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Lipid bilayer of the cell membrane is composed of phospholipids, which are amphiphilic (that is contains both the hydrophobic and hydrophilic ends). The hydrophilic ends points or heads outwards from the bilayer, whereas the hydrophobic tails point inwards (And towards each other). The lipid bilayer allows selective permeability to the cell and only allows certain molecules to pass through it. In general, a molecule's size and polarity decides whether it can pass through the bilayer or not. Polar molecules and large molecules have very low permeability through the bilayer, same for ions. Lipids can pass through the lipid bilayer and water can also pass through.
The transport through the bilayer can be passive (i.e. without the need to spend energy) and can take place on its own (through diffusion) or can be assisted by membrane proteins. The transport can also be active, where energy must be spent for entry into the cell.
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