Skip to content
Login Register

#active transport

14 public questions tagged with this topic.

Basics of Cell Biology

Which of the following types of transport requires ATP?

Membrane transport classification depends on thermodynamics and mechanism. Simple diffusion of lipophilic gases O2, CO2, steroids occurs directly through hydrocarbon core partitioning driven solely by concentration gradient without protein. Facilitated diffusion via channels like aquaporins and carriers like GLUT1 enhances polar solute permeation down gradient, increasing rate and specificity, still entropy-driven, saturable, no ATP hydrolysis. Osmosis is water movement across semipermeable membrane toward solute, also passive. Active transport moves substrates against electrochemical potential requiring energy coupling. Primary active transport directly hydrolyzes ATP by P-type ATPases Na+/K+ ATPase with phosphorylation intermediate pumping 3 Na+ outward and 2 K+ inward maintaining resting potential -70 mV, SERCA pumping Ca2+ into sarcoplasmic reticulum, V-type V-ATPase acidifying lysosomes. Secondary active transport uses sodium gradient energy to drive glucose uptake SGLT1 symport or Ca2+/Na+ exchange. ABC transporters also primary. Consequently active transport uniquely requires ATP or preexisting ion gradient powered originally by ATP, distinguishing it from passive modalities essential for nerve impulse and volume regulation.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 11: Active Transport Requires ATP.

View Details
Plasma Membrane

Which type of membrane transport is facilitated by transmembrane proteins?

Passive entry of polar solutes across 4 nm hydrophobic core is energetically prohibited, necessitating membrane proteins that provide facilitated diffusion pathway without ATP input. Transmembrane proteins create two mechanisms: channel proteins like aquaporin-1, potassium channels with selectivity filter TVGYG, and porins with beta-barrels that form continuous aqueous pores allowing diffusion at rates approaching 10^8 ions per second down electrochemical gradient, gating regulated by voltage, ligand, or mechanical force. Carrier proteins like GLUT1 glucose transporter and AE1 anion exchanger bind solute specifically, undergo conformational inversion from outward-open to inward-open, increasing permeability and specificity while still moving down gradient. Simple diffusion of O2, CO2, and steroid hormones occurs directly through lipid matrix independent of proteins due to high partition coefficient. Passive osmosis follows water activity gradient but accelerated by aquaporins. Lipid bilayer flipping of polar lipids requires flippases and is not spontaneous transport. Hence transmembrane proteins convert impermeable barrier into selective gateway enabling facilitated diffusion essential for nutrient uptake and excitability.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: Facilitated Diffusion and Transporters.

View Details
Membrane Transportation

Which of the following ions is not actively transported in most cells?

Cellular ion homeostasis requires continuous energy expenditure to maintain steep gradients. Sodium gradient high outside 145 millimolar low inside 12 millimolar and potassium opposite 4 outside 140 inside established by Na+/K+ ATPase hydrolyzing ATP forming phosphoenzyme E1P E2P cycling three sodium out two potassium in per ATP. Calcium gradient extreme low cytosolic 100 nanomolar versus 1 to 2 millimolar extracellular maintained by plasma membrane Ca2+ ATPase PMCA and sarcoplasmic endoplasmic reticulum Ca2+ ATPase SERCA, both P-type pumps transporting two calcium per ATP against ten thousand fold gradient and thapsigargin sensitive. These gradients underlie excitability and signaling. Chloride in contrast distribution of about 110 millimolar extracellular versus 4 to 20 millimolar intracellular in many cells largely approximates passive Donnan equilibrium governed by membrane potential and net impermeable intracellular anions. Transport achieved via KCC potassium chloride cotransporter and NKCC sodium potassium two chloride secondary active, and ClC chloride channels providing conductance, but no mammalian primary ATP dependent pump directly hydrolyzes ATP to pump chloride uphill as main mechanism. Hence chloride considered not primary actively transported in most animal cells.

Ref: Alberts et al., Molecular Biology of the Cell, Chapter 11: Ion Distributions - Cl- Passive vs Active Transport.

View Details
Membrane Transportation

The Na+/glucose transporter (SGLT1) is an example of:

Sodium-dependent glucose transport exemplifies secondary active symport coupling uphill absorption to downhill sodium entry. SGLT1 gene SLC5A1 located apically in intestinal villus enterocytes and kidney proximal tubule contains fourteen transmembrane helices in LeuT fold forming cavity with two sodium sites Na1 and Na2 plus glucose pocket recognizing hydroxyls. Cycle has ordered binding: two sodium ions bind first increasing affinity for D-glucose by opening outer gate then protein transitions from outward open to occluded to inward open releasing cargo inside. Both substrates move same direction into cell hence symporter opposite of antiport exchanger. Energy for concentrative accumulation up to thousandfold in intestine derives from inward sodium electrochemical gradient established by basolateral Na+/K+ ATPase maintaining low intracellular sodium near twelve millimolar. Stoichiometric coupling allows luminal glucose at low concentration to be concentrated inside important during starvation and renal reabsorption preventing glycosuria and calorie loss. Inhibitors like phlorizin from apple bark block SGLT non-selectively used as scaffold for antidiabetic gliflozins empagliflozin dapagliflozin selectively inhibiting SGLT2 isoform lowering plasma glucose via promoting urinary glucose excretion.

Ref: Wright et al., Journal of Biological Chemistry 2004: SGLT1 as Sodium Glucose Symport.

View Details
Membrane Transportation

Which process requires ATP?

