Establishing Equilibrium in Passive Transport
Facilitated Diffusion in Cells
Factors That Influence Diffusion
Several factors affect the rate of diffusion. One factor is the nature of the diffusing substance. Heavier molecules diffuse much more slowly than lighter molecules. Nonpolar substances, those that have the same charge on both sides, diffuse across a membrane at a higher rate than polar substances, which are less soluble in the cell membrane's phospholipid bilayer.
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.
Diffusion describes the passive transport of any material across a membrane. Osmosis is a type of diffusion that only involves the movement of water molecules across a semipermeable membrane from an area of high concentration to one of low concentration. Water molecules can diffuse directly through a cell membrane via osmosis, and it is constantly occurring in living things.Water dissolves substances, so it is a solvent. The substance that is dissolved is called a solute. The cell membrane limits the diffusion of solutes dissolved in water. However, the water molecules themselves can freely diffuse across the cell membrane through channel proteins called aquaporins. An aquaporin is a transport protein in a cell membrane that allows for osmosis, the movement of water back and forth. When osmosis occurs, the direction of water movement is based on the relative concentration of solutes on either side of the membrane. During osmosis, water moves from an area of low solute concentration to area of high solute concentration. In a cell, water will always move to reach an equal concentration of solute on both sides of the cell membrane. For example, if the salt concentration inside a cell is higher than the salt concentration outside of the cell, water will move into the cell until the salt concentration is the same on both sides of the membrane. Cell volume may change due to osmosis when the extracellular environment changes. Osmolarity refers to the concentration of solutes in a solution. The osmolarity of the cell can be compared to the osmolarity of the extracellular fluid that surrounds the cell. This relationship is described by three different conditions. In isotonic conditions, the extracellular fluid has the same osmolarity as the cell. Movement of water into the cell exactly balances the amount of water moving out of the cell. These are the ideal conditions for most animal cells.
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."