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{{Short description|Cellular molecule transport mechanism against the concentration gradient}} {{about|transport in cellular biology|human systems|active mobility}} In [[Cellular Biology|cellular biology]], '''active transport''' is the movement of molecules or ions across a [[cell membrane]] [[Second law of thermodynamics|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, with energy. Active transport is essential for various physiological processes, such as nutrient uptake, hormone secretion, and nig impulse transmission. For example, the [[Sodium–potassium pump|sodium-potassium pump]] uses ATP to pump sodium ions out of the cell and potassium ions into the cell, maintaining a concentration gradient essential for cellular function. Active transport is highly selective and regulated, with different transporters specific to different molecules or ions. Dysregulation of active transport can lead to various disorders, including cystic fibrosis, caused by a malfunctioning chloride channel, and diabetes, resulting from defects in glucose transport into cells. == Active cellular transportation (ACT) == Unlike [[passive transport]], which uses the [[kinetic energy]] and natural [[entropy]] of molecules moving down a gradient, active transport uses cellular energy to move them against a gradient, polar repulsion, or other resistance. Active transport is usually associated with accumulating high concentrations of molecules that the cell needs, such as [[ion]]s, [[glucose]] and [[amino acid]]s. Examples of active transport include the uptake of glucose in the intestines in humans and the uptake of mineral ions into [[root hair]] cells of plants.<ref name="Active Transport Process">{{cite web|title=The importance of homeostasis |work=Science |publisher=me |url=http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_pre_2011/homeostasis/importancerev6.shtml |access-date=23 April 2013}}</ref> == History == In 1848, the [[Germany|German]] physiologist [[Emil du Bois-Reymond]] suggested the possibility of active transport of substances across membranes.<ref>Du Bois-Reymond, E. (1848–84). ''Untersuchungen über thierische Elektricität'' Berlin: Reimer. (Vol. 1, Part 1, 1848; Vol. 1, Part 2, 1849; Vol. 2, Part 1, 1860; Vol. 2, Part 2, 1884).</ref> In 1926, [[Dennis Robert Hoagland]] investigated the ability of [[plant]]s to absorb [[Salt (chemistry)|salts]] against a [[concentration]] gradient and discovered the dependence of [[Plant nutrition|nutrient]] absorption and [[Xylem|translocation]] on [[Metabolism|metabolic energy]] using innovative [[model organism|model systems]] under controlled experimental conditions.<ref>{{cite journal | last1 = Hoagland | first1 = D R | last2 = Hibbard | first2 = P L |last3 = Davis | first3 = A R| year = 1926 |title = The influence of light, temperature, and other conditions on the ability of ''Nitella'' cells to concentrate halogens in the cell sap | journal = J. Gen. Physiol. | volume = 10 | issue = 1 | pages = 121–126 | doi=10.1085/jgp.10.1.121| pmid = 19872303 | pmc = 2140878 | doi-access = free }}</ref> Rosenberg (1948) formulated the concept of active transport based on energetic considerations,<ref>{{cite journal | last1 = Rosenberg | first1 = T | year = 1948 | title = On accumulation and active transport in biological systems. I. Thermodynamic considerations | journal = Acta Chem. Scand. | volume = 2 | pages = 14–33 | doi=10.3891/acta.chem.scand.02-0014| doi-access = free }}</ref> but later it would be redefined. In 1997, [[Jens Christian Skou]], a Danish [[physician]]<ref name="Jens C 2014">"Jens C. Skou - Biographical". Nobelprize.org. Nobel Media AB 2014. Web. 11 Nov 2017</ref> received the [[Nobel Prize in Chemistry]] for his research regarding the [[sodium-potassium pump]].