This carrier is expressed at the distal part of the small intestine and operates at neutral pH Said, The proton-coupled folate transporter mediates the transport of folic acid and 5-methyl- and formyl-tetrahydrofolates.
The transporter localizes to the proximal small intestine, operates at an acidic pH Said, ; Zhao et al. They mediate the unidirectional influx of folate, whereby the entire folate-receptor complex is internalized Ball, ; Zhao et al.
A reversible transport has also been suggested Zempleni et al. The basolateral folate transporter has not yet been characterized at the molecular level. However, the presence of a specific carrier protein mediating sodium-independent but pH-dependent folic acid transport has been shown in rats Hamid et al.
In the case of humans, 5-methyl-tetrahydrofolate has been identified in the portal blood Ball, , but the corresponding transport mechanisms and proteins have not been elucidated. Seven ABC transporters expressed in the plasma membrane in different epithelial and non-epithelial cells have shown affinity toward folate and its derivatives Matherly and Goldman, ; Toyoda and Ishikawa, ; Zhao et al. Numerous solute carrier organic anion transporters OAT transport methotrexate a structural analog of folic acid and are also relevant folate transporters in the liver and kidneys Matherly and Goldman, ; Zhao and Goldman, One can refer to Zhao et al.
Vitamin B12 is the precursor for two coenzymes, adenosylcobalamin and methylcobalamin. Adenosylcobalamin is required for methylmalonyl CoA-mutase activity E. Methylcobalamin is required for methionine synthase activity E. Interestingly, this vitamin depends on a carrier-mediated transport when administered in pharmacological doses Ball, Cobalamin is transported into intestinal epithelial cells by cubilin-mediated absorption. The latter serve as an anchor for the receptor and aids cobalamin uptake.
In addition, proteins, such as megalin and receptor-associated protein, can interact with CUBN. The protein responsible for the basolateral efflux of cobalamin has not been experimentally validated. However, multidrug resistance-associated protein 1 ABCC1 , GeneID: has been shown to transport cobalamin in prokaryotes and eukaryotes, including mice Green, In addition, SMVT transports lipoic acid and is called a multi-vitamin transporter. The basolateral release of biotin is mediated by a yet uncharacterized carrier protein in a sodium-independent manner Said, The mechanism of basolateral pantothenate efflux remains to be elucidated Said, The membrane location has been confirmed in rats Boyer et al.
SVCT2 is ubiquitously expressed except in the lungs and skeletal muscle , whereas SVCT1 is confined to the intestines, liver, kidneys, colon, ovaries, and prostrate Liang et al.
Ascorbate export has been assumed via volume-sensitive anion channels Wilson, Alternatively, intracellular vitamin C can be oxidized to dehydroascorbate, which can freely diffuse into the blood stream Gropper et al. Transport systems for water-soluble vitamins have been more intensively investigated than FSVs Reboul and Borel, FSV transport was not well represented in Recon 2, but the transport of water-soluble vitamins was fairly well captured.
However, the genes encoding for proteins transporting fat-soluble vitamins, including those discussed for vitamins A, D, and E i. The transport protein encoded by the ABCA1 gene is so far only associated with cholesterol, but not vitamin E transport Table 3.
OAT1—OAT4-mediated transport can be added, via the module, to completely capture the current knowledge about folate transporters. The vitamin B12 transport proteins i. See Supplemental Table S1 for the vitamin transporters and their properties.
Water moves across biological membranes via different mechanisms. Apart from diffusing through the lipid bilayer, co-transporters in the form of protein channels exist in the membrane, through which water can diffuse. The movement of water molecules through such channels, called aquaporins, is driven by osmosis Macaulay et al.
Water is also a substrate for co-transporters, such as excitatory amino acid transporter 1 EAAT1 SLC1A3 , GeneID: , which is expressed in the brain and moves both urea and water along with glutamate Vandenberg et al. For details on the various water co-transporters, specifically those operating in the brain, one may refer to Macaulay et al. Aquaporins are a family of membrane channel proteins that allow the passage of water molecules, neutral molecules e.
