List the following compounds in order of increasing lipid bilayer permeability: RNA, Ca^2+, gluc

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List the following compounds in order of increasing lipid...

Key Concepts

Lipid Bilayer Structure

The lipid bilayer is composed of two layers of phospholipids with hydrophilic heads and hydrophobic tails, creating a selective barrier that separates the interior of a cell from its external environment. This structure determines which molecules can passively diffuse through, based on their compatibility with the hydrophobic core of the membrane.

Passive Diffusion

Passive diffusion is the process by which molecules move across a cell membrane down their concentration gradient without the need for energy input. The efficiency of passive diffusion depends on factors such as molecular size, polarity, and lipid solubility, with nonpolar, smaller molecules typically diffusing more readily through the hydrophobic core of the membrane.

Molecular Size

The size of a molecule is a key factor in determining its capacity to cross the lipid bilayer. Smaller molecules can navigate through the tight packing of lipid tails more easily than larger ones, which often results in increased permeability for small molecules compared to bulky macromolecules.

Molecular Polarity and Hydrophobicity

Molecular polarity influences a compound’s affinity for either an aqueous environment or the hydrophobic interior of a lipid bilayer. Nonpolar, lipophilic molecules tend to have higher permeability as they can dissolve in the hydrophobic core, while polar compounds have reduced permeability due to unfavorable interactions with the nonpolar lipid tails.

Charge Effects

Charged molecules, including ions, experience significant electrostatic repulsion when interacting with the hydrophobic region of the lipid bilayer. This results in very low passive permeability for ions, often necessitating specific transport proteins or channels to facilitate their movement across membranes.

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