Osmosis

Osmosis is referred to as a spontaneous process which includes the net movement of the solvent molecules across a permeable membrane which allows selective movement through it, into a higher region of solute concentration towards a direction which allows to equate the concentration of solute molecules on both the sides. It may also be described as a process within which the solvent travels across a permeable membrane providing a solution for different concentrations. Osmosis can be put to work. Osmotic pressure on the other hand, refers to a pressure exerted externally which is required to ensure that there is no movement of solvent across the membrane. Osmotic pressure carries a colligative property, which tends to mean that it depends entirely upon the molar concentration of the solute which is present leaving the identity on its side.

Osmosis plays a vital role within the biological systems, as biological membranes tend to show the features of a semipermeable membrane. Generally, all of these membranes are impermeable to molecules which are large and polar, such as those of ions, proteins and polysaccharides, on the other hand are permeable to non-polar or hydrophobic molecules such as that of lipids along with smaller molecules like of oxygen, carbon dioxide, nitrogen and nitric oxide. Permeability varies upon various properties such as solubility, charge, chemistry along with the size of the solute molecules. Molecules of the water travel across the plasma membrane, the tonoplast membrane or the protoplast by diffusing across the phospholipid bilayer with the utilization of aquaporins. Osmosis provides the provision for the primary means of the transport of water in and out across the cells. The turgor pressure again, is maintained by the process of osmosis across the cell membrane within the cell interior and its relative hypotonic environment.

In general words, Osmosis is the movement of the solvent across a semipermeable membrane towards a solute having a much higher concentration. Within the biological systems, the solvent is generally water but again the osmosis can take place in other liquids as well such as supercritical liquids and even within gases.   

The time a cell is submerged in water, the water molecules make movement across the cell membrane from an area which has a low concentration of solute to a place possessing a higher concentration of the same solute molecules. Hypothetically speaking, if a cell is submerged in saltwater, the water molecules exit the cell. While, if the same cell is submerged in freshwater then the water molecules move right into the cell.

When the cell membrane is placed in a region where there exists pure water on both the sides, water molecules travel in and out across the cell in both the direction exactly at the same rate. Hence, there exists no net flow of water across the membrane be it in or out of the cell. The mechanism which is completely responsible for the process of osmosis, is commonly referred in the field of biology or chemistry as either the dilution of water by solute or by a solute’s attraction of water. Both of these statements however, has been completely refused.

The model of diffusion related to the process of osmosis is rendered by the fact that osmosis has the ability to drive water across the membrane having a tendency of moving towards a higher concentration of water. The model namely referred to as the “Bound water” is disapproved by the fact that osmosis has no dependency upon the size of the solute molecules which is again a colligative property of this process.

However, it is certainly difficult to describe the process of osmosis without the help of a mechanical or thermodynamic explanation, but is essential to describe that there is an interaction between the solute and the water that counteracts the existing pressure in order to disdain the solute molecules from exerting that. A fact to be noted down is that, the heat from the surrounding carries the ability to convert itself into mechanical energy.

Various thermodynamic explanations can be taken into consideration with the concept of the chemical potential and how the functionality of water within the solution side completely differs from that of the pure water due to the higher pressure along with the presence of the solute molecules which are interacting keeping the chemical potential of the system unchanged. The osmotic pressure is the main reason behind the support in most of the plants. The entry of water due to osmosis helps raising the turgor pressure of the cell which is exerted upon the cell wall, till the time it equals the osmotic pressure giving birth to a steady state.

Some of the examples of osmosis within the real world are,

i) When a saline solution is used in case of contact lenses, it is done for a reason because if they are placed in pure water. It is uncomfortable to worn the next day. On the other hand, if one forgets to rinse the lens before wearing then they become too salty. Saline solution for the contact lenses is of the same concentration that of the salt water in our eye.

ii) Saline water which is injected into the body of the patient is of the same concentration as that of the patient’s body.

iii) The making of French fries includes the first step of keeping the freshly cut potato immersed in salt water which makes the French fries a lot crispier.

iv) Another example of osmosis related to the real life is that, when raisins are kept immersed into water overnight. They tend to puff in size and hence explaining the movement of the water molecules into the raisins as a reason of change in concentration across the cell membrane of the raisin.

v) When humans sit in the bathtub or have their fingers submerged under water for a long time, after a certain period of time they start getting wrinkled. This phenomenon occurs all due to the process of osmosis. There occurs a travelling of water molecules across the cell membrane of the cells present on the fingers and hence, it is evident of change in concentration across the cell membrane.