Osmosis

The process by which solvent molecules pass from a solution of lower concentration to a solution of higher concentration through a semipermeable membrane. It is a passive process that takes place without any expenditure of energy. The transport of solvent molecules continues from low to high concentration regions till the concentration on either side of the membrane is equal. Wilhelm Pfeffer, a German plant physiologist, was the first to investigate the process in depth in 1877. Previous research on leaky membranes (e.g., animal bladders) and the transport of water and escape chemicals through them had been less precise. A British scientist, Thomas Graham, coined the term osmose (now osmosis) in 1854. 

When a solution is separated from a pure solvent by a membrane that is permeable to the solvent but not to the solute, the solution will tend to grow more dilute as the solvent is absorbed through the membrane. By increasing the fluid's osmotic pressure by a given amount, this process can be slowed.  In 1886, the Dutch-born chemist Jacobus Henricus van 't Hoff demonstrated that if the solute is so dilute that its partial vapor pressure above the solution obeys Henry's law (i.e., is proportional to its concentration in the solution), then osmotic pressure varies with concentration and temperature in a manner similar to that of a gas occupying the same volume. This relationship led to formulae for estimating the molecular weights of solutes in dilute solutions based on the solvent's freezing, boiling, and vapor pressure.

Osmotic Pressure

The osmotic pressure is the minimal pressure required to prevent the inward flow of a solution's pure solvent across a semipermeable barrier. It can also be described as a measurement of a solution's proclivity to absorb a pure solvent via osmosis. The highest osmotic pressure that could develop in a solution if it were removed from its pure solvent by a semipermeable membrane is known as potential osmotic pressure.

When two solutions with varying amounts of solute are separated by a selectively permeable membrane, osmosis occurs. From the low-concentration solution to the solution with increasing solute concentration, solvent molecules pass across the membrane preferentially. The movement of solvent molecules will continue until equilibrium is achieved. 

What is a Semipermeable Membrane?

It is a thin barrier between two solutions that permits certain components of the solutions, generally the solvent, to pass through. A semipermeable membrane is a barrier that allows certain molecules to cross but prevents others from doing so. A semipermeable barrier functions as a filter in essence. Semipermeable membranes of various sorts can block molecules of varying sizes. Biological or synthetic materials can be used to create a semipermeable membrane.

A partially permeable membrane or a deferentially permeable membrane are other names for a semipermeable membrane.

Types of Osmosis

Generally, there are two types of osmosis. These are

Endosmosis

If a cell is placed in a hypotonic solution, water moves inside the cell making it swell or plasmolyze. This happens because the solute concentration of the solution is less than the concentration inside the cell. This process is known as endosmosis. The osmosis toward the inner of a cell or vessel is known as endosmosis. It happens when the water potential outside the cell is higher than the water potential inside the cell. As a result, the surrounding solution's solute concentration is lower than that of the cytoplasm. Hypotonic solutions are the name for this sort of solution. In endosmosis, water molecules pass through the cell membrane and inside the cell. The passage of water into cells causes them to swell. 

Example: Raisins swell when placed in normal water.

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Exosmosis

If a cell is placed in a hypertonic solution, the water inside the cell moves outside, and thus the cell plasmolysis (becomes flaccid). This happens because the solute concentration in the solution is more than the concentration inside the cytoplasm. This process is known as exosmosis. Exosmosis is the osmosis of a cell or vessel toward the outside. It happens when the water potential outside the cell is lower than the water potential inside the cell. As a result, the surrounding solution's solute concentration is higher than that of the cytoplasm. Hypertonic solutions are the name for these types of solutions. Exosmosis is the movement of water molecules out of the cell across the cell membrane. The migration of water out of cells causes cells to shrink. 

Example: Raisins placed in a concentrated salt solution shrivel.

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Reverse Osmosis 

It can be defined as a separation process that uses pressure to force a solvent through a semipermeable membrane that retains the solute on one side and makes the solvent pass through the other side.  It uses pressure to force the solvent to move from a high solute concentration region to a low solute concentration region. So reverse osmosis can be referred to as the opposite of general osmosis.

Application

It is used to remove major contaminants from water by pushing water through a semipermeable membrane under pressure.

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Forward Osmosis: 

It is a natural phenomenon that uses a semi-permeable membrane to separate dissolved solutes from water. The forward osmosis technique is beneficial for a variety of industrial water treatment applications, including wastewater management, product concentration, and water recycling, because of its very effective filtration process, which ensures that only pure water is recovered from the feed solution. It uses less energy than other hydraulic pressure-based water treatment systems since it relies on the natural energy of osmotic pressure.

Application: Desalination of water, waste-water treatment, osmotic power generation.

