Final answer:
The sentence is referring to proteins that function as pumps in active transport, with the sodium-potassium pump being a primary example that uses ATP to move ions against their concentration gradient.
Step-by-step explanation:
This is usually accomplished through the use of proteins called pumps, such as the sodium-potassium pump. One of the most common types of active transport involves proteins that work exactly like these pumps. Similar to how energy is used to pump air into a bicycle tire, ATP (Adenosine Triphosphate) is required by these membrane proteins to transport substances, including molecules and ions, across the cellular membrane. This often occurs against their concentration gradient, which is an uphill process from areas of lower concentration to areas of higher concentration. A well-known example of a pump for active transport is the Na+-K+ ATPase, which transports sodium and potassium ions across the cell membrane.