What must be present in order for substances to move across a cell membrane?

While negatively charged molecules can move across membranes, their movement is not a universal requirement for transport across cell membranes. The presence of negatively charged molecules does not inherently create the necessary conditions for transport, as other factors like concentration gradients and membrane permeability are more critical.

Similarly to negatively charged molecules, positively charged molecules do not constitute a necessary condition for transport across the membrane. The movement of ions, whether positively or negatively charged, is influenced by concentration gradients and electrochemical gradients, but their mere presence does not guarantee transport.

Although protein channels facilitate the movement of certain substances across the membrane, they are not always required for transport to occur. Small, nonpolar molecules can diffuse directly through the lipid bilayer without the assistance of protein channels, provided a concentration gradient exists.