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.
1. Which of these proteins is involved in nuclear transport?
HEY.all the above
Answer to question #1
Heyit's correct. Imports mediate movement into the nucleus, exports help the movement of molecules out of the nucleus. RanGTP is involved in both processes.
2. Which of these molecules diffuses freely through small pores in the cell membrane?
HEY.none of the above
Answer to question #2
doit's correct. The pores are composed of transmembrane proteins called aquaporins. Glucose is transported by facilitated diffusion or active transport, against its concentration gradient. ATP is a charged molecule and therefore cannot freely diffuse through the lipid bilayer.
3. Select the molecule that does NOT require active transport to move across the cell membrane.
Answer to question #3
siit's correct. It can even diffuse through aquaporin channels. The other three are charged particles and cannot diffuse through the cell membrane.
The selective permeability of the cell membrane refers to the ability of the cell membrane to control the flow of substances in and out of the cell. It helps maintain a cell's internal environment, and to regulate its internal conditions, such as osmotic pressure, pH, and ion concentration.What is the function of the selective permeability? ›
Selective permeability is mediated by special proteins that traverse the cellular membrane. They are involved in the movement of ions and small molecules as well as large polymers such as RNA and proteins. This movement can be passive or active – with or without the expenditure of energy.What is selective permeability in biology quizlet? ›
Selective permeability. Ability of the cell membrane to allow certain substances to pass through while keeping others out.What is selective permeability with an example? ›
What is an example of selective permeability? In selective permeability, certain substances are able to cross the membrane, while other substances are not able to cross. In animal cells, lipid-soluble materials, hydrocarbons, and oxygen are examples of molecules the cell allows to pass through.What is an example of permeability in biology? ›
Permeability of a cell membrane is the passive diffusion through the cell membrane. It depends on the factors like the molecule's charge, molecular mass, etc. Example- small hydrophobic gases like oxygen and carbon dioxide can permeate easily through the cell membrane.What is selective permeability a property of quizlet? ›
The cell membrane is selectively permeable. A property of biological membranes that allows some substances to cross more easily than others.What is the function of a selectively permeable membrane quizlet? ›
A selectively permeable cell membrane is one that allows certain molecules or ions to pass through it by means of active or passive transport.What is the function of the cell membrane's selective permeability quizlet? ›
Selective permeability ensures that only certain substances are allowed to pass through the plasma membrane. This ensures a constant internal environment for the cell as well as allows only certain substances to pass from the external environment to the inside of the cell, depending on the function of the cell.Is selective permeability important? ›
This process of only letting certain molecules into or out of the cell is called selectively permeable. The selectively permeability is important for transporting glucose into and out of cells because it is too big to fit through the phospholipids, as well as creating electrical impulses in neurons.Which statement best describes selective permeability? ›
Expert-Verified Answer. the phrase which best describes the property of selective permeability is some molecules . the membrane is said to be selective permeable when it allows the movement of the some selective molecules while do not allow the moving of other molecules through it.
The cell membrane is called selectively permeable as it only allows specific molecules to pass. Only specific molecules like water and gaseous molecules can pass through the cell membrane directly. It stops the flow of other molecules towards the two sides.What does selectively permeable mean ______? ›
A selectively-permeable membrane is a membrane that allows only some substances and molecules to pass into or leave the cell. An example of a selectively-permeable membrane is the cell membrane. It allows the passage of only certain types of molecules through diffusion and occasionally by facilitated diffusion.Where does selective permeability occur? ›
The plasma membrane is semi-permeable or half permeable because it allows free passage of water molecules and regulates the passage of solute molecules. As It allows only selected solute particles to pass through the plasma membrane is also called as selectively permeable or differentially permeable.Which of the following is selective permeability? ›
Cell membrane or plasma membrane is described as a selectively permeable membrane.What would be the best definition of permeability? ›
Permeability is the quality or state of being permeable—able to be penetrated or passed through, especially by a liquid or gas. The verb permeate means to penetrate, pass through, and often become widespread throughout something. Similar words are pervade and saturate.What is permeability in simple terms? ›
Permeability is a measure of the ease of passage of liquids or gases or specific chemicals through the material. Permeability is determined by applying a head and determining the depth of penetration or the amount of liquid or gas passing through the sample.What best describes permeability? ›
Permeability is a measure of the ease of flow of a fluid through a porous solid. A rock may be extremely porous, but if the pores are not connected, it will have no permeability.What does selective permeability depend on? ›
Context for Understanding Selective Permeability
If the cell needs a particular molecule inside the cell, that molecule is permitted to pass through the membrane. However, this depends on the size and charge of the molecule, either positive or negative, as well as the solubility of the molecule in water.
The permeability of a membrane is affected by temperature, the types of solutes present and the level of cell hydration. Increasing temperature makes the membrane more unstable and very fluid. Decreasing the temperature will slow the membrane.Why is the most important function of A cell membrane to be selectively permeable? ›
The membrane is selectively permeable because substances do not cross it indiscriminately. Some molecules, such as hydrocarbons and oxygen can cross the membrane. Many large molecules (such as glucose and other sugars) cannot. Water can pass through between the lipids.
It allows the entry of only useful molecules, such as substances like food molecules, water,salts, and oxygen; and regulatory substances like vitamins and hormones. It permits the exit of waste materials from the cell.How the cell membrane functions as A selectively permeable barrier for the cell? ›
Cell membranes serve as barriers and gatekeepers. They are semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot. Small hydrophobic molecules and gases like oxygen and carbon dioxide cross membranes rapidly.Which is essential for selective permeability of cell membrane? ›
Calcium is an essential macronutrient for plants. It is required in cell wall synthesis as calcium pectate, development of root and stem apices, cell membrane permeability, activator of some enzymes and for organisation of mitotic spindle.What is meant by a selectively permeable membrane quizlet? ›
What does selectively permeable (semi-permeable) mean? It is a property of cell membranes that allows some substances to pass through, while others cannot.What would happen without selective permeability? ›
In other words, plasma membranes are selectively permeable—they allow some substances through but not others (Figure 1). If the membrane were to lose this selectivity, the cell would no longer be able to maintain homeostasis, or to sustain itself, and it would be destroyed.What does it mean when a cell membrane is selectively permeable? ›
The cell membrane is called selectively permeable as it only allows specific molecules to pass. Only specific molecules like water and gaseous molecules can pass through the cell membrane directly. It stops the flow of other molecules towards the two sides.What is selective vs semi permeability? ›
Note that a semipermeable membrane is not the same as a selectively permeable membrane. Semipermeable membrane describes a membrane that allows some particles to pass through (by size), whereas the selectively permeable membrane "chooses" what passes through (size is not a factor).What does it mean for a membrane to be selectively permeable? ›
The membrane is selectively permeable because substances do not cross it indiscriminately. Some molecules, such as hydrocarbons and oxygen can cross the membrane. Many large molecules (such as glucose and other sugars) cannot. Water can pass through between the lipids.What is selectively permeable in simple terms? ›
Selective permeability of the cell membrane refers to its ability to differentiate between different types of molecules, only allowing some molecules through while blocking others. Some of this selective property stems from the intrinsic diffusion rates for different molecules across a membrane.What is a selectively permeable membrane definition quizlet? ›
A selectively permeable cell membrane is one that allows certain molecules or ions to pass through it by means of active or passive transport.
The cell membrane permeability governs the rate of solute transport into and out of the cell, significantly affecting the cell's metabolic processes, viability, and potential usefulness in both biotechnological applications and physiological systems.What things have selective permeability? ›
The main components of selectively permeable are proteins, lipids (cholesterol and phospholipids), and carbohydrates.