Symporter

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Comparison of transport proteins Porters.PNG
Comparison of transport proteins

A symporter is an integral membrane protein that is involved in the transport of two (or more) different molecules across the cell membrane in the same direction. The symporter works in the plasma membrane and molecules are transported across the cell membrane at the same time, and is, therefore, a type of cotransporter. The transporter is called a symporter, because the molecules will travel in the same direction in relation to each other. This is in contrast to the antiport transporter. Typically, the ion(s) will move down the electrochemical gradient, allowing the other molecule(s) to move against the concentration gradient. The movement of the ion(s) across the membrane is facilitated diffusion, and is coupled with the active transport of the molecule(s). In symport, two molecule move in a 'similar direction' at the 'same time'. For example, the movement of glucose along with sodium ions. It exploits the uphill movement of other molecules from low to high concentration, which is against the electrochemical gradient for the transport of solute molecules downhill from higher to lower concentration.

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Examples

Robert K. Crane and his sketch of the sodium-glucose symporter Dr Robert K. Crane and his sketch for coupled cotransport.png
Robert K. Crane and his sketch of the sodium-glucose symporter

SGLT1 in the intestinal epithelium transports sodium ions (Na+) and glucose across luminal membrane of the epithelial cells so that it can be absorbed into the bloodstream. This is the basis of oral rehydration therapy. If this symporter did not exist, individual sodium channels and glucose uniporters would not be able to transfer glucose against the concentration gradient and into the bloodstream.

Na+/K+/2Cl symporter in the loop of Henle in the renal tubules of the kidney transports 4 molecules of 3 different types; a sodium ion (Na+), a potassium ion (K+) and two chloride ions (2Cl). Loop diuretics such as furosemide (Lasix) act on this protein.

Marine invertebrates use symporters to transport against strong chemical gradients. Amino acids and sugars are taken up from sea water in the presence of extracellular sodium and is driven by the NA+/K+-ATPase pump. [1]

In the roots of plants, the H+/K+ symporters are only one member of a group of several symporters/antiporters that specifically allow only one charged hydrogen ion (more commonly known as a proton) and one charged K+ ion. This group of carriers all contribute to modulate the chemiosmotic potential inside the cell. Initially H+ is pumped into the area outside the root by H+ ATPase. This change in both the pH and electrochemical potential gradient between the inside of the cell and the outside produces a proton-motive force, as the protons will want to naturally flow back into the area of low concentration and with a voltage closer to zero from their current situation of being in an area of high concentration of positively charged protons.

The reasons for this are twofold. For one, substances in nature have a tendency to move from areas of high concentration to areas of low concentration, as is evident by dropping a drop of food coloring in a glass of water. It does not aggregate, but begins to move from the highly concentrated areas (the colored areas) to the areas of low concentration (clear areas). Second, large groups of predominantly positively charged or negatively charged particles will naturally repel each other. This is demonstrated by attempting to push the two positive poles or two negative poles of a magnet together. Depending on the strength of the magnet, the repulsion may be so strong that it is impossible to push the magnets together unless aided by machinery. Proton-motive force does work on the system by bringing ions back towards the epidermis of the root or surface of a root hair along with the protons. From the surface of the soil/root interface, specific carriers, like H+/K+ symporters allow the specific ions to come into the cell and the out the plasmodesmata/symporters/antiporters of the side of the cell facing away from the soil so that the essential element can make its way up the plant to the area it is needed so that it may supply the plant with important nutrients that are vital to the plant's being able to reach maturity.

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Related Research Articles

In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellular energy to achieve this movement. There are two types of active transport: primary active transport that uses adenosine triphosphate (ATP), and secondary active transport that uses an electrochemical gradient. This process is in contrast to passive transport, which allows molecules or ions to move down their concentration gradient, from an area of high concentration to an area of low concentration, without energy.