In total, 13 members of this family have been characterized at the molecular level, and they are expressed in a wide variety of tissues abundantly in the epithelial layer of the kidneys, intestine, lungs, and brain Verkman, Interestingly, these proteins have been associated with various cellular functions, including skin hydration Dumas et al. Aquaporins are believed to hold therapeutic potential for congestive heart failure, hypertension, glaucoma, brain swelling, epilepsy, obesity, and cancer Verkman, , ; Tradtrantip et al.
Heme forms the prosthetic group of hemoglobin and other heme-containing proteins, such as myoglobin, cytochromes P, cytochrome C, tryptophan pyrrolase, and catalase Murray et al. In addition, heme degradation serves as a source for the essential micronutrient iron Iannotti et al. These transport proteins directly transfer extracellular heme into the cell. While the proton-coupled folate transporter acts at the apical surface, the feline leukemia virus subgroup C receptor-related protein 1 is believed to have an active transport mechanism Uc et al.
The hemopexin protein directly interacts with feline leukemia virus subgroup C receptor-related protein 1, hence increasing heme efflux, which is perceived to be a cellular protection against heme toxicity Yang et al. Heme transport can also occur via receptor-mediated endocytosis, by prolow-density lipoprotein receptor-related protein 1 LRP1 , GeneID: , which has been proposed to play a role in inflammation Hvidberg et al.
In addition to essential nutrients, there are certain other conditionally essential nutrients CEN , which are usually synthesized by the body in almost sufficient amounts.
However, under conditions of increased need, such as tissue injury or neonatal conditions, these nutrients may need to be derived from the diet.
CEN includes compounds, such as arginine, CoQ10, carnitine, propionyl carnitine, taurine, lipoic acid, betaine, ribose, cysteine, chondroitin sulfate, and glutamine Kendler, ; Soghier and Brion, In this section, we will focus only on carnitine, taurine and betaine because the transport of arginine, cysteine, glutamine, and ribose has already been discussed in the relevant sections above also see Supplemental Table S1.
Carnitine transports fatty acyl-CoAs i. A positive effect of carnitine supplementation has been demonstrated for neuro-regeneration in rats McKay Hart et al. Carnitine further exerts protective effects in corneal epithelial cells preventing the deleterious effects of dry eye syndrome Xu et al. While carnitine synthesis occurs using methionine and lysine in the liver and kidney Flanagan et al.
One of the end products of methionine and cysteine metabolism is taurine, which plays an important role in a number of tissues. In the brain, taurine acts as a neuromodulator, neurotransmitter, and membrane stabilizer Tamai et al. High taurine concentrations in the heart and muscles support its contractile function and osmo-regulation, and taurine can also exert antioxidant action by neutralizing hypochlorous acid and regulating mitochondrial protein synthesis in these tissues Schaffer et al.
Additional evidence for the importance of this amino acid in human health suggests its positive effect on growth in low birth weight infants, promotion of biliary flow, and prevention of cholestasis Guertin et al.
Disruption of taurine transport causes retinal degeneration in mice Heller-Stilb et al. The stoichiometry is 1 taurine: 2 sodium: 1 chloride, but limited transport activity has also been observed without chloride Tamai et al.
Although the transport directionality remains to be confirmed, the movement of taurine through the blood-brain barrier was shown to occur from the blood into the brain Tamai et al. This high-capacity but low-affinity transporter, which also transports beta-alanine, is highly expressed on the apical membrane of enterocytes Anderson et al. Betaine is another important molecule involved in methionine metabolism. Once synthesized from choline, betaine donates its methyl group to regenerate methionine from homocysteine and helps to conserve the cellular methionine level Craig, In addition, betaine acts as an osmolyte, particularly helpful for normal physiological functions of the kidneys, intestinal epithelium, red blood cells, and skeletal muscle.
