Answer
The smaller, non-polar molecules such as oxygen and carbon dioxide could diffuse across the membrane unaided, while somewhat larger, charged molecules such as amino acids would need to have help in the form of membrane proteins. Large things such as enzymes would need to be secreted by exocytosis. All of this is decided by the hydrophobic interior of the membrane, which prevents diffusion across it of charged molecules, and by the very slow diffusion of very large things like ezymes. In both of these cases, help is needed for transport to occur.
Work Step by Step
This question is asking you to review a number of concepts from the chapter--skim through and look for how the different kinds of molecules listed in the question move across membranes, and apply what is found in the chapter to each of those specific cases (non-polar molecules, polar and charged molecules, large molecules).
