[ Home ] [ Up ]

Figure 1. If we put human red blood cells (RBCs) in a solution having the same water concentration as the inside of the cell (an isotonic solution), the cells look like this. They have smooth surfaces and are thinner in their centers than on their edges. These are normal red blood cells and is how they look when they are in our blood vessels. Water is moving into and out of these cells at the same rate. With no net change in water volume, the cells retain their normal shapes.

Figure 2. The cells in this picture have been left in a solution of lower water concentration (higher dissolved solids, hypertonic), and the net movement of water was out of the cells, from the area of higher water concentration (inside the cells) to the area of lower water concentration (outside the cells). This has caused the cells to deflate and their cell membranes to wrinkle.

The magnification here is a little lower and the lighting is a little different; however, the important difference to notice is the shape of the membranes. As the cells lost water by osmosis their membranes wrinkled, sort of the way a mylar balloon wrinkles as it loses air.

Figure 3. These are normal onion cells. Each is bounded by a darkly stained cell wall. Just inside the cell wall is a cell membrane, pressed up against the wall, like an inner tube in a tire. (The cell membrane is too thin to see at this magnification.) You can see nuclei (light blue) in some cells. The rest of the material in the cell is called cytoplasm. The cells look like this when they are in distilled water or in a water solution where the water concentration matches that in the cell.

Figure 4. If we change the solution around the cells to one that has a lower water concentration (higher in dissolved solids), the net movement of water will be out of the cell, by osmosis. As it loses water, the cytoplasm will shrink and the cell membrane will follow it, being pulled away from the cell wall. With less water in the cytoplasm to support them, the cells lose their shape -- they shorten, thicken, and lose rigidity. This onion has become shriveled, soft, and pliable -- dehydrated.