6.11: Osmosis and living with and without a cell wall

Cells are packed full of molecules. These molecules take up space, space that cannot be occupied by water molecules. The concentration of water outside of the cell [H₂O]_out will generally be higher than the concentration of water inside the cell [H₂O]_in. This water concentration gradient leads to the net movement of water molecules into the cell, a process known as osmosis.^[255] This movement occurs across the cell's surface membrane, a membrane that is somewhat permeable to water (see above). A surprising finding that earned Peter Agre a share of the 2003 Noble prize in chemistry was that the membrane also contains water channels, known as aquaporins.^[256] Follow the video link (→) to a molecular simulation of a water molecule (yellow) moving across a membrane through an aquaporin protein. Without aquaporins the rate of osmotic movement of water is dramatically slower. In addition to water, aquaporin-type proteins can facilitate the movement of other small uncharged molecules across cellular membranes.

The gradient in water concentration across the cell membrane, together with the presence of aquaporins, generates a system that can do work. The water gradient, can lift a solution against the force of gravity, a process involved in how plants stand up straight. How is this possible? If we think of a particular molecule in solution, it moves through collisions with its neighbors. These collisions drive the stochastic movement of particles. But if there is a higher concentration of molecules on one side of a membrane compared to the other, then the random movement of molecules will result in a net flux of molecules from the area of high concentration to that of low concentration, even though each molecule, on its own moves unpredictably. This video is a good illustration of the movement of a water molecule across a membrane. At steady state, the force generated by the net flux of water moving down its concentration gradient is balanced by other forces (e.g. the weight of the solution) acting in the other direction.

The water concentration gradient across the plasma membrane of most free-living organisms leads to an influx of water into the cell. As water enters, the plasma membrane expands. If the influx of water continues unopposed, the membrane will eventually burst like an over-inflated balloon, killing the cell. One strategy to avoid this lethal outcome, adopted by a range of organisms, is to build a semi-rigid “cell wall” external to the plasma membrane (↓). The synthesis of this cell wall involves the controlled assembly of macromolecules secreted by the cell. As osmosis "drives" water through the plasma membrane and into the cell, the plasma membrane is pressed up against the cell wall. The force exerted by the rigid cell wall on the membrane balances the force of water entering the cell. When the two forces are equal, the net influx of water into the cell stops, the systems arrives at a steady state. Conversely, if [H₂O]_outside decreases, this pressure is reduced, the membrane moves away from the cell wall and, because they are only semi-rigid, the walls flex. It is this behavior that causes plants to wilt when they do not get enough water. These are passive behaviors, based on the structure of the cell wall; they are built into the wall as it is assembled. Once the cell wall has been built, a cell with a cell wall does not need to expend energy to resist osmotic effects. Plants, fungi, bacteria and archaea all have cell walls. A number of antibiotics work by disrupting the assembly of cell walls. This leaves the cells osmotically sensitive, water enters these cells until they burst and die.

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