Active transport

In cellular biology, active transport is the movement of molecules across a membrane from a region of their lower concentration to a region of their 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 ATP, and secondary active transport that uses an electrochemical gradient. An example of active transport in human physiology is the uptake of glucose in the intestines.

Cellular transportation mechanisms

Active transport is the movement of molecules across a membrane from a region of their lower concentration to a region of their higher concentration—against the concentration gradient or other obstructing factor.

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 ions, glucose and amino acids. If the process uses chemical energy, such as from adenosine triphosphate (ATP), it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient. 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.[1]

History

In 1848, the German physiologist Emil du Bois-Reymond suggested the possibility of active transport of substances across membranes.[2]

Rosenberg (1948) formulated the concept of active transport based on energetic considerations,[3] but later it would be redefined.

In 1997, Jens Christian Skou, a Danish physician [4] received the Nobel Prize in Chemistry for his research regarding the sodium-potassium pump. [4]

One category of cotransporters that is especially prominent in research regarding diabetes treatment [5] is sodium glucose cotransporters. These transporters were discovered by scientists at the National Health Institute.[6] 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 SGLT1 followed by the discovery of SGLT2.[6] Robert Krane also played a prominent role in this field.

Background

Specialized transmembrane proteins recognize the 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.[7] 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.[8] 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 energy and moves substances in the direction of their respective concentration gradient.[9]

In an antiporter, one substrate is transported in one direction across the membrane while another is cotransported 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+, K+ or H+ ions) down its concentration gradient.

If substrate molecules are moving from areas of lower concentration to areas of higher concentration[10] (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., 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. Active transport enables these cells to take up salts from this dilute solution against the direction of the concentration gradient. For example, the molecules chlorine (Cl^-) and nitrate NO3- 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[11]

Primary active transport

Scheme sodium-potassium pump-en
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.[12] Substances that are transported across the cell membrane by primary active transport include metal ions, such as Na+, K+, Mg2+, and Ca2+. These charged particles require ion pumps or ion channels to cross membranes and distribute through the body.

Most of the enzymes that perform this type of transport are transmembrane ATPases. A primary ATPase universal to all animal life is the sodium-potassium pump, which helps to maintain the cell potential. The sodium-potassium pump maintains the membrane potential by moving three Na+ ions out of the cell for every two [13] K+ 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.

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.[14]

Types of primary active transporters

  1. P-type ATPase: sodium potassium pump, calcium pump, proton pump
  2. F-ATPase: mitochondrial ATP synthase, chloroplast ATP synthase
  3. V-ATPase: vacuolar ATPase
  4. ABC (ATP binding cassette) transporter: MDR , CFTR, etc.

Adenosine Triphosphate-binding cassette transporters (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.[15]

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.[15] Furthermore, certain plant ABC transporters may function in actively exporting volatile compounds[16] and antimicrobial metabolites.[17]

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 benezyl alcohol and methylbenzoate, across the plasma membrane.[16]

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.[17]

Secondary active transport

Scheme secundary active transport-en
Secondary active transport

In secondary active transport, also known as coupled transport or cotransport, energy is used to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP; instead it relies upon the electrochemical potential difference created by pumping ions in/out of the cell.[18] 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+) is a commonly co-transported 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.[19] In bacteria and small yeast cells, a commonly cotransported ion is hydrogen.[19] 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.[20]

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.[21] Crane's discovery of cotransport was the first ever proposal of flux coupling in biology.[22][23]

Cotransporters can be classified as symporters and antiporters depending on whether the substances move in the same or opposite directions.

Antiporter

Porters
Function of symporters and antiporters.

In an antiporter two species of ion 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 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.[24] This antiporter mechanism is important within the membranes of cardiac muscle cells in order to keep the calcium concentration in the cytoplasm low.[8] Many cells also possess calcium ATPases, which can operate at lower intracellular concentrations of calcium and sets the normal or resting concentration of this important second messenger.[25] 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.[26] 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 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 SGLT1, which co-transports one glucose (or galactose) molecule into the cell for every two sodium ions it imports into the cell.[27] This symporter is located in the small intestines,[28] heart,[29] and brain.[30] It is also located in the S3 segment of the proximal tubule in each nephron in the kidneys.[31] Its mechanism is exploited in glucose rehydration therapy[32] This mechanism uses the absorption of sugar through the walls of the intestine to pull water in along with it.[33] Defects in SGLT2 prevent effective reabsorption of glucose, causing familial renal glucosuria.[34]

Bulk transport

Endocytosis and exocytosis are both forms of bulk transport that move materials into and out of cells, respectively, via vesicles.[35] In the case of endocytosis, the cellular membrane folds around the desired materials outside the cell.[36] The ingested particle becomes trapped within a pouch, known as a vesicle, inside the cytoplasm. Often enzymes from lysosomes 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.[37] 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.[38]

Biologists distinguish two main types of endocytosis: pinocytosis and phagocytosis.[39]

  • In pinocytosis, cells engulf liquid particles (in humans this process occurs in the small intestine, where cells engulf fat droplets).[40]
  • In phagocytosis, cells engulf solid particles.[41]

Exocytosis involves the removal of substances through the fusion of the outer cell membrane and a vesicle membrane[38] An example of exocytosis would be the transmission of neurotransmitters across a synapse between brain cells.

