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The cell. 3. Cell membrane.

TRANSPORT

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Pumps, channels and transporters are proteins that allow electrically charged molecules and ions to cross cell membranes.

Pumps create gradients of ions by using energy. Other molecules, such as ATPase can use gradients to produce ATP.

Transporters are able to translocate charged molecules from one side of the membrane to the other by using concentration gradients.

Channels have hydrophilic passages, and ions can move throughout these passages. It is a passive flux of ions ruled by membrane electrochemical gradients.

Membranes are barriers against free diffusion of ions and electrically charged molecules. However, ions and many charged molecules are needed inside de cell or in membrane-limited compartments, so that they have to cross the cell membranes. For example, glucose has to enter in the cell to be used as energy supply, and ions are needed for electrochemical gradients. As we mentioned in the previous pages, concentration gradients in membranes are used for many cell functions, and cells need to create, regulate, and break such gradients. There are many types of proteins inserted in membranes which are specialized in the use of these gradients, for example, for transporting molecules needed for metabolism, or allowing the interchange of ions between both sides of membranes to create and modify gradients. All of them are transmembrane proteins: pumps, transporters and channels.

Pumps
Some examples of ion transport, mediated by proteins, propelled by different energy sources. These proteins are known as pumps. The first example (on the left) shows a protein complex from the respiratory chain of mitochondria. The next one is a bacteriorhodopsin which uses visible light to move protons across the membrane. On the right, a pump is shown, which interchanges sodium and potassium across membrane, and contributes to create gradients in the plasma membrane (modified from Alberts et al., 2002).

Pumps are transmembrane proteins for moving ions or small molecules against their concentration gradient from one side of the membrane to the other. The energy for these movements may be obtained from different sources, such as light, oxidoreduction chemical reactions, and from ATP hydrolysis, which is the most frequent source. Pumps transform chemical or electromagnetic energy in electrochemical gradients that are used by many cellular processes. There are not many different classes of pumps, and can be classified according to their source of energy. a) Light. For example, bacteriorhodopsin uses light to create a proton gradient in membranes of some prokaryotes. b) Oxidoreduction potential. NADPH dehydrogenase complex of the electron transport chain of mitochondria uses the oxidation of the NADPH to transport protons from the mitochondrial matriz to the intermembrane space, so the gradient is created in the inner mitochondrial membrane. c) ATP. There are several types of pumps in this group. Some of them introduce protons in organelles, such as those present in the lysosomal membranes, a process that produces acidification of the lysosomal lumen and facilitates the degradation of molecules by the acid hydrolases. To this group also belong both mitochondrial and chloroplast ATPases, and they use the proton gradient for synthesizing ATP, although can also perform the opposite process (generate proton gradient by ATP) in some circumstances. Another type of pumps uses ATP for transporting several ions. The sodium/potassium pump is in charge of the concentration gradients of the plasma membrane that make possible neuronal excitability, cell muscle contraction, absorption of digested molecules from the intestine, and many more. We can imagine how important this pump is considering that it consumes up to 25 % of the total cellular ATP of animal cells. To this family of pumps also belong those pumps responsible for the transport of cations, such as calcium. For example, one of them takes calcium out of the endoplasmic reticulum during every muscle cell contraction. Finally, there are the ABC pumps that use ATP to move a large variety of molecules across membranes. They are present in all studied eukaryotic cells, and are able to transport ions, carbohydrates, amino acids, and small polypeptides.

Transporters
Drawing showing the type of transport that is carried out by transporters. It can be uniport if they move molecules in only one direction with the energy of their concentration gradient. Another type is cotransport, where two types of molecules are moved across membranes, usually propelled by concentration gradient of one of them. Cotransport may be symport if the two type of molecules are move in the same direction or antiport if they are moved in opposite directions (modified from Alberts et al., 2002).

Transporters are transmembrane proteins that use electrochemical gradients to move molecules across membranes. This type of movement is known as facilitated diffusion. It is passive because the movement is propelled by the existing electrochemical gradient and it is facilitated because the transporter is the way that allows molecules to cross the membranes. Transporters are very abundant, more than 100 families, and can be found in all cell membranes. During the molecular movement across the membrane, a recognition of the molecule by the transporter is first needed, and then a conformational change of the transporter occurs, allowing the transfer of the molecule across the membrane. The movement may be in both directions and more than one type of transported molecule may be involved. Uniport transport is when one molecule type is moved by concentration gradient. Cotransport is when two types of molecules are transported at the same time. Cotransport may be antiport, when the two types of molecules travel across membrane in opposite directions, and symport, when they both are moving in the same direction. During co-transportation, one type of molecule is propelled by gradient concentration, whereas the other uses this energy to move against its own gradient concentration. For example, cardiomyocytes use the Na+/Ca2+ transporter to move Ca2+ against gradient concentration from the cytosol to the extracellular space by using the energy of the incoming movement of Na+ from the extracellular space to the cytosol by gradient concentration. During cotransport, similar elements are interchanged: cation for cation, anion for anion, sugar for sugar, and so on. However, different types of elements can be transported during symport processes. For example, in intestine epithelium, enterocytes use incoming stream of Na+ to enter D-glucose.

Channels
Ionic channels contain hydrophilic passages that allow ions to cross membranes moved by concentration gradient. These channels can be opened and closed. For example, voltage dependent channels are opened or closed depending on membrane potential (electric charges distribution at both sides of the membrane), but others open and let ions pass the membrane when a ligand, such as a neurotransmitter, is recognized by their extracellular domain (modified from Alberts et al., 2002).

Channels are transmembrane proteins containing hydrophilic passages that communicate both sides of the membrane. They have the ability to open and close the passage depending on certain signals. The main function of channels is to handle the ionic gradients for modifying the membrane potential, so that changes turn into information for the cell. They are also needed for secretion and absorption of substances, as in the kidneys. Anyway, it is always a passive movement of ions through the channels, since ions move by gradient concentration. The selection of ions going through the channels depends of the diameter of the hydrophilic passage. There is wide variety of channels: Na+ channels, Ca2+ channels, K+ channels, Cl-, and some others. Aquaporin is a different type of channel that allows water to cross the outer mitochondrial membrane without restrictions. As we mentioned above, channel closing and opening may be regulated. If this regulation is produced by changes in the value of the membrane potential, channels are known as voltage gated channels. They may be also ruled by ligands (ligand-gated channels) or by chemical modifications, such as phosphorylation. Channels carry out essential functions for organisms such as neuronal excitability, polyspermy prevention (avoiding more than one sperm fuses with the oocyte during fertilization), etcetera.


Bibliography

Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. 2002. Molecular Biology of the Cell, 4th edition. New York: Garland Science. SBN-10: 0-8153-3218-1


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Updated: 23-04-2016. 18:04