Moreover, its protective role has been observed in the heart and liver cells Craig, Details regarding the directionality of BGTmediated transport remain unknown. The remainder of the aquaporins can be accounted for by expanding the GPRs of the corresponding reactions Supplemental Table S2. Recon 2 lacks the heme transporter FLVCR1 because additional biochemical experiments needed to clarify the precise transport mechanism.
All of the above discussed carnitine transport proteins, except for CT2, are present in Recon 2. The function of CT2 is captured in the transport module. The transport reactions catalyzed by BGT-1 [i. The transport module was assembled according to the established reconstruction protocol Thiele and Palsson, a using rBioNet as a reconstruction tool Thorleifsson and Thiele, The functionality of reactions in the module, in conjunction with Recon 2, was subsequently tested.
All of the discussed modifications and additions are provided through a transport module, which comprises of 71 metabolites, 70 reactions, and 41 genes including 19 newly added genes. These additional transport reactions are for amino acids 27 reactions , lipids 16 reactions , nucleosides 6 reactions , vitamins and minerals 8 reactions , hormones 6 reactions , and others 7 reactions. In addition, 24 Recon 2 reactions need to be updated with respect to their gene-protein-reaction associations provided in Supplemental Table S2.
Overall, the transport module summarizes in a computer-readable, structured manner all transport systems, and their corresponding reactions, that we discovered to be missing from Recon 2 Figure 1H. This module is thus an extension to Recon 2, which can be added to the existing reconstruction if desired. Transporters fulfill a broad range of functions, which go far beyond the sole movement of metabolites.
In our discussion on the transport of distinct metabolite classes, many of these functions have been mentioned. Targeting specific transport proteins to combat disease conditions, such as cholestasis Wagner and Trauner, , neurodegenerative disorders Hinoi et al. Herein, we will focus on the discussion of transport proteins in disease groups concerning IEMs and cancer.
Metabolic disorders are associated with disrupted cellular metabolism. Although rare mostly observed in the Finish and Japanese populations, with incidence of , live births , this IEM has a clinical picture of recurrent diarrhea, vomiting, and in the long-term affects the immune system, skeletal system, and pulmonary and renal function, which can even lead to the death if left untreated.
Specific dietary recommendations include protein restriction and citrulline and lysine supplementation. Recon 1 captured 22 of the 45 plasma membrane transport protein-associated IEMs genes. The remaining 20 transport protein-associated IEMs could not be mapped onto Recon 2 due to missing genes see Table 4 for details. The non-inclusion of the ABC transport proteins into human GENREs is because they have been shown to transport mainly medically important drugs and their derivatives e.
Cancer cells reprogram metabolic pathways to support their increased need for energy and biosynthetic precursors Cairns et al. The metabolic characteristics of cancer cells are the high uptake of glucose, aerobic glycolysis, secretion of lactate Warburg effect , and a high rate of glutaminolysis to compensate for the efflux of TCA cycle intermediates into biosynthetic pathways Feron, Alternations in metabolite uptake e.
As discussed above, redundancy and overlapping substrate specificity exist within and between transporter families. Cancer cells have to operate sets of transporters that best nourish their metabolic dependencies.
In fact, the distinctive transporter expression between cancerous and normal cells could provide good opportunities for targeted treatment Ganapathy et al. The contribution of transporters of the metabolite classes in cancer discussed above has been reviewed elsewhere Fuchs and Bode, ; Verkman et al.
Table 5. Metabolite transporters relevant to cancer and their current coverage in Recon 2. Coverage and accurate representation of transport systems are essential to perform valuable simulations using COBRA. Recon 1 has been used for the generation and analysis of cancer-specific metabolic models Folger et al.
Of the 20 extracellular transporters Table 5 that play a role in cancer metabolic reprogramming and proliferation, 13 transporters are correctly represented in Recon 2 Table 5 , three need to be modified, and four are still missing or require further curation.
