What part of the membrane do small uncharged particles diffuse through?
The lipid bilayer
The cell membrane is made of phospholipids. The head is hydrophilic (water-loving) and the tail hydrophobic( water-hating). This is why the heads are facing the watery exterior of inside and outside the cell, and the tails are tucked away from the water. An example of this is when you add oil and water.
Small, uncharged polar molecules such as oxygen and water(at uni, you learn that there are special pores called aquaporins that are used for water molecules to pass through the phospholipid bilayer) can pass through the lipid bilayer and into the cell. However, larger molecules such as glucose and ions like sodium and potassium ions cannot pass through the phospholipid bilayer and so special protein channels and carrier proteins that can span throughout the bilayer do the business of moving glucose and ions to and fro the cell.
Cholesterol maintains the membrane's fluidity so that it can withstand changes in temperature.
Glycoproteins act as receptors to hormones such as oestrogen, cortisol, adrenaline and more.
Glycolipids act as "antigens" so that the Immune System recognizes the cell as one of its own, basically to not attack it. That is why people with autoimmune disorders, the Immune system recognises its own cells as an enemy and cause inflammation and all sorts of problems to the host.