21. Because the interior of the lipid bilayer contains the nonpolar , only small nonpolar substances are able to diffuse directly across the cell membrane.

Pls me i need this ! each cell has genes activated depending on it's job and what kind of cell it is. it is the presence of that causes the repressor protein to fall off and unblock the gene on the lac operon. if a gene is turned on then it is being an additional circular chromosome found in some bacteria that is used in genetic engineering.