Problem 4 - Biochemistry: A Short Course

Question

Arrange the following substances in order of increasing permeability through a lipid bilayer: (a) glucose; (b) glycerol; (c) \(\mathrm{Cl}^{-} ;\) (d) indole; (e) tryptophan.

Step-by-Step Solution

Answer

Order: \( \mathrm{Cl}^{-} < \text{glucose} < \text{tryptophan} < \text{glycerol} < \text{indole} \).

Step 1: Understand Permeability Concepts

Permeability through a lipid bilayer depends on the polarity, size, and charge of the molecule. Nonpolar substances generally pass through more easily, while charged particles find it more difficult. Additionally, smaller molecules tend to pass more readily than larger ones.

Step 2: Evaluate Molecules for Polarity and Charge

Identify the polarity and charge of each substance: glucose and glycerol are polar but uncharged, \( \mathrm{Cl}^{-} \) is a charged ion, indole and tryptophan are nonpolar, with tryptophan being larger due to its amino acid structure.

Step 3: Rank by Size and Charge

Given that charged molecules like \( \mathrm{Cl}^{-} \) have the least permeability, start from least permeable (\( \mathrm{Cl}^{-} \)) to most permeable. Larger or more complex molecules follow (glucose and tryptophan), while smaller, less polar molecules, glycerol and indole, come last.

Step 4: Determine Order

Arrange them from least to most permeable: \( \mathrm{Cl}^{-} \) (charged), glucose (polar and large), tryptophan (nonpolar but large), glycerol (polar but small), indole (nonpolar and small).

Key Concepts

Molecule Polarity

Molecule polarity is an essential factor in determining how substances interact with the lipid bilayer of cell membranes. Polarity arises from a difference in electronegativity between atoms in a molecule, leading to the distribution of electrical charge. This distribution results in polar molecules having partially positive and negative regions. When it comes to permeability through a lipid bilayer:

• Polar molecules, which possess significant charge separation, face more difficulty passing through the nonpolar core of a lipid bilayer.
• The lipid bilayer effectively forms a barrier to these hydrophilic substances, making them less permeable.

In our example, glucose and glycerol are polar molecules, which is why they encounter more resistance moving through the bilayer compared to nonpolar substances.

Molecular Size

Molecular size plays a critical role in the permeability of molecules through lipid bilayers. Generally, smaller molecules can more easily traverse the lipid bilayer than larger ones. This is because the bilayer is composed of tightly packed lipid groups that form a semi-permeable membrane. Crucial points on size and permeability:

• Smaller molecules diffuse more rapidly through the membrane because they fit more easily into the bilayer.
• Larger molecules struggle more due to their bulkiness, making it harder for them to slip between the tightly packed lipids.

For our substances, glucose and tryptophan are larger compared to small molecules like glycerol and indole, impacting their ability to permeate the membrane efficiently.

Ion Permeability

Ion permeability is primarily influenced by the charge of the molecule. Ions, with their full charges, are among the least permeable substances through lipid bilayers. Why do ions have trouble crossing?

• Lipid bilayers are primarily nonpolar, creating a hydrophobic environment that is unfavorable for charged ions.
• Ions, such as chloride \(\mathrm{Cl}^{-}\), need specialized protein channels or transporters to pass through the membrane.

Thus, \(\mathrm{Cl}^{-}\) ions have much lower permeability through the lipid bilayer than neutral molecules, because they are repelled by the hydrophobic interior of the membrane.

Nonpolar Molecules

Nonpolar molecules generally have high permeability through lipid bilayers. This is because they do not have charge separation, making them compatible with the hydrophobic core of the lipid bilayer. Features of nonpolar molecule permeability:

• Nonpolar molecules can dissolve in the lipid core, allowing them to move through the membrane with ease.
• They face less resistance than polar or charged entities, resulting in higher permeability rates.

In the given substances, indole and tryptophan are nonpolar, making them more capable of penetrating the lipid bilayer compared to polar molecules such as glucose and glycerol.