Diffusion : Particles in a liquid-filled beaker are initially concentrated in one area, but diffuse from their area of high concentration to the areas of low concentration until they are distributed evenly throughout the liquid. Not only do gaseous particles move with high kinetic energy, but their small size enables them to move through small openings as well; this process is known as effusion.
The opening of the hole must be smaller than the mean free path because otherwise, the gas could move back and forth through the hole. Effusion is explained by the continuous random motion of particles; over time, this random motion guarantees that some particles will eventually pass through the hole. Set the temperature, then remove the barrier, and measure the amount of time it takes the blue molecules to reach the gas sensor.
When the gas sensor has detected three blue molecules, it will stop the experiment. Compare the diffusion rates at low, medium and high temperatures. Trace an individual molecule to see the path it takes. This is written as follows:. What is the ratio of the rate of effusion of ammonia, NH 3 , to that of hydrogen chloride, HCl? Recall that a result of the Kinetic Theory of Gases is that the temperature, in degrees Kelvin, is directly proportional to the average kinetic energy of the molecules.
Therefore, equating the kinetic energy of molecules 1 and 2, we obtain:. The rate of effusion is determined by the number of molecules that diffuse through the hole in a unit of time, and therefore by the average molecular velocity of the gas molecules.
Osmosis is the movement of water across a membrane from an area of low solute concentration to an area of high solute concentration. Osmosis is the movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes.
Semipermeable membranes, also termed selectively permeable membranes or partially permeable membranes, allow certain molecules or ions to pass through by diffusion. Not surprisingly, the aquaporin proteins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules. Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high concentration to one of low concentration.
An obvious question is what makes water move at all? Imagine a beaker with a semipermeable membrane separating the two sides or halves. On both sides of the membrane the water level is the same, but there are different concentrations of a dissolved substance, or solute, that cannot cross the membrane otherwise the concentrations on each side would be balanced by the solute crossing the membrane.
If the volume of the solution on both sides of the membrane is the same but the concentrations of solute are different, then there are different amounts of water, the solvent, on either side of the membrane. If there is more solute in one area, then there is less water; if there is less solute in one area, then there must be more water. To illustrate this, imagine two full glasses of water. One has a single teaspoon of sugar in it, whereas the second one contains one-quarter cup of sugar.
If the total volume of the solutions in both cups is the same, which cup contains more water? Because the large amount of sugar in the second cup takes up much more space than the teaspoon of sugar in the first cup, the first cup has more water in it.
Osmosis : In osmosis, water always moves from an area of higher water concentration to one of lower concentration. In the diagram shown, the solute cannot pass through the selectively permeable membrane, but the water can. Returning to the beaker example, recall that it has a mixture of solutes on either side of the membrane. A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of passing through the membrane will diffuse through it.
In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system.
Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure.
In the beaker example, this means that the level of fluid in the side with a higher solute concentration will go up. Tonicity, which is directly related to the osmolarity of a solution, affects osmosis by determining the direction of water flow. Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis.
Osmolarity describes the total solute concentration of the solution. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles; a solution with high osmolarity has fewer water molecules with respect to solute particles. In a situation in which solutions of two different osmolarities are separated by a membrane permeable to water, though not to the solute, water will move from the side of the membrane with lower osmolarity and more water to the side with higher osmolarity and less water.
This effect makes sense if you remember that the solute cannot move across the membrane, and thus the only component in the system that can move—the water—moves along its own concentration gradient.
An important distinction that concerns living systems is that osmolarity measures the number of particles which may be molecules in a solution. Therefore, a solution that is cloudy with cells may have a lower osmolarity than a solution that is clear if the second solution contains more dissolved molecules than there are cells. Three terms—hypotonic, isotonic, and hypertonic—are used to relate the osmolarity of a cell to the osmolarity of the extracellular fluid that contains the cells.
In a hypotonic situation, the extracellular fluid has lower osmolarity than the fluid inside the cell, and water enters the cell. In living systems, the point of reference is always the cytoplasm, so the prefix hypo- means that the extracellular fluid has a lower concentration of solutes, or a lower osmolarity, than the cell cytoplasm.
