Cell membrane

The fluid mosaic model of the cell membrane was proposed by Singer and Nicolson (1972).
$Na^+$ and $K^+$ are moved by active transport (the $Na^+/K^+$ pump); in the human RBC membrane proteins are about 52 per cent; and the polar heads of the lipids face outwards while the hydrophobic tails face inwards.

According to the fluid mosaic model of Singer and Nicolson (1972), the cell membrane is made of a lipid bilayer in which proteins are embedded.
The lipids are mainly phospholipids, arranged with their polar (hydrophilic) heads towards the outer sides and their non-polar (hydrophobic) tails towards the inside.
Proteins are either peripheral (on the surface) or integral (partially or totally buried in the bilayer); the quasi-fluid nature of the lipid allows lateral movement of proteins.
Carbohydrates are present only in small amounts as glycoproteins and glycolipids; they do not form the bilayer.
Hence the plasma membrane consists mainly of proteins embedded in a phospholipid bilayer.