Overview: What Are Solutions?
A solution is a mixture in which molecules of one substance are distributed evenly in another. For example, in a solution of sugar water, molecules of sugar from sugar crystals are dispersed in molecules of water. If sugar is invisible in the solution because the molecules are so well mixed, the sugar is dissolved in water. In the solution of sugar water, water is the solvent and sugar is the solute. Water is the solvent in most cellular activities, and that is why water is so important to life.
What Is Diffusion?
All the molecules in liquids, solids, and gases move constantly, and the movement in liquids and gases are random, called Brownian movement. Observe a drop of food coloring in a container of plain, clear water. The molecules of food coloring disperse randomly throughout the water until they are dissolved throughout the water, coloring it. In this example, the highly concentrated molecules of food coloring have been diffused throughout the water, going from a high concentration to a low concentration. The color in a drop of food coloring is darker and more intense than the resulting shade after it has been diffused in a large volume of water.
What Affects Diffusion?
The rate of diffusion is affected by the concentration of molecules from higher to lower. Molecules always move from higher concentrations to lower concentrations within a gradient. The rate can be affected by the temperature of the solvent, as well as the pressure of the solvent. A rise in temperature increases the rate of diffusion, as does a rise in pressure. This is because molecules move more rapidly within liquids as they heat and as pressure rises.
Diffusion through Cell Membranes
A cell membrane forms a barrier that allow materials to pass into and out of the cell. Water can easily pass through it, but only some of the molecules that are dissolved in water can get beyond the barrier. Molecules have to be small enough to move through the gaps in the double layer of proteins and lipids. Carbon dioxide and oxygen dissolve in the lipid molecules and easily pass though.
Concentrations of Solutions and Equilibrium
Living cells are made up of water that flows freely in and out across cell membranes. The concentration of water and solutes across either side of the membrane constantly changes until the cell and its outer environment reach equilibrium. Nutrients enter the cell and are digested, creating waste products that must exit the cell. Consequently, the cell and its environment go in and out of equilibrium. If solutes are the same outside as well as inside the cell, the cell is in an isotonic solution. Blood plasma is a typical isotonic solution to blood cells. In hypotonic solutions, the concentration of water is higher in the solution than in the cell. Water will flow into the cell and be stored inside the cell to make it isotonic with the surrounding solution. In contrast, hypertonic solutions are those in which the concentration of solutes are higher than that inside the cell. For example, cells placed in salt water will shrivel, because the water inside the cells will flow out of the cell to make the solution isotonic.
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