<ref name="Jens C 2014"/> One category of cotransporters that is especially prominent in research regarding [[diabetes]] treatment<ref>Inzucchi, Silvio E et al. "SGLT-2 Inhibitors and Cardiovascular Risk: Proposed Pathways and Review of Ongoing Outcome Trials." Diabetes & Vascular Disease Research 12.2 (2015): 90–100. PMC. Web. 11 Nov. 2017</ref> is sodium-glucose cotransporters. These transporters were discovered by scientists at the National Health Institute.<ref name="niddk.nih.gov">Story of Discovery: SGLT2 Inhibitors: Harnessing the Kidneys to Help Treat Diabetes." National Institute of Diabetes and Digestive and Kidney Diseases, U.S. Department of Health and Human Services, www.niddk.nih.gov/news/research-updates/Pages/story-discovery-SGLT2-inhibitors-harnessing-kidneys-help-treat-diabetes.aspx.</ref> These scientists had noticed a discrepancy in the absorption of glucose at different points in the kidney tubule of a rat. The gene was then discovered for intestinal glucose transport protein and linked to these membrane sodium glucose cotransport systems. The first of these membrane transport proteins was named [[SLC5A1|SGLT1]] followed by the discovery of [[SGLT2]].<ref name="niddk.nih.gov"/> [[Robert K. Crane#Discovery of cotransport|Robert Krane]] also played a prominent role in this field. ==Background== Specialized [[transmembrane protein]]s recognize the [[Chemical substance|substance]] and allow it to move across the membrane when it otherwise would not, either because the [[phospholipid bilayer]] of the membrane is impermeable to the substance moved or because the substance is moved against the direction of its [[concentration gradient]].<ref>{{usurped|1=[https://web.archive.org/web/20120120234037/http://www.buzzle.com/articles/active-transport-process.html Active Transport Process]}}. Buzzle.com (2010-05-14). Retrieved on 2011-12-05.</ref> There are two forms of active transport, primary active transport and secondary active transport. In primary active transport, the proteins involved are pumps that normally use chemical energy in the form of ATP. Secondary active transport, however, makes use of potential energy, which is usually derived through exploitation of an [[electrochemical]] gradient. The energy created from one ion moving down its electrochemical gradient is used to power the transport of another ion moving against its electrochemical gradient.<ref name="ncbi.nlm.nih.gov">Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 15.6, [https://www.ncbi.nlm.nih.gov/books/NBK21687/ Cotransport by Symporters and Antiporters].</ref> This involves pore-forming [[proteins]] that form channels across the [[cell membrane]]. The difference between passive transport and active transport is that the active transport requires energy, and moves substances against their respective concentration gradient, whereas passive transport requires no cellular energy and moves substances in the direction of their respective concentration gradient.<ref>Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Chapter 15, [https://www.ncbi.nlm.nih.gov/books/NBK21525/ Transport across Cell Membranes].</ref> In an [[antiporter]], one substrate is transported in one direction across the membrane while another is [[cotransport]]ed in the opposite direction. In a [[symporter]], two substrates are transported in the same direction across the membrane. Antiport and symport processes are associated with [[secondary active transport]], meaning that one of the two substances is transported against its concentration gradient, utilizing the energy derived from the transport of another ion (mostly Na{{Sup|+}}, K{{Sup|+}} or H{{Sup|+}} ions) down its concentration gradient. {{cn|date=January 2025}} If substrate molecules are moving from areas of lower concentration to areas of higher concentration<ref>[http://www.biologycorner.com/bio1/active.html Active Transport] {{webarchive |url=https://web.archive.org/web/20110824003030/http://www.biologycorner.com/bio1/active.html |date=August 24, 2011 }}. Biologycorner.com. Retrieved on 2011-12-05.