Energy coupling distinguishes passive and active transport. Simple diffusion of gases, facilitated diffusion via carriers like GLUT and channels like aquaporin, and osmosis driven by water potential are passive, relying on thermal motion and existing gradients without ATP input, negative Gibbs free energy toward equilibrium. Primary active transport directly uses chemical energy from ATP hydrolysis within same protein to move solute against gradient. P-type ATPases like Na+/K+ ATPase exchanging 3 Na+ out for 2 K+ in, V-type H+ ATPase acidifying lysosomes, F-type ATP synthase reversible, and ABC superfamily including CFTR, MDR1, TAP hydrolyze two ATP to pump ions, drugs or peptides uphill creating gradients for secondary use. Because it maintains Na+, K+, Ca2+ and H+ gradients primary active consumes up to forty percent of ATP in neurons and kidney. Secondary active transport uses pre-existing ion gradients, for example Na+/glucose symport. Tertiary active further couples gradients. Inhibitors ouabain and bafilomycin collapse gradients impairing nutrient uptake, pH regulation and volume control rapidly demonstrating energetic dependence.

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 11: Primary Active Transport – ATP Requirement.

View Details
Membrane Transportation

The Na+/K+ ATPase pump is an example of:

Coupled transporters classified as antiporters exchange substrates in opposite directions driven by gradients. Sodium potassium ATPase exemplifies primary active antiporter using ATP rather than secondary gradient. Alpha subunit has ten transmembrane helices forming pocket with three sodium sites and two potassium sites. It binds three intracellular Na+ in high affinity E1 and exports them while binding two extracellular K+ and importing them, so Na+ and K+ move opposite in same cycle. Exchange is electrogenic due to 3:2 stoichiometry, contributing about minus four millivolts to potential and producing outward current measurable in voltage clamp. Antiport contrasts with symport where solutes travel same direction as Na+-glucose symport and uniport where single solute moves passively. Mechanistically antiporters use alternating-access rocker-switch where binding of one substrate lowers affinity for other enforcing exchange in many secondary transporters, while ATPase cycle allows slippage under nonphysiological conditions. E1 to E2 conformational change alternates binding site exposure, coupling ATP hydrolysis to opposing movements essential for excitability and volume control.

Ref: Skou & Esmann, Journal of Bioenergetics 1992: Na+,K+-ATPase as Antiporter Mechanism.

View Details
Membrane Transportation

The Na+/K+ ATPase pump transports:

Sodium-potassium pump is archetypal P-type ATPase maintaining electrochemical gradients vital for volume control and excitability. Each cycle hydrolyzes one ATP to ADP and phosphate, driving conformational shift from E1 high Na+ affinity inward to E2 low affinity outward via phosphorylated Asp369 intermediate. In E1 facing cytosol, cavity binds three sodium ions, triggering autophosphorylation. Transition to E2-P exposes cavity extracellularly where Na+ affinity drops, sodium released and potassium sites exposed. Two extracellular potassium ions bind with high affinity in E2-P, inducing dephosphorylation returning to E1 where potassium released inside due to low affinity. Net result is outward movement of three Na+ and inward two K+ per ATP, generating net outward positive current and inside-negative potential around minus seventy millivolts, preventing swelling and providing high internal K+ favoring ribosome function. This gradient fuels secondary transporters like Na+/Ca2+ and Na+/H+ exchangers and action potential recovery. Cardiac glycosides ouabain and digoxin inhibit by binding E2-P state, raising intracellular Na+.

Ref: Skou & Esmann, Journal of Bioenergetics 1992: Na+,K+-ATPase Stoichiometry – 3 Na+ out, 2 K+ in.

View Details
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.

View Details
Membrane Transportation

Which of the following statements about membrane transport is true?

Plasma membrane selectively controls solute entry through distinct physico-chemical mechanisms dictated by molecular properties. Small non-polar gases such as O2, CO2, N2 and NO partition readily into hydrophobic core and cross by simple diffusion governed by Fick's law, without assistance, explaining lung alveolar exchange and mitochondrial oxygen supply. However many essential nutrients are hydrophilic or charged and face high energetic barrier. Facilitated diffusion provides protein-assisted route down concentration gradient without direct energy coupling, accelerating equilibration thousandsfold. Carriers such as GLUT family for glucose and channels such as aquaporins for water or ion channels for K+ increase effective permeability by lowering activation energy through transient binding or aqueous pore formation. Both display saturability, specificity, competitive inhibition and regulation by gating or translocation, distinguishing them from simple diffusion which is linear, non-saturable and non-selective beyond partition coefficient. Active transport moves solutes against gradients requiring energy, either primary via ATP hydrolysis as with Na+/K+ ATPase and V-ATPase or secondary using coupled ion gradients such as Na+/glucose symport. Osmosis represents water diffusion across semipermeable membranes, also energy independent. Therefore hallmark of facilitated diffusion is requirement for integral membrane protein while remaining passive and driven by gradient.

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 11: Membrane Transport – Facilitated Diffusion Principles.

View Details
Endomembrane System - ER Golgi Lysosome Vacuole

Consider the following statements about transport across the plasma membrane: I. A neutral solute may move from higher t

Neutral solutes may diffuse down their concentration gradient without energy. Polar molecules require carrier proteins to cross the non-polar bilayer. Movement against a gradient is active transport and requires energy supplied through ATP.

Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Endomembrane System - ER Golgi Lysosome Vacuole

View Details