The sodium-potassium pump is the classic example of active molecular transport across a membrane. The hydrolysis of ATP induces conformational changes in the transporter protein, which move the molecules across the membrane. More information on the [Wikipedia page](https://en.m.wikipedia.org/wiki/Sodium%E2%80%93potassium_pump)
A proteins can take multiple shapes depending on a number of factors. This is called the proteins “conformation”. For active transport you typically have a molecule that is called “transmembrane” – it has one part sitting outside the cell and another part sitting inside a cell. Some transmembrane proteins will bind to a molecule and then use an energy source – typically ATP to undergo a conformational change which shuttles the molecule into the cell and unbind it from the protein.
ELI5:
Protein binds to molecule, protein uses energy (ATP) to make the protein change shape, protein has now moved the molecule inside the cell.
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