Tonicity describes the amount of solute in a solution. Osmosis proceeds constantly in living systems. This diffusion of water through the membrane- osmosis -will continue until the concentration gradient of water goes to zero. Therefore, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. Water has a concentration gradient in this system. In this example, the solute cannot diffuse through the membrane, but the water can. However, only the material capable of getting through the membrane will diffuse through it. In this system, the solute cannot pass through the selectively permeable membrane.Ī principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. Figure 3 In osmosis, water always moves from an area of higher concentration (of water) to one of lower concentration (of water). If the volume of the water is the same, but the concentrations of solute are different, then there are also different concentrations of water, the solvent, on either side of the membrane. On both sides of the membrane, the water level is the same, but there are different concentrations on each side of a dissolved substance, or solute, that cannot cross the membrane. Imagine a beaker with a semipermeable membrane, separating the two sides or halves ( Figure 3). Water, like other substances, moves from an area of higher concentration to one of lower concentration. Whereas diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the diffusion of solutes in the water. Osmosis is the diffusion of water through a semipermeable membrane according to the concentration gradient of water across the membrane.
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