Question

proteins do not pass through plasma membranes because ______. a the membrane is made of protein b they contain nitrogen c they are very large molecules d they cause emulsification

Explanation:

• Option A: Plasma membranes are mainly composed of phospholipids and proteins (as integral/ peripheral proteins), but the membrane's composition of protein doesn't prevent proteins from passing through. The structure of the membrane (like the phospholipid bilayer with small pores/ channels for small molecules) is the key, not just the presence of protein. So A is incorrect.
• Option B: Many molecules contain nitrogen (e.g., amino acids, nucleic acids), and the presence of nitrogen doesn't relate to the inability of proteins to cross the plasma membrane. So B is incorrect.
• Option C: Proteins are large macromolecules (polymers of amino acids with complex 3D structures). Plasma membranes have a phospholipid bilayer with small hydrophobic cores and limited pore sizes. Large molecules like proteins can't pass through the membrane freely because their size exceeds the capacity of the membrane's transport mechanisms (except via specific protein - mediated transport for some, but generally, free diffusion is not possible due to size). So C is correct.
• Option D: Emulsification is a process related to breaking down fats into small droplets (e.g., by bile salts), and it has nothing to do with proteins passing through plasma membranes. So D is incorrect.

Answer:

C. they are very large molecules