Plant Cell Walls - 4  
311 Syllabus

Functional Overview

The Cell Wall is an Exoskeleton (External)

It is Required for Water Relations (Turgor Pressure would not develop without a Cell Wall).

It is Necessary for Specialized Cell Differentiation. Otherwise, all cells would be spherical.

Turgor Pressure is the driving force for cell expansion.

Turgor Pressure is a positive pressure that develops because the Vacuole contains more dissolved chemicals than the cytoplasm and the external environment which is almost pure water. Water molecules enter the Vacuole so that an equilibrium may be established. If cell walls are removed, the typical plant cell bursts because the influx of water causes an enormous increase in cell volume. The Vacuole Membrane (Tonoplast) and the Plasma Membrane (Plasmalemma) dissipate because they have virtually no strength to withstand the increased cell volume.

However, Cell Walls can withstand the increase in Pressure because they are strong and thus constrain the cell's Volume. Since the cell's volume can not Increase, a pressure develops due to the influx of water. This is Turgor Pressure.

Imagine a limp balloon. If we hook it up to a faucet it will expand as water fills it. If we continue to add water, the balloon will eventually burst. The internal space is the Vacuole and the skin of the balloon in the Plasmalemma.

If we place the balloon in a narrow box and open the faucet. The balloon will expand and conform to the shape of the box. Pressure will develop within the balloon until it matches the pressure of the incoming water. When these two pressures are equal, there will be no net change in the Volume of the balloon!