#glucose transport

Membrane Transportation

Which of the following describes SGLT2?

Sodium glucose cotransporters SGLT1 SLC5A1 and SGLT2 SLC5A2 are secondary active members using sodium gradient but differ in location, affinity and role. SGLT1 expressed predominantly in apical brush border of small intestine enterocytes and early kidney proximal tubule S3 is high-affinity low-capacity with 2 Na+ to 1 glucose stoichiometry handling dietary glucose and galactose absorption and some glucose reabsorption. SGLT2 localized exclusively in S1 and S2 segments of proximal tubule is low-affinity high-capacity with 1 Na+ to 1 glucose stoichiometry accounting for roughly 90 percent renal glucose reabsorption of filtered load near 180 g per day. Dysfunction produces renal glycosuria. SGLT2 inhibitors gliflozins empagliflozin and dapagliflozin block renal reabsorption promoting glycosuria, used for type 2 diabetes with cardiovascular benefits. Describing SGLT2 as intestinal transporter conflates families; SGLT1 dominates gut while SGLT2 dominates kidney. Neither functions as proton pump or cystic fibrosis related channel. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.

Ref: Wright et al., Physiol Rev 2011, SGLT1 vs SGLT2 distribution and physiology.

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Membrane Transportation

Which type of transport does SGLT1 use?

SGLT1, SLC5A1, is apical sodium-glucose cotransporter highly expressed in brush border of small intestinal enterocytes and S3 segment of kidney proximal tubule. It operates as secondary active transporter, not primary ATPase nor simple facilitator. Stoichiometry of two Na+ to one glucose allows accumulation of glucose against its concentration gradient using electrochemical sodium gradient established by basolateral Na+/K+ ATPase that maintains low intracellular Na+. Transport follows alternating access mechanism: outward-open binds Na+ increasing glucose affinity, sugar binding triggers outward-to-inward switch releasing Na+ and glucose inside. Imported glucose exits basolaterally via facilitative GLUT2. This coupling explains oral rehydration therapy where sodium and glucose coadministration drives water absorption. Inhibitors include phlorizin natural glucoside and selective SGLT2 inhibitors gliflozins used for diabetes. Genetic defects cause glucose-galactose malabsorption with severe diarrhea, highlighting nutritional importance of sodium-coupled concentrative uptake mechanism. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.

Ref: Wright et al., Physiol Rev 2011, Sodium-glucose cotransporters SLC5 family.

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Membrane Transportation

Which GLUT transporter has the lowest affinity for glucose?

GLUT isoforms display divergent Michaelis constants reflecting tissue metabolic roles and sensing. GLUT2 product of SLC2A2 on 3q26 expressed strongly in hepatocyte sinusoidal membrane pancreatic beta cell plasma membrane intestinal basolateral side and kidney tubule exhibits unusually high Km fifteen to twenty millimolar for glucose also transporting fructose galactose low affinity corresponding to lowest affinity among transporters. This high Km enables effective glucose sensor rather than supply line where rate varies roughly linearly across physiological plasma glucose three to fifteen millimolar without early saturation allowing liver to take up proportionally after meals for glycogen synthesis when sugar high and release glucose during fasting via glucose-6-phosphatase. In beta cells low affinity coupled with high Km glucokinase ensures metabolism proportional to blood glucose linking insulin secretion to glycemia via ATP sensitive K+ channel closure. In intestine high capacity efflux of concentrated glucose via basolateral GLUT2 prevents buildup. By contrast GLUT1 and GLUT3 low Km near one and one point five millimolar provide constitutive high affinity uptake ensuring constant brain glucose despite fluctuations and GLUT4 intermediate Km five millimolar regulated by insulin for muscle storage.

Ref: Thorens, Molecular Aspects of Medicine 2015: GLUT2 – Low Affinity Sensor Transporter.

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Membrane Transportation

What happens to glucose transport in GLUT1-deficient cells?

