Permeability is the property of a material that describes the ease with which a fluid or solute can pass through it. This applies across fields, from the flow of groundwater through rock layers to the transit of molecules across a cell membrane. In a biological context, the selective permeability of cell membranes allows a cell to maintain its internal environment, regulating the intake of nutrients and the expulsion of waste. Understanding permeability guides the development of drug delivery systems and is integral to environmental filtration processes.
Characteristics of the Permeating Substance
The chemical and physical properties of the molecule attempting to cross a barrier are the first determinants of its ability to permeate. A primary factor is the molecule’s size, which is inversely related to its permeability across a lipid bilayer. Smaller molecules, such as oxygen and carbon dioxide, can navigate the tight packing of the barrier more easily than larger ones. For unassisted passive diffusion across a biological membrane, permeability decreases sharply for molecules with a molecular weight above approximately 100 to 150 g/mol.
The second major factor is the substance’s lipid solubility, or hydrophobicity. Since the core of a cell membrane is composed of nonpolar lipid tails, highly lipid-soluble, nonpolar molecules can readily dissolve into and diffuse through this environment. Substances like steroid hormones, which are nonpolar, pass through with relative ease, making lipid solubility a strong predictor of membrane permeability.
Electrical charge and polarity present significant resistance to permeation through the nonpolar membrane interior. Charged ions, such as sodium or potassium, and large polar molecules, like glucose, are repelled by the hydrophobic environment of the lipid bilayer. These molecules require the assistance of dedicated transport proteins embedded within the barrier to facilitate their passage. Even a small increase in polarity can drastically reduce a molecule’s ability to cross the membrane without specialized help.
The Nature of the Barrier
The physical and chemical composition of the barrier itself is a crucial element in determining permeability. In biological systems, the composition of the phospholipid bilayer directly influences its fluidity and, consequently, its permeability. Membranes rich in saturated fatty acids have straight hydrocarbon chains that pack tightly together, creating a more rigid and less permeable barrier.
Conversely, the presence of unsaturated fatty acids introduces kinks into the hydrocarbon chains, which prevents close packing and increases the overall membrane fluidity. This less-ordered arrangement creates more space between the lipids, allowing molecules to pass through the membrane more easily. Cholesterol, a lipid component in many eukaryotic membranes, acts as a fluidity buffer, generally reducing the permeability to small water-soluble molecules by filling gaps between phospholipids.
The thickness of the barrier also plays a role, as a substance must travel a greater distance through a thicker membrane to cross it. A thicker barrier increases the time required for a molecule to diffuse across, although minor variations in thickness may not significantly alter permeability. Furthermore, the presence of specific protein structures within the barrier can fundamentally change its selective permeability.
These embedded proteins function as selective gates, channels, or carriers that facilitate the transport of specific molecules that cannot cross the lipid core on their own. For example, aquaporins are channel proteins that permit the rapid, selective passage of water molecules across the membrane. The number and activity of these specialized transport proteins dictate which polar or charged substances can enter or exit the cell, making the protein profile a defining characteristic of the barrier’s permeability.
Influence of the External Environment
Factors external to the molecule and the static barrier structure can modify the rate at which permeation occurs. Temperature is a significant environmental variable that affects the physical state of the barrier. As temperature increases, the kinetic energy of the phospholipid molecules in a membrane rises, causing them to move more vigorously.
This increased movement leads to greater membrane fluidity, which typically results in an increase in permeability. However, if the temperature becomes excessively high, the proteins embedded in the membrane may begin to denature, causing structural damage that can lead to an uncontrolled, drastic increase in permeability. Conversely, very low temperatures reduce the kinetic energy of the lipids, causing the membrane to become rigid and decreasing its permeability.
The pH of the surrounding environment can also influence permeability by altering the charge of a permeating substance. A change in pH can shift a molecule’s ionization state, causing a neutral, nonpolar molecule to become a charged, polar ion, or vice versa. Since charged species have difficulty crossing the hydrophobic barrier, this external pH change fundamentally modifies the substance’s ability to permeate. Extreme pH levels can also damage the membrane proteins themselves, affecting their ability to function as selective transporters.
Finally, the concentration gradient acts as the primary energetic driving force for passive movement across any permeable barrier. A steep concentration gradient drives molecules from the area of higher concentration to the area of lower concentration, resulting in a faster rate of passive permeation. Without a sufficient gradient, even highly permeable substances will move slowly or not at all.