# How does the tail-to-tail arrangement affect the process of simple diffusion across the plasma membrane?

Oct 22, 2016

It provides a nonpolar region through which ions and polar molecules cannot pass.

#### Explanation:

The plasma membrane

The plasma membrane consists of a phospholipid bilayer, in which the nonpolar tails of the lipids are arranged head-to-head.

Thus, the membrane forms a stable barrier between two aqueous areas — the inside and the outside of the cell.

The bilayer has a fluid consistency, similar to that of light oil.

Simple diffusion

The plasma membrane is selectively permeable.

Ions and charged amino acids cannot move through the membrane because they cannot get through the nonpolar region formed by the lipid tails.

Large molecules like glucose cannot move through the membrane because they are too large (and also too polar).

Nonpolar molecules such as ${\text{O}}_{2}$ and ${\text{CO}}_{2}$ can easily diffuse through the membrane.

Even small polar molecules like ethanol and water can pass through the membrane.

If there is a difference in concentration on either side of the cell wall, the membrane will let the molecules pass through.

For example, ${\text{O}}_{2}$ is more concentrated outside the cell, because the cell uses ${\text{O}}_{2}$ during cellular respiration. The ${\text{O}}_{2}$ automatically moves inside to the area of lower concentration.

Similarly, ${\text{CO}}_{2}$ is more concentrated inside the cell, because the cell produces ${\text{CO}}_{2}$ during cellular respiration. The ${\text{CO}}_{2}$ automatically diffuses to the area of lower concentration outside the cell.