GLUT1 encoded by SLC2A1 on 1p34 is prototype basal transporter responsible for constitutive glucose uptake in erythrocytes where it comprises about five percent of membrane protein, endothelial cells of blood-brain and retinal barriers, astrocytes and fetal tissues requiring continuous supply. It functions as twelve transmembrane uniporter via alternating access low Km 1-2 mM ensuring efficient transport even at low plasma glucose near three millimolar during fasting delivering substrate for glycolysis and pentose phosphate producing NADPH. Genetic haploinsufficiency from heterozygous missense or nonsense mutations causes GLUT1 deficiency syndrome De Vivo disease autosomal dominant characterized by infantile refractory seizures starting early, acquired microcephaly, developmental delay, ataxia and hypoglycorrhachia with cerebrospinal fluid glucose below 3.3 mM while blood normal. In patient cells glucose uptake declines markedly in 2-deoxyglucose assay forcing reliance on ketone bodies and lactate alternative fuels for brain. Compensation by GLUT4 limited because expression restricted to insulin-responsive muscle and adipose not cerebrovascular endothelium and cannot be upregulated. Ketogenic diet producing beta-hydroxybutyrate crossing via MCT1 benefits by bypassing defect restoring energy and reducing seizures.

Ref: Seidner et al., Nature Genetics 1998: GLUT1 Deficiency – Reduced Glucose Uptake.

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Membrane Transportation

Which type of transport is used by the GLUT4 transporter?

Facilitative GLUT transporters use alternating access without ion coupling. GLUT4 encoded by SLC2A4 is insulin-responsive isoform in skeletal, cardiac muscle and adipose, handling major postprandial glucose disposal. In basal fasting state most GLUT4 sequestered in storage vesicles containing IRAP, sortilin, LRP1 and VAMP2 retained by TUG tether and AS160 GAP keeping Rabs GDP-bound. Insulin triggers receptor tyrosine autophosphorylation, IRS recruitment, PI3K activation producing PIP3 recruiting Akt2 which phosphorylates AS160 relieving Rab8A, Rab10, Rab14 inhibition, mobilizing vesicles along actin tracks to plasma membrane increasing surface density twentyfold within minutes. Once inserted, GLUT4 works as uniporter moving D-glucose down gradient without Na+ coupling or ATP, stereospecific, saturable Km near 5 mM close to plasma glucose, inhibited by cytochalasin B. Entry followed by hexokinase II phosphorylation trapping glucose for glycogen synthesis. Insulin resistance in type 2 diabetes impairs translocation despite preserved total protein due to defective Akt signaling and lipid-induced serine phosphorylation. Recruitment also involves myosin motors and actin remodeling that facilitate vesicle movement toward cortex.

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 11: GLUT4 and Facilitated Diffusion.

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Membrane Transportation

The transport of glucose into intestinal epithelial cells via SGLT1 is an example of:

Intestinal glucose absorption across absorptive enterocytes lining duodenum and jejunum illustrates elegant coupling of electrochemical gradients for nutrient uptake against concentration difference. Luminal sodium concentration remains high around 140 millimolar due to continuous activity of Na+/K+ ATPase on basolateral membrane pumping three Na+ out and two K+ in per ATP hydrolyzed, creating inward sodium electrochemical gradient with both chemical and electrical components. Sodium glucose cotransporter 1, SGLT1, SLC5A1 gene, located apically in brush border with fourteen transmembrane segments, exploits this gradient by symporting two Na+ ions together with one D-glucose molecule in same direction across apical membrane. Transport is classified secondary active because glucose is driven uphill against its concentration gradient without direct ATP hydrolysis by transporter itself, but energy originates indirectly from primary active Na+/K+ ATPase maintaining Na+ gradient. After accumulation, glucose exits basolaterally via GLUT2 facilitated diffusion uniporter down its gradient into interstitial fluid and portal blood. Similar mechanism concentrates glucose in kidney proximal tubule S1 segment. Symport stoichiometry of 2:1 allows concentrative capacity over thirtyfold and coupling ratio determines efficiency, demonstrating indirect energy coupling and vectorial transport for efficient absorption and preventing loss of calories.

Ref: Wright et al., Physiology Reviews 2011: Sodium-Glucose Cotransporter SGLT1 – Secondary Active Transport.

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Excretory System

Exit of glucose from PCT cells into blood occurs through:

Correct option is B, GLUT transporter. In Excretory System, the accurate description is GLUT transporter, and that is what you should commit to memory. Keep the definition tight and the role clear — GLUT transporter is the option that correctly names the structure, process, or principle asked for in Excretory System. Then GLUT transporter is the answer you can defend. Check yourself by covering the options and writing the answer first, then matching the letter — that builds confidence. If the wording feels dense, translate it into everyday language first, then map that plain sentence back onto GLUT transporter. Remember that physiology questions reward precise language: name the process, the location, and the outcome, then match that to the option text.

Ref: Animal physiology is Unit 7 of the CSIR NET Life Science Syllabus, covering core body systems. It accounts for roughly 10% of the total marks in Sections B and C.

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