Im pretty sure its guanine
<span>phospholipid bilayer, which is two layers of phospholipids back-to-back</span>
I don't know witch definition you mean, but here: https://www.google.com/search?q=google+dictionary&rlz=1C1CHBF_enUS871US871&oq=google+dic&aqs=chrome.0.0j69i57j0l6.3972j0j7&sourceid=chrome&ie=UTF-8#dobs=blowout
They help work together because when the heart pmps blood through the body and the cells flow through the blood non stop its like a barrier of protection