Final answer:
Small molecules like O2 or CO2 can cross cellular membranes unassisted, while most molecules require the help of membrane transport proteins to get in or out of cells. Transport proteins can act as gates that permit the diffusion of molecules along a concentration gradient. Some transport proteins are pumps that use energy to move molecules against a concentration gradient.
Step-by-step explanation:
Small molecules like O2 or CO2 can cross cellular membranes unassisted; neither the hydrophilic surfaces nor the hydrophobic interior of the phospholipid bilayer are barriers to their transit. On the other hand, most molecules (even water!) need the help of membrane transport proteins to get in or out of cells and organelles. Transport proteins can act as gates that might be open or closed. When open, they permit diffusion of molecules into or out of cells along a concentration gradient so that their concentrations equalize across the membrane. Like the passive diffusion of small gasses, facilitated diffusion by membrane proteins does not require an input of energy.
In contrast, some transport proteins are actually pumps, using chemical energy to move molecules across membranes against a concentration gradient. The result of this active transport is to concentrate solutes on one side of a membrane.