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Fig. 3.3 Distilled water 1 When a salt crystal (green) is placed
Copyright © McGraw-Hill Education. Permission required for reproduction or display. Distilled water 1 When a salt crystal (green) is placed into a beaker of water, a concentration gradient exists between the salt from the salt crystal and the water that surrounds it. 2 Salt ions (green) move down their concentration gradient into the water. 3 Salt ions and water molecules are distributed evenly throughout the solution. Even though the salt ions and water molecules continue to move randomly, an equilibrium exists, and no net movement occurs because no concentration gradient exists.
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Fig. 3.6 Copyright © McGraw-Hill Education. Permission required for reproduction or display. * Because the tube contains salt ions (green and pink spheres) as well as water molecules (blue spheres), there is proportionately less water in the tube than in the beaker, which contains only water. The water molecules diffuse with their concentration gradient into the tube (blue arrows). Because the salt ions cannot leave the tube, the total fluid volume inside the tube increases, and fluid moves up the glass tube (black arrow) as a result of osmosis. 3% salt solution Weight of water column Selectively permeable membrane Salt solution rising The solution stops rising when the weight of the water column prevents further movement of water into the tube by osmosis. Distilled water Osmosis Water 1 The end of a tube containing a 3% salt solution (green) is closed at one end with a selectively permeable membrane, which allows water molecules to pass through but retains the salt ions within the tube. 2 The tube is immersed in distilled water. Water moves into the tube by osmosis (see inset above*). The concentration of salt in the tube decreases as water rises in the tube (lighter green color). 3 Water moves by osmosis into the tube until the weight of the column of water in the tube (hydrostatic pressure) prevents further movement of water into the tube. The hydrostatic pressure that prevents net movement of water into the tube is equal to the osmotic pressure of the solution in the tube.
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a-c: ©David M. Phillips/ Science Source
Fig. 3.7 Copyright © McGraw-Hill Education. Permission required for reproduction or display. Red blood cell H2O Hypotonic solution Isotonic solution Hypertonic solution (a) When a red blood cell is placed in a hypotonic solution (one having a low solute concentration), water enters the cell by osmosis (black arrows), causing the cell to swell or even burst (lyse; puff of red in lower part of cell). (b) When a red blood cell is placed in an isotonic solution (one having a concentration of solutes equal to that inside the cell), water moves into and out of the cell at the same rate (black arrows). No net water movement occurs, and the cell shape remains normal. (c) When a red blood cell is placed in a hypertonic solution (one having a high solute concentration), water moves by osmosis out of the cell and into the solution (black arrows), resulting in shrinkage (crenation). a-c: ©David M. Phillips/ Science Source
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