See also

References

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  5. ^ 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
  6. ^ a b 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.
  7. ^ Active Transport Process. Buzzle.com (2010-05-14). Retrieved on 2011-12-05.
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  19. ^ a b Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Carrier Proteins and Active Membrane Transport. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26896/
  20. ^ Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Electron-Transport Chains and Their Proton Pumps. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26904/
  21. ^ Crane, Robert K.; Miller, D.; Bihler, I. (1961). "The restrictions on possible mechanisms of intestinal transport of sugars". In Kleinzeller, A.; Kotyk, A. Membrane Transport and Metabolism. Proceedings of a Symposium held in Prague, August 22–27, 1960. Prague: Czech Academy of Sciences. pp. 439–449.
  22. ^ Wright EM, Turk E (February 2004). "The sodium/glucose cotransport family SLC5". Pflügers Arch. 447 (5): 510–8. doi:10.1007/s00424-003-1063-6. PMID 12748858. 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 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.
  23. ^ Boyd CA (March 2008). "Facts, fantasies and fun in epithelial physiology". Exp. Physiol. 93 (3): 303–14. doi:10.1113/expphysiol.2007.037523. PMID 18192340. p. 304. “the insight from this time that remains in all current text books is the notion of Robert Crane published originally as an appendix to a symposium paper published in 1960 (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.
  24. ^ Yu, SP; Choi, DW (June 1997). "Na(+)-Ca2+ exchange currents in cortical neurons: concomitant forward and reverse operation and effect of glutamate". The European Journal of Neuroscience. 9 (6): 1273–81. doi:10.1111/j.1460-9568.1997.tb01482.x. PMID 9215711.
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  27. ^ Wright, EM; Loo, DD; Panayotova-Heiermann, M; Lostao, MP; Hirayama, BH; Mackenzie, B; Boorer, K; Zampighi, G (November 1994). "'Active' sugar transport in eukaryotes". The Journal of Experimental Biology. 196: 197–212. PMID 7823022.
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  29. ^ Zhou, L; Cryan, EV; D'Andrea, MR; Belkowski, S; Conway, BR; Demarest, KT (1 October 2003). "Human cardiomyocytes express high level of Na+/glucose cotransporter 1 (SGLT2)". Journal of Cellular Biochemistry. 90 (2): 339–46. doi:10.1002/jcb.10631. PMID 14505350.
  30. ^ Poppe, R; Karbach, U; Gambaryan, S; Wiesinger, H; Lutzenburg, M; Kraemer, M; Witte, OW; Koepsell, H (July 1997). "Expression of the Na+-D-glucose cotransporter SGLT1 in neurons". Journal of Neurochemistry. 69 (1): 84–94. doi:10.1046/j.1471-4159.1997.69010084.x. PMID 9202297.
  31. ^ Wright EM (2001). "Renal Na+-glucose cotransporters". Am J Physiol Renal Physiol. 280 (1): F10–8. PMID 11133510.
  32. ^ Loo, DD; Zeuthen, T; Chandy, G; Wright, EM (12 November 1996). "Cotransport of water by the Na+/glucose cotransporter". Proceedings of the National Academy of Sciences of the United States of America. 93 (23): 13367–70. doi:10.1073/pnas.93.23.13367. PMC 24099. PMID 8917597.
  33. ^ Loo, Donald D.; Zeuthan, Thomas; Chandy, Grischa; Wright, Ernest M. (1996-11-12). "Cotransport of water by Na+/glucose cotransporter" Proceedings of the National Academy of Sciences. 93 (23): 13367-13370. ISSN 0027-8424. PMID 8917597.
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Notes

External links

Antiporter

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. Na+/H+ antiporters have been reviewed.In secondary active transport, one species of solute moves along its electrochemical gradient, allowing a different species to move against its own electrochemical gradient. This movement is in contrast to primary active transport, in which all solutes are moved against their concentration gradients, fueled by ATP.

