1/24/2018Chapter 4 - Cellular Regulation: BSC2023 RVC 11811/12Chapter 4 - Cellular Regula±onOrganization and RegulationWater and Solute MovementCell membranes act as barriers to most, but not all, molecules. Development of a cell membrane thatcould allow some materials to pass while constraining the movement of other molecules was a majorstep in the evolution of the cell. Cell membranes are differentially (or semi-) permeable barriersseparating the inner cellular environment from the outer cellular (or external) environment.Water potential is the tendency of water to move from an area of higher concentration to one of lowerconcentration. Energy exists in two forms: potential and kinetic. Water molecules move according todifferences in potential energy between where they are and where they are going. Gravity and pressureare two enabling forces for this movement. These forces also operate in the hydrologic (water) cycle.Remember in the hydrologic cycle that water runs downhill (likewise it falls from the sky, to get into thesky it must be acted on by the sun and evaporated, thus needing energy input to power the cycle).
1/24/2018Chapter 4 - Cellular Regulation: BSC2023 RVC 11812/12The hydrologic cycle.Diffusion is the net movement of a substance (liquid or gas) from an area of higher concentration to oneof lower concentration. You are on a large (10 ft x 10 ft x10 ft) elevator. An obnoxious individual with a litcigar gets on at the third floor with the cigar still burning. You are also unfortunate enough to be in a verytall building and the person says "Hey we're both going to the 62nd floor!" Disliking smoke you move tothe farthest corner you can. Eventually you are unable to escape the smoke! An example of diffusion inaction. Nearer the source the concentration of a given substance increases. You probably experiencethis in class when someone arrives freshly doused in perfume or cologne, especially the cheap stuff.Since the molecules of any substance (solid, liquid, or gas) are in motion when that substance is aboveabsolute zero (0 degrees Kelvin or -273 degrees C), energy is available for movement of the moleculesfrom a higher potential state to a lower potential state, just as in the case of the water discussed above.The majority of the molecules move from higher to lower concentration, although there will be some thatmove from low to high. The overall (or net) movement is thus from high to low concentration. Eventually,if no energy is input into the system the molecules will reach a state of equilibrium where they will bedistributed equally throughout the system.