Final answer:
Carrier proteins in cell membranes are specific for certain molecules, and they work by changing shape to transport these molecules across the membrane. An example includes the glucose transporter, which facilitates the entry of glucose into cells through both passive and active transport mechanisms.
Step-by-step explanation:
Proteins function as transport carriers in cell membranes by moving substances across the membrane. This is particularly important for molecules that cannot pass through the membrane channels due to their size. Carrier proteins bind to these molecules; this binding causes a change in the protein's shape, which allows the molecule to be transported across the membrane. An example of a carrier protein at work is the glucose transporter, which facilitates the transport of glucose molecules into cells. This process can occur through passive transport, such as with facilitated diffusion, or through active transport, where ATP is required to move molecules against their concentration gradient.