Do you wonder how molecules are transported inside cells?
The transport of molecules across the cell membrane or in the cell follows two pathways: passive transport and active transport. Active transport is the transport of molecules against a concentration gradient across the cell membrane using transport proteins and energy. In contrast, passive transport is a direct transfer of molecules across the membrane down the concentration gradient.
The transport rate of molecules down the concentration gradient depends on the permeability and fluidity of the cell membrane. These two properties, membrane permeability, and fluidity further depend on the physiological and structural characteristics of the cell membrane.
This article covers cell membrane permeability, methods to measure the rate of permeation of molecules, and the factors that affect the permeability of the cell. You will also get a glimpse of membrane fluidity and the factors that influence this property.
What Are Cell Membrane Permeability and Cell Fluidity?
Cell Membrane Permeability
Permeability of a membrane is the rate of passive diffusion of molecules across the membrane. It’s the ease with which molecules pass through the membrane barrier.
The cell membrane is selectively permeable and only allows specific molecules to enter the cell. This function is essential for the normal functioning of organisms.
Fluidity is defined as the ability or ease of molecules to move in the membrane. It refers to the viscosity of the cell membrane and affects the diffusion of proteins and other molecules inside cells, thus, affecting their function. This property is affected by phospholipid structure, cholesterol composition in the membrane, and temperature.
Cell Membrane Structure
The selective permeability and cell fluidity of the membrane are due to its structural composition. The cell membrane, also known as the plasma membrane or biological membrane, separates the interior from the outer surroundings.
It acts as a structural control barrier in transporting molecules inside the cells for metabolic regulations or other functions of the body. The membrane consists of lipids, protein, carbohydrates, and cholesterol.
The three classes of lipids that compose the membrane’s structure are phospholipids, glycolipids, and sterols. A phospholipid is composed of alcohol (glycerol and sphingosine), fatty acids, phosphate, and alcohol, attached to phosphates.
The two main classes of phospholipids include glycerophospholipids and sphingophospholipids. Other than these two, glycolipids and sterols also have significant contributions to building the membrane structure.
Glycolipids are carbohydrates covalently attached to lipids, while sterols are multi-hydroxy alcohols with multiple ring structures. The principal sterol present in the biological membrane is cholesterol.
It’s an integral part of the cell membrane. The two types of proteins involved in the structural formation of membranes are intrinsic membrane protein and extrinsic membrane protein.
Intrinsic membrane proteins are either completely or partially embedded in the lipid bilayer of the membrane. In contrast, extrinsic membrane proteins are loosely bound proteins attached to the surface of the membrane.
They are present on the surface of the membrane. They are either glycoproteins (carbohydrates attached to proteins) or glycolipids (carbohydrates attached to lipids).
Carbohydrates on the surface of plasma membranes consist of 2 to 60 monosaccharide units that can be straight or branched. They coat the cell surface and protect it from any damage or harm and mediate cell-cell adhesion events.
Figure: An illustrative diagram of cell membrane showing its different structural elements.
Factors Affecting Cell Membrane Permeability
1. Cell Membrane Composition
The two main players in the cell membrane that affects its permeability are:
- The predominance of saturated or unsaturated fatty acids in the membrane: Shorter chain lengths and higher unsaturated fatty acids increase the membrane’s permeability. The short chains have less surface area because of which the interaction between hydrocarbon chains is reduced. While the unsaturated fatty acids have kinks that lead to lesser Van der Waals interaction with other lipids.
The saturated fatty acid tightly packs the protein bilayer, making it difficult for the molecules to pass across the membrane. And, the bulky hydrophilic parts of the hydrophilic groups of phospholipids trap the polar and charged molecules, restricting their movement through passive diffusion.
- The amount of cholesterol in the membrane: In eukaryotes, the higher the cholesterol concentration, the lower the membrane permeability.
- Similarly in the case of fluidity, shorter fatty acid chains and a higher amount of unsaturated bonds between carbon atoms of fatty acids increase the membrane fluidity. Also, an increase in cholesterol concentration decreases membrane fluidity by restricting the motions of membrane molecules. This property is also dependent on temperature. For example, low temperature decreases the effect of high cholesterol concentration and permits better membrane fluidity.
Figure: An illustration of the effect of saturated and unsaturated carbons on membrane fluidity.
An increase in the temperature increases membrane permeability. At a freezing 0 degrees temperature, the phospholipids in the membrane are tightly packed and become rigid, and this decreases the permeability. However, increasing the temperature up to 45 degrees celsius increases the permeability because of the loosening of the phospholipid structures in the membrane.
- But increasing the temperature above 45 degrees celsius denatures the membrane protein, increases the water expansion inside the cell, and exerts pressure on the membrane. This deformation of the membrane structure makes the membrane lose its control over what enters or leaves the cell.
- For membrane fluidity: At higher temperatures, the membrane is more fluid than at lower temperatures. At low temperature, the membrane solidifies or becomes static because of the restriction in the movement of molecules that compose the membrane.
