Final answer:
Nonpolar molecules such as oxygen and carbon dioxide can pass through the plasma membrane without transport proteins due to their compatibility with the hydrophobic interior of the membrane. This process is part of passive transport. Polar molecules and ions, however, require facilitated diffusion involving transport proteins.
Step-by-step explanation:
The substances most likely to pass through the plasma membrane without the help of a transport protein are nonpolar molecules such as oxygen and carbon dioxide. These gases are able to diffuse freely through the lipid bilayer because they are nonpolar and thus can interact favorably with the hydrophobic tails of the lipids within the membrane. In contrast, polar substances and ions, such as glucose, amino acids, and charged particles like sodium or potassium, encounter resistance due to the hydrophobic nature of the membrane's interior and require assistance from special proteins, like channel or carrier proteins, in a process known as facilitated diffusion.
Hydrophobic substances like lipids and molecules like alcohol can also pass through the membrane without the help of transport proteins due to their nonpolar nature. This type of movement across the membrane where no energy is expended is known as passive transport. On the other hand, substances that cannot pass freely often utilize transport proteins through facilitated diffusion or require energy in a process called active transport to cross the plasma membrane.