Question

how do carrier proteins transport substances across cell membranes?

Answer 1

they get a hold of them and carry them to their destination, just as the name says.

Answer 2

Carrier Proteins transport molecules from an area of higher concentration on one side of the membrane to an area of lower concentration on the other side. To be more descriptive, carrier proteins binds to a specific substance on one side of the cell membrane. This binding causes the protein to change shape. as the protein's shape changes, the substance is moved across the membrane and is released on the other side.