Final answer:
Small, nonpolar molecules such as oxygen and carbon dioxide can pass through a cell membrane by simple diffusion without the need for specialized pores. Polar substances and ions must utilize facilitated transport involving protein channels to cross the membrane.
Step-by-step explanation:
Certain small and nonpolar molecules can pass through a cell membrane without the need for specialized pores. Simple diffusion allows these molecules to move across the membrane due to a concentration gradient, from an area of higher concentration to one of lower concentration. The most common examples of such molecules include oxygen, carbon dioxide, and to a certain extent, water. While water can pass through the lipid bilayer, it often makes use of specialized channels known as aquaporins for more efficient transport.
Polar substances and ions, however, cannot easily pass through the hydrophobic lipid core of the membrane due to their charged nature. Molecules like glucose and amino acids require facilitated transport using specific protein channels to penetrate the plasma membrane. This process, while also utilizing the concentration gradient, does not require energy and is part of passive transport.