Molecules a and b come in contact with the cell membrane of the same cell. molecule a passes through the membrane readily, but molecule b does not. which statement could describe molecules a and b?

Molecule A is a simple sugar or amino acid, while molecule B is a starch or protein.

Explanation:

Molecule A, as a simple sugar or amino acid, consists of small molecules which can readily pass through the cell membrane, while Molecule B, as a starch or protein, consists of large molecules, thus, can't pass across the cell membrane.

This is the reason why the cell membrane is considered to be selectively permeable.