One type of active transport involves molecules or ions being moved across a membrane by

I belive you're looking for "carrier proteins".

Carrier proteins can change their shapes in order to "grab" the ions needed and transfer them to the other side of the membrane, which has a higher concentration of that kind of substance. At the same time, they can ensure the molecules already in the other side from not diffusing back out.

This process usually requires extra energy provided by the organism, such as from cellular respiration. Examples of active transport requiring carrier proteins include absorbing nutrient in the small intestine, or absorbing mineral ions from soil in the plant root hair cells.

One type of active transport involves molecules or ions being moved across a membrane by active transport.