Movement in and out of Cells - Osmosis

Osmosis is defined as the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.

Water potential ($\\Psi$) is a measure of the tendency of water molecules to move from one area to another. Pure water has the highest potential, defined as $0\text{ kPa}$. All solutions have a negative water potential.

In plant cells, water entering via osmosis creates turgor pressure. This pressure pushes the cell membrane against the rigid cell wall, making the cell 'turgid', which provides structural support to the plant.

When plant cells lose water, they become 'flaccid'. If water loss continues, the cell membrane pulls away from the cell wall, a process known as plasmolysis.

Animal cells lack a cell wall. In a hypotonic solution (high water potential), water enters the cell until it bursts, a process called lysis. In a hypertonic solution (low water potential), the cell loses water and shrivels, known as crenation.

A partially permeable membrane (e.g., the cell membrane) allows small molecules like $H_2O$ to pass through but prevents larger solute molecules like glucose or sucrose from crossing easily.