substances which can both enter and come out of the cell
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cell membranes act as barriers to most, but not all, molecules. Development of a cell membrane that could allow some materials to pass while constraining the movement of other molecules was a major step in the evolution of the cell. Cell membranes are differentially (or semi-) permeable barriers separating 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 lower concentration. Energy exists in two forms: potential and kinetic. Water molecules move according to differences in potential energy between where they are and where they are going. Gravity and pressure are 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 the sky it must be acted on by the sun and evaporated, thus needing energy input to power the cycle).
Water potential is the tendency of water to move from an area of higher concentration to one of lower concentration. Energy exists in two forms: potential and kinetic. Water molecules move according to differences in potential energy between where they are and where they are going. Gravity and pressure are 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 the sky it must be acted on by the sun and evaporated, thus needing energy input to power the cycle).
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