cell walls
about how that occurs, in terms of membrane structure. If the influx of water continued unopposed, the membrane would 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 exterior to the plasma membrane . The synthesis of this cell wall is based on the controlled assembly of macromolecules secreted by the cell through the process of exocytosis see above . As water passes through the plasma membrane and into the cell driven by osmosis , 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. Conversely, if the [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 bacterial cell walls. This leaves the bacteria osmotically sensitive, water enters these cells until they burst and die. Questions to answer & to ponder: Make a graph of the water concentration across a typical cellular membrane for an organism living in fresh water; explain what factors influenced your drawing. Look at this video: https://www. youtube. com/watch?v=VctALENHyyI&feature=youtu. How could you use reverse osmosis to purify water? Where does the energy involved in moving molecules come from? Plants and animals are both eukaryotes; how would you decide whether the common ancestor of the eukaryotes had a cell wall. Why does an aquaporin channel not allow a Na+ ion to pass through it? If there is no net flux of A, even if there is a concentration gradient between two points, what can we conclude?.