Question

cells are protected from external environments by macromolecular membranes. what characteristic of the lipids allows this seperation to occur

Answer 1

Well the lipids have a hydrophilic side and a hydrophobic side.
hydrophobic means water fearing, hydrophobic parts don't like water - (water-hating)
hydrophilic means water loving, those parts touch water - (water-loving)
so the lipid membrane is made up of 2 layers, they face outwards so that the middle is hydropobic and the outside is hydrophilic - this is nice because it creates such a nice seal; water is not going to go THROUGH the membrane because water is repelled by the inside.

-- That's actually one of the biggest aspects that make this membrane work so well.

Answer 2

so that's pretty much all when we're talking about acting as a barrier in water.