5.7
Osmosis is the movement of water across a semipermeable membrane, such as a cell membrane, from an area of lower dissolved solute concentration to an area of higher dissolved solute concentration.
The dissolved solutes cannot cross this semipermeable membrane on their own and remain separated, which establishes a concentration gradient across the membrane.
Imagine one liter of a solution. The total number of dissolved particles in it is its osmolarity. As the number of solute particles increases, osmolarity also increases. The same idea applies to biological cells. Osmolarity determines osmotic pressure, the minimum pressure required to prevent net water movement across a semipermeable membrane.
When the solute concentration inside the cell is higher than the concentration outside, an osmotic gradient forms, with relatively more water outside the cell.
As a result, water slowly moves from the region of lower osmolarity to the region of higher osmolarity. Small channel proteins called aquaporins help water move across the membrane much faster. This movement of water continues until osmotic equilibrium is reached.
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.
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.
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.
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 water across a semipermeable membrane, such as a cell membrane, from an area of lower dissolved solute concentration to an area of higher dissolved solute concentration.
The dissolved solutes cannot cross this semipermeable membrane on their own and remain separated, which establishes a concentration gradient across the membrane.
Imagine one liter of a solution. The total number of dissolved particles in it is its osmolarity. As the number of solute particles increases, osmolarity also increases. The same idea applies to biological cells. Osmolarity determines osmotic pressure, the minimum pressure required to prevent net water movement across a semipermeable membrane.
When the solute concentration inside the cell is higher than the concentration outside, an osmotic gradient forms, with relatively more water outside the cell.
As a result, water slowly moves from the region of lower osmolarity to the region of higher osmolarity. Small channel proteins called aquaporins help water move across the membrane much faster. This movement of water continues until osmotic equilibrium is reached.
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