Passive Transport Through the Plasma Membrane

What types of molecules can rapidly pass directly through the phospholipids of the plasma membrane without the help of a transport protein?

Small, nonpolar molecules such as oxygen, carbon dioxide, and some lipids can rapidly pass directly through the phospholipids of the plasma membrane without the help of a transport protein.

Main Answer

Small, nonpolar molecules can rapidly pass directly through the phospholipids of the plasma membrane, without the help of a transport protein.

The phospholipid bilayer of the plasma membrane has a selective permeability. It allows some molecules to pass through while blocking others. The phospholipids that make up the plasma membrane are arranged in a double layer known as the bilayer. The hydrophilic heads of the phospholipids are exposed to the aqueous environment on the outside and inside of the cell. The hydrophobic tails of the phospholipids face each other on the inside of the bilayer. They create a nonpolar, hydrophobic region between them that acts as a barrier to most polar and charged molecules.

Small, nonpolar molecules such as oxygen, carbon dioxide, and some lipids can pass through the phospholipids of the plasma membrane without the help of a transport protein. They are small enough to fit through the hydrophobic region of the phospholipid bilayer. They are also nonpolar, which means they do not interact with the hydrophilic heads of the phospholipids. As a result, they can move freely through the membrane by simple diffusion without the need for a transport protein.

Final Answer

Nonpolar substances like lipids, gases, and alcohol can rapidly pass through the plasma membrane, while water-soluble substances like glucose and amino acids need assistance to cross the membrane.

Substances that can rapidly pass directly through the phospholipids of the plasma membrane without the help of a transport protein are nonpolar molecules such as other lipids, oxygen and carbon dioxide gases, and alcohol. These nonpolar substances can diffuse through the lipid bilayer of the membrane due to their hydrophobic nature. On the other hand, water-soluble materials like glucose, amino acids, and electrolytes require assistance to cross the membrane as they are repelled by the hydrophobic tails of the phospholipid bilayer and they use facilitated transport through specific protein channels in the membrane.