose levels in the intestine are high.
Hence, glucose enters cells against its concentration gradient, using energy in the form of ATP indirectly, via the sodium gradient established by the sodium-potassium ATPase pump.