What Keeps Plant Cells From Bursting in High Concentrations of Water?

Diffusion

• Water is one of the essential substances that are needed for life. Most compounds essential for life dissolve easily in water, so it is important to provide a way to move water in and out of the cell. Usually this is done through diffusion. Most gaseous and liquid substances tend to flow from an area of high concentration to an area of low concentration. If pure water is on one side of a permeable membrane, and saltwater is on the other, some of the pure water will flow to the saltwater side, since the water on the saltwater side is less concentrated.

Osmosis

• The diffusion of water through the cell membrane is called osmosis. If the cell is surrounded by pure water, water will tend to flow into the cell, but if the cell is surrounded by water that is heavily salted, the water will tend to leave the cell. This is why most plants shrivel up when placed in saltwater, but they become more rigid if placed in fresh water. In saltwater, there is less pressure pushing out on the cell membrane and cell wall. In fresh water, the water entering the cell puts more outward pressure on the cell membrane and cell wall.

Cell Membrane

• The cell membrane is semipermeable. It allows substances with small molecules, such as water, to go back and forth freely through the cell, but does not normally allow the passage of larger molecules. In animal cells, there is no natural mechanism for limiting the diffusion of water into or out of the cell. If a red blood cell is placed in a water solution that is heavily salted, the cell will lose most of its water content, shrivel up and die. If a red blood cell is placed in pure water, the cell will absorb water until it expands and bursts. The cell membrane is not strong enough to keep the cell from bursting.

Cell Walls

• Plant cells are different in that the cell membrane is surrounded by a cell wall. In saltwater, the cell membrane shrinks and pulls away from the cell wall as the fresh water leaves. In fresh water, the cell membrane pushes out against the cell wall as the water is absorbed into the cell. The rigid cell wall limits the degree to which the membrane can expand, so the cell does not burst. Once the cell membrane is pressed firmly against the cell wall, no more water can enter the cell.