3. The pH of the cell surrounding/membrane
The normal pH of the cell is 7. Increasing or decreasing the pH of a cell surrounding by 1 unit, that is, to 6 and 8, disrupts the structural forms of the membrane, reduces the permeability of the membrane, and disrupts its function.
4. Polarity, electric charge, and molecular mass of molecules passing through the membrane
Smaller molecules without any charges, such as CO2, N2, O2, and molecules with high solubility in fat such as ethanol, have the fastest rate of diffusion. These molecules can easily move across the membrane through passive diffusion without any membrane protein or energy.
The permeability decreases for molecules having charges or those bigger in size. These structural features of molecules don’t have any effect on membrane fluidity.
Figure: A representation of permeability of molecules across the membrane, based on their size and charges.
Scientific Approaches to Artificially Improve Membrane Permeability
The permeability of the cell membrane plays an essential role in the transport of molecules. Scientists are working to alter the membrane’s permeability for its application in enhancing and smoothing the process of drug delivery in cells at the target location.
That’s why scientists have designed certain approaches to enhance membrane permeation and achieve drug delivery, some of which are explained below:
1. Small Molecule Cargo
Membrane permeation is an essential factor considered while designing principles that maximize obtaining a drug and its distribution in the organisms. Better permeability increases the bioavailability (concentration of a chemical that is available for biological action) of the drug molecule.
In such efforts, conjugating compounds to known transporter substrates are designed to have improved permeation, releasing the drug molecule in the membrane.
2. Peptide Cargo
Peptides can not cross the membrane passively, and that’s why scientists are trying to alter their physical properties like conformational flexibility and polarity to improve their permeability across the membrane.
For example, scientists proposed that cyclizing a given peptide and methylating nitrogen involved in the amide bond can improve the permeation of the peptide.
Furthermore, in some cases, cyclization by changing the peptide’s α-helical content and the presence of arginine residue within α-helices is also found to have increased permeability.
3. Protein Cargo
The size and polarity of protein molecules make it difficult for them to cross the membrane passively. Due to the vast differences between different proteins’ physicochemical properties, there’s no generalized technique to enhance their permeability. Given below are some approaches introduced by scientists to ease the delivery of proteins across the membrane:
- Mechanical disruption of the membrane: Various physical methods disrupt the cell membrane to deliver the proteins across the membrane. It includes microinjection, electroporation, a microfluidic device that disrupts the plasma membrane through physical constriction, and nanowires that pierce the cell membrane.
- Peptide-based strategies: Cell-penetrating peptides are short peptides with 30 or fewer amino acid residues and have membrane penetrating functions. It can transport molecules like small macromolecular drugs, nucleic acids, proteins, viruses, and imaging agents across plasma membranes of eukaryotic cells through energy-independent pathways. Some examples of such peptides are the TAT peptide that translocates covalently coupled peptides and an amphiphilic CPP Pep-1 peptide that forms a noncovalent complex with other deliverable peptides and proteins.
- Protein-based strategies: It’s an approach to translocate exogenous proteins using various pore- or channel-forming proteins of bacterial origin. Examples include an engineered bacterial channel (MscL) and cholesterol-dependent cytolysin (CDC) family of pore-forming toxins. Also, certain human proteins and supercharged GFP (Green Fluorescent Proteins) having high net positive charges can translocate proteins across membranes.
- Virus-based strategies: This strategy involves packaging proteins in virus-like particles and attaching them to an engineered bacteriophage T4 head to enhance the delivery of proteins to the cytoplasm.
- Lipid- and polymer-based strategies: This method encapsulates protein cargo in liposomes or complexes with proteins. For example, a mixture of cationic and neutral lipids has been found to translocate negatively charged proteins across the plasma membrane. Moreover, polymer-based compounds like polyethyleneimine (PEI) can deliver proteins through the proton sponge effect. In this effect, the polymer-based compound rich in protonable amines causes buffering of protons and osmotic swelling in endosomes, which stalls its maturation and rupture to deliver the protein molecules. Nanocapsules have also been reported to deliver protein and transcription factors by degrading themselves in response to protease activity or changes in pH or redox potential.
- Inorganic material-based strategies: Different inorganic materials, such as silica, carbon nanotubes, quantum dots, and gold nanoparticles, have been reported to translocate protein cargo.
Cell permeability and cell fluidity are two essential properties, having roles in transporting molecules across the membrane. These properties are affected by physiological factors like temperature, pH, and the membrane’s composition. Any extremity of these factors disrupts the membrane molecules that build the membrane, thus, affecting the normal functioning of the organism.
The permeability of molecules is a trending research topic among scientists because of its role in drug delivery. They are working to alter the properties of the membrane or create certain approaches that can enhance drug delivery in cells and treat certain diseases.
Some of the methods that have been or are being tested include mechanical disruption of the membrane, virus-based strategies, protein-based strategies, and lipid and polymer-based strategies.
These approaches are still not frequent in practice and need serious consideration and in-depth analysis before bringing them in for actual medical practices. But they do show promising potential in the medical field.
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