Membrane transports principle

Membrane transports principle

In cellular biology, the term membrane transport refers to the collection of mechanisms that regulate the passage of the solutes such as the ions and small molecules through biological membranes, which are the lipid bilayers and that contain the proteins embedded in them. The regulation of passage through the membrane is due to selective membranes permeability- a characteristic of biological membranes which allow them to separate substances of distinct chemical nature. In other words, they are permeable to certain substances but not other. The movement of most solutes through the membrane are mediated by the membrane transport proteins which are specialized to varying degree in the transport of the specific molecules. As the diversity and the physiology of the distinct cells are highly related to their capacities to attract the different external elements, it is postulated that there is a group of specific transport proteins for each cell type and for the specific physiological stage. This differential expression is regulated through the differential transcription of the genes coding for these proteins and its translation, for instance, through the genetic-molecular mechanisms, but it also at the cell biology level: the production of these proteins can be activated by cellular signaling the pathways, at the biochemical level or by being situated in cytoplasmic vesicles. Thermodynamically the flow of substances from the one compartment to the another can occur in the direction of a concentration or electrochemical gradient or against it. If the exchange of substances occurs in the direction of the gradient, that is in the direction of decreasing the potential, there is no requirement for an input of energy from outside the system; if however, the transport is against the gradient, it will be required the input of energy, the metabolic energy in this case .For example, classic chemical mechanisms for separation that does not require the addition of the external energy in dialysis. In this system, a semi-permeable membrane separates two solutions of different concentrations of the same solute. If the membrane allows the passage of the water but not the solute. The water will move into the compartment with the greatest solute concentration in order to maintain an equilibrium in which the energy of the system is at a minimum. Because of this, the water moves from a high solvent concentration to a low one (in terms of the solute, the opposite occurs) and the water is moving along a gradient there is no need for an external input of energy.

The nature of biological especially that of its lipid, is amphiphilic as they form bilayers that contain an internal hydrophobic layer and the external hydrophilic layer. This structure makes transport possible by the simple or passive diffusion, that consists of the diffusion of substances through the membrane without expending metabolic energy and without the aid of the transport proteins. If the transported substances have a net electrical charge it will move only in response to the concentration gradient, but also to an electrochemical gradient due to the membrane potential. Relative permeability of the phospholipid bilayer to various substances are types of uncharged polar molecules Urea, water,methanol, permeable,that totally or partially large uncharged polar molecules glucose, fructose. Not permeable ions are K⁺, Na⁺, Cl^-, HCO₃^-

Not permeable charged polar molecule ATP, amino acid , glucose-6- phosphate.

Not permeable but few molecules are able to diffuse through the lipid membrane the majority of the transport process a large number of the alpha helices immersed in the lipid matrix. In bacteria, these proteins are present in the beta lamina form. This structure probably involves the conduit through the hydrophilic protein environments that cause a disruption in the highly hydrophobic medium formed by the lipids. That protein can be involved in transport in a number of ways: they act as pumps driven by the ATP that is by metabolic energy, or as channels of facilitated diffusion.

The membrane also maintains the cell potential and the cell membrane thus works as a selective filter that allows only certain things to come inside or go outside the cell. The cell employs the number of transport mechanisms that involve biological membranes.

1. Passive osmosis and diffusion: Some substances such as the carbon dioxide and the oxygen can move across the plasma membrane acts as a barrier for certain molecules and the ions, they occur in different concentrations on the two sides of the membrane. Such as a concentration gradient across the semipermeable membrane sets up the osmotic flow of the water.
2. Transmembrane protein channels and transporters: The nutrients, such as the sugars or amino acids must enter the cell and certain products of metabolism must leave the cell. Such molecules diffuse passively through the protein channels such as aquaporins in facilitated diffusion or are pumped across the membrane by the transmembrane transporters. Protein channel proteins also called permeases are usually quite specific recognizing and transporting the limited food group of chemical substances, often even only a single substance.
3. Endocytosis:The Endocytosis is the process by which cells absorb molecules by engulfing them. The plasma membrane creates the small deformation inward and it is called an invagination, in which the substances to be transported is captured. The deformation that pinches off from the membrane on the outside of the cell, creating a vesicle containing the captured substance. Endocytosis is the pathway for the internalizing solid particles, small molecules and ions and macromolecules. Endocytosis requires energy and is thus a form of active transport.
4. Exocytosis: Just as the material can be brought into the cell by invagination and formation of vesicles, the membrane of a vesicle can be fused with the plasma membrane, extruding its content to the surrounding medium. This is the process of exocytosis. Exocytosis occurs in various cells to remove the undigested residues of substances brought in by endocytosis, to secrete substances such as hormones and enzymes and to transport the substance completely across the cellular barrier. In this process of exocytosis, the undigested waste-containing food vacuole or the secretory vesicle budded from the Golgi apparatus is first moved by cytoskeleton from the interior of the cell to the surface. The vesicle membrane comes in contact with the plasma membrane. The lipid molecule of the two bilayers rearranges themselves as the two membranes are thus fused. A Passage is formed in the fused membrane and the vesicles discharge it contents outside the cell.

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