Which phrase best describes the property of selective permeability?
The Correct Answer and Explanation is :
The phrase that best describes the property of selective permeability is: “Allows certain molecules or ions to pass through a membrane while blocking others.”
Explanation:
Selective permeability refers to the ability of a biological membrane, such as the cell membrane, to regulate the passage of substances into and out of the cell or organelle. This characteristic is crucial for maintaining homeostasis within the cell, as it controls the internal environment by allowing essential nutrients, ions, and gases to enter while preventing harmful substances from entering and waste products from accumulating.
The cell membrane is primarily composed of a phospholipid bilayer, with hydrophobic (water-repelling) tails and hydrophilic (water-attracting) heads. Embedded within this bilayer are proteins that aid in the transport of molecules across the membrane. These proteins can function as channels, carriers, or pumps to facilitate the movement of specific substances.
Selective permeability is achieved through several mechanisms:
- Size and Charge of Molecules: Small, nonpolar molecules (like oxygen and carbon dioxide) can diffuse freely across the lipid bilayer, while larger, charged molecules (like ions) cannot pass through as easily. This creates a natural barrier that prevents some substances from entering the cell without assistance.
- Transport Proteins: Certain proteins embedded in the membrane are selective for specific substances. For example, ion channels allow only certain ions (such as sodium or potassium) to pass through, while carrier proteins bind to molecules (such as glucose) and change shape to transport them into or out of the cell.
- Active and Passive Transport: In passive transport, substances move along their concentration gradient without energy input (e.g., diffusion or facilitated diffusion). In contrast, active transport requires energy (usually in the form of ATP) to move substances against their concentration gradient, ensuring that important molecules are concentrated within the cell.
By carefully controlling what enters and exits, selective permeability ensures the cell can perform vital functions like nutrient uptake, waste removal, and maintaining appropriate ion concentrations.