A symporter is an

integral membrane protein An integral, or intrinsic, membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane. All ''transmembrane proteins'' are IMPs, but not all IMPs are transmembrane proteins. IMPs comprise a sign ...

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 The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...

and molecules are transported across the cell membrane at the same time, and is, therefore, a type of

cotransporter 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 ...

. 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 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 membr ...

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 Facilitated diffusion (also known as facilitated transport or passive-mediated transport) is the process of spontaneous passive transport (as opposed to active transport) of molecules or ions across a biological membrane via specific transmembra ...

, and is coupled with the

active transport 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 cellul ...

of the molecule(s). In symport, two molecule move in 'similar direction' at the 'same time' Example of symport:- movement of

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

along with sodium ion

Examples

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 Oral rehydration therapy (ORT) is a type of fluid replacement used to prevent and treat dehydration, especially due to diarrhea. It involves drinking water with modest amounts of sugar and salts, specifically sodium and potassium. Oral rehydrat ...

. 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 The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...

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 Furosemide is a loop diuretic medication used to treat fluid build-up due to heart failure, liver scarring, or kidney disease. It may also be used for the treatment of high blood pressure. It can be taken by injection into a vein or by mo ...

(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. In the

root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...

s of

plants Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...

, 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 Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membra ...

, 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.

References

External links

* {{Membrane transport Integral membrane proteins

Examples

See also

References

External links