Osmosis is often defined macroscopically as the net flow of solvent from a region of lower solute concentration (dilute solution) to a region of higher solute concentration (concentrated solution). However, at the molecular level, solvent molecules are in constant random motion and pass through the semi-permeable membrane from both sides.

1. Bidirectional Movement: Solvent molecules strike the membrane pores from both the dilute side and the concentrated side.
2. Unequal Rates: The flow rate from the dilute side (higher solvent concentration/vapor pressure) to the concentrated side is greater than the flow rate from the concentrated side to the dilute side.
3. Net Flow: This difference in flow rates results in a net flow of solvent into the concentrated solution. Options suggesting flow from 'only' one side ignore the kinetic nature of liquid molecules.