#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 fl

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

View Details

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 i

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

View Details

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.

View Details

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

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

View Details