</ref> (i.e., in the opposite direction as, or ''against'' the concentration gradient), specific transmembrane carrier proteins are required. These proteins have receptors that bind to specific molecules (e.g., [[Sodium-glucose transport proteins|glucose]]) and transport them across the cell membrane. Because energy is required in this process, it is known as 'active' transport. Examples of active transport include the transportation of [[sodium]] out of the cell and [[potassium]] into the cell by the sodium-potassium pump. Active transport often takes place in the internal lining of the [[small intestine]]. Plants need to absorb mineral salts from the soil or other sources, but these salts exist in very dilute [[Solution (chemistry)|solution]]. Active transport enables these cells to take up salts from this dilute solution against the direction of the [[concentration gradient]]. For example, [[chloride]] (Cl<sup>−</sup>) and [[nitrate]] (NO<sub>3</sub><sup>−</sup>) ions exist in the cytosol of plant cells, and need to be transported into the vacuole. While the vacuole has channels for these ions, transportation of them is against the concentration gradient, and thus movement of these ions is driven by hydrogen pumps, or proton pumps.<ref name="ncbi.nlm.nih.gov"/> ==Primary active transport== [[Image:Scheme sodium-potassium pump-en.svg|thumb|The action of the [[sodium-potassium pump]] is an example of primary active transport.]] Primary active transport, also called direct active transport, directly uses metabolic energy to transport molecules across a membrane.<ref>{{cite book| title= Essentials of Human Physiology| first= Thomas M. |last= Nosek| chapter=Section 7/7ch05/7ch05p11 |chapter-url=http://humanphysiology.tuars.com/program/section7/7ch05/7ch05p11.htm |archive-url=https://web.archive.org/web/20160324124828/http://humanphysiology.tuars.com/program/section7/7ch05/7ch05p11.htm|archive-date=2016-03-24}}</ref> Substances that are transported across the cell membrane by primary active transport include metal ions, such as [[sodium|Na]]<sup>+</sup>, [[potassium|K]]<sup>+</sup>, [[magnesium|Mg]]<sup>2+</sup>, and [[calcium|Ca]]<sup>2+</sup>. These charged particles require [[Ion transporter|ion pump]]s or [[ion channel]]s to cross membranes and distribute through the body. {{cn|date=January 2025}} Most of the [[enzyme]]s that perform this type of transport are transmembrane [[ATPase]]s. A primary ATPase universal to all animal life is the [[Na+/K+-ATPase|sodium-potassium pump]], which helps to maintain the [[Membrane potential|cell potential]]. The sodium-potassium pump maintains the membrane potential by moving three Na<sup>+</sup> ions out of the cell for every two<ref>{{Cite book|title=Tenth Edition, Campbell's Biology|last1=Reese|first1=Jane B.|last2=Urry|first2=Lisa A.|last3=Cain|first3=Michael L.|last4=Wasserman|first4=Steven A.|last5=Minorsky|first5=Peter V.|last6=Jackson|first6=Robert B.|publisher=Pearson Education Inc.|year=2014|isbn=978-0-321-77565-8|location=United States|pages=135|edition=Tenth}}</ref> K<sup>+</sup> ions moved into the cell. Other sources of energy for primary active transport are [[redox]] energy and [[photon]] energy ([[light]]). An example of primary active transport using redox energy is the mitochondrial [[electron transport chain]] that uses the reduction energy of [[NADH]] to move protons across the inner mitochondrial membrane against their concentration gradient. An example of primary active transport using light energy are the proteins involved in [[photosynthesis]] that use the energy of photons to create a proton gradient across the [[thylakoid membrane]] and also to create reduction power in the form of [[NADPH]]. {{cn|date=January 2025}} ===Model of active transport=== [[ATP hydrolysis]] is used to transport hydrogen ions against the [[electrochemical gradient]] (from low to high hydrogen ion concentration). [[Phosphorylation]] of the [[carrier protein]] and the binding of a [[hydrogen ion]] induce a conformational (shape) change that drives the hydrogen ions to transport against the electrochemical gradient. [[Hydrolysis]] of the bound [[phosphate group]] and release of hydrogen ion then restores the carrier to its original conformation.