Biological membraneA biological membrane or biomembrane is an enclosing or separating tissue which acts as a barrier within or around a cell. It is a lipid bilayer, composed of a double layer of lipid-class molecules, specifically phospholipids, with occasional proteins intertwined, some of which function as channels. In organism, all biological membranes are in a liquid crystalline condition.

Phospholipids bilayer Phospholipids contain a hydrophilic head and a

nonpolar hydrophobic tail. Hydrogen bonds form between the phospholipids "heads" and the watery environment inside and outside of the cell. Hydrophobic interactions force the "tails" to face inward. Phospholipids are not bonded to each other, which makes the double layer fluid. Cholesterol embedded in the membrane makes it stronger and less fluid

Proteins embedded in membrane serve different functions

1. Channel Proteins - form small openings for molecules to diffuse through

2. Carrier Proteins- binding site on protein surface "grabs" certain molecules and pulls them into the cell

3. Receptor Proteins - molecular triggers that set off cell responses (such as release of hormones or opening of channel proteins)

4. Cell Recognition Proteins - ID tags, to identify cells to the body's immune system

5. Enzymatic Proteins - carry out metabolic reactions

Properties The membranes typically define enclosed

spaces or compartments in which cells may maintain a chemical or biochemical environment that differs from the outside.

• The most important feature of a biomembrane is that it is a selectively permeable structure. This means that the size, charge and other chemical properties of the atoms and molecules attempting to cross it will determine whether they succeed to do so. Selective permeability is essential for effective separation of a cell or organelle from its surroundings.

• Biological membranes also have certain mechanical (elastic) and electric properties

Passive transportPassive transport is a moving atomic molecular and ion substances, across membranes. This process does not involve chemical energy. The four main kinds of passive transport are diffusion, facilitated diffusion, filtration and osmosis.

Diffusion is the net movement of material from an area of high concentration of that material to an area with lower concentration. The difference of concentration between the two areas is often termed as the concentration gradient, and diffusion will continue until this gradient has been eliminated. This processes is described by Fick’s law:

where J is the diffusion flux, D is the diffusion coefficient, c is the concentration, x is the position.

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Moving of the charged particles in solute through membrane depends not only from , but also of electric gradient. Set of concentration and electric gradients refers to as an electrochemical gradient. According to the Nernst-Plank equation:

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where J is a flux of substance, A is an area of diffusion, c is a concentration of substance, F is a Faraday's const, Z is a valence of substance, φ is an electric potential.

Facilitated diffusion is movement of molecules across the cell membrane via special transport proteins that are embedded within the cellular membrane. Many large molecules, such as glucose are insoluble in lipids and too large to cross through the membrane. Therefore, they are binds with its specific carrier proteins, and the complex will then be moved through the cellular membrane.

Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the cardiovascular system. Depending on the size of the membrane pores, only solutes of a certain size may pass through it.

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Osmosis is the diffusion of a solvent across a membrane to a region of higher solute concentration. (In biological processes then, it usually is diffusion of water molecules). Most cell membranes are permeable to water, and since the diffusion of water plays such an important role in the biological functioning of any living being.

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In red blood cells, the cytoplasmic membrane is permeable to both Na+ and K+ ions but the K+ ion concentration inside the membrane is greater than outside. For Na+ the reverse is true. Thus an ionic gradient is maintained by a specific transport system known as the Na+ - K+ pump. This ionic gradient is crucial for signal transmission through nerves.

Active transport

This difference in K+ and Na+ ion concentration can be maintained only by active transport and in fact the Na+ - K+ pump maintains the gradient at the expense of hydrolysis of ATP. Thus active transport is coupled with the process that supplies energy. Under suitable conditions, this pump can be reversed to synthesis ATP from ADP and Pi. A model has been proposed for the Na+ - K+ pump.

The model takes into account the discovery, by Jens Skou, of an enzyme which hydrolyses ATP and which is present among the molecules that go to make up the pump mechanism. According to this model, hydrolyzing enzyme in Na+ - K+ pump must be in a position to adopt two conformations, and the affinity for transported species must be different for the two conformations. These are such that the cavity, or the site at which the species to be transported binds, is on the outside for one conformation and on inside for the other.

Sodium-Potassium pump, an example of Primary active transport

Active transport is the transport of biochemical, and other atomic/molecular substances, across membranes. Unlike passive transport, this process requires the expenditure of chemical energy. In this form of transport, molecules move against either an electrical or concentration gradient (collectively termed an electrochemical gradient).

The active transport of small molecules or ions across a cell membrane is generally carried out by transport proteins that are found in the membrane. Larger molecules such as starch can also be actively transported across the cell membrane by processes known as endocytosis and exocytosis.

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The free energy spent on carry of one asking substance through a membrane in a direction of higher electrochemical potential:

Any system of active transport will consist of a source of free energy, a carrier of the given substance and a regulator. In the systems of active transport a source of free energy is АТP. Carriers can be small albuminous molecules or large proteins molecules.

Resting potentialIn most cells the resting potential has a negative value, because the negative charge inside compared to outside. The resting potential is mostly determined by the concentrations of the ions in the fluids on both sides of the cell membrane and the ion transport proteins that are in the cell membrane. The resting potential can be calculated with the Goldman-Hodgkin-Katz equation using the concentrations of ions:

Em is the membrane potential, R is the universal gas constant, T is the absolute temperature,

F is the Faraday constant, P is the ppermeability of membrane for:

[Na+]0 is the extracellular concentration of Na,[Na+]i is the intracellular concentration of Na,[K+]o is the extracellular concentration of K, [K+]i is the intracellular concentration of K, [Cl-]o is the extracellular concentration of Cl[Cl-]i is the intracellular concentration of Cl.

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An action potential is a wave of electrical discharge that travels along the membrane of a cell. Action potentials can be created by many types of cells, but are used most extensively by the nervous system for communication between neurons.An action potential is a rapid swing in the polarity of the voltage from negative to positive and back, the entire cycle lasting on the order of milliseconds. Each cycle—and therefore each action potential—has a rising phase, a falling phase, and finally an undershoot.The action potential does not stayl in one location of the cell's membrane, but travels along the membrane