Continuing on our theme of applying the solubility theme in the biological context, let us introduce a favorite MCAT example: Peptide hormones are water soluble while steroid hormones are not.

This is an interesting distinction because the difference determines the signaling mode. While peptide hormones cannot pass through the nonpolar interior of the phospholipid bilayer, steroid hormones can pass through cell membranes without a channel. Because they cannot pass through membranes, unlike steroid hormones, peptide hormones typically rely on second messenger systems for signal transduction into the cell.