Which of the following is always true about molecules?

Not all molecules can freely pass through cell membranes; factors such as size, polarity, and charge influence a molecule's ability to cross a membrane. For instance, large or polar molecules often require specific transport proteins to enter or exit cells, thus limiting their movement through membranes.

This statement is inaccurate because moving molecules from low concentration to high concentration is known as active transport, which requires energy input to occur. Passive transport, including diffusion, only allows movement from high to low concentration without energy expenditure.

This choice is incorrect as molecules are in constant motion due to thermal energy, allowing them to diffuse throughout their environment. The ability of molecules to move is fundamental to many physical and biological processes, such as gas exchange and nutrient absorption.