What determines the direction of the net movement of water across a membrane?
Water will only flow across a membrane firstly if the membrane is permeable to water. There is no such thing as water flow across an impermeable plastic membrane!
Now, we need to determine what factors would cause water to flow across a permeable membrane. Generally, these factors are based on two things: hydrostatic pressure and osmotic pressure.
Hydrostatic pressure would cause water to flow from an area of high pressure to an area of low pressure. For example, if you pressurize the tank of a squirt gun and pull the trigger, the elevated pressure in the tank relative to outside the gun causes water to flow onto the person you are squirting. The same principle causes water to flow across membranes in the body in heart failure, causing pulmonary edema in the lungs.
Osmosis is based on a different principle. If you have different solute concentrations across a membrane, and those solutes are unable to cross, then water will cross to even out the osmolarity. For example, if you have a tank with a high concentration of protein mixed in water connected to a tank with a lower concentration of protein, then water will flow from the lower concentration to the higher concentration until the concentrations are equal. A good example of this is when you put cells into a highly concentrated salt solution. Water will flow out of the cells and cause them to shrivel. Conversely, if you put cells into water with a very low salt concentration, water may flow into the cells to the point of bursting!
Remember the sentence:
"Water flows from high to low."
Essentially, this means that water will go from higher concentrations to lower concentrations. This is why when vegetables are placed in a container with salt, water will flow out the plant cells. There is a higher amount of water within the cell than outside.
Chemically, you can refer to Le Chatelier's Principle, which states that equilibrium will shift when a stress is added. The stress in this case would be a difference in concentration of water. To relieve the stress, equilibrium would shift to produce more H2O.
Concentration difference is the phenomenon that makes water moves through semi permeable or permeable membrane.
This process is called as osmosis. Consider two different concentrated solutions divided by semi permeable membrane (
which allows only water to flow). Initially there is concentration gradient. Then water flows from lower concentrated
solution to higher concentrated solution to dilute it. Thus after some times equilibrium will be established where
concentration on both sides will be same