In order to fully understand the movement of particles across a membrane, one must be knowledgeable of basic concepts concerning diffusion and osmosis. Diffusion can be defined as solutes moving an area having a high concentration to an area having a lower concentration. Diffusion is the outcome of the constant, random motion is the outcome of the constant, random motion of solutes. The Random Thermal Motion (RTM) theory states that particles are constantly taking on and giving off energy. Because osmotically active particles move randomly they move from higher to lower concentration. As concentration increases, the rate of this movement increases. Just as concentration decreases, the rate decreases. Temperature is also a factor that influences the movement of molecules. As temperature increases the rate increases and as temperature decreases the rate also decreases. When at equilibrium, the net movement of solutes no longer undergoes this movement but the random motion continues. A concentration gradient also plays a role in controlling diffusion. A concentration gradient can be described as the difference in concentrations between a solvent and a solute. Diffusion, along with osmosis, move down, a concentration gradient from higher to lower. Osmosis is the diffusion of water across a membrane. This membrane is relatively impermeable to a solute that forms a solution with the solvent, water. This type of movement also moves down a concentration gradient, from a higher water concentration which has fewer solute molecules, to a lower water concentration with more solute molecules. Water moves into a cell, for example, by osmosis when it is placed into a hypotonic solution because the solution usually has a higher concentration of water and fewer non-penetrating particles. This results in swelling as molecules of water move into the cell.
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