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#passive transport

5 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.

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Membrane Transportation

Which of the following processes does not require ATP?

Membrane transport energetics divide conceptually into active requiring cellular energy input and passive driven solely by existing electrochemical gradients toward equilibrium. Primary active transport directly couples exergonic ATP hydrolysis to ion movement against gradient: Na+/K+ ATPase phosphorylates aspartate forming E1P E2P cycle extruding three sodium importing two potassium, V-type proton pumps rotate c-ring acidifying lysosomes using ATP, ABC transporters dimerize nucleotide binding domains consuming ATP to expel drugs and lipids. Secondary active cotransporters such as SGLT1 sodium glucose symporter and NCX sodium calcium exchanger use previously established sodium gradient to accumulate glucose or extrude calcium without directly hydrolyzing ATP. Vesicular routes endocytosis and exocytosis require ATP for actin polymerization, dynamin GTPase activity, clathrin uncoating by Hsc70 and NSF mediated SNARE disassembly. In contrast facilitated diffusion includes GLUT uniporters operating via alternating access rocker switch and channel proteins like voltage gated ion channels and aquaporins providing continuous aqueous pathway. Both show saturation kinetics Michaelis-Menten like but operate down gradient, no phosphorylation intermediate or nucleotide consumption, powered solely by concentration difference. Therefore facilitated diffusion distinguishes itself by independence from ATP hydrolysis and uphill energy coupling.

Ref: Lodish et al., Molecular Cell Biology, Chapter 11: Classification of Transport - ATP Requirement.

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Membrane Transportation

Which of the following is not a property of facilitated diffusion?

Facilitated diffusion encompasses carrier and channel mediated passive movement strictly down electrochemical gradient sharing discriminating features versus simple diffusion and active transport. Transport is obligately mediated by integral proteins with defined binding sites or selectivity filters conferring high specificity for instance GLUT1 preferring D-glucose over L-glucose tenfold and aquaporin excluding ions while passing water at billion per second via NPA motif. It is saturable because limited carriers or channels create Vmax plateau when all sites occupied enabling competitive inhibition by cytochalasin B or phloretin and Michaelis-Menten kinetics. However it requires no direct ATP hydrolysis or coupling to sodium or proton gradients; driving force is solute's own chemical potential difference distinguishing it from primary pumps and secondary symporters that accumulate against gradient. Once gradient abolished net flux ceases although exchange continues. Regulation occurs via gating phosphorylation altering open probability or vesicular trafficking such as insulin induced GLUT4 translocation and vasopressin induced aquaporin insertion but energy controls availability not transport step itself. Thus energy dependence is not property of facilitated diffusion rather it conserves energy while enhancing selectivity speed and regulation for polar nutrients unable to cross lipid rapidly and needing control.

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

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Membrane Transportation

Which membrane transport process is energy-independent?

Transport classification distinguishes energy-dependent versus independent processes based on need for free energy beyond ambient thermal energy and existing gradients. Simplest criterion examines whether net flux can occur spontaneously down electrochemical potential or requires coupling to external energy store. Simple diffusion of hydrophobic molecules like steroid hormones and non-polar gases occurs directly across lipid without protein and without ATP, driven by entropy. Facilitated diffusion remains energy-independent but overcomes barrier for polar solutes using integral membrane proteins to provide aqueous pore or carrier binding site; flux occurs strictly down gradient without nucleotide hydrolysis, though proteins confer selectivity for glucose versus fructose and saturability following Michaelis-Menten kinetics. Examples include GLUT1 glucose uniport, aquaporin-1 water channel, and CFTR chloride channel. These systems can be regulated by gating, phosphorylation or vesicular trafficking such as vasopressin induced AQP2 insertion, but regulation controls availability not thermodynamic drive. In contrast primary active transport like Na+/K+ ATPase, V-ATPase acidifying lysosomes, F-ATP synthase operating reverse, and ABC exporters hydrolyze ATP directly, while secondary active uses ion gradients. Facilitated diffusion therefore occupies middle ground as protein-assisted passive route enabling efficient nutrient uptake without energy cost.

Ref: Lodish et al., Molecular Cell Biology, 8th ed., Chapter 11: Energy-Independent Transport – Facilitated Diffusion.

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