Selective Permeability Definition
Selective permeability is a property of cell membranes that only allows certain molecules to enter or leave the cell. This is important for the cell to maintain its internal order regardless of changes in the environment. For example, depending on metabolic activity, water, ions, glucose, and carbon dioxide must be imported or exported from the cell. Similarly, signaling molecules may need to enter the cell and proteins may need to be released into the extracellular matrix. The presence of a selectively permeable membrane allows the cell to control the amount, time, and speed of movement of these molecules.
Movement through a selectively permeable membrane can be active or passive. For example, water molecules can passively move through tiny pores in the membrane. Carbon dioxide released as a by-product of respiration also quickly diffuses out of the cell. Some molecules are actively transported. For example, kidney cells use energy to absorb all the glucose, amino acids, and vitamins from the glomerular filtration, even against the concentration gradient. Failure of this process leads to the presence of glucose or the by-products of protein metabolism in the urine; a telltale sign of diabetes.
About Structure of Selectively Permeable Membranes
Cell membranes are not easily visible with light microscopes. Therefore, hypotheses about its existence did not emerge until the end of the 19th century, almost two hundred years after the first cells were observed. In different places, different models have tried to explain how the membrane’s structure supports its function. Originally, the membrane was supposed to be a simple lipid layer that separates the cytosol from the extracellular region. The models then included semipermeable gel-like regions in a sea of lipids to explain the movement of water but not charged particles. Thereafter, the presence of pores that allow small molecules to move freely was suggested.
At present, the cell membrane should consist of a selectively permeable phospholipid bilayer whose hydrophilic domains face the aqueous medium inside and outside the cell and whose hydrophobic domains face each other to form a bilayer. This lipid bilayer is interrupted by cholesterol, glycolipids, and protein molecules that are anchored or cross the entire membrane. These proteins form channels, pores, or gates to maintain the selective permeability of ions, signaling molecules, and macromolecules depending on the requirements of the cell.
The nuclear membrane has a structure, unlike any other cell membrane. It has nuclear pore complexes, basket-shaped multiprotein complexes that are freely permeable to water, but strictly mediate the nuclear transport of macromolecules. Importin and exportin are two classes of proteins that actively participate in nuclear transport. Both are energy-intensive and each transport event involves the hydrolysis of an energy-rich phosphate bond into a guanosine triphosphate. The directionality of the movement also requires the presence of a small molecule called Ran, which has a different affinity for its substrates depending on whether it is bound to GTP or GDP.
What is the Function of Selective Permeability?
The selective permeability is crucial in order to create a significantly different environment within the cell compared to the extracellular matrix. It is equally relevant to maintaining the integrity of various organelles within the cell. Each organelle is a small compartment with a specialized function that requires optimal concentrations of proteins, small molecules, and ions. For example, cellular respiration within a mitochondrion requires that the proteins that support this process be selectively imported into the organelle, and their internal chemistry must not be affected by the other metabolic processes in the cytoplasm. Similarly, after a neuron has transmitted an electrochemical signal, it must recover and return to its resting potential in order to allow the next round of excitatory activity. The same thing happens in all cells of the heart muscle with every heartbeat. These large-area, rapid changes in the electrochemical properties of these cells are necessary for their function and require the presence of a selectively permeable membrane.
The selective permeability of membranes is particularly important for transport across the nuclear membrane in eukaryotic cells. Proteins, nucleic acids, and nucleotides involved in transcription must be selectively and efficiently transported to the cell nucleus and transcription products must be exported promptly. The nucleus has a different microenvironment compared to the cytoplasm, and active transport mechanisms work to maintain this distinction.
Proteins Mediating Selective Permeability
Selective permeability is mediated by special proteins that cross the cell membrane. They are involved in the movement of ions and small molecules, as well as large polymers like RNA and proteins. This movement can be passive or active, with or without energy expenditure.
For example, ions are transported through channels and pumps through selectively permeable membranes. While channels are used for passive transport, ion pumps mediate primary active transport against a concentration gradient with the hydrolysis of a high-energy phosphate bond.
Active transport can also be coupled to the movement of another molecule. This can be done by means of a symporter protein, in which two molecules are transported in the same direction, or by an anti-carrier protein, in which the molecules move in opposite directions. The principle is the same in both cases: potential energy stored in an electrochemical gradient is used to drive the transport of another molecule.
Active And Passive Transport Across Selectively Permeable Membranes
Hay dos tipos de transporte pasivo, difusión libre o difusión facilitada, y el movimiento siempre se realiza a lo largo de un gradiente de concentración. La difusión libre se observa con mayor frecuencia cuando moléculas sin carga como el dióxido de carbono o el etanol se mueven a través de la membrana celular sin la participación de otras moléculas.
Facilitated diffusion requires the presence of another molecule, usually a protein, which acts as a carrier and helps the substrate to cross the cell membrane. Carrier proteins bind to the substrate on one side of the membrane and change their conformation to release the substrate on the other side. Classic examples of facilitated diffusion are the movement of oxygen by binding to hemoglobin or the transport of water through small pores formed by aquaporins.
The diffusion of water can also be observed at the macroscopic level. For example, when the seeds swell after soaking in water, we see the general effect of the water entering the cell. Likewise, fruits that are kept in a dry environment like a refrigerator will shrink and shrink when they lose water. Many organisms, including humans, have a waxy coating on the skin to minimize water loss from their cells in a dry environment.
Transmembrane transport can also be carried out actively with energy expenditure. Active transport involves the hydrolysis of the terminal phosphate group in ATP or GTP to drive the movement of molecules against their concentration gradient. For example, in most cells, there is a large excess of sodium ions in the extracellular environment along with an excess of potassium ions within the cell. This is accomplished through a transmembrane enzyme called Na + / K + ATPase, which catalyzes the movement of three Na + ions out of the cell along with the importation of two K + ions. For each of these transport cycles, the enzyme uses the energy released by converting a molecule of ATP to ADP. This is known as primary active transport, where the movement is directly coupled to the hydrolysis of an energy-rich phosphate bond. A similar process is used to pump protons against their concentration gradient, and this is a crucial part of both photosynthesis and cellular respiration.
Gradients of H +, Na +, and K + ions are used to drive other processes through secondary active transport, with different electrochemical concentrations being the driving force for other energy-intensive processes such as the transport of amino acids or glucose. For example, glucose uptake in the intestine is related to the transport of Na + ions. This is an example of importation in which both the sodium ion and the glucose molecule are imported into the cell. Sodium ions also participate in the movement of another charged molecule: Ca2 +. The sodium-calcium exchanger uses the movement of Na + along its gradient to drive the counter-transport of Ca2 +. This is especially important in the movement of calcium ions in large quantities, such as in neurons, heart cells, and to maintain a low concentration of calcium in the mitochondria.
Selective Permeability Definition, &Function