<span class="mw-page-title-main">ATPase</span> Dephosphorylation enzyme

ATPases (EC 3.6.1.3, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3-ATPase, adenosine triphosphatase) are a class of enzymes that catalyze the decomposition of ATP into ADP and a free phosphate ion or the inverse reaction. This dephosphorylation reaction releases energy, which the enzyme (in most cases) harnesses to drive other chemical reactions that would not otherwise occur. This process is widely used in all known forms of life.

<span class="mw-page-title-main">Distal convoluted tubule</span> Feature of kidney anatomy

The distal convoluted tubule (DCT) is a portion of kidney nephron between the loop of Henle and the collecting tubule.

<span class="mw-page-title-main">Mediated transport</span> Transportation of substances via membrane

Mediated transport refers to transport mediated by a membrane transport protein. Substances in the human body may be hydrophobic, electrophilic, contain a positively or negatively charge, or have another property. As such there are times when those substances may not be able to pass over the cell membrane using protein-independent movement. The cell membrane is imbedded with many membrane transport proteins that allow such molecules to travel in and out of the cell. There are three types of mediated transporters: uniport, symport, and antiport. Things that can be transported are nutrients, ions, glucose, etc, all depending on the needs of the cell. One example of a uniport mediated transport protein is GLUT1. GLUT1 is a transmembrane protein, which means it spans the entire width of the cell membrane, connecting the extracellular and intracellular region. It is a uniport system because it specifically transports glucose in only one direction, down its concentration gradient across the cell membrane.

A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane. Transport proteins are integral transmembrane proteins; that is they exist permanently within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion, active transport, osmosis, or reverse diffusion. The two main types of proteins involved in such transport are broadly categorized as either channels or carriers. Examples of channel/carrier proteins include the GLUT 1 uniporter, sodium channels, and potassium channels. The solute carriers and atypical SLCs are secondary active or facilitative transporters in humans. Collectively membrane transporters and channels are known as the transportome. Transportomes govern cellular influx and efflux of not only ions and nutrients but drugs as well.

<span class="mw-page-title-main">Loop of Henle</span> Part of kidney tissue

In the kidney, the loop of Henle is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer, the German anatomist Friedrich Gustav Jakob Henle, the loop of Henle's main function is to create a concentration gradient in the medulla of the kidney.

In cellular biology, membrane transport refers to the collection of mechanisms that regulate the passage of solutes such as ions and small molecules through biological membranes, which are lipid bilayers that contain proteins embedded in them. The regulation of passage through the membrane is due to selective membrane permeability – a characteristic of biological membranes which allows them to separate substances of distinct chemical nature. In other words, they can be permeable to certain substances but not to others.

<span class="mw-page-title-main">Uniporter</span>

Uniporters, also known as solute carriers or facilitated transporters, are a type of membrane transport protein that passively transports solutes across a cell membrane. It uses facilitated diffusion for the movement of solutes down their concentration gradient from an area of high concentration to an area of low concentration. Unlike active transport, it does not require energy in the form of ATP to function. Uniporters are specialized to carry one specific ion or molecule and can be categorized as either channels or carriers. Facilitated diffusion may occur through three mechanisms: uniport, symport, or antiport. The difference between each mechanism depends on the direction of transport, in which uniport is the only transport not coupled to the transport of another solute.

<span class="mw-page-title-main">Antiporter</span> Class of transmembrane transporter protein

An antiporter (also called exchanger or counter-transporter) is a cotransporter and integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma membrane in opposite directions, one into the cell and one out of the cell. Na+/H+ antiporters have been reviewed.

<span class="mw-page-title-main">Cotransporter</span> Type of membrane transport proteins

Cotransporters are a subcategory of membrane transport proteins (transporters) that couple the favorable movement of one molecule with its concentration gradient and unfavorable movement of another molecule against its concentration gradient. They enable coupled or cotransport and include antiporters and symporters. In general, cotransporters consist of two out of the three classes of integral membrane proteins known as transporters that move molecules and ions across biomembranes. Uniporters are also transporters but move only one type of molecule down its concentration gradient and are not classified as cotransporters.

