Establishing Equilibrium in Passive Transport
Facilitated Diffusion in Cells
Factors That Influence Diffusion
Another factor is the physical environment in which diffusion is occurring. If the temperature or pressure of the environment increases, the kinetic energy of the molecules increases, causing the rate of diffusion to increase.
The cellular conditions in which diffusion is occurring can influence the rate of diffusion. The higher the density of the solvent, the slower diffusion will progress. When cells are dehydrated, the density of the cytoplasm is higher, reducing the ability of material to diffuse. Thick membranes, or membranes with a higher density of glycoproteins and glycolipids, can impede diffusion. A larger membrane surface area will increase the rate of diffusion because there are more places where the molecules can diffuse across.
When the concentration gradient increases (i.e., the difference in concentration on either side of the cell membrane is high), the rate of diffusion increases. The degree to which the rate of facilitated diffusion can increase may be limited by the density of transport proteins in the membrane. If all transport proteins are active and operating at maximum efficiency, an increase in concentration will not increase the rate of diffusion.
When a cell is in a hypotonic solution, the extracellular fluid has a lower osmolarity than the fluid inside the cell. In this case, solute concentration in the extracellular fluid is lower than the solute concentration inside the cell. Water flows to the region with the highest solute concentration, inside the cell. In hypotonic conditions, there is a net water movement into the cells and as a result, cells will swell. If the concentration difference is extreme and excess water is not removed, cells may burst, or lyse.In hypertonic conditions, the extracellular fluid has a higher osmolarity than the inside of the cell. Because there is more solute outside the cell, water will flow in this direction until equilibrium is achieved. As a result, the cell shrinks as it loses water. This impairs a cell's ability to function or divide. If the solute concentration difference is extreme, the cell may lose so much water that it "dies."