Final answer:
The CFTR protein is a channel protein involved in the active transport of Cl- ions out of cells. It requires ATP to function and uniquely allows Cl- ions to flow down their concentration gradient.
Step-by-step explanation:
The cystic fibrosis transmembrane conductance regulator, commonly known as CFTR, falls into the category of membrane proteins known as channel proteins. The CFTR protein is an integral membrane protein responsible for the active transport of Cl- ions out of cells, and it operates by allowing these ions to flow down their concentration gradient. While active transport typically involves the movement of substances against their concentration gradient, CFTR is a unique case because it requires ATP to function, yet the Cl- ions move with the gradient, a characteristic that initially puzzled researchers.
Ion channels like CFTR are embedded within the membrane and are crucial for facilitating the movement of ions across the cell membrane. There are different types of transport proteins, including aquaporins which allow water to pass through, electrogenic ion pumps, and others, but based on the given options and the function of CFTR, it is categorized as a channel protein which provides a path for ions to swiftly move through the membrane.