A polarized membrane is a

lipid membrane The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes form a continuous barrier around all cells. The cell membranes of almost all organisms and many viruses are made of a l ...

that has a positive

electrical charge Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

on one side and a negative charge on another side, which produces the

resting potential The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as opposed to the specific dynamic electrochemical phenomena called action potential and graded membrane potential. The re ...

in living cells. Whether or not a membrane is polarized is determined by the distribution of dissociable

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

s and permeant

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s inside and outside the membrane that travel ''passively'' through

ion channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by Gating (electrophysiol ...

or ''actively'' via ion pump, creating an

action potential An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

Structure and composition

Polarized membranes consist of a

phospholipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes form a continuous barrier around all cells. The cell membranes of almost all organisms and many viruses are made of a l ...

, with embedded membrane proteins that aid in molecular transport and membrane stability as well as

lipids Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E, E and Vitamin K, K), monoglycerides, diglycerides, phospholipids, and others. The fu ...

that primarily aid in structure and compartmentalization of membrane proteins. The

amphiphilic In chemistry, an amphiphile (), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'', nonpolar) properties. Such a compound is called amphiphilic or amphipathic. Amphiphilic c ...

nature of the

phospholipid Phospholipids are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...

s creates the bilayer structure of the membrane. These

phospholipids Phospholipids are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typi ...

contain a

hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are n ...

head region with a

phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

bonded to a variety of

functional groups In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...

. This head region is localized to face the extracellular space outside of the cell as well as the intracellular,

cytosolic The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

region of the cell. The

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

phospholipid tail region consists of a chain of carbon molecules bound to hydrogen with two categories: saturated or unsaturated. Illustration of Polarized Membrane

The polarization of cellular membranes are established and maintained through the ''active'' and ''passive'' transport of ions across the membrane through membrane proteins, specifically channel proteins and ion pumps. These proteins maintain an

electrochemical gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts: * The chemical gradient, or difference in Concentration, solute concentration across ...

by pumping certain

s in and out of the cell. This

gradient In vector calculus, the gradient of a scalar-valued differentiable function f of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p gives the direction and the rate of fastest increase. The g ...

of ions lead to a positive charge on one side and a negative charge on the other. The primary mechanism for generating this electrochemical gradient is the activity of the sodium-potassium pump (Na/K ATPase), which utilizes ''active'' transport to pump two

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

(K+) ions into the cell and three

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

(Na+) ions out of the cell per cycle. This is a P-class protein, meaning it is

phosphorylated In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writt ...

in the process and utilizes

adenosine triphosphate Adenosine triphosphate (ATP) is a nucleoside triphosphate that provides energy to drive and support many processes in living cell (biology), cells, such as muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all known ...

(ATP) as an energy source. Ion channels, which are specific in which ions are allowed to pass through them, are also crucial to polarization and maintaining polarization. Voltage-gated ion channels activate or deactivate in response to changes in membrane potential, allowing various ions to flow down their concentration gradient according to the channel's specificity. These channels are crucial in the propagation and transduction of

s in the nervous system, when transient activation and deactivation of said ion channels enable signal transduction.

Role in Cellular Functions

Polarized membranes are key cellular components that aid in facilitating compartmentalization, cell-to-cell communication, and signaling. Cells actively utilize polarized membranes to form and maintain electrochemical gradients and regulate both intracellular and extracellular environments. Polarization is crucial in a variety of cells, but especially important in neurons. In neurons of the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

and

peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system of Bilateria, bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside t ...

, polarized membranes allow for the propagation and transduction of

s. As explained above, this polarization is maintained by sodium-potassium pumps and a host of other ion channels ensuring an electrochemical gradient is sustained. These action potentials signal neurons to release

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotra ...

s leading to further polarization of afferent neurons or a direct response to a signal. Polarized membranes also function to maintain intracellular homeostasis in

lysosome A lysosome () is a membrane-bound organelle that is found in all mammalian cells, with the exception of red blood cells (erythrocytes). There are normally hundreds of lysosomes in the cytosol, where they function as the cell’s degradation cent ...

s. Lysosomes are cell organelles that are highly acidic and store

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalysis, catalyzes proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of new protein products ...

s that aid in degradation of spent organelles or various biological polymers. The acidic nature of the inside of the lysosome is maintained by

proton pump A proton pump is an integral membrane protein pump that builds up a proton gradient across a biological membrane. Proton pumps catalyze the following reaction: : n one side of a biological membrane/sub> + energy n the other side of the m ...

s which actively pump hydrogen ions into the lysosome. This mass movement of ions in and out of the lysosome lead to a polarized lysosomal membrane, with a negative resting potential.