Transport may involve one or more of each type of solute. For example, the Na+/Ca2+ exchanger, used by many cells to remove cytoplasmic calcium, exchanges one calcium ion for three sodium ions.

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 enable cotransport (secondary active transport) 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.

Dihydropteroate synthase

Dihydropteroate synthase is an enzyme classified under EC 2.5.1.15. It produces dihydropteroate in bacteria, but it is not expressed in most eukaryotes including humans. This makes it a useful target for sulfonamide antibiotics, which compete with the PABA precursor.

All organisms require reduced folate cofactors for the synthesis of a variety of metabolites. Most microorganisms must synthesize folate de novo because they lack the active transport system of higher vertebrate cells that allows these organisms to use dietary folates. Proteins containing this domain include dihydropteroate synthase (EC 2.5.1.15) as well as a group of methyltransferase enzymes including methyltetrahydrofolate, corrinoid iron-sulphur protein methyltransferase (MeTr) that catalyses a key step in the Wood-Ljungdahl pathway of carbon dioxide fixation.

Dihydropteroate synthase (EC 2.5.1.15) (DHPS) catalyses the condensation of 6-hydroxymethyl-7,8-dihydropteridine pyrophosphate to para-aminobenzoic acid to form 7,8-dihydropteroate. This is the second step in the three-step pathway leading from 6-hydroxymethyl-7,8-dihydropterin to 7,8-dihydrofolate. DHPS is the target of sulfonamides, which are substrate analogues that compete with para-aminobenzoic acid. Bacterial DHPS (gene sul or folP) is a protein of about 275 to 315 amino acid residues that is either chromosomally encoded or found on various antibiotic resistance plasmids. In the lower eukaryote Pneumocystis jirovecii (previously P. carinii) DHPS is the C-terminal domain of a multifunctional folate synthesis enzyme (gene fas).

Endocytosis

Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of plasma membrane, which then buds off inside the cell to form a vesicle containing the ingested material. Endocytosis includes pinocytosis (cell drinking) and phagocytosis (cell eating). It is a form of active transport.

Exocytosis

Exocytosis () is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell (exo- + cytosis) by expelling them through an energy-dependent process. Exocytosis and its counterpart, endocytosis, are used by all cells because most chemical substances important to them are large polar molecules that cannot pass through the hydrophobic portion of the cell membrane by passive means. It is a process of which the contents of a vacuole are released, since the plant cell has large amounts of content in its vacuole, when exocytosis is in process a large amount of molecules are released thus making it a form of bulk transport.

In exocytosis, membrane-bound secretory vesicles are carried to the cell membrane, and their contents (i.e., water-soluble molecules) are secreted into the extracellular environment. This secretion is possible because the vesicle transiently fuses with the plasma membrane. In the context of neurotransmission, neurotransmitters are typically released from synaptic vesicles into the synaptic cleft via exocytosis; however, neurotransmitters can also be released via reverse transport through membrane transport proteins.

Exocytosis is also a mechanism by which cells are able to insert membrane proteins (such as ion channels and cell surface receptors), lipids, and other components into the cell membrane. Vesicles containing these membrane components fully fuse with and become part of the outer cell membrane.

Heterotroph

A heterotroph (; Ancient Greek ἕτερος héteros = "other" plus trophe = "nutrition") is an organism that cannot produce its own food, relying instead on the intake of nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but not producers. Ninety-five percent or more of all types of living organisms are heterotrophic, including all animals and fungi and some bacteria and protists. The term heterotroph arose in microbiology in 1946 as part of a classification of microorganisms based on their type of nutrition. The term is now used in many fields, such as ecology in describing the food chain.

Heterotrophs may be subdivided according to their energy source. If the heterotroph uses chemical energy, it is a chemoheterotroph (e.g., humans and mushrooms). If it uses light for energy, then it is a photoheterotroph (e.g., green non-sulfur bacteria).

Heterotrophs represent one of the two mechanisms of nutrition (trophic levels), the other being autotrophs (auto = self, troph = nutrition). Autotrophs use energy from sunlight (photoautotrophs) or inorganic compounds (lithoautotrophs) to convert inorganic carbon dioxide to organic carbon compounds and energy to sustain their life. Comparing the two in layman's terms, heterotrophs (such as animals) eat either autotrophs (such as plants) or other heterotrophs, or both.

Detritivores are heterotrophs which obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). Saprotrophs (also called lysotrophs) are chemoheterotrophs that use extracellular digestion in processing decayed organic matter. It is a term most often associated with fungi. The process is most often facilitated through the active transport of such materials through endocytosis within the internal mycelium and its constituent hyphae.