It also means that the extracellular fluid has a higher concentration of water in the solution than does the cell. In this situation, water will follow its concentration gradient and enter the cell, causing the cell to expand. Changes in Cell Shape Due to Dissolved Solutes : Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions.
Because the cell has a relatively higher concentration of water, water will leave the cell, and the cell will shrink.
In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the osmolarity of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell, although water will still move in and out.
Blood cells and plant cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances. Cells in an isotonic solution retain their shape. Cells in a hypotonic solution swell as water enters the cell, and may burst if the concentration gradient is large enough between the inside and outside of the cell. Cells in a hypertonic solution shrink as water exits the cell, becoming shriveled. Osmoregulation is the process by which living things regulate the effects of osmosis in order to protect cellular integrity.
Tonicity is the ability of a solution to exert an osmotic pressure upon a membrane. There are three types of tonicity: hypotonic, hypertonic, and isotonic.
In a hypotonic environment, water enters a cell, and the cell swells. In a hypertonic solution, water leaves a cell and the cell shrinks. In an isotonic condition, the relative concentrations of solute and solvent are equal on both sides of the membrane. There is no net water movement; therefore, there is no change in the size of the cell.
The membrane resembles a mosaic with discrete spaces between the molecules comprising it. If the cell swells and the spaces between the lipids and proteins become too large, the cell will break apart. In contrast, when excessive amounts of water leave a red blood cell, the cell shrinks, or crenates. This has the effect of concentrating the solutes left in the cell, making the cytosol denser and interfering with diffusion within the cell.
Turgor Pressure and Tonicity in a Plant Cell : The turgor pressure within a plant cell depends on the tonicity of the solution in which it is bathed. Various living things have ways of controlling the effects of osmosis —a mechanism called osmoregulation. Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis in a hypotonic solution.
The plasma membrane can only expand to the limit of the cell wall, so the cell will not lyse. In fact, in plants, the cellular environment is always slightly hypotonic to the cytoplasm, and water will always enter a cell if water is available. This inflow of water produces turgor pressure, which stiffens the cell walls of the plant. In nonwoody plants, turgor pressure supports the plant. Conversely, if the plant is not watered, the extracellular fluid will become hypertonic, causing water to leave the cell.
When dye is added to the solution it diffuses over time. At first you see streaks of blue moving through the solution until finally the entire solution becomes blue because the concentration of dye is the same everywhere.
At this point, although the dye molecules are still moving around, you will not be able to perceive it since the blue dye has diffused and colored the entire volume of liquid. Diffusion is thus a passive process meaning that it does not require the input of energy.
A substance moves from an area of high concentration to an area of lower concentration. This movement continues until the concentration of the substance evens out. Once the concentration has evened out, the substance still moves but will no longer have a concentration gradient. This state is called dynamic equilibrium. Molecules are constantly moving around due to the amount of thermal energy they have. This movement is affected by the size of the particle and the environment the particle is in.
Particles will always move around in a medium but the overall rate of diffusion can be affected by many factors. Concentration : Diffusion of molecules is entirely dependent on moving from an area of higher concentration to an area of lower concentration. After a substance has diffused completely through a space removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another.
This lack of a concentration gradient in which there is no net movement of a substance is known as dynamic equilibrium. While diffusion will go forward in the presence of a concentration gradient of a substance, several factors affect the rate of diffusion:. A variation of diffusion is the process of filtration.
In filtration, material moves according to its concentration gradient through a membrane; sometimes the rate of diffusion is enhanced by pressure, causing the substances to filter more rapidly. This occurs in the kidney where blood pressure forces large amounts of water and accompanying dissolved substances, or solutes, out of the blood and into the renal tubules.
The rate of diffusion in this instance is almost totally dependent on pressure. Learning Objectives Describe diffusion and the factors that affect how materials move across the cell membrane. Key Points Substances diffuse according to their concentration gradient; within a system, different substances in the medium will each diffuse at different rates according to their individual gradients. After a substance has diffused completely through a space, removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another, a state known as dynamic equilibrium.
Several factors affect the rate of diffusion of a solute including the mass of the solute, the temperature of the environment, the solvent density, and the distance traveled.
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