So these peptide hormones can travel in the bloodstream by themselves. Now eventually these peptide hormones will arrive at the target cell. Now these peptide hormones are water soluble, they are not lipid soluble. And that means they cannot actually pass across the plasma membrane of the target cell because the plasma membrane is made predominantly of hydrophobic tails of hydrophobic fatty acids. So that basically means peptide hormones bind on to receptor proteins found on the plasma membrane of the target cell. And once they bind, they can create some type of change.