Ion transporter

In biology, an ion transporter (or ion pump) is a transmembrane protein that moves ions across a plasma membrane against their concentration gradient through active transport. These primary transporters are enzymes that convert energy from various sources—including adenosine triphosphate (ATP), sunlight, and other redox reactions—to potential energy stored in an electrochemical gradient. This potential energy is then used by secondary transporters, including ion carriers and ion channels, to drive vital cellular processes, such as ATP synthesis.

Iontophoresis

Iontophoresis is a process of transdermal drug delivery by use of a voltage gradient on the skin. Molecules are transported across the stratum corneum by electrophoresis and electroosmosis and the electric field can also increase the permeability of the skin. These phenomena, directly and indirectly, constitute active transport of matter due to an applied electric current. The transport is measured in units of chemical flux, commonly µmol/(cm2*hour).

Iontophoresis has experimental, therapeutic and diagnostic applications.

Membrane transport protein

A membrane transport protein (or simply transporter) is a membrane protein involved in the movement of ions, small molecules, or 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 or active transport. The two main types of proteins involved in such transport are broadly categorized as either channels or carriers. The solute carriers and atypical SLCs are secondary active or facilitative transporters in humans.

Mycelium

Mycelium is the vegetative part of a fungus or fungus-like bacterial colony, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates into a homokaryotic mycelium, which cannot reproduce sexually; when two compatible homokaryotic mycelia join and form a dikaryotic mycelium, that mycelium may form fruiting bodies such as mushrooms. A mycelium may be minute, forming a colony that is too small to see, or it may be extensive, as in Armillaria ostoyae:

Is this the largest organism in the world? This 2,400-acre [970-hectare] site in eastern Oregon had a contiguous growth of mycelium before logging roads cut through it. ... Mushroom-forming forest fungi are unique in that their mycelial mats can achieve such massive proportions.

Through the mycelium, a fungus absorbs nutrients from its environment. It does this in a two-stage process. First, the hyphae secrete enzymes onto or into the food source, which break down biological polymers into smaller units such as monomers. These monomers are then absorbed into the mycelium by facilitated diffusion and active transport.

Mycelium is vital in terrestrial and aquatic ecosystems for their role in the decomposition of plant material. They contribute to the organic fraction of soil, and their growth releases carbon dioxide back into the atmosphere (see carbon cycle). Ectomycorrhizal extramatrical mycelium, as well as the mycelium of Arbuscular mycorrhizal fungi increase the efficiency of water and nutrient absorption of most plants and confers resistance to some plant pathogens. Mycelium is an important food source for many soil invertebrates.

"Mycelium", like "fungus", can be considered a mass noun, a word that can be either singular or plural. The term "mycelia", though, like "fungi", is often used as the preferred plural form.

Sclerotia are compact or hard masses of mycelium.

Na-K-Cl cotransporter

The Na-K-Cl cotransporter (NKCC) is a protein that aids in the 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 (SLC12A2 and SLC12A1 respectively). Two isoforms of the NKCC1/Slc12a2 gene result from keeping (isoform 1) or skipping (isoform 2) exon 21 in the final gene product.NKCC1 is widely distributed throughout the human body; it has important functions in organs that secrete fluids. NKCC2 is found specifically in the kidney, where it serves to extract sodium, potassium, and chloride from the urine so that they can be reabsorbed into the blood.

Parietal cell

Parietal cells (also known as oxyntic or delomorphous cells) are the epithelial cells that secrete hydrochloric acid (HCl) and intrinsic factor. These cells are located in the gastric glands found in the lining of the fundus and in the cardia of the stomach. They contain an extensive secretory network (called canaliculi) from which the HCl is secreted by active transport into the stomach. The enzyme hydrogen potassium ATPase (H+/K+ ATPase) is unique to the parietal cells and transports the H+ against a concentration gradient of about 3 million to 1, which is the steepest ion gradient formed in the human body. Parietal cells are primarily regulated via histamine, acetylcholine and gastrin signaling from both central and local modulators (see 'Regulation').

Passive transport

Passive transport is a movement of ions and other atomic or molecular substances across cell membranes without need of energy input. Unlike active transport, it does not require an input of cellular energy because it is instead driven by the tendency of the system to grow in entropy. The rate of passive transport depends on the permeability of the cell membrane, which, in turn, depends on the organization and characteristics of the membrane lipids and proteins. The four main kinds of passive transport are simple diffusion, facilitated diffusion, filtration, and/or osmosis.