This section discusses the cancer relevant transporters currently missing or requiring revision Table 5. The pyruvate to lactate conversion is necessary to sustain a high glycolytic flux Feron, The accumulation of lactate and a decreasing pyruvate level put cell survival at risk due to increasing acidification of the cytoplasm.
Cancer cells counteract the decrease in intracellular pH by specific ion transport i. For example, SLC5A8 is silenced by methylation in human astrocytomas and oligodendrogliomas Hong et al. In addition to its transporter function, the SLC5A8 protein has a demonstrated role in tumor suppression through the active import of endogenous inhibitors of histone acetylases HDACs [i.
Recently, SLC5A8 was shown to counteract tumor progression independent from its transport function. Instead, SLC5A8 acts through an unknown mechanism involving a decrease in the anti-apoptotic protein survivin Coothankandaswamy et al.
Hence, these data were added in the transport module Supplemental Table S2. SLC5A8 was not included in Recon 2, most likely because this protein has been mainly discussed in the context of cancer.
ABC transporters mediate the efflux of cytotoxic drugs, causing multidrug resistance MDR and chemotherapy failure Fletcher et al. Both are known to be overexpressed in different cancer types Fletcher et al. A subpopulation of cancer cells with enriched stem cell activity, so called side populations SPs , have been extracted from six human lung cancer cell lines H, H23, HTB, A, H, and H All six showed resistance to exposure to different chemotherapeutic drugs.
The survival of such cells with stem cell activity upon drug treatment could be connected to a relapse in vivo Ho et al. Strong expression of aquaporins has been observed in various tumors, especially aggressive tumors Verkman et al.
Some aquaporins are exclusively expressed in malignant tissue Verkman et al. The aquaglyceroporin aquaporin-3, AQP3 AQP3 , GeneID: , which also transports glycerin in addition to water, is expressed in normal epidermis and overexpressed in basal cell carcinoma and human skin squamous cell carcinomas Hara-Chikuma and Verkman, AQP3-facilitated glycerol transport was found to determine cellular ATP levels and therefore be important for hyperproliferation and tumor cell proliferation in epidermal mice cells Hara-Chikuma and Verkman, Correspondingly, the resistance of AQP3 null-mice toward skin tumors might arise through reduced tumor cell glycerol metabolism and ATP generation Hara-Chikuma and Verkman, This property renders AQP3 inhibition a possible target for the prevention and treatment of skin, and possibly other cancers associated with aquaglyceroporin overexpression Hara-Chikuma and Verkman, AQP3 is currently missing in Recon 2 and covered in the transport module.
Although many of the transporters associated with cancer are present in Recon 2 Table 5 , important mediators of intra- and extracellular pH, drug resistance, and proliferative energy metabolism are still missing. A great deal of work in the field of constraint-based modeling has focused on the generation of highly curated GENREs and their usage for the generation of tissue-specific metabolic models for biomedical applications. Transporters not only maintain the connectivity of metabolites across different cell types but also determine the uptake and secretion profile of individual cells.
The metabolite exchanges of individual cell types with the corresponding extracellular compartment are inevitably connected to their internal biochemical pathways and cell functions. The inclusion of the cell type-specific transporters is important for enabling the use of the human metabolic reconstruction as a template for the generation of more accurate and physiologically relevant cell type-specific sub-networks and ultimately function-representative models.
Moreover, information regarding distinct transporter function at different locations, as is the case for polarized cells, is crucial for such an effort and has thus been noted throughout this review. We identified numerous gaps through our literature review, many of which could be filled and are provided in the accompanying transport module.
