Final answer:
Large polar or ionic molecules, both of which are hydrophilic, will diffuse slowest across a phospholipid bilayer as they cannot easily bypass the membrane's hydrophobic core. Charged molecules also cannot cross easily due to repulsion from the hydrophobic tails in the bilayer.
Step-by-step explanation:
The molecules that will diffuse slowest across an artificial phospholipid bilayer are large polar or ionic molecules. These substances are hydrophilic and therefore do not easily pass through the hydrophobic interior of the bilayer. Furthermore, charged atoms or molecules, regardless of their size, face difficulties in crossing via simple diffusion due to repulsion from the hydrophobic tails. Instead, these substances typically require facilitated diffusion using specialized transport proteins to enter or exit the cell.
Hydrophobic molecules like O2 and CO2, on the other hand, can readily diffuse through the lipid bilayer. Small and uncharged molecules, such as water, also have an easier time traversing the bilayer even though they are polar, thanks to their small size. In contrast, molecules such as glucose, which are both large and polar, must rely on transport proteins to facilitate their movement across the membrane.