<ref>{{cite book|last=Cooper|first=Geoffrey|title=The Cell: A Molecular Approach|year=2009|publisher=ASK PRESS|location=Washington, DC|isbn=9780878933006|page=65}}</ref> == Types of primary active transporters == # [[P-type ATPase]]: [[sodium potassium pump]], [[calcium pump]], [[proton pump]] # [[F-ATPase]]: mitochondrial ATP synthase, chloroplast ATP synthase # [[V-ATPase]]: vacuolar ATPase # ABC ([[ATP binding cassette]]) transporter: MDR, [[CFTR]], etc. Adenosine triphosphate-binding cassette transporters ([[ATP-binding cassette transporter|ABC transporters]]) comprise a large and diverse protein family, often functioning as ATP-driven pumps. Usually, there are several domains involved in the overall transporter protein's structure, including two nucleotide-binding domains that constitute the ATP-binding motif and two hydrophobic transmembrane domains that create the "pore" component. In broad terms, ABC transporters are involved in the import or export of molecules across a cell membrane; yet within the protein family there is an extensive range of function.<ref name=":0">{{Cite journal|last1=Kang|first1=Joohyun|last2=Park|first2=Jiyoung|date=December 6, 2011|title=Plant ABC Transporters|journal= The Arabidopsis Book|volume=9|pages=e0153|doi=10.1199/tab.0153|pmid=22303277|pmc=3268509}}</ref> In plants, ABC transporters are often found within cell and organelle membranes, such as the mitochondria, chloroplast, and plasma membrane. There is evidence to support that plant ABC transporters play a direct role in pathogen response, phytohormone transport, and detoxification.<ref name=":0" /> Furthermore, certain plant ABC transporters may function in actively exporting volatile compounds<ref name=":1">{{Cite journal|last=Adebesin|first=Funmilayo|date=June 30, 2017|title=Emission of volatile organic compounds from petunia flowers is facilitated by an ABC transporter|journal=Plant Science|volume=356|issue=6345|pages=1386–1388|doi=10.1126/science.aan0826|pmid=28663500|bibcode=2017Sci...356.1386A|s2cid=206658803|doi-access=free|hdl=11245.1/2a6bd9dd-ea94-4c25-95b8-7b16bea44e92|hdl-access=free}}</ref> and antimicrobial metabolites.<ref name=":2">{{Cite journal|last=Crouzet|first=Jerome|date=April 7, 2013|title=NtPDR1, a plasma membrane ABC transporter from Nicotiana tabacum, is involved in diterpene transport|url=https://www.science.org/doi/pdf/10.1126/science.aan0826|journal=Plant Molecular Biology|volume=82|issue=1–2|pages=181–192|via=SpringerLink|doi=10.1007/s11103-013-0053-0|pmid=23564360|s2cid=12276939|url-access=subscription}}</ref> In petunia flowers (''Petunia hybrida''), the ABC transporter PhABCG1 is involved in the active transport of volatile organic compounds. PhABCG1 is expressed in the petals of open flowers. In general, volatile compounds may promote the attraction of seed-dispersal organisms and pollinators, as well as aid in defense, signaling, allelopathy, and protection. To study the protein PhABCG1, transgenic petunia [[RNA interference]] lines were created with decreased ''PhABCG1'' expression levels. In these transgenic lines, a decrease in emission of volatile compounds was observed. Thus, PhABCG1 is likely involved in the export of volatile compounds. Subsequent experiments involved incubating control and transgenic lines that expressed ''PhABCG1'' to test for transport activity involving different substrates. Ultimately, PhABCG1 is responsible for the protein-mediated transport of volatile organic compounds, such as benzyl alcohol and methylbenzoate, across the plasma membrane.