Osmosis

Osmosis 

The movement of water molecules from higher concentration to lower concentration region through a semi-permeable membrane is known osmosis.
The cytoplasm of a cell contains ions and molecules, such as sugars and amino acids, dissolved in water. The mixture of these substances and water is called an aqueous solution. Water is termed the solvent, and the substances dissolved in the water are solutes.

Both water and solutes tend to diffuse from regions of high concentration to ones of low concentration; that is, they diffuse down their concentration gradient.

When two regions are separated by a membrane, what happens depends on whether the solutes can pass freely through that membrane. Most solutes, including ions and sugars, are not lipid-soluble and therefore are unable to cross the lipid bilayer. 

Importantly, a concentration gradient of these solutes can lead to the net movement of water across the membrane.
Water molecules interact with dissolved solutes by forming hydration shells around the charged or polar solute molecules. As a direct result of this, when a membrane separates two solutions with different concentrations of charged or polar solutes, the concentrations of free water molecules on the two sides of the membrane also differ—the side with higher solute concentration has tied up more water molecules in hydration shells and so has fewer free water molecules.

As a consequence of this difference, free water molecules move down their concentration gradient, toward the higher 

solute concentration. This net diffusion of water across a membrane toward a higher solute concentration is called osmosis.

Figure:Concentration differences in charged 

or polar molecules that cannot cross a selectively permeable membrane result in the movement of water, which can cross the membrane. Water molecules form hydrogen bonds with charged or polar molecules, creating a hydration shell around them in solution. A higher concentration of polar molecules (urea), shown on the left side of the membrane, leads to water molecules gathering around each urea molecule. These water molecules are no longer free to diffuse across the membrane. The polar solute has reduced the concentration of free water molecules, creating a gradient. This causes a net movement of water by diffusion from right to left in the U-tube, raising the level on the left and lowering it on the right.

The concentration of all solutes in a solution determines the osmotic concentration of the solution. If two solutions have unequal osmotic concentrations, the solution with the higher concentration is said to be hypertonic (Greek hyper, “more than”), and the solution with the lower concentration is said to be hypotonic (Greek hypo, “less than”). When two solutions have the same osmotic concentration, the solutions are isotonic (Greek iso,“equal”). The terms hyperosmotic, hypoosmotic, and isosmoticare also used to describe these conditions.

A cell in any environment can be thought of as a plasma membrane separating two solutions: the cytoplasm and the extracellular fluid. The direction and extent of any diffusion of water across the plasma membrane are determined by comparing the osmotic strength of these solutions. Put another way, water diffuses out of a cell in a hypertonic solution (that is, the cytoplasm of the cell is hypotonic, compared with the extracellular fluid). 

This loss of water causes the cell to shrink until the osmotic concentrations of the cytoplasm and the extracellular fluid become equal.

 Water Channels                        

As we have discussed osmosis, have you been wondering how water molecules, which are polar, are able to freely diffuse across the lipid bilayer of membranes? This question puzzled biologists for a long time. The solution to the puzzle came with the discovery of specialized protein channels for water called aquaporins.

A simple experiment demonstrates the key role of aquaporins in admitting water into cells. If an amphibian egg is placed in hypotonic spring water (the solute concentration in the cell is higher than that of the surrounding water), the egg does not swell. 

Within an as-yet-undeveloped egg, the genes encoding aquaporins have not yet been expressed. If aquaporin mRNA is then injected into the egg, the amphibian channel proteins are expressed and appear in the egg’s plasma membrane.Water can now diffuse into the egg, causing it to swell.

More than 11 kinds of aquaporins have been found in mammals. These fall into two general classes: those that are specific for only water and those that allow other small hydrophilic molecules, such as glycerol or urea, to cross the membrane as well. This latter post explains how some membranes allow the easy passage of small hydrophilic substances.
Hereditary (nephrogenic) diabetes insipidus (NDI), a human genetic disease, has been shown to be caused by a nonfunctional aquaporin protein. This disease causes the excretion of large volumes of dilute urine, illustrating the importance of aquaporins to our physiology.

Osmotic Pressure

What happens to a cell in a hypotonic solution (that is, where the cell’s cytoplasm is hypertonic relative to the extracellular fluid)? In this situation, water diffuses into the cell from the extracellular fluid, causing the cell to swell. The pressure of the cytoplasm pushing out against the cell membrane, or hydrostatic pressure, increases. The amount of water that enters the cell depends on the difference in solute concentration between the cell and the extracellular fluid. This is measured as osmotic pressure, defined as the force needed to stop osmotic flow.

If the membrane is strong enough, the cell reaches an equilibrium, where the osmotic pressure, which tends to drive water into the cell, is exactly counterbalanced by the hydrostatic pressure, which tends to drive water back out of the cell. However, a plasma membrane by itself cannot withstand large internal pressures, and an isolated cell under such conditions would burst like an overinflated balloon.

Accordingly, it is important for animal cells, which are encased only within plasma membranes, to maintain osmotic balance. In contrast, the cells of prokaryotes, fungi, plants, and many protists are surrounded by strong cell walls that can withstand high internal pressures without bursting.

                         

Maintaining osmotic balance

Organisms have developed many strategies for solving the dilemma posed by being hypertonic to their environment and therefore exposed to a steady influx of water by osmosis.

1.Extrusion

Some single-celled eukaryotes, such as the protist Paramecium, use organelles called contractile vacuoles to remove water. Each vacuole collects water from various parts of the cytoplasm and transports it to the central part of the vacuole, near the cell surface. The vacuole possesses a small pore that opens to the outside of the cell. By contracting rhythmically, the vacuole pumps out (extrudes) through this pore the water that is continuously drawn into the cell by osmotic forces.

2.Isosmotic regulation

Some organisms that live in the ocean adjust their internal concentration of solutes to match that of the surrounding seawater. Because they are isosmotic with respect to their environment, no net flow of water occurs into or out of these cells.

Many terrestrial animals solve the problem in a similar way, by circulating a fluid through their bodies that bathes cells in an isotonic solution. The blood in your body, for example, contains a high concentration of the protein albumin, which elevates the solute concentration of the blood to match that of your cells’ cytoplasm.

3.Turgor. Most plant cells are hypertonic to their immediate environment, containing a high concentration of solutes in their central vacuoles. The resulting internal hydrostatic pressure, known as turgor pressure, presses the plasma membrane firmly against the interior of the cell wall, making the cell rigid.

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