However, some knowledge gaps still remain because the responsible transporter or transport mechanism is unknown. Such a knowledge update for the GENRE needs to be performed periodically because of the important implications on their predictive potential Thiele and Palsson, b and thus biomedical applications.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The authors thank the anonymous reviewers for their comments and suggestions. The authors are thankful to Prof. Franzson, Mrs. Heinken, and Mrs. Haraldsdottir for valuable discussions. This work was supported by the Icelandic Research Fund No. Adibi, S.
Regulation of expression of the intestinal oligopeptide transporter Pept-1 in health and disease. Liver Physiol. Agren, R. Reconstruction of genome-scale active metabolic networks for 69 human cell types and 16 cancer types using INIT. PLoS Comput. Agu, R. Proton-coupled oligopeptide transporter POT family expression in human nasal epithelium and their drug transport potential. Alberts, B. Molecular Biology of the Cell. Amaral, M. Molecular targeting of CFTR as a therapeutic approach to cystic fibrosis.
Trends Pharmacol. Amin, K. Effect of carnitine and herbal mixture extract on obesity induced by high fat diet in rats. Amiry-Moghaddam, M. Anderson, C. Augustin, R. The protein family of glucose transport facilitators: It's not only about glucose after all. Badagnani, I. Interaction of methotrexate with organic-anion transporting polypeptide 1A2 and its genetic variants. Baird, F. Tertiary active transport of amino acids reconstituted by coexpression of System A and L transporters in Xenopus oocytes.
Bakirtzi, K. Cerebellar neurons possess a vesicular compartment structurally and functionally similar to Glut4-storage vesicles from peripheral insulin-sensitive tissues.
Baldwin, S. The equilibrative nucleoside transporter family, SLC Pflugers Arch. Ball, G. Vitamins: Their Role in the Human Body. Hoboken, NJ: Blackwell publishing. Barnes, K. Distribution and functional characterization of equilibrative nucleoside transporter-4, a novel cardiac adenosine transporter activated at acidic pH.
Bates, C. Bioavailability of riboflavin. Pubmed Abstract Pubmed Full Text. Berry, D. Signaling by vitamin A and retinol-binding protein regulates gene expression to inhibit insulin responses. Bianchi, L. A single amino acid change converts the sugar sensor SGLT3 into a sugar transporter.
Biegel, A. Amino Acids 31, — Bordbar, A. A multi-tissue type genome-scale metabolic network for analysis of whole-body systems physiology. BMC Syst. Insight into human alveolar macrophage and M. Boyer, J. Broer, S. Amino acid transport across mammalian intestinal and renal epithelia. The role of amino acid transporters in inherited and acquired diseases. Brunton, L. Burke, T. Allelic and phenotypic heterogeneity in ABCA4 mutations.
Ophthalmic Genet. Cairns, R. Regulation of cancer cell metabolism. Cancer 11, 85— Calvo, M. Potential role of sugar transporters in cancer and their relationship with anticancer therapy. Camacho, J. Hyperornithinemia-Hyperammonemia-Homocitrullinuria Syndrome. Seattle, WA: University of Washington. Chabowski, A. Prostaglandins Leukot. Fatty Acids 77, — Chang, R. Drug off-target effects predicted using structural analysis in the context of a metabolic network model. Charrier, L.
The oligopeptide transporter hPepT1: gateway to the innate immune response. Chen, J. Diabetes Ther. Closs, E. Structure and function of cationic amino acid transporters CATs. Cooper, R. Glucose transporter-1 GLUT-1 : a potential marker of prognosis in rectal carcinoma?
Cancer 89, — Coothankandaswamy, V. The plasma membrane transporter SLC5A8 suppresses tumour progression through depletion of survivin without involving its transport function.
Corpe, C. Craig, S. Betaine in human nutrition. Cura, A. The role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism and homeostasis. Czeizel, A. Reduction of urinary tract and cardiovascular defects by periconceptional multivitamin supplementation.
