Final answer:
Small, uncharged particles typically move across lipid bilayers by passive diffusion, which doesn't require energy and moves particles down their concentration gradient.
Step-by-step explanation:
The movement of small, uncharged particles across lipid bilayers is typically accomplished by passive diffusion. This natural process does not require energy; instead, it relies on the concentration gradient, moving particles from an area of higher concentration to one of lower concentration. Examples include the diffusion of gases like O₂ and CO₂, which are essential for physiological and cellular respiration. Water can also move across the membrane through a similar process called osmosis. However, for many substances, including ions and larger or polar molecules, facilitated diffusion may be necessary, involving transport proteins to aid their passage through the membrane.