Selective Permeability - Definition and Function | Dictionary of Biology (2023)

Definition of selective permeability

Selective permeability is a property of cell membranes that allows only certain molecules to enter or leave the cell. This is important for the cell to maintain its internal order regardless of environmental changes. For example, water, ions, glucose, and carbon dioxide may need to move in or out of the cell, depending on its metabolic activity. 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 quantum, timing and speed of movement of these molecules.

Movement across a selectively permeable membrane can be active or passive. For example, water molecules can move passively through tiny pores in the membrane. Similarly, carbon dioxide released as a byproduct of respiration diffuses rapidly out of the cell. Some molecules are actively transported. For example, kidney cells expend energy to reabsorb all glucose, amino acids, and vitamins from the glomerular filtrate, 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.

Structure of selectively permeable membranes

Cell membranes are not easily visible using light microscopes. Therefore, hypotheses about their existence did not arise until the end of the 19th century, almost two hundred years after the first cells were observed. At various points, different models have attempted to explain how the structure of the membrane supports its function. Originally, the membrane was assumed to be a simple lipid layer that bounded the cytoplasm from the extracellular region. After that, models include gel-like semipermeable regions in a lipid sea to account for the movement of water, but not charged particles. The presence of pores was then proposed, allowing small molecules to move freely.

Currently, the cell membrane is said to be constructed of a selectively permeable phospholipid bilayer whose hydrophilic regions face the aqueous environments inside and outside the cell and the hydrophobic regions face each other to form a bilayer. This lipid bilayer is interrupted by cholesterol molecules, glycolipids, and proteins that are anchored or span the entire membrane. These proteins form channels, pores, or gates to maintain the selective permeability of ions, signaling molecules, and macromolecules based on the cell's requirements.

The nuclear membrane has a different structure than all other cell membranes. It has nuclear pore complexes - basket-like complexes of polyproteins that are freely permeable to water, but strictly mediate the nuclear transport of macromolecules. Importins and exportins are two classes of proteins actively involved in nuclear transport. Both are energy-intensive, and each transport event involves the hydrolysis of a high-energy phosphate bond to a guanosine triphosphate. The directionality of movement also requires the presence of a small molecule called Ran, which has a differential affinity for its substrates based on whether it is bound to GTP or GDP.

Selective permeability mode

Selective permeability is crucial to create a distinctly different environment inside the cell compared to the extracellular matrix. It is equally important for maintaining the integrity of the various organelles in the cell. Each organelle is a small compartment with a specialized function, requiring optimal concentrations of proteins, small molecules and ions. For example, cellular respiration in a mitochondrion requires that the proteins that support this process be selectively imported into the organelle, and its internal chemistry must remain unaffected by the other metabolic processes of the cytoplasm. Similarly, after a neuron sends an electrochemical signal, it must recover and return to its resting potential to allow the next round of excitatory activity. The same thing happens to each heart muscle cell every time the heart beats. These rapid and large-scale changes in the electrochemical properties of these cells are essential for their function and require the presence of a membrane that is selectively permeable.

Selective membrane permeability 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 into the nucleus and the products of transcription exported in a timely manner. The nucleus has a distinct microenvironment compared to the cytoplasm, and transport mechanisms act to maintain this distinction.

Selective permeability is mediated by specific proteins that cross the cell membrane. They are involved in the movement of ions and small molecules, but also in large polymers such as RNA and proteins. This movement can be passive or active - with or without energy consumption.

For example, ions are transported across selectively permeable membranes via channels and pumps. While channels are passive transport, ion pumps primarily mediate active transport over a concentration gradient, by hydrolysis of a high-energy phosphate bond.

Active transport can also be linked to the movement of another molecule. This can be done through a cotransporter protein - where two molecules are transported in the same direction - or through an antitransporter protein - where the molecules are transported in opposite directions. The principle is the same in both cases: the potential energy stored in an electrochemical gradient is used to drive the transport of another molecule.

Active and passive transport through selectively permeable membranes

There are two types of passive transport—free diffusion or facilitated diffusion—and movement always occurs along a concentration gradient. Free diffusion is most often seen in the movement of uncharged molecules such as carbon dioxide or ethanol across the cell membrane, without the involvement of other molecules.

Facilitated diffusion requires the presence of another molecule, usually a protein, that acts as a carrier and helps the substrate cross the cell membrane. Carrier proteins bind to the substrate on one side of the membrane and change conformation to release the substrate on the other side. Classic examples of facilitated diffusion are the movement of oxygen through binding to hemoglobin or the transport of water through microscopic pores formed by aquaporins.

Water diffusion can also be observed at a macroscopic level. For example, if seeds swell after being soaked in water, we see the overall effect of water entering the cell. Similarly, fruit left in a dry environment such as the refrigerator will shrivel and shrink as it loses water. Many organisms, including humans, have a waxy coating on their skin to minimize water loss from their cells in dry environments.

Transmembrane transport can also take place actively, expending energy. Active transport involves the hydrolysis of the terminal phosphate group to ATP or GTP to initiate 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 inside the cell. This is accomplished by a transmembrane enzyme called Na+/K+ ATPase, which catalyzes the movement of three Na+ ions out of the cell, along with the entry of two K+ ions. For each transport cycle, the enzyme uses the energy released by converting one molecule of ATP to ADP. This is called primary active transport, where movement is directly coupled to the hydrolysis of a high-energy phosphate bond. A similar process is used to pump protons against their concentration gradient and is a critical part of both photosynthesis and cellular respiration.

H+, Na+ and K+ ion gradients are used to drive other processes, via secondary active transport, where differential electrochemical concentrations drive other energy-consuming processes, such as amino acid or glucose transport. For example, the absorption of glucose in the intestine is linked to the transport of Na+ ions. This is an example of a signal, where both the sodium ion and the glucose molecule enter the cell. Sodium ions are also involved in the movement of another charged molecule - Ca2+. The sodium-calcium exchanger uses the movement of Na+ along its gradient to drive the countertransport of Ca2+. This is especially important for the movement of calcium ions in large quantities, such as in neurons, heart cells, and for maintaining a low concentration of calcium in the mitochondria.

• Electron microscope- A microscope that uses an electron beam to illuminate the specimen and achieve extremely high magnification and resolution.
• Extracellular matrix- Non-cellular component of tissues and organs consisting of water, proteins and polysaccharides, which provides physical, industrial and biochemical support to cells.
• Hydrophilic- A molecule that is attracted to water.
• he walked- A small protein that directs transport across the nuclear membrane based on its binding to guanosine dinucleotides and trinucleotides.

Quiz

1. Which of these proteins is involved in nuclear transport?
ONE.Introduction
SI.Export
DO.RanGTP
HEY.all the above

2. Which of these molecules diffuses freely through small pores in the cell membrane?
ONE.Glucose
SI.ATP
DO.Water
HEY.none of the above

3. Select the molecule that does NOT require active transport to move across the cell membrane.
ONE.sodium ions
SI.Carbon dioxide
DO.Amino acids
HEY.potassium ions