<ref name=":1" /> Additionally in plants, ABC transporters may be involved in the transport of cellular metabolites. Pleiotropic Drug Resistance ABC transporters are hypothesized to be involved in stress response and export antimicrobial metabolites. One example of this type of ABC transporter is the protein NtPDR1. This unique ABC transporter is found in ''Nicotiana tabacum'' BY2 cells and is expressed in the presence of microbial elicitors. NtPDR1 is localized in the root epidermis and aerial trichomes of the plant. Experiments using antibodies specifically targeting NtPDR1 followed by Western blotting allowed for this determination of localization. Furthermore, it is likely that the protein NtPDR1 actively transports out antimicrobial diterpene molecules, which are toxic to the cell at high levels.<ref name=":2" /> ==Secondary active transport== In secondary active transport, also known as [[Cotransporter|cotransport or coupled transport]], energy is used to transport molecules across a membrane; however, in contrast to [[primary active transport]], there is no direct coupling of [[Adenosine triphosphate|ATP]]. Instead, it relies upon the [[electrochemical potential|electrochemical potential difference]] created by pumping ions in/out of the cell.<ref>{{cite book| title= Essentials of Human Physiology| first= Thomas M. |last= Nosek| chapter=Section 7/7ch05/7ch05p12 |chapter-url=http://humanphysiology.tuars.com/program/section7/7ch05/7ch05p12.htm |archive-url=https://web.archive.org/web/20160324124828/http://humanphysiology.tuars.com/program/section7/7ch05/7ch05p12.htm|archive-date=2016-03-24}}</ref> Permitting one ion or molecule to move down an electrochemical gradient, but possibly against the concentration gradient where it is more concentrated to that where it is less concentrated, increases [[entropy]] and can serve as a source of [[energy]] for [[metabolism]] (e.g. in [[ATP synthase]]). The energy derived from the pumping of protons across a cell membrane is frequently used as the energy source in secondary active transport. In humans, sodium (Na<sup>+</sup>) is a commonly cotransported ion across the plasma membrane, whose electrochemical gradient is then used to power the active transport of a second ion or molecule against its gradient.<ref name="Alberts B 2002">Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. [https://www.ncbi.nlm.nih.gov/books/NBK26896/ Carrier Proteins and Active Membrane Transport].</ref> In bacteria and small yeast cells, a commonly cotransported ion is hydrogen.<ref name="Alberts B 2002"/> Hydrogen pumps are also used to create an electrochemical gradient to carry out processes within cells such as in the [[electron transport chain]], an important function of [[cellular respiration]] that happens in the [[mitochondrion]] of the cell.<ref>Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. [https://www.ncbi.nlm.nih.gov/books/NBK26904/ Electron-Transport Chains and Their Proton Pumps].</ref> In August 1960, in Prague, [[Robert K. Crane]] presented for the first time his discovery of the sodium-glucose cotransport as the mechanism for intestinal glucose absorption.<ref>{{cite book |author-link=Robert K. Crane |first1=Robert K. |last1=Crane |first2=D. |last2=Miller |first3=I. |last3=Bihler |chapter=The restrictions on possible mechanisms of intestinal transport of sugars |editor=Kleinzeller, A. |editor2=Kotyk, A. |title=Membrane Transport and Metabolism. Proceedings of a Symposium held in Prague, August 22–27, 1960 |publisher=[[Academy of Sciences of the Czech Republic|Czech Academy of Sciences]] |location=Prague |year=1961 |pages=439–449 }}</ref> Crane's discovery of cotransport was the first ever proposal of flux coupling in biology.