Types of Polarized Membranes

Plasma Membranes

Plasma membranes exhibit electrochemical polarity through establishment and maintenance of a resting membrane potential. Cells with polarized plasma membranes must buffer and adequately distribute certain ions, such as sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and chloride (Cl⁻) to establish and maintain this polarity. Integral channel proteins such as the sodium-potassium pump actively maintain the electrochemical gradient through movement of sodium and potassium ions. Voltage-gated ion channels in

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

s allow for transient changes in membrane potential, giving way for signal transduction through

s and/or

release.

Organelle Membranes

Mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

present in all cells in the human body require a resting membrane potential of the

inner mitochondrial membrane The inner mitochondrial membrane (IMM) is the mitochondrial membrane which separates the mitochondrial matrix from the intermembrane space. Structure The structure of the inner mitochondrial membrane is extensively folded and compartmentalized. T ...

to synthesize

(ATP). This membrane polarity is established through a series of

s transporting hydrogen ions into the mitochondrion. This transport of hydrogen ions is a crucial component of energy storage during

oxidative phosphorylation Oxidative phosphorylation(UK , US : or electron transport-linked phosphorylation or terminal oxidation, is the metabolic pathway in which Cell (biology), cells use enzymes to Redox, oxidize nutrients, thereby releasing chemical energy in order ...

. The same mechanism is used to acidify

s and acidify the intraorganellar space of the lysosome.

Membranes in Excitable Cells

Excitable cells, or cells that have the unique ability to generate and transduce electrical impulses, employ polarized membranes with highly fluctuant electrochemical gradients. Neurons and muscle fiber cells are the primary examples of excitable cells. Neuronal cells generate electrical signals through activation of voltage-gated ion channels, while muscle fiber cells use similar methods to coordinate muscle contraction.

Photosynthetic Membranes

Polarized membranes are not uniquely localized to humans or mammals alone. In plants, thylakoid membranes within the chloroplast generate and maintain electrochemical gradients vital for the photosynthetic pathway. Light-dependent reactions generate a proton gradient similar to that found in

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

, providing the necessary gradient needed to power ATP synthesis. This polarity generated by light-dependent reactions are converted into ATP and NADPH during the

Calvin cycle The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into ...

Technology and Techniques for Studying Polarized Membranes

Researchers and scientists alike use various techniques in order to study the properties and behaviors of these polarized membranes. These techniques and tools allow for precise measurements of individual ion channels and electrochemical gradients of certain ions across a cell's membrane. This area of research is highly beneficial in understanding the principles of excitable cells and allowing innovations in biotechnology and medical treatments.

Patch-Clamp Electrophysiology

Patch-clamp The patch clamp technique is a laboratory technique in electrophysiology used to study ionic currents in individual Cell isolation, isolated living cells, tissue sections, or patches of cell membrane. The technique is especially useful in the st ...

recording was a major innovation in the scientific community as it allowed for the measurement of the properties of one or a few ion channels in real time. This technique employs a glass micropipette with a tiny internal diameter that forms a tight seal on the surface of the cell's membrane (gigaohm seal). This gigaohm seal allows for measurement of current flow across a very small patch of membrane. Voltage clamp is a variation of

patch clamp The patch clamp technique is a laboratory technique in electrophysiology used to study ionic currents in individual Cell isolation, isolated living cells, tissue sections, or patches of cell membrane. The technique is especially useful in the st ...

recording in which the membrane is held at a constant potential and measuring the current required to do so. This information is used to characterize the currents that underlie the action potential.

Fluorescent Imaging Techniques

Ion sensitive fluorescent dyes provide both spatial and temporal imaging of electrochemical gradients of certain ions (e.g. calcium or potassium). These

fluorescent Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with color ...

dyes change in intensity depending on concentrations of certain ions which are directly correlated to membrane potential. Fluo-4 is a calcium imaging fluorescent dye used to track intracellular calcium concentrations in living cells. This tracking is especially helpful in measuring growth and maturation of neuronal networks in learning as calcium is a key signaling and regulating molecule.

References

{{DEFAULTSORT:Polar Membrane Membrane biology Electrophysiology