Cells possess a variety of membrane proteins to carry solutes across the membrane. Carrier proteins bind to a solute and change shape as they shuttle the solute molecule from one side of the membrane to another. One type of carrier protein, called a uniporter, carries only one type of solute. In this example, there are three types of solute present. They are represented by a circle, a triangle, and a hexagon. Notice how a uniporter selectively binds only one type of solute, the hexagon, and shuttles it from one side of the membrane to another. Let's consider another kind of a carrier protein, called a symporter. A symporter carries two types of solutes in the same direction across the membrane. In this example, the symporter ferries both the solutes represented by the green circles and the solutes represented by the purple hexagons into the cell. Note that there are more of the green circles outside the cell than inside the cell. On the other hand, there are more of the purple hexagons inside the cell than outside. The movement of the green circle solutes down their concentration gradient releases energy. This symporter uses the energy released by moving the green circles down their concentration gradient to pump the purple hexagons against their concentration gradient. Let's consider a third type of carrier protein, called an antiporter. Similar to symporters, antiporters also carry two types of solutes. However, antiporters couple the transport of two solutes in opposite directions across the membrane. In this example, the antiporter ferries the solutes represented by the green circles out of the cell and the solutes represented by the brown triangles into the cell. Note that this antiporter is moving both solutes against their concentration gradients, making this an example of active transport, which requires an input of energy. For example, antiporters often hydrolyze ATP to pump solutes against their concentration gradients. In sum, carrier proteins bind to solute molecules and change conformations as they transport solutes across the membrane. Carrier proteins can be grouped into 3 types. Uniporters move one type of solute molecule. Symporters carry 2 types of solute molecules in the same direction and Antiporters move 2 types of solutes in opposite directions.