<ref>{{cite journal |vauthors=Wright EM, Turk E |title=The sodium/glucose cotransport family SLC5 |journal=Pflügers Arch. |volume=447 |issue=5 |pages=510–8 |date=February 2004 |pmid=12748858 |doi= 10.1007/s00424-003-1063-6 |s2cid=41985805 |quote=[[Robert K. Crane|Crane]] in 1961 was the first to formulate the cotransport concept to explain active transport [7]. Specifically, he proposed that the accumulation of glucose in the intestinal epithelium across the brush border membrane was coupled to downhill {{chem|Na|+}} transport cross the brush border. This hypothesis was rapidly tested, refined and extended [to] encompass the active transport of a diverse range of molecules and ions into virtually every cell type.}}</ref><ref>{{cite journal |author =Boyd CA |title=Facts, fantasies and fun in epithelial physiology |journal=Exp. Physiol. |volume=93 |issue=3 |pages=303–14 (304) |date=March 2008 |pmid=18192340 |doi=10.1113/expphysiol.2007.037523 |quote= the insight from this time that remains in all current text books is the notion of [[Robert K. Crane|Robert Crane]] published originally as an appendix to a symposium paper published in 1960 ([[Robert K. Crane|Crane]] et al. 1960). The key point here was 'flux coupling', the cotransport of sodium and glucose in the apical membrane of the small intestinal epithelial cell. Half a century later this idea has turned into one of the most studied of all transporter proteins (SGLT1), the sodium–glucose cotransporter.|doi-access=free }}</ref> [[Cotransporter]]s can be classified as [[symporter]]s and [[antiporter]]s depending on whether the substances move in the same or opposite directions. ===Antiporter=== [[Image:Porters.PNG|right|thumb|Function of [[symporter]]s and [[antiporter]]s.]] In an antiporter two species of ions or other solutes are pumped in opposite directions across a membrane. One of these species is allowed to flow from high to low concentration, which yields the [[entropy|entropic energy]] to drive the transport of the other solute from a low concentration region to a high one. An example is the [[sodium-calcium exchanger]] or [[antiporter]], which allows three sodium ions into the cell to transport one calcium out.<ref>{{cite journal|last1=Yu|first1=SP|last2=Choi|first2=DW|title=Na<sup>+</sup>-Ca<sup>2+</sup> exchange currents in cortical neurons: concomitant forward and reverse operation and effect of glutamate.|journal=The European Journal of Neuroscience|date=June 1997|volume=9|issue=6|pages=1273–81|pmid=9215711|doi=10.1111/j.1460-9568.1997.tb01482.x|s2cid=23146698}}</ref> This antiporter mechanism is important within the membranes of cardiac muscle cells in order to keep the calcium concentration in the cytoplasm low.<ref name="ncbi.nlm.nih.gov"/> Many cells also possess [[calcium ATPase]]s, which can operate at lower intracellular concentrations of calcium and sets the normal or resting concentration of this important [[second messenger]].<ref>{{cite journal|last1=Strehler|first1=EE|last2=Zacharias|first2=DA|s2cid=9062253|title=Role of alternative splicing in generating isoform diversity among plasma membrane calcium pumps.|journal=Physiological Reviews|date=January 2001|volume=81|issue=1|pages=21–50|pmid=11152753|doi=10.1152/physrev.2001.81.1.21}}</ref> But the ATPase exports calcium ions more slowly: only 30 per second versus 2000 per second by the exchanger. The exchanger comes into service when the calcium concentration rises steeply or "spikes" and enables rapid recovery.<ref>{{cite journal|last1=Patterson|first1=M|last2=Sneyd|first2=J|last3=Friel|first3=DD|title=Depolarization-induced calcium responses in sympathetic neurons: relative contributions from Ca<sup>2+</sup> entry, extrusion, ER/mitochondrial Ca<sup>2+</sup> uptake and release, and Ca<sup>2+</sup> buffering.