Final answer:
The second major constituents of cell membranes and intracellular membranes are proteins. These proteins can be either integral or peripheral proteins, which serve various functions such as channels or pumps, enzymes, or structural attachments.
Step-by-step explanation:
The second major constituents of cell membranes and intracellular membranes are proteins. These proteins can be either integral proteins, which are completely integrated into the membrane structure and interact with the hydrophobic region of the phospholipid bilayer, or peripheral proteins, which are found on the exterior or interior surfaces of membranes. Both integral and peripheral proteins can serve various functions such as channels or pumps, enzymes, or structural attachments.