#transport proteins

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 flux because sites saturated. This yields specificity and competitive inhibition where glucose competes with analogs raising apparent Km. Simple diffusion never saturates. Vmax estimation proxies expression level, while Km reflects affinity. Regulatory hormone insulin recruitment of GLUT4 raises Vmax by increasing surface number without changing Km, enhancing uptake capacity after meals independent of affinity modulation, allowing muscle to clear glucose rapidly. Distinguishing Km and Vmax changes aids interpretation of mutations affecting binding versus trafficking defects in diabetes.

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

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Membrane Transportation

Which of the following does not use specific transport proteins?

Permeability of lipid bilayer correlates closely with solute oil-water partition coefficient, molecular size, and hydrogen bonding potential in solubility-diffusion framework. Gases such as oxygen, molecular nitrogen, carbon dioxide and nitric oxide possess small molecular weight under 50 Daltons, non-polar character lacking hydrogen bond donors or acceptors and high solubility in hydrocarbon, allowing direct dissolution into hydrophobic acyl chain core and rapid transverse diffusion without assistance, permeability coefficient on order of ten to hundred centimeters per second. This explains instantaneous alveolar gas exchange, tissue oxygenation for oxidative phosphorylation, CO2 elimination and NO signaling paracrine diffusion to smooth muscle activating guanylate cyclase. Larger polar molecules like glucose with five hydroxyl groups, amino acids, nucleosides and phosphorylated metabolites face high energetic barrier for dehydration requiring transient breaking of many hydrogen bonds, drastically low permeability needing carriers. Even water despite small size diffuses slowly through pure lipid, so aquaporins accelerate flux fiftyfold for kidney water reabsorption. Charged ions face even larger Born energy barrier essentially impermeant. Therefore oxygen movement relies strictly on simple diffusion governed by Fick's law and partial pressure gradients, demonstrating that membrane protein assistance is selective for hydrophilic solutes and not required for lipophilic gases supporting respiration and signaling.

Ref: Lodish et al., Molecular Cell Biology, 8th ed., Chapter 11: Lipid Bilayer Permeability – Nonpolar Gases.

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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 galactose, or water over ions via NPA motifs and size exclusion. Saturability occurs because finite number of transporters exists in membrane, so flux follows Michaelis-Menten-like kinetics approaching plateau Vmax when all binding sites continuously occupied and cycling at maximal turnover, unlike simple diffusion which increases linearly with gradient indefinitely. Transport remains thermodynamically passive, does not use ATP directly, cannot move solute against its electrochemical gradient, though gradient itself may be maintained by active pumps. Regulation can occur via transporter trafficking, as insulin-stimulated GLUT4 recruitment to plasma membrane increases Vmax, or allosteric gating. This distinguishes facilitated diffusion from primary and secondary active transport and simple lipid diffusion, providing controlled uptake of polar nutrients while conserving energy and allowing rapid response to metabolic demands during feeding and fasting cycles.

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

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Membrane Transportation

Which of the following statements about channel proteins is incorrect?

Channel and carrier proteins represent two mechanistically distinct classes of facilitated transport differing in kinetics and conformational dynamics. Channels form transmembrane aqueous pores where ions flow rapidly without strong binding, reaching ten to hundred million per second, gated by voltage, ligand or mechanical stimuli switching between open and closed states. Their selectivity arises from narrow filters where carbonyl oxygens replace hydration, as in potassium channel TVGYG motif, but they avoid large alternating-access changes; binding is weak and transient. Carrier proteins use alternating-access mechanism, alternately exposing binding sites to either side through major helix movements, binding solute with micromolar to millimolar affinity, providing saturable Michaelis-Menten kinetics, competitive inhibition and slower turnover of hundred to ten thousand per second. Saying channels bind and ferry solutes via large conformational change confuses them with carriers. Channels are generally less selective, though highly selective ion channels exist, and their high throughput supports rapid electrical signaling in neurons and epithelial transport requiring fast flux.

Ref: Lodish et al., Molecular Cell Biology, 8th ed., Chapter 11: Carrier vs Channel Mechanisms.

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