Secondary Active Transport -

The process works as follows:

In antiport, the driving ion and the driven molecule move in across the membrane.

| Approach | What it tests | | :--- | :--- | | | Remove external Na⁺ → transport stops (proves Na⁺ dependence) | | Oubain (Na⁺/K⁺ pump inhibitor) | Transport gradually stops as Na⁺ gradient collapses (indirect proof) | | Valinomycin (K⁺ ionophore) | Changes membrane potential; affects electrogenic transport | | Vesicle assays | Load vesicles with high Na⁺ inside, then add external substrate – uptake occurs without ATP | secondary active transport

| | | Secondary Active Transport | Facilitated Diffusion | | :--- | :--- | :--- | :--- | | Direct energy source | ATP | Ion gradient | None (kinetic energy) | | Can move against gradient? | Yes | Yes | No | | Requires membrane protein? | Yes (pump) | Yes (cotransporter/exchanger) | Yes (carrier/channel) | | Example | Na⁺/K⁺ ATPase | SGLT1, NCX | GLUT1 glucose transporter |

Glucose concentration inside the cell can be 10–100× higher than outside – all without direct ATP use. The process works as follows: In antiport, the

Reclaiming essential minerals and nutrients from urine before they leave the body.

Outside (High Na⁺) → [Na⁺ + Glucose] → Inside (Low Na⁺) | Yes (pump) | Yes (cotransporter/exchanger) | Yes

Outside (High Na⁺) → Na⁺ enters; Ca²⁺ exits ← Inside (High Ca²⁺)

Secondary active transport is the at the bottom of that dam. As the sodium ions naturally want to flow back "downhill" into the cell (where their concentration is low), the transport protein captures that kinetic energy. It uses the momentum of the falling sodium ion to "carry" another molecule—like glucose or an amino acid—along with it, even if that second molecule is moving against its own gradient.