Peritubular capillaries

In the renal system, peritubular capillaries are tiny blood vessels, supplied by the efferent arteriole, that travel alongside nephrons allowing reabsorption and secretion between blood and the inner lumen of the nephron. Peritubular capillaries surround the proximal and distal tubules, as well as the loop of Henle, where they are known as vasa recta.Ions and minerals that need to be saved in the body are reabsorbed into the peritubular capillaries through active transport, secondary active transport, or transcytosis.

The ions that need to be excreted as waste are secreted from the capillaries into the nephron to be sent towards the bladder and out of the body.

Basically, they reabsorb useful substances such as glucose and amino acids and secrete certain mineral ions and excess water into the tubule.

The majority of exchange through the peritubular capillaries occurs because of chemical gradients osmosis and hydrostatic pressure. Movement of water into the peritubular capillaries is due to the loss of water from the glomerulus during filtration, which increases the colloid osmotic pressure of the blood. This blood leaves the glomerulus via the efferent arteriole, which supplies the peritubular capillaries. The higher osmolarity of the blood in the peritubular capillaries creates an osmotic pressure which causes the uptake of water. Other ions can be taken up by the peritubular capillaries via solvent drag. Water is also driven into the peritubular capillaries due to the higher fluid pressure of the interstitium, driven by reabsorption of fluid and electrolytes via active transport, and the low fluid pressure of blood entering the peritubular capillaries due to the narrowness of the efferent arteriole.

Physical activity

Physical activity is defined as any bodily movement produced by skeletal muscles that requires energy expenditure. Physical activity encompasses all activities, at any intensity, performed during the 24 hour day. It includes exercise and incidental activity integrated into daily activity. This integrated activity may not be planned, structured, repetitive or purposeful for the improvement of fitness, and may include activities such as walking to the local shop, cleaning, working, active transport etc.

Renal glucose reabsorption

Renal glucose reabsorption is the part of kidney (renal) physiology that deals with the retrieval of filtered glucose, preventing it from disappearing from the body through the urine.

If glucose is not reabsorbed by the kidney, it appears in the urine, in a condition known as glucosuria. This is associated with diabetes mellitus.Firstly, the glucose in the proximal tubule is co-transported with sodium ions into the proximal convoluted tubule walls via the SGLT2 cotransporter. Some (typically smaller) amino acids are also transported in this way.

Once in the tubule wall, the glucose and amino acids diffuse directly into the blood capillaries along a concentration gradient. This blood is flowing, so the gradient is maintained.

Lastly, sodium/potassium ion active transport pumps remove sodium from the tubule wall and the sodium is put back into the blood. This maintains a sodium concentration gradient in the proximal tubule lining, so the first step continues to happen.

Gliflozins such as canagliflozin inhibit renal glucose reabsorption, and are used in diabetes mellitus to lower blood glucose.

Saprotrophic nutrition

Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs and heterotrophs, and is most often associated with fungi (for example Mucor) and soil bacteria. Saprotrophic microscopic fungi are sometimes called saprobes; saprotrophic plants or bacterial flora are called saprophytes (sapro- + -phyte, "rotten material" + "plant"), though it is now believed that all plants previously thought to be saprotrophic are in fact parasites of microscopic fungi or other plants. The process is most often facilitated through the active transport of such materials through endocytosis within the internal mycelium and its constituent hyphae.Various word roots relating to decayed matter (detritus, sapro-), eating and nutrition (-vore, -phage), and plants or life forms (-phyte, -obe) produce various terms, such as detritivore, detritophage, saprotroph, saprophyte, saprophage, and saprobe; their meanings overlap, although technical distinctions (based on physiologic mechanisms) narrow the senses. For example, usage distinctions can be made based on macroscopic swallowing of detritus (as an earthworm does) versus microscopic lysis of detritus (as a mushroom does).

A facultative saprophyte appears on stressed or dying plants and may combine with the live pathogens..

Symporter

A symporter is an integral membrane protein that is involved in the transport of many differing types of molecules across the cell membrane. 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).

Transport protein

A transport protein (variously referred to as a transmembrane pump, transporter, escort protein, acid transport protein, cation transport protein, or anion transport protein) is a protein that serves the function of moving other materials within an organism. Transport proteins are vital to the growth and life of all living things. There are several different kinds of transport proteins.

Carrier proteins are proteins involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane. Carrier proteins are integral membrane proteins; that is, they exist within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion (i.e., passive transport) or active transport. These mechanisms of movement are known as carrier-mediated transport. Each carrier protein is designed to recognize only one substance or one group of very similar substances. Research has correlated defects in specific carrier proteins with specific diseases. A membrane transport protein (or simply transporter) is a membrane protein that acts as such a carrier.

A vesicular transport protein is a transmembrane or membrane associated protein. It regulates or facilitates the movement by vesicles of the contents of the cell.

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