Why is the presence of an integral protein necessary for a charged particle, such as an ion, to cross a cell membrane?

Im is in a crowd of people at the mall looking for his girlfriend tammy, who was supposed to meet him for dinner. jim realizes tammy is across the room as he catches a whiff of her overpowering perfume, 'paris #6'. which process explains why jim is able to smell the perfume?

The smallest biological units that can evolve are

Darwin’s studies of finches on the galápagos islands suggest that the finches’ differences in beak structure were most likely directly due to: