Turgid and Flaccid Plant Cells

Understanding the difference between turgid and flaccid plant cells is simple. A turgid cell is one that is full of water, swollen and rigid. A flaccid cell is one that has lost water due to osmosis and is limp and unable to support the plant. However, to fully understand the cycle of turgor pressure and osmotic flow (the forces that determine whether a plant cell becomes turgid of flaccid) there are several terms you also need to remember:

Diffusion

Diffusion is the natural movement of molecules from a higher concentrated solution to a lower one to achieve balance through a semi permeable barrier such as the cell wall. So if there are more molecules of the same type on one side of the barrier than the other, more will move from the side with more molecules (the hypertonic side) to the side with fewer molecules (the hypotonic side) until there are the same amount of molecules on each side, and they are isotonic.

Osmosis

Osmosis is a special type of diffusion in which water molecules move through the cell wall or barrier. Because water is the solution in most cases of diffusion, the movement of water from the side of the barrier with more molecules to the side with less will be in the opposite direction of the movement of other diffusing molecules.

Osmotic pressure

We call difference between the hypertonic and hypotonic solutions the diffusion concentration gradient, or the diffusion gradient. A solute flows down the concentration gradient. While water flows down its own concentration gradient, you may see or hear it described as flowing up the diffusion gradient of the solute moving in the opposite direction. The pressure exerted by the flow of water into the cell is osmotic pressure.

Turgor Pressure

Turgor pressure is that inside the cell pushing against the cell wall. While the cell is flaccid and empty of water, there is little to no turgor pressure. When the cell is full of water and the water molecules are pressing against the cell wall, there is a lot of turgor pressure. This is when the cell swells to its full size, and we say it is turgid. When the turgor pressure exceeds the osmotic pressure, water will stop flowing into the cell, although solutes may continue to flow out of it.

Plasmolysis

Normally as solutes and water molecules flow out of a turgid cell, osmosis will occur sporadically as the cell loses turgor pressure to maintain balance. However if water is in short supply so that the cells contain more water molecules than the surrounding environment, the direction of osmotic flow is reversed, and the cell will lose water through osmosis. When the cell loses enough water, its insides contract and the cell membrane pulls away from the cell wall. At this point the cell is plasmolysed, and this is when it becomes limp and flaccid.

While individual cells naturally cycle from turgid to flaccid states, if too many cells in the plant become flaccid die to water shortages, the plant will wilt and eventually die.

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