A cell membrane is very specific about what it allows to cross. how does this help the cell?

The cell membrane is selectively permeable . It lets some substances pass through rapidly and some substances pass through more slowly, but prevents other substances passing through it at all. Some small molecules such as water, oxygen and carbon dioxide can pass directly through the phospholipids in the cell membrane

haploid cells;

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.