Osmosis is all about the movement of water. Specifically, it is the diffusion of free water molecules from an area of high water potential to an area of low water potential across a partially permeable membrane. As we saw in diffusion, the water molecules move both ways, but the overall net movement is down the water potential gradient.
Osmosis and water potential
What is water potential? Simply, it is the potential that water molecules have to diffuse out of a particular solution. So if there is a high concentration of water molecules in a solution (and therefore a low concentration of dissolved solute), there is more potential for them to diffuse out. This is better explained by a diagram.
If two solutions have the same water potential they are isotonic, and there is no net movement of water molecules between the two. Pure water has the highest water potential possible.
As with diffusion, there are a few things that can affect the rate of osmosis:
- Thickness: the thinner the exchange surface (the partially permeable membrane), the faster the rate.
- Surface area: the large the surface area of the exchange surface, the faster the rate.
- The ‘steepness’ of the water potential gradient: a steeper gradient means a faster rate, but it will slow down as the water potentials even out.
Effect of water potential on cells
We mentioned osmosis and water potential when we were looking at the storage of glucose as polysaccharides. Starch and glycogen are insoluble so do not affect water potential, making them ideal for storage. Let’s take a look at what would happen to cells if glucose (a water soluble molecule) was stored as it is.
If the inside of the cell has a lower water potential than outside the cell, for example by having a higher concentration of glucose, water will move in across the cell membrane by osmosis, and the cell will swell and possibly burst. When a cell is sitting in a solution with a higher water potential than inside, we say the solution is hypotonic. The reverse is a hypertonic solution.
Plant cells are a little different. They have a vacuole and a cell wall. In a hypotonic solution, any water that moves in by osmosis ends up in the vacuole which swells and pushes against the cell wall. The cell becomes turgid (rigid). If water moves out of the vacuole (when the cell is in a hypertonic solution), the cytoplasm pulls away from the cell wall in a sort of implosion – this is called plasmolysis. The cell becomes flaccid (limp), which is what happens when plants wilt.
- Osmosis is diffusion of free water molecules from an area of high water potential to an area of low water potential across a partially permeable membrane.
- The rate of osmosis depends on the water potential gradients, and the thickness and surface area of the exchange surface (e.g the cell membrane).
- If two solutions have the same water potential they are isotonic.
- If a cell is in a solution where the water potential is higher than inside the cell, the solution is hypotonic.
- If a cell is in a solution where the water potential is lower than inside the cell, the solution is hypertonic.