Illustration demonstrating the process of ion transport across a cell membrane by a transporter in four stages. A transporter is a transmembrane protein that moves ions across a membrane to accomplish various biological functions including cellular communication and energy production. Proteins involved in moving ions are called ionophores. Step 1 shows an ionophore (blue and red) in a cell membrane in the vicinity of an ion (green). Step 2 shows the ion binding to the ionophore. Step 3 shows the ionophore switching to the opposite direction. Step 4 shows the ionophore releasing the ion into the cytoplasm.