Osmosis is a directed flow of molecular particles through a semipermeable membrane. In biology, it is central to the regulation of water balance in cells.
What is osmosis?
Osmosis is a directed flow of molecular particles through a semipermeable membrane. In biology, it is central to the regulation of water balance in cells. Osmosis means “penetration” in Greek. It is described as spontaneous passage for solvents such as water through a selectively permeable membrane. The membrane is permeable only to the solvent but not to the solute. The selective diffusion of only one component results in an equalization of the chemical potential on both sides of the membrane. Osmosis is frequently encountered in nature. Especially in biological membranes, selective mass transfer is necessary for biological transport processes to take place. However, active, energy-consuming transport processes also ensure here that the passively developing osmotic pressure is not destructive to the cell. While no reversal is possible in normal diffusion processes, osmosis is a reversible process.
Function and task
In osmosis, molecules of a solution or pure solvent selectively diffuse through a membrane until the chemical potential is balanced on both sides of that membrane. For example, a concentrated solution is diluted on the other side by the solvent until the built-up hydrostatic pressure prevents further diffusion. Molecules can migrate through the membrane, regardless of which side they come from. However, they are always more likely to diffuse in the direction of the largest potential difference. When the chemical potential is balanced, the same number of particles migrate from left to right as from right to left. Thus, nothing changes externally. However, due to the desired dilution of the concentrated solution, a higher amount of liquid has accumulated on one side, which has built up a high pressure (the osmotic pressure). If the membrane can no longer withstand the pressure, the cell may be destroyed. Active transport processes through the membrane ensure that certain substances are removed with the expenditure of energy. An illustrative example of an osmotic process is the swelling of ripe cherries when water is added to them. In this process, the water penetrates through the outer skin of the fruit, while the sugar cannot escape. The dilution process within the fruit continues until it bursts. Within the body, the combination of osmotic and active energy-consuming transport processes ensures the smooth running of biochemical processes in spaces separated by biomembranes. Thus, cells can exist that are separated from the external environment but are in constant metabolic exchange with it. There are also organelles within the cell where separate reactions can take place. To prevent osmotic pressure from increasing to the point of rupturing biomembranes, molecules are expelled by active transport processes. In mammalian cells, the protein NFAT5 is increasingly produced when osmotic pressure increases. It provides a number of counter mechanisms to protect the cell from hypertonic stress (overpressure). In this process, transport proteins are produced which, with the expenditure of energy, channel certain substances out of the cell. Among other things, urinary substances such as glucose and excess electrolytes are excreted by the kidneys to regulate the osmotic pressure in the body.
Diseases and ailments
Osmosis also plays an important role in regulating 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 both inside the cell (intracellular), outside the cells (interstitial) or inside the bloodstream (intravascular). The concentration differences generate the electrical tension on the cell membranes, triggering a variety of processes at the cellular level. If the concentration differences are disturbed, the entire electrolyte balance is also upset.The kidneys regulate this electrolyte balance through various mechanisms such as thirst mechanisms, hormonal processes or kidney-acting peptides. In cases of severe diarrhea, vomiting, blood loss or renal failure, the water and electrolyte balance may be disturbed. Each electrolyte may be present in either too high or too low concentrations. Disturbances in water and electrolyte balance are sometimes life-threatening, depending on their severity. Examples of such conditions include dehydration, hyperhydration, hyper- and hypovolemia (increased or decreased blood volume), hypo- and hypernatremia, hypo- and hyperkalemia, and 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 disturbed by renal insufficiency or another disease, chronic electrolyte imbalances may occur. As a result, edema, cardiovascular disease, cerebral edema, confusional states or seizures occur. The interrelationships of water and electrolyte balance with biological processes in the body are so complex that similar symptoms are often observed for all forms of electrolyte disorders. Determination of electrolyte balance should be a standard investigation when these symptoms are chronic.
