The crossing over is a process that is essential to human genetic diversity. During the crossing over parts of the homologous chromosomes ( two chromosomes identical by structure, but different in origin, one inherited from the mother and one from the father) randomly exchange parts of their sequences. If the crossing-over didn't exist, this would greatly decrease the genetic diversity, because the chromosomes would always bee passed o the next generations unchanged.
A cell's lipid bilayer allows to small uncharged substances such as: ions, hydrophobic molecules, oxygen and carbone dioxide to travel through the membrane by simple diffusion
