Body processes

Osmosis – Function, Task & Diseases


Osmosis is a directional flow of molecular particles across a semipermeable membrane . In biology it is of central importance for the regulation of the water balance in the cells .

What is osmosis?

Osmosis means “penetration” in Greek. It is described as the spontaneous passage of solvents such as water through a selectively permeable membrane. The membrane is only permeable to the solvent but not to the dissolved substances. The selective diffusion of only one component results in an equalization of the chemical potential on both sides of the membrane.

Osmosis is common in nature. In biological membranes in particular, a selective exchange of substances is necessary so that biological transport processes can take place. However, active, energy-consuming transport processes also ensure that the passively occurring osmotic pressure does not have a destructive effect on the cell.

While normal diffusion processes cannot be reversed, osmosis is a reversible process.

function & task

In osmosis, molecules of a solution or a pure solvent selectively diffuse through a membrane until the chemical potential on both sides of this membrane is balanced. For example, a concentrated solution on the other side is diluted by the solvent until the hydrostatic pressure that has built up prevents further diffusion. 

The molecules can migrate through the membrane, regardless of which side they come from. However, the probability of diffusing in the direction of the largest potential difference is increasing for them.

When the chemical potential is balanced, the same number of particles move from left to right as from right to left. So nothing changes on the outside. However, due to the desired dilution of the concentrated solution, a larger amount of liquid has accumulated on the one hand, which has built up a high pressure (the osmotic pressure). If the membrane can no longer withstand the pressure, the cell can be destroyed.

Active transport processes through the membrane ensure that certain substances are transported away using energy. A vivid example of an osmotic process is the swelling of ripe cherries when water is added to them. The water penetrates through the outer skin of the fruit, while the sugar cannot escape. The thinning process within the fruit continues until it bursts.

Within the body, the combination of osmotic and active, energy-consuming transport processes ensures that biochemical processes run smoothly in spaces separated by biomembranes. Cells can exist that separate themselves from the external environment but are in constant exchange of substances with it.

There are also organelles within the cell where separate reactions can take place. To ensure that the osmotic pressure does not increase so much that the biomembranes burst open, molecules are ejected through active transport processes.

The protein NFAT5 is increasingly produced in mammalian cells when the osmotic pressure increases . It provides a number of counter-mechanisms to protect the cell from hypertonic stress (overpressure). In the process, transport proteins are produced, which smuggle certain substances out of the cell using energy. Among other things, substances that have to be excreted in the urine, such as glucose and excess electrolytes , are excreted via the kidneys in order to regulate the osmotic pressure in the body.

Diseases & Ailments

Osmosis also plays an important role in the regulation of electrolyte balance. Electrolytes are dissolved salts and consist of positively charged metal ions such as sodium , potassium , magnesium or calcium ions and negatively charged anions such as chloride, bicarbonate or phosphate anions. 

They are present in different concentrations inside the cell (intracellular), outside the cells (interstitial) or inside the bloodstream (intravascular). The differences in concentration generate the electrical voltage on the cell membranes and thus trigger a large number of processes at the cell level. If the concentration differences are disturbed, the entire electrolyte balance is also disturbed.

The kidneys regulate this electrolyte balance through various mechanisms such as thirst mechanisms, hormonal processes or kidney-active peptides. With severe diarrhea , vomiting , blood loss or kidney failure , the water and electrolyte balance can be disturbed. Each electrolyte can be present in concentrations that are either too high or too low.

Depending on the severity, disorders in the water and electrolyte balance can sometimes be life-threatening. Examples of such states include dehydration , hyperhydration , hyper- and hypovolemia (blood volume is increased or decreased), hypo- and hypernatremia, hypo- and hyperkalemia , or also hypo- and hypercalcemia .

Each of these conditions requires intensive treatment. As a rule, the water and electrolyte balance is quickly rebalanced. However, if the regulatory mechanism between active transport processes and osmotic processes is disrupted by renal insufficiency or another disease, chronic electrolyte imbalances can occur. As a result, edema , cardiovascular disease , cerebral edema , states of confusion or seizures occur.

The connections between the water and electrolyte balance and the biological processes in the body are so complex that similar symptoms are often observed for all forms of electrolyte disorders. Determining the electrolyte balance should be part of the standard examinations if these symptoms persist chronically.

Lisa Newlon
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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.