Simple diffusion allows very small, non-polar, fat-soluble molecules to cross the plasma membrane into or out of the cell through a concentration gradient. In addition, it is necessary that these molecules cannot have electrical charges, which prevents sodium, potassium, calcium and chloride ions from crossing the plasma membrane, even if they are very small.
Simple diffusion only allows small molecules to pass through
the cell membrane. It is unlikely for dissolved sodium ions to cross the
membrane as it is too large. Simple diffusion only occurs for non-polar small
molecules that can pass through the membrane.