<span class="mw-page-title-main">Electrochemical gradient</span> Gradient of electrochemical potential, usually for an ion that can move across a membrane

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:

<span class="mw-page-title-main">Ion transporter</span> Transmembrane protein that moves ions across a biological membrane

In biology, a transporter is a transmembrane protein that moves ions across a biological membrane to accomplish many different biological functions including, cellular communication, maintaining homeostasis, energy production, etc. There are different types of transporters including, pumps, uniporters, antiporters, and symporters. Active transporters or ion pumps are transporters that convert energy from various sources—including adenosine triphosphate (ATP), sunlight, and other redox reactions—to potential energy by pumping an ion up its concentration gradient. This potential energy could then be used by secondary transporters, including ion carriers and ion channels, to drive vital cellular processes, such as ATP synthesis.

Method of glucose uptake differs throughout tissues depending on two factors; the metabolic needs of the tissue and availability of glucose. The two ways in which glucose uptake can take place are facilitated diffusion and secondary active transport. Active transport is the movement of ions or molecules going against the concentration gradient.

The galactose permease or GalP found in Escherichia coli is an integral membrane protein involved in the transport of monosaccharides, primarily hexoses, for utilization by E. coli in glycolysis and other metabolic and catabolic pathways (3,4). It is a member of the Major Facilitator Super Family (MFS) and is homologue of the human GLUT1 transporter (4). Below you will find descriptions of the structure, specificity, effects on homeostasis, expression, and regulation of GalP along with examples of several of its homologues.

Sodium-dependent glucose cotransporters are a family of glucose transporter found in the intestinal mucosa (enterocytes) of the small intestine (SGLT1) and the proximal tubule of the nephron. They contribute to renal glucose reabsorption. In the kidneys, 100% of the filtered glucose in the glomerulus has to be reabsorbed along the nephron. If the plasma glucose concentration is too high (hyperglycemia), glucose passes into the urine (glucosuria) because SGLT are saturated with the filtered glucose.

The Na–K–Cl cotransporter (NKCC) is a transport protein that aids in the secondary active transport of sodium, potassium, and chloride into cells. In humans there are two isoforms of this membrane transport protein, NKCC1 and NKCC2, encoded by two different genes. Two isoforms of the NKCC1/Slc12a2 gene result from keeping or skipping exon 21 in the final gene product.

Renal reabsorption of sodium (Na+) is a part of renal physiology. It uses Na-H antiport, Na-glucose symport, sodium ion channels (minor). It is stimulated by angiotensin II and aldosterone, and inhibited by atrial natriuretic peptide.

The chloride potassium symporter is a membrane transport protein of the solute carrier family 12 that is present in the S3-segment of the renal proximal tubule and in the neuron. It functions in renal chloride reabsorption to transport chloride across the basolateral membrane. Chloride potassium symporter can lower intracellular chloride concentrations below the electrochemical equilibrium potential.

<span class="mw-page-title-main">Sodium in biology</span> Use of Sodium by organisms

Sodium ions are necessary in small amounts for some types of plants, but sodium as a nutrient is more generally needed in larger amounts by animals, due to their use of it for generation of nerve impulses and for maintenance of electrolyte balance and fluid balance. In animals, sodium ions are necessary for the aforementioned functions and for heart activity and certain metabolic functions. The health effects of salt reflect what happens when the body has too much or too little sodium. Characteristic concentrations of sodium in model organisms are: 10 mM in E. coli, 30 mM in budding yeast, 10 mM in mammalian cell and 100 mM in blood plasma.

The P-type plasma membrane H+
-ATPase
is found in plants and fungi. For the gastric H+
/K+
ATPase, see Hydrogen potassium ATPase.

References

  1. Willmer, Pat (2009). Environmental Physiology of Animals. Wiley-Blackwell.