|journal=The Journal of General Physiology|date=January 2007|volume=129|issue=1|pages=29–56|pmid=17190902|doi=10.1085/jgp.200609660|pmc=2151609}}</ref> This shows that a single type of ion can be transported by several enzymes, which need not be active all the time (constitutively), but may exist to meet specific, intermittent needs. === Symporter === A [[symporter]] uses the downhill movement of one [[Solution (chemistry)|solute species]] from high to low concentration to move another molecule uphill from low concentration to high concentration (against its [[concentration gradient]]). Both molecules are transported in the same direction. An example is the glucose symporter [[Sodium-glucose transport proteins|SGLT1]], which co-transports one [[glucose]] (or [[galactose]]) molecule into the cell for every two sodium ions it imports into the cell.<ref>{{cite journal|last1=Wright|first1=EM|last2=Loo|first2=DD|last3=Panayotova-Heiermann|first3=M|last4=Lostao|first4=MP|last5=Hirayama|first5=BH|last6=Mackenzie|first6=B|last7=Boorer|first7=K|last8=Zampighi|first8=G|title='Active' sugar transport in eukaryotes.|journal=The Journal of Experimental Biology|date=November 1994|volume=196|pages=197–212|doi=10.1242/jeb.196.1.197|pmid=7823022}}</ref> This [[symporter]] is located in the small intestines,<ref>{{cite journal|last1=Dyer|first1=J|last2=Hosie|first2=KB|last3=Shirazi-Beechey|first3=SP|title=Nutrient regulation of human intestinal sugar transporter (SGLT2) expression.|journal=Gut|date=July 1997|volume=41|issue=1|pages=56–9|pmid=9274472|doi=10.1136/gut.41.1.56|pmc=1027228}}</ref> heart,<ref>{{cite journal|last1=Zhou|first1=L|last2=Cryan|first2=EV|last3=D'Andrea|first3=MR|last4=Belkowski|first4=S|last5=Conway|first5=BR|last6=Demarest|first6=KT|title=Human cardiomyocytes express high level of Na+/glucose cotransporter 1 (SGLT2).|journal=Journal of Cellular Biochemistry|date=1 October 2003|volume=90|issue=2|pages=339–46|pmid=14505350|doi=10.1002/jcb.10631|s2cid=21908010}}</ref> and brain.<ref>{{cite journal|last1=Poppe|first1=R|last2=Karbach|first2=U|last3=Gambaryan|first3=S|last4=Wiesinger|first4=H|last5=Lutzenburg|first5=M|last6=Kraemer|first6=M|last7=Witte|first7=OW|last8=Koepsell|first8=H|title=Expression of the Na+-D-glucose cotransporter SGLT1 in neurons.|journal=Journal of Neurochemistry|date=July 1997|volume=69|issue=1|pages=84–94|pmid=9202297|doi=10.1046/j.1471-4159.1997.69010084.x|s2cid=34558770|doi-access=free}}</ref> It is also located in the S3 segment of the [[proximal tubule]] in each [[nephron]] in the [[kidney]]s.<ref>{{cite journal|author =Wright EM|year=2001|title=Renal Na<sup>+</sup>-glucose cotransporters|journal=Am J Physiol Renal Physiol|pmid=11133510|volume=280|issue=1|pages=F10–8|doi=10.1152/ajprenal.2001.280.1.F10}}</ref> Its mechanism is exploited in [[Oral rehydration therapy|glucose rehydration therapy]]<ref name="pmid8917597">{{cite journal|last1=Loo|first1=DD|last2=Zeuthen|first2=T|last3=Chandy|first3=G|last4=Wright|first4=EM|title=Cotransport of water by the Na+/glucose cotransporter.|journal=Proceedings of the National Academy of Sciences of the United States of America|date=12 November 1996|volume=93|issue=23|pages=13367–70|pmid=8917597|doi=10.1073/pnas.93.23.13367|pmc=24099|bibcode=1996PNAS...9313367L|doi-access=free}}</ref> This mechanism uses the absorption of sugar through the walls of the intestine to pull water in along with it.<ref name="pmid8917597" /> Defects in SGLT2 prevent effective reabsorption of glucose, causing [[Glucosuria|familial renal glucosuria]].<ref>{{cite journal|vauthors=Wright EM, Hirayama BA, Loo DF |year=2007|title=Active sugar transport in health and disease|pmid=17222166|volume=261|issue=1|pages=32–43|doi=10.1111/j.1365-2796.2006.01746.x|journal=Journal of Internal Medicine|doi-access=}}</ref> ==Bulk transport== {{Main|Endocytosis|Exocytosis}} [[Endocytosis]] and [[exocytosis]] are both forms of [[Solvent drag|bulk transport]] that move materials into and out of cells, respectively, via [[Vesicle (biology and chemistry)|vesicles]].<ref>{{Cite book|title=Tenth Addition Campbell Biology|last1=Reece|first1=Jane|last2=Urry|first2=Lisa|last3=Cain|first3=Michael|last4=Wasserman|first4=Steven|last5=Minorsky|first5=Peter|last6=Jackson|first6=Robert|publisher=Pearson Education, Inc|year=2014|isbn=978-0-321-77565-8|location=United States of America|pages=137|edition=Tenth Addition}}</ref> In the case of endocytosis, the cellular membrane folds around the desired materials outside the cell.