Prevention of the first occurrence of neural-tube defects by periconceptional vitamin supplementation. Devaux, P. How lipid flippases can modulate membrane structure. Acta , — Diez-Sampedro, A. A glucose sensor hiding in a family of transporters. Diop-Bove, N. Hereditary Folate Malabsorption. Doblado, M. Facilitative glucose transporter 9, a unique hexose and urate transporter.
Duarte, N. Global reconstruction of the human metabolic network based on genomic and bibliomic data. Dumas, M. Hydrating skin by stimulating biosynthesis of aquaporins. Drugs Dermatol. Dusso, A. Vitamin D. Renal Physiol. Dutta-Roy, A. Life Sci.
Enomoto, A. Molecular identification of a novel carnitine transporter specific to human testis. Insights into the mechanism of carnitine recognition.
Falasca, M. Investigational ABC transporter inhibitors. Expert Opin. Drugs 21, — Febbraio, M. CD implications in cardiovascular disease. Cell Biol. Feron, O. Pyruvate into lactate and back: from the Warburg effect to symbiotic energy fuel exchange in cancer cells.
Flanagan, J. Role of carnitine in disease. Fletcher, J. ABC transporters in cancer: more than just drug efflux pumps. Cancer 10, — Folger, O. Predicting selective drug targets in cancer through metabolic networks. Forrest, L. The structural basis of secondary active transport mechanisms. Frezza, C.
Haem oxygenase is synthetically lethal with the tumour suppressor fumarate hydratase. Nature , — Fuchs, B. Cancer Biol. Fukasawa, Y. Fukuda, Y. ABC transporters and their role in nucleoside and nucleotide drug resistance. Furuhashi, M. Fatty acid-binding proteins: role in metabolic diseases and potential as drug targets. Drug Discov. These processes involve the substance passing though the luminal barrier and the basolateral membrane, two plasma membranes of the kidney epithelial cells, and into the peri-tubular capillaries on the other side.
Some substances can also pass through tiny spaces in between the renal epithelial cells, called tight junctions. As filtrate passes through the nephron, its osmolarity ion concentration changes as ions and water are reabsorbed. Finally, in the distal convoluted tubule and collecting duct, a variable amount of ions and water are reabsorbed depending on hormonal stimulus.
The final osmolarity of urine is therefore dependent on whether or not the final collecting tubules and ducts are permeable to water or not, which is regulated by homeostasis.
Reabsorption throughout the nephron : A diagram of the nephron that shows the mechanisms of reabsorption. Learning Objectives Describe the process of tubular reabsorption in kidney physiology. Key Points Proper function of the kidney requires that it receives and adequately filters blood.
In the vasa recta particularly, this rate of flow is important for two additional reasons. The flow must be slow to allow blood cells to lose and regain water without either crenating or bursting. Approximately 80 percent of filtered water has been recovered by the time the dilute forming urine enters the DCT. The DCT will recover another 10—15 percent before the forming urine enters the collecting ducts.
Peritubular capillaries receive the solutes and water, returning them to the circulation. Finally, calcitriol 1,25 dihydroxyvitamin D, the active form of vitamin D is very important for calcium recovery. These binding proteins are also important for the movement of calcium inside the cell and aid in exocytosis of calcium across the basolateral membrane.
Solutes move across the membranes of the collecting ducts, which contain two distinct cell types, principal cells and intercalated cells. A principal cell possesses channels for the recovery or loss of sodium and potassium. An intercalated cell secretes or absorbs acid or bicarbonate. As in other portions of the nephron, there is an array of micromachines pumps and channels on display in the membranes of these cells.
Regulation of urine volume and osmolarity are major functions of the collecting ducts. If the blood becomes hyperosmotic, the collecting ducts recover more water to dilute the blood; if the blood becomes hyposmotic, the collecting ducts recover less of the water, leading to concentration of the blood.
Another way of saying this is: If plasma osmolarity rises, more water is recovered and urine volume decreases; if plasma osmolarity decreases, less water is recovered and urine volume increases. This function is regulated by the posterior pituitary hormone ADH vasopressin. With mild dehydration, plasma osmolarity rises slightly.
