二次能動輸送

Secondary active transport uses energy stored in ion electrochemical gradients, such as the sodium gradient established by primary active transport, instead of using ATP directly. These gradients help move solutes like glucose against their concentration gradients.

One protein that shows secondary active transport is Sodium-glucose cotransporter 1, or SGLT1. Initially, this transporter is positioned so that the cytoplasm-facing side is closed, but the extracellular end is open. This exposes sodium-binding sites that favor the binding of positively charged sodium ions from the outside.

Since sodium levels are higher outside the cell than inside, and the cytoplasm is more negative than the extracellular space, sodium ions bound to the transporter move down their electrochemical gradient.

This releases energy, enabling the protein to change conformation and gain a higher affinity for glucose, even when glucose levels outside the cell are lower than inside.

A glucose molecule then binds to the transporter. The binding of both sodium and glucose causes the protein to close on the extracellular side and open on the cytoplasm-facing side.

Once sodium detaches from the transporter and enters the cytoplasm, the transporter’s affinity for glucose decreases, causing glucose to be released into the cytoplasm, moving against its concentration gradient.

Once empty, the transporter returns to its original outward-facing position and is ready for another transport cycle.