5.6: Osmosis

Approximately 60% to 95% of the weight of living organisms is attributed to water. Therefore, maintaining appropriate water balance within cells is of paramount importance. Osmosis is the movement of water across a semipermeable membrane, such as a cell’s plasma membrane. In living organisms, water plays a crucial role as a solvent—a molecule that dissolves other molecules.

Diffusion Versus Osmosis

Both diffusion and osmosis are types of passive transport—cellular transport that does not require additional energy. Diffusion is the transport of a substance (solute) dissolved in a liquid (solvent) from an area of high concentration to an area of low concentration. Diffusion may also occur across a membrane if the membrane is permeable for that solute. A membrane that hinders the passage of a specific solute is a semipermeable membrane. While the semipermeable membrane stops the flow of the solute, the solvent moves freely—a process called osmosis.

Mechanism

Osmosis occurs when there is more solute on one side of the semipermeable membrane than on the other. The ratio of water to solute is called osmolarity. During osmosis, water flows from the side with low osmolarity (more water relative to solute) to the side with high osmolarity (less water relative to solute) until the osmolarity on both sides is approximately equal. For instance, a cell surrounded by a semipermeable membrane has water flowing in, if there is a higher concentration of solute inside the cell compared to the outside.

Osmotic Imbalance in Cells

Whenever osmotic balance is disrupted, cells may expand or shrink. The underlying mechanisms of osmotic imbalance and prevention of dangerous outcomes are further discussed in subsequent sections.

Osmosis is the movement of only water across a semipermeable membrane, like a cell's plasma membrane, down its concentration gradient. The solute, any dissolved substances, cannot cross.

More solute inside the cell compared to outside results in there being less water, thereby setting up a concentration gradient with more water outside.

Over time, water will move from the side with the lower osmolarity, a greater ratio of water to solute, into the side with higher osmolarity, a smaller ratio of water to solute, until the gradient equalizes.