This increase is detected by osmoreceptors in the hypothalamus, which stimulates the release of ADH from the posterior pituitary. If plasma osmolarity decreases slightly, the opposite occurs. When stimulated by ADH, aquaporin channels are inserted into the apical membrane of principal cells, which line the collecting ducts. As the ducts descend through the medulla, the osmolarity surrounding them increases due to the countercurrent mechanisms described above. If aquaporin water channels are present, water will be osmotically pulled from the collecting duct into the surrounding interstitial space and into the peritubular capillaries.
Therefore, the final urine will be more concentrated. If less ADH is secreted, fewer aquaporin channels are inserted and less water is recovered, resulting in dilute urine. By altering the number of aquaporin channels, the volume of water recovered or lost is altered.
This, in turn, regulates the blood osmolarity, blood pressure, and osmolarity of the urine. Aldosterone is secreted by the adrenal cortex in response to angiotensin II stimulation. As an extremely potent vasoconstrictor, angiotensin II functions immediately to increase blood pressure.
By also stimulating aldosterone production, it provides a longer-lasting mechanism to support blood pressure by maintaining vascular volume water recovery. In addition to receptors for ADH, principal cells have receptors for the steroid hormone aldosterone. Intercalated cells play significant roles in regulating blood pH. This function lowers the acidity of the plasma while increasing the acidity of the urine.
The kidney regulates water recovery and blood pressure by producing the enzyme renin. It is renin that starts a series of reactions, leading to the production of the vasoconstrictor angiotensin II and the salt-retaining steroid aldosterone.
Water recovery is also powerfully and directly influenced by the hormone ADH. Even so, it only influences the last 10 percent of water available for recovery after filtration at the glomerulus, because 90 percent of water is recovered before reaching the collecting ducts.
Mechanisms of solute recovery include active transport, simple diffusion, and facilitated diffusion. Most filtered substances are reabsorbed. Urea, NH 3 , creatinine, and some drugs are filtered or secreted as wastes. Movement of water from the glomerulus is primarily due to pressure, whereas that of peritubular capillaries and vasa recta is due to osmolarity and concentration gradients.
The PCT is the most metabolically active part of the nephron and uses a wide array of protein micromachines to maintain homeostasis—symporters, antiporters, and ATPase active transporters—in conjunction with diffusion, both simple and facilitated. Almost percent of glucose, amino acids, and vitamins are recovered in the PCT.
Bicarbonate HCO 3 — is recovered using the same enzyme, carbonic anhydrase CA , found in erythrocytes. The recovery of solutes creates an osmotic gradient to promote the recovery of water.
The collecting ducts actively pump urea into the medulla, further contributing to the high osmotic environment. The vasa recta recover the solute and water in the medulla, returning them to the circulation.
Nearly 90 percent of water is recovered before the forming urine reaches the DCT, which will recover another 10 percent. In the collecting ducts, ADH stimulates aquaporin channel insertion to increase water recovery and thereby regulate osmolarity of the blood. Answer the question s below to see how well you understand the topics covered in the previous section. Skip to main content. Module 9: The Urinary System. Search for:. Tubular Reabsorption Learning Objectives By the end of this section, you will be able to: List specific transport mechanisms occurring in different parts of the nephron, including active transport, osmosis, facilitated diffusion, and passive electrochemical gradients List the different membrane proteins of the nephron, including channels, transporters, and ATPase pumps Compare and contrast passive and active tubular reabsorption Explain why the differential permeability or impermeability of specific sections of the nephron tubules is necessary for urine formation Describe how and where water, organic compounds, and ions are reabsorbed in the nephron Explain the role of the loop of Henle, the vasa recta, and the countercurrent multiplication mechanisms in the concentration of urine List the locations in the nephron where tubular secretion occurs.
Figure 1.
0コメント