Final answer:
Osmosis requires a concentration gradient and a selectively permeable membrane to occur, with water moving from higher to lower water concentrations until equilibrium is reached or pressure balances the system.
Step-by-step explanation:
The two conditions that must be present for osmosis to occur are a concentration gradient and a selectively permeable membrane. Osmosis is the diffusion of water through a semipermeable membrane, where water moves from an area of high water concentration (which corresponds to low solute concentration) to an area of low water concentration (high solute concentration). Water can move readily across most membranes due to the presence of aquaporins, but the membrane limits the diffusion of solutes. This process continues until the concentration gradient of water goes to zero or until hydrostatic pressure balances the osmotic pressure.