Substances that can move by diffusion directly through the phospholipid bilayer of the plasma membrane include: - sodium ions - VCE - SSCE Biology - Question 1 - 2018 - Paper 1

Sodium ions:

• Sodium ions are charged and cannot pass through the hydrophobic lipid bilayer directly. They require ion channels for facilitated diffusion.

Oxygen molecules:

• Oxygen is a small, non-polar molecule, allowing it to diffuse freely across the phospholipid bilayer.

Polar protein molecules:

• Polar protein molecules cannot diffuse directly through the bilayer due to their size and hydrophilicity; they require transport proteins.

Ribonucleic acid molecules:

• RNA molecules are also larger and polar, thus cannot diffuse directly and require specific transport mechanisms.