Osmosis

Osmosis:
Osmosis is the passage of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
The water potential of pure water is zero.
Water potential affects movement, as it will cause water to move from an area of high water potential (the highest being zero) to an area of lower water potential (the more negative value).
With an animal cell, if the water potential outside is higher, then water will enter the cell and it will swell and burst. If the water potential is the same, no water will move in or out. If the water potential outside is lower, water leaves, and the animal cell shrinks.
With a plant cell, the cell wall makes it slightly different. When the water potential outside is higher, the water enters the cell, the protoplast (inside bit) swell, and pushes up against the cell wall. The cell is said to be turgid. If it is the same, no water enters or leaves, but the protoplast is slightly pulled away from the wall. If the water potential outside the cell is lower, then water will leave the plant cell, and the protoplast will shrink and become plasmolyzed.