Which of the following is NOT a type of transport that allows small molecules to cross the cell membrane?

The answer D, Polymerization, is correct because polymerization refers to the process of joining small molecules, or monomers, together to form larger molecules, or polymers, such as proteins, nucleic acids, and plastics. This process is not associated with the movement of small molecules across the cell membrane.

On the other hand, diffusion, osmosis, and active transport are all processes specifically involved in transporting molecules across cell membranes. Diffusion is the spontaneous movement of molecules from an area of higher concentration to an area of lower concentration. Osmosis is a specific type of diffusion, focusing on the movement of water across a semi-permeable membrane. Active transport, distinct from diffusion and osmosis, involves the movement of molecules against their concentration gradient, requiring energy, often in the form of ATP.

The inclusion of polymerization among these transport types is inaccurate, as it does not pertain to the movement of molecules across the membrane but rather to the creation of larger complex structures.