Answer the following question given this paragraph: 

A carrier protein is a transport protein that is specific for an ion, molecule, or group of substances. Carrier proteins "carry" the ion or molecule across the membrane by changing shape after the binding of the ion or molecule. Carrier proteins are involved in passive and active transport. A model of a channel protein and carrier proteins is shown in Figure below .


Q: Where do carrier proteins carry ions or molecules?

 Choices:

- between nuclei

- across isotopes

- back to nucleus

- across the membrane

A:
across the membrane
