Annex B - Research Data

How does water move in and out of cells.

What Causes Water Movement Into or Out of Cells? Diffusion is the movement of molecules from a region of high free energy to a region of lower free energy. Under standard conditions of temperature and pressure, this is simply movement from a region of high concentration to a region of lower concentration. Temperature, pressure, and concentration all affect free energy, so all three factors should be considered if they differ between two different regions. To experience diffusion, have a friend open a bottle of perfume across the room from you. The volatile molecules in the perfume will enter the air, where their concentration is lower, and your friend will soon smell them. As their concentration builds up in the air around your friend, the molecules will travel to where the concentration is lower, eventually reaching your nose. Osmosis is the diffusion of water across a differentially permeable membrane. Cell membranes are differentially permeable, since water passes across them more easily than do most solutes. If a cell is placed in a hypertonic solution, one in which the solute concentration is higher than in the cytoplasm, water will diffuse out of the cell, causing plasmolysis or cell shriveling. If a plant cell is placed in a hypotonic solution, one in which the solute concentration is lower than in the cytoplasm, water will move into the cell. Animal cells may rupture under such conditions. In a plant cell, the additional water creates pressure against the cell wall, and, as described earlier, the cell becomes turgid. Solute concentration is the same as in the cell in an isotonic solution, and no net movement of water occurs.

 How Do Plants Move Water?

 Osmosis drives the transport of water and many solutes between plant cells. While this process occurs at or below the cellular level, plants must move water from their roots all the way to the tallest parts of the plant.

Movement is driven by transpiration, the evaporation of water from plant leaves, stems, and flowers. While plant tissues are mostly made up of water, about 90% of the water a plant’s roots take up is ultimately lost to transpiration! As water vapor passes from cells to the atmosphere via pores called stomata (see below), the relative amount of solutes in those cells increases, making them slightly more hypertonic than the cells more interior to the plant. Osmosis results in the movement of water from the more interior cells to the ones that have lost water, ultimately forming a gradient all the way back to the roots. Due to water’s strong surface tension and adhesion to tube walls, some water transport in plants is also accounted for by capillary action within the long, tube-like cells of the vascular system.

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