Which of the following is NOT an important structural feature on the surface membrane of an animal cell? A) lipid bilayer B) sugar molecules C) cholesterol D) glycoproteins E) cell wall What is the most important barrier protecting the inner contents of an animal cell from its exterior environment? A) the cellular membrane B) the cell wall C) the nucleus D) the cytoskeleton E) Both a) and b) are correct. The lipids within the fluid mosaic of the plasma membranes are held in place by: A) covalent bonds. B) disulfide bridges. C) hydrogen bonds. D) ionic bonds. E) hydrophobic interactions. Multiple atoms linked together are collectively called: A) an element. B) a reactant. C) a molecule. D) a substrate. E) an isotope. A nucleotide consists of a phosphate group, a pentose sugar, and _____, all linked together by covalent bonds. A) a phosphorus-containing base B) a polypeptide C) a potassium-containing base D) a triglyceride E) a nitrogen-containing base