Active Transport and Sodium–Glucose Co-transport

Active transport moves a substance against its concentration gradient, using a carrier mechanism coupled to an energy source. In a common ATP-driven mechanism, ATP hydrolysis and phosphorylation change a transport protein’s conformation, moving bound ions across the membrane before the carrier resets.

This allows a cell to maintain concentration differences that diffusion alone would dissipate. More carriers can increase transport capacity, but only if the required energy source and substrates remain available. Saying “the membrane needs energy” is less precise than identifying the ATP-dependent protein process.

In an epithelial cell lining the ileum, the sodium–potassium pump in the membrane facing tissue fluid uses ATP to move sodium ions out of the cell, helping maintain a low intracellular sodium concentration. Sodium therefore has a tendency to enter from the intestinal lumen through suitable transport proteins.

A sodium–glucose co-transporter in the lumen-facing membrane binds sodium and glucose. Sodium movement down its electrochemical gradient is coupled to glucose uptake, which can occur against the glucose concentration gradient. The co-transporter is not simply hydrolysing one ATP directly each time it binds glucose; its energy dependence is indirect through the maintained sodium gradient.

Scroll the diagram to see the full view.

Glucose can leave the epithelial cell into tissue fluid through a different transporter by facilitated diffusion when the concentration gradient favours that direction. It can then enter the circulation. Microvilli increase the membrane area available for transport proteins, while mitochondria support the ATP demand of maintaining ion gradients.

Blocking ATP production can eventually reduce glucose uptake by allowing the sodium gradient to weaken. It need not stop every glucose molecule instantly because an existing gradient can persist briefly. Locate each membrane and transport direction clearly in a diagram; mixing the lumen-facing co-transporter with the tissue-facing pump loses the causal sequence.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.