Ions can't diffuse across membranes, they must used channels to transport across
glucose
Nonpolar molecules like lipid-soluble substances (e.g., steroid hormones, oxygen, and carbon dioxide) are most likely to passively diffuse across the plasma membrane by dissolving in the lipid bilayer. This type of diffusion does not require a specific transport protein and can occur directly through the phospholipid bilayer due to the molecules' hydrophobic nature.
Most small non polar molecules can diffuse across the membrane. Water is polar but diffuses across the cell through aquaporins. There are also proteins that actively transport substances across the cell membrane such as ions (ex.The Na/K pump). It can get a little confusing. I tried to be as simple as possible. I don't know if this is what you were looking for.
Diffusion is simply the movement of one molecule from an area of high concentration to low concentration. It is semi related to the plasma membrane because plasma membrane does allow diffusion, to an extent. Plasma membrane is semipermeable and only allows certain things to diffuse in or diffuse out.
Water molecules freely diffuse across a semipermeable membrane.
Sodium and potassium diffuse across the plasma membrane of cells through ion channels called voltage-gated channels. These channels open and close in response to changes in membrane potential, allowing sodium and potassium ions to flow down their electrochemical gradients.
Nitric oxide can diffuse freely across the plasma membrane due to its small size and lipophilic nature. It does not require a specific transporter or receptor for entry into cells.
Oxygen molecules diffuse across the membrane.
protiens
selectively permeable
selectively permeable
They are too large and usually contain hydrophilic regions which prevent movement across the hydrophobic core of the plasma membrane.