<ref>[https://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mboc4&part=A2383 Transport into the Cell from the Plasma Membrane: Endocytosis – Molecular Biology of the Cell – NCBI Bookshelf]. Ncbi.nlm.nih.gov (2011-10-03). Retrieved on 2011-12-05.</ref> The ingested particle becomes trapped within a pouch, known as a vesicle, inside the [[cytoplasm]]. Often enzymes from [[lysosome]]s are then used to digest the molecules absorbed by this process. Substances that enter the cell via signal mediated electrolysis include proteins, hormones and growth and stabilization factors.<ref>Paston, Ira; Willingham, Mark C. (1985). Endocytosis. Springer, Boston, MA. pp 1–44. doi: 10.1007/978-1-4615-6904-6_1. {{ISBN|9781461569060}}.</ref> Viruses enter cells through a form of endocytosis that involves their outer membrane fusing with the membrane of the cell. This forces the viral DNA into the host cell.<ref>{{Cite journal|last1=Jahn|first1=Reinhard|last2=Südhof|first2=Thomas C.|date=1999|title=Membrane Fusion and Exocytosis|journal=Annual Review of Biochemistry|language=en|volume=68|issue=1|pages=863–911|doi=10.1146/annurev.biochem.68.1.863|pmid=10872468|issn=0066-4154}}</ref> Biologists distinguish two main types of endocytosis: [[pinocytosis]] and [[phagocytosis]].<ref>[http://www.takdangaralin.com/science/biology-science/cell-two-major-process-in-exchange-of-materials-between-cell-and-environment/ Cell : Two Major Process in Exchange Of Materials Between Cell And Environment] {{webarchive|url=https://web.archive.org/web/20100811173330/http://www.takdangaralin.com/science/biology-science/cell-two-major-process-in-exchange-of-materials-between-cell-and-environment/|date=August 11, 2010}}. Takdang Aralin (2009-10-26). Retrieved on 2011-12-05.</ref> * In pinocytosis, cells engulf liquid particles (in humans this process occurs in the small intestine, where cells engulf fat droplets).<ref>[http://www.biology-online.org/dictionary/Pinocytosis Pinocytosis: Definition]. biology-online.org</ref> * In phagocytosis, cells engulf solid particles.<ref>[http://courses.washington.edu/conj/bloodcells/phagocytosis.htm Phagocytosis]. Courses.washington.edu. Retrieved on 2011-12-05.</ref> Exocytosis involves the removal of substances through the fusion of the outer cell membrane and a vesicle membrane.<ref name="annualreviews.org">{{cite journal|doi=10.1146/annurev.biochem.68.1.863|title=Membrane Fusion and Exocytosis|year=1999|last1=Jahn|first1=Reinhard|last2=Südhof|first2=Thomas C.|journal=Annual Review of Biochemistry|volume=68|pages=863–911|pmid=10872468}}</ref> An example of exocytosis would be the transmission of neurotransmitters across a synapse between brain cells. ==See also== * [[ATP-binding cassette transporter]] * [[Countercurrent exchange]] * [[Protein targeting]] * [[Translocation (disambiguation)|Translocation]] ==References== {{reflist|2}} {{Refbegin}} ==Notes== * {{cite book |author1=Lodish H.|author2=Berk A. |author3=Zipursky S.L. |author4=Matsudaira P. |author5=Baltimore D. |author6=Darnell J. |author7=López D.|chapter=Section 15.6 Cotransport by Symporters and Antiporters |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK21687/ |title=Molecular Cell Biology |publisher=W.H. Freeman |location=New York |year=2000 |isbn=978-0-7167-3136-8 |url=https://archive.org/details/molecularcellbio00lodi |edition=4th |url-access=registration }} {{Refend}} ==External links== * {{Commons category-inline}} * [http://www.physiologyweb.com/lecture_notes/membrane_transport/secondary_active_transport.html Secondary Active Transport] {{Membrane transport}} {{Authority control}} [[Category:Membrane biology]] [[Category:Biological matter]] [[de:Membrantransport#Aktiver Transport]]
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