#transport mechanisms

Membrane Transportation

Which of the following is not a characteristic of secondary active transport?

Secondary active transport refers to movement of solute against its concentration gradient energized indirectly by ion electrochemical gradient established by primary active pumps, not by direct ATP binding and hydrolysis at transport protein itself. Hallmarks include dependence on Na+/K+ ATPase or H+-ATPase to maintain Na+ or H+ motive force, inhibition by collapse of gradient with ionophores or ouabain, and involvement of symport or antiport mechanisms where downhill flow of driving ion drags substrate uphill. Protein operates by alternating access rocker-switch mechanism without Walker A ATP-binding motifs. Therefore direct ATP consumption at transporter characterizes primary active class exemplified by P-type ATPases such as Na+/K+ ATPase, Ca2+ ATPase SERCA, and ABC exporters where ATP hydrolysis at NBDs directly powers movement. Assigning direct ATP utilization to secondary systems is inaccurate; they utilize gradient, so feature of primary pumps cannot be defining for secondary transport. Symport and antiport, gradient utilization remain defining features. 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: Alberts et al., 7th ed., Chapter 11, Distinction primary vs secondary active transport.

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Membrane Transportation

Which of the following is a feature of ABC ATPase pumps?

A unifying structural signature distinguishes ABC ATPase pumps from other transporter classes like P-type or SLC carriers. Functionally active unit requires two hydrophobic transmembrane domains, each generally composed of six alpha-helices that create substrate-binding chamber accessible alternately to cytoplasm or extracellular milieu, and two peripherally located nucleotide-binding domains in cytoplasm that dimerize upon ATP ligation. The nucleotide-binding domains contain highly conserved Walker A, Walker B, Q-loop, D-loop, H-loop and ABC signature LSGGQ motif that contacts ATP across dimer interface. In many eukaryotic exporters such as P-glycoprotein ABCB1, CFTR ABCC7 and BSEP ABCB11, all four domains fuse into single polypeptide with two homologous halves connected by linker containing regulatory insertions. Bacterial importers often split domains into separate subunits plus extracellular solute-binding proteins delivering cargo. Intracellular coupling helices in TMDs dock into grooves of NBDs transducing ATP-driven motions to transmembrane reorientation enabling alternating access independent of ion gradient or membrane voltage, fundamental for energetic coupling.

Ref: Locher, Nature 2016, Mechanistic diversity ABC transporters; Alberts, 7th ed., Chapter 11 structure.

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Membrane Transportation

Which of the following is not an example of facilitated diffusion?

Facilitated diffusion denotes protein mediated passive transport operating down electrochemical gradient, characterized by saturation kinetics and competitive inhibition but lacking ATP hydrolysis. Several protein families exemplify it. GLUT uniporters possess twelve transmembrane helices organized into two bundles that alternate between outward open and inward open via rocker switch, transporting glucose along blood tissue gradients. Ion channels such as voltage gated sodium and potassium channels form aqueous pores with selectivity filters containing carbonyl oxygens and charged rings permitting millions of ions per second. Aquaporins are tetramers where each monomer forms single file water pore with NPA motifs and aromatic arginine constriction blocking protons while allowing water at billion per second rates. All operate spontaneously without nucleotide. In contrast Na+/K+ ATPase is primary active transporter of P-type family hydrolyzing ATP at DKTGT aspartate forming phosphoenzyme E1P to E2P, extruding three sodium against gradient and importing two potassium, maintained by ouabain sensitivity. ABC and V-ATPases also hydrolyze ATP. Due to direct ATP dependence and uphill ion movement, Na+/K+ ATPase does not represent facilitated diffusion.

Ref: Cooper, The Cell, 8th ed., Chapter: Facilitated Diffusion vs Active Transport - Na+/K+ ATPase Distinction.

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Membrane Transportation

Which of the following uses primary active transport?

Primary active transport is defined by direct coupling of ATP hydrolysis within same protein to drive solute against gradient uphill requiring work. Sodium potassium ATPase exemplifies true primary pump prototype with ten transmembrane helices and three cytosolic domains forming P-type family characterized by transient phosphorylation of Asp369 during cycle. One ATP hydrolyzed phosphorylates P-domain inducing rearrangement from E1 to phosphorylated E2 state that translocates three sodium outward and two potassium inward per cycle consuming about quarter of ATP at rest generating electrogenic potential. In contrast GLUT1 and GLUT2 encoded by SLC2A are facilitative uniporters operating by alternating access rocker switch without ATP moving glucose down gradient passively saturable Km one to twenty millimolar stereoselective not energy linked expressed in erythrocytes brain liver beta cells for basal uptake. Aquaporins are tetrameric water channels each monomer forming pore allowing passive water diffusion at billion per second following osmotic gradient without energy gating by pH. Distinguishing categories crucial for pharmacology since ouabain digoxin specifically inhibit Na+/K+ ATPase binding extracellular side of E2-P while cytochalasin B inhibits GLUTs. Therefore primary active example among membrane proteins is sodium potassium ATPase requiring ATP directly.

Ref: Pirch et al., Annual Review of Physiology 2020: Primary Active Transport – Na+/K+ ATPase as P-type ATPase.

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