What Is Osmosis
Kenneth Calimlim
In the diagram above, the higher concentration of water molecules to the left of the partially permeable membrane makes it likely that a large number of water molecules will collide with the membrane and pass through it.
The lower concentration of water molecules on the right-hand side of the partially permeable membrane in the diagram makes it likely that fewer water molecules will collide with the membrane and pass through it.
This means that more water molecules move from left to right on this diagram than move from right to left, and so the overall movement (net movement) is to the right. It is important, though, to stress to students that water molecules are moving in both directions.
If a plant cell is surrounded by a solution that contains a higher concentration of water molecules than the solution inside the cell, water will enter the cell by osmosis and the plant cell will become turgid (firm). The pressure that develops inside a plant cell when it becomes turgid is called turgor pressure. Turgid plant cells help a stem to stay upright.
If a plant cell is surrounded by a solution that contains a lower concentration of water molecules than the solution inside the plant cell, water will leave the cell by osmosis and the plant cell will become flaccid (soft). If the cells in a plant stem become flaccid the turgor pressure inside them will decrease and the stem will wilt.
If a plant cell is surrounded by a solution that contains the same concentration of water molecules as the solution inside the plant cell, there is no overall net flow of water. The movement of water molecules into and out of the cell, through the partially permeable membrane, balances out.
In plants, water enters the root cells by osmosis and moves into tubes called xylem vessels to be transported to the leaves. Water molecules inside the xylem cells are strongly attracted to each other because of hydrogen bonding (this is called cohesion). When water evaporates from the leaves (through tiny pores called stomata), more water is drawn up from the root xylem cells to replace that which has been lost. A continuous column of water is, therefore, pulled up the stem in the xylem vessels by evaporation from the leaves. This is called transpiration.
In physiology, osmosis (Greek for push) is the net movement of water across a semipermeable membrane.[1][2] Across this membrane, water will tend to move from an area of high concentration to an area of low concentration. It is important to emphasize that ideal osmosis requires only the movement of pure water across the membrane without any movement of solute particles across the semipermeable membrane. Osmosis can still occur with some permeability of solute particles, but the osmotic effect becomes reduced with greater solute permeability across the semipermeable membrane. It is also true that, at a specific moment in time, water molecules can move towards either the higher or lower concentration solutions, but the net movement of water will be towards the higher solute concentration. The compartment with the highest solute and lowest water concentration has the greatest osmotic pressure. Osmotic pressure can be calculated with the van 't Hoff equation, which states that osmotic pressure depends on the number of solute particles, temperature, and how well a solute particle can move across a membrane. Its measured osmolality can describe the osmotic pressure of a solution. The osmolality of a solution describes how many particles are dissolved in the solution. The reflection coefficient of a semipermeable membrane describes how well solutes permeate the membrane. This coefficient ranges from 0 to 1. A reflection coefficient of 1 means a solute is impermeable. A reflection coefficient of 0 means a solute can freely permeable, and the solute can no generate osmotic pressure across the membrane.[2] The compartment with the greatest osmotic pressure will pull water in and tend to equalize the solute concentration difference between the compartments. The physical driving force of osmosis is the increase in entropy generated by the movement of free water molecules. There is also thought that the interaction of solute particles with membrane pores is involved in generating a negative pressure, which is the osmotic pressure driving the flow of water.[3] Reverse osmosis occurs when water is forced to flow in the opposite direction. In reverse osmosis, water flows into the compartment with lower osmotic pressure and higher water concentration. This flow is only possible with the application of an external force to the system. Reverse osmosis is commonly used to purify drinking water and requires the input of energy. [4] The concept of osmosis should not be confused with diffusion. Diffusion is the net movement of particles from an area of high to low concentration. One can think of osmosis as a specific type of diffusion. Both osmosis and diffusion are passive processes and involve the movement of particles from an area of high to low concentration.[2][5]
The rate of osmosis always depends on the concentration of solute. The process is illustrated by comparing an environmental or external solution to the internal concentration found in the body. A hypertonic solution is any external solution that has a high solute concentration and low water concentration compared to body fluids. In a hypertonic solution, the net movement of water will be out of the body and into the solution. A cell placed into a hypertonic solution will shrivel and die by a process known as plasmolysis. An isotonic solution is any external solution that has the same solute concentration and water concentration compared to body fluids. In an isotonic solution, no net movement of water will take place. A hypotonic tonic solution is any external solution that has a low solute concentration and high water concentration compared to body fluids. In hypotonic solutions, there is a net movement of water from the solution into the body. A cell placed into a hypotonic solution will swell and expand until it eventually burst through a process known as cytolysis. These three examples of different solute concentrations provide an illustration of the spectrum of water movement based on solute concentration through the process of osmosis. The body, therefore, must regulate solute concentrations to prevent cell damage and control the movement of water where needed.
A hypertonic solution has a higher solute concentration compared to the intracellular solute concentration. When placing a red blood cell in any hypertonic solution, there will be a movement of free water out of the cell and into the solution. This movement occurs through osmosis because the cell has more free water than the solution. After the solutions are allowed to equilibrate, the result will be a cell with a lower overall volume. The remaining volume inside the cell will have a higher solute concentration, and the cell will appear shriveled under the microscope. The solution will be more dilute than originally. The overall process is known as plasmolysis.
An isotonic solution has the same solute concentration compared to the intracellular solute concentration. When a red blood cell is placed in an isotonic solution, there will be no net movement of water. Both the concentration of solute and water are equal both intracellularly and extracellularly; therefore, there will be no net movement of water towards the solution or the cell. The cell and the environment around it are in equilibrium, and the cell should remain unchanged under the microscope.
A hypotonic solution has a lower solute concentration compared to the intracellular solute concentration. When a red blood cell is placed in a hypotonic solution, there will be a net movement of free water into the cell. This situation will result in an increased intracellular volume with a lower intracellular solute concentration. The solution will end up with a higher overall solute concentration. Under the microscope, the cell may appear engorged, and the cell membrane may eventually rupture. This overall process is known as cytolysis.
Note that osmosis is a dynamic equilibrium, so at any given moment, water molecular can momentarily flow toward any direction across the semipermeable membrane, but the overall net movement of all water molecules will be from an area of high free water concentration to an area of low free water concentration.[5][6]
Water is known as the "universal solvent," and almost all known life depends on it for survival. Therefore, the principle of osmosis, though seemingly simple, plays a large role in almost all physiological processes. Osmosis is specifically important in maintaining homeostasis, which is the tendency of systems toward a relatively stable dynamic equilibrium. Biological membranes act as semipermeable barriers and allow for the process of osmosis to occur. Osmosis underlies almost all major processes in the body, including digestion, kidney function, nerve conduction, etc. It allows for water and nutrient concentrations to be at equilibrium in all of the cells of the body. It is the underlying physical process that regulates solute concentration in and out of cells, and aids in excreting excess water out of the body.[2][7][8][9][10][11]
Osmosis is a passive process and happens without any expenditure of energy. It involves the movement of molecules from a region of higher concentration to lower concentration until the concentrations become equal on either side of the membrane.
Osmosis is important for the cells for many reasons. It helps in the movement of important materials inside and out of the cell. The nutrients, water and other solutes move in and out of the cell by the process of osmosis.
Osmosis is a process of movement of solvents through a semi-permeable membrane from a region of lower solute concentration to higher solute concentration. On the contrary, diffusion does not require a semi-permeable membrane to occur and the molecules move from a region of higher concentration to lower concentration.
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