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Osmotic Concentration

A solution's osmolarity is a measurement of how concentrated the solute is inside one litre of the solution. Osmoles (Osm) are used to measure osmotic concentration, which is represented as osmoles per litre (Osm/L). In some cases, osmotic concentration is also expressed in millimoles per litre (mmol/L). The osmotic concentration of the solute rises as the amount of water or solvent decreases. Similarly, increasing the amount of solvent in a solution decreases the solute's osmotic concentration.

Isotonic Solution

In an isotonic solution, a cell is in equilibrium with its surroundings when the concentrations of solutes inside and outside are the same (iso means equal in Latin). There is no concentration gradient in this state, which means there is no significant water movement in or out. However, water molecules are free to enter and exit the cell at the same pace in both directions.

Hypotonic Solution

The concentration of solutes in a hypotonic fluid is lower than inside the cell (the prefix hypo is Latin for under or below). Water enters the cell due to the concentration differential between the compartments. Animal cells are less tolerant of this condition than plant cells. Water pours into the intercellular space as the huge central vacuole fills with water in plants. Turgor pressure is caused by the combination of these two actions, which pushes against the cell wall, causing it to bulge out. The cell wall acts as a barrier to prevent the cell from bursting. An animal cell, on the other hand, will enlarge until it bursts and dies if left in a very hypertonic solution.

Hypertonic Solution

The prefix hyper indicates "above" or "above" in Latin. The concentration of solutes in hypertonic fluids is higher than inside the cell. Water rushes out, causing the cell to wrinkle or shrivel. This is seen in red blood cells that are undergoing a process known as crenation. Because of what happens within, plant cells in a hypertonic solution can resemble a pincushion. The cell membrane slips away from the cell wall, but at plasmodesmata, it remains connected. Plasmodesmata are small tubes that transport and communicate information between plant cells. Plasmolysis occurs when the inner membrane contracts, causing the plasmodesmata to constrict.

Osmosis in a Plant Cell

If a cell is put in a hypertonic solution that is more concentrated than the cell, it will shrink due to loss of water and eventually die. For example, if a piece of carrot is put in a solution of salty water it will become soft and limp as the cells would shrivel.  In contrast, if the carrot piece is put in an isotonic solution, it would swell and expand. Generally, a normal cell would burst, but the rigid cell wall in the carrot cell protects it from rupturing. As the water enters the cell, it expands, until it creates a maximum pressure on the cell wall to expand more. However, the cell wall pushes back with equal pressure and no more water can enter.

Osmosis plays an important role in the transport of water in plants. Solute concentrations increase as they move from soil to root cells and then to leaf cells. The difference in pressure helps to push water upwards. Osmosis also controls the evaporation of water from leaves by regulating the size of the stomata on the leaf surface.

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The Importance of Osmosis:

In Plants:

• Osmosis helps in maintaining water content within a plant cell

• It provides turgidity to softer cells of a plant body.

• Osmosis controls the absorption of water by root hairs from the soil.

• It controls the conduction of water from xylem elements to adjacent cells

• Higher osmotic pressure provides resistance to plants against drought injury

In Animals:

• Osmosis regulates the flow of dissolved solids, liquids, and gases across cells.

• The semi-permeable membrane that encloses the cell selectively allows substances to pass in and out of the cell. This assists in releasing toxic metabolic waste products such as urea.

• Osmosis also helps in absorbing water from the intestines to the blood.

The Significance of Osmosis:

• Osmosis plays an important role in the transportation of nutrients and the release of metabolic waste products within a living cell.

• It stabilizes the internal movement of water and intracellular fluid levels within a cell.

• Osmosis also controls cell-to-cell diffusion and maintains the mechanical structure of a cell.

• In plants, growing root tips remain turgid and can penetrate easily into the soil because of osmosis.

• Osmosis plays a major role in the germination of seeds.

Examples of Osmosis

• The absorption of water by plant roots from the soil.

• The guard cells of a plant cell are affected by osmosis. When a plant cell is filled with water the guard cells swell up for the stomata to open and let out excess water

• If you keep your fingers in water for a long time, they become prunes. The reason behind this is that the skin absorbs water and expands.

What is Diffusion?

Diffusion is a process that results from the random motion of molecules and results in a net movement of matter from a high-concentration region to a low-concentration zone. A common example is the scent of a flower that swiftly pervades a room's still air.

In fluids, heat conduction refers to the transfer of thermal energy from a higher to a lower temperature. The operation of a nuclear reactor necessitates the diffusion of neutrons through a material that frequently scatters neutrons but only rarely absorbs them.

Examples of Diffusion

• A drop of food colouring diffuses throughout the water in a glass.

• If you put a sugar cube in water it will eventually dissolve and sweeten the water.

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