Membrane Structure and Function

Selective Permeability of Biological Membranes

Biological membranes possess selective permeability, a property that allows some substances to cross more easily than others. This selectivity is essential for regulating the transport of materials across cellular boundaries, which is crucial for cell survival and function.

• Selective permeability enables cells to control the internal environment by regulating the entry and exit of substances.

• The fluid mosaic model describes the structure of cell membranes, highlighting the dynamic arrangement of lipids and proteins that regulate molecular traffic.

• Small molecules and ions move across the plasma membrane in both directions, facilitating essential chemical exchanges.

• Examples of exchanges include:

  • Uptake of sugars, amino acids, and nutrients into the cell.

  • Removal of metabolic waste products from the cell.

  • Intake of O2 for cellular respiration and expulsion of CO2.

  • Regulation of inorganic ion concentrations such as Na+, K+, Ca2+, and Cl-.

• Despite high traffic, membranes are selective, allowing only certain molecules and ions to pass while excluding others.

The Permeability of the Lipid Bilayer

The lipid bilayer forms the fundamental structure of biological membranes and determines much of their permeability properties.

• Nonpolar molecules (e.g., hydrocarbons, O2, CO2) are hydrophobic and can dissolve in the lipid bilayer, crossing the membrane easily without the aid of membrane proteins.

• The hydrophobic interior of the membrane impedes the passage of ions and polar molecules, which are hydrophilic.

• Polar molecules (e.g., glucose, other sugars) pass through the lipid bilayer slowly.

• Water, though small and polar, crosses the membrane less rapidly than nonpolar molecules.

• Charged atoms or molecules (ions) with their hydration shells are even less likely to penetrate the hydrophobic core of the membrane.

• Membrane proteins are essential for regulating the transport of many substances, contributing to selective permeability.

Transport Proteins

Transport proteins facilitate the movement of specific ions and polar molecules across cell membranes, allowing them to avoid direct contact with the hydrophobic lipid bilayer.

• Channel proteins provide hydrophilic channels that certain molecules or ions use as tunnels through the membrane.

• Aquaporins are a type of channel protein that specifically facilitate the passage of water molecules.

  • Each aquaporin consists of four identical polypeptide subunits, each forming a channel for water to pass through in single file.

  • Aquaporins can allow the entry of up to 3 billion water molecules per second (inferred typical value).

  • Without aquaporins, only a tiny fraction of water molecules would cross the membrane.

• Carrier proteins bind to their passengers and change shape to shuttle them across the membrane.

• Transport proteins are specific for the substances they move, allowing only certain molecules or closely related groups to cross.

• Example: The glucose carrier protein in red blood cells transports glucose across the membrane 50,000 times faster than it would diffuse on its own, and is highly selective (rejects fructose, a structural isomer of glucose).

Determinants of Selective Permeability

• The selective permeability of a membrane depends on:

  • The discriminating barrier of the lipid bilayer.

  • The specific transport proteins embedded in the membrane.

• The direction and mechanism of molecular traffic across membranes involve two main modes:

  • Passive transport (does not require energy input).

  • Active transport (requires energy input, usually from ATP).

Concept Check: Membrane Permeability and Transport Proteins

• O2 and CO2 can cross the lipid bilayer without membrane proteins due to their small size and nonpolar nature, allowing them to diffuse through the hydrophobic core.

• Transport proteins are necessary for the rapid movement of water molecules across the membrane because water is polar and cannot easily pass through the hydrophobic lipid bilayer. Aquaporins facilitate this process.

• Aquaporins exclude hydronium ions (H3O+) but allow glycerol and H2O due to the specific size and charge properties of the channel. The selectivity is based on the channel's preference for neutral molecules over charged ions.

Summary Table: Types of Molecules and Membrane Permeability

Additional info: Typical aquaporin water transport rates and the distinction between passive and active transport were inferred for completeness.