#facilitated diffusion

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

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

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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 permit

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 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 inhi

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

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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

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

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Membrane Transportation

What is the Vmax in facilitated diffusion?

Facilitated carriers show enzyme-like saturation kinetics because transporter number is finite and each must cycle. At low substrate concentration occupancy low, rate rises near linearly as collisions produce binding, first-order regime. As concentration rises fractional occupancy climbs, rate limited by isomerization steps approaching plateau where all carriers cycle at maximal turnover kcat typically hundred to ten thousand per second for GLUTs. Maximal velocity Vmax reflects total functional transporters times turnover measured per minute. Beyond Vmax increasing gradient no longer raises fl

Ref: Widdas, Journal of Physiology 1952: Facilitated Diffusion – Vmax as Saturation of Carriers.

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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 protei

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

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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

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

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Membrane Transportation

In facilitated diffusion, transport proteins help move molecules down their concentration gradient. This type of transpo

Facilitated diffusion accelerates equilibration of membrane-impermeant polar molecules without expending cellular ATP, harnessing existing chemical gradients and thermal motion. Integral membrane proteins such as glucose uniporters GLUT1-4, amino acid transporters LAT1, urea transporter and aquaporins provide hydrophilic pathways lowering activation barrier for crossing hydrophobic core. Two hallmarks define this process: selectivity and saturability. Selectivity emerges from specific binding pockets, steric fit or selectivity filter discriminating D-glucose over L-glucose, glucose over galact

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

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