You're right to question this! It's not entirely accurate to say that cellular energy is *never* required for carrier molecules to transport passengers across the cell membrane. Here's a breakdown:

Passive Transport (No Energy Required):

* Facilitated Diffusion: This is the type of transport where carrier molecules help molecules cross the membrane *down* their concentration gradient. This means the molecule is moving from an area of high concentration to an area of low concentration. This movement doesn't require the cell to expend energy. Think of it like a helper guiding a ball rolling downhill.

* Examples: Glucose entering red blood cells using a glucose transporter protein.

Active Transport (Energy Required):

* Carrier Proteins: Some carrier proteins help molecules move *against* their concentration gradient (from low to high concentration). This requires the cell to expend energy, usually in the form of ATP. Think of it like a helper pushing a ball uphill.

* Examples: The sodium-potassium pump is a vital example. It pumps sodium ions out of the cell and potassium ions into the cell, maintaining the cell's electrical potential.

Key Points:

* Concentration Gradient: The direction of movement (with or against the gradient) determines if energy is needed.

* Carrier Protein Type: Some carrier proteins are passive, while others are active.

So, the statement that cellular energy is *never* required is incorrect. It depends on the specific type of transport and the direction of movement.