PART II: WATER CONTENT OF CELLS
Although living tissue is about 75-85% water, the effective concentration of solutes inside cells is not 15- 25%. Most of the non-water components of cells are bound into cell structures and are not in solution, so do not influence osmotic pressure. It is the concentration of solutes (substances actually dissolved in the fluid portion of the cell) rather than overall water content that determines whether water will diffuse into or out of a cell. Just what is the actual concentration of solutes (or “salts”) in intracellular fluids, the internal fluids of cells? Is it anything like sea water (97% water, 3% salts) in which the first cells originated and evolved? Let’s find out. In Part II you will learn a technique to estimate the concentration of solutes and water in living cells without killing them. You will use this technique to determine and compare cells from different environments — fresh water, marine, and terrestrial. The direction of osmosis Whether osmosis will move water into a cell or out of it depends on the “environment” surrounding the cell. Unfortunately, environments are not described in terms of their water content (e.g., 97% water). Instead environments are described in terms of their “solute concentration,” meaning the concentration of substances dissolved in the water (e.g., 3% salt). What is most important is the relative concentration of solutes in the environment compared to the solutes inside the cell. For example, when the environment’s solute concentration is higher than inside the cell, its water concentrations will be lower than in inside the cell, so water will move by osmosis out of the cell, dehydrating the cell. There are three possible environmental conditions:
Environment Solute concentration Water concentration Water movement
Hypertonic Higher than in cell Lower than in cell Out of the cell
Hypotonic Lower than in cell Higher than in cell Into the cell
Isotonic Same as in cell Same as in cell No net movement
1. In hypertonic environments, solute concentrations are higher in the environment than they are inside the cell. (Hyper means more, in this case more solute.) That means water moves out of the cell. 2. In hypotonic environments, solute concentrations are lower outside than inside the cell, and water moves into the cell. (Hypo means less, in this case less solute.) 3. In isotonic environments, solute concentrations are the same outside as inside, so there is no net movement of water. Water moves out at the same rate it moves in. (Iso means equal.) Turgor and plasmolysis Everyone knows that cut flowers need to be kept in water. But why? In plant cells, the cell membrane is surrounded by a rigid cell wall. When plant cells are placed in water — a hypotonic environment — water diffuses into the cell, pressure builds inside, and the cell’s plasma membrane becomes pressed tightly
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against the cell wall. The cell does not burst, but instead becomes turgid. Turgor pressure is the pressure exerted on the cell wall by the fluid contents of the cell, keeping the flowers erect. When a plant cell is placed in a hypertonic environment, water diffuses out of the cell. Having lost its turgor pressure, the cell’s plasma membrane collapses and pulls away from the cell wall. This shrinking of the cell membrane away from the cell wall is called plasmolysis. Figure 2. Turgid (left) and plasmolyzed plant cells.