The host says that the polar compounds get "stuck" at the hydrophobic core as the reason why polar compounds cannot cross the membrane. Sometimes people even describe this as the hydrophobic core "repels" polar compounds. This reasoning is WRONG! 1. Polar compounds are forming bonds with the water on the outside of the membrane. These bonds are called hydrogen 2. Can polar compounds interact in this same way with the hydrophobic tails of the phospholipids? (yes or no) no 3. Do interactions form between polar compounds and the hydrophobic tails? (yes or no) no 4. In order for a polar compound to enter into the hydrophobic space of the phospholipid tails, it would need to break the many bonds it has formed with the water and enter into a space where these bonds cannot form. Therefore, it is not a repelling action but a "lack of attraction."

-Bridge hydrogen bond: It is formed between a hydrogen attached to a very electronegative element (such as oxygen in water) and another very electronegative element (such as oxygen, fluorine). Polar molecules that contain electronegative elements in their structure may form this junction with water.

-Dipole-dipole union: it is formed between polar molecules where the zone with positive charge density of one molecule approaches the zone with negative charge density of another.

-Dipole-induced dipole union: it is formed between polar molecules where the zone with positive charge density of a molecule causes a non-polar molecule to partially polarize. It is a weak union but becomes important in long hydrophobic chains. It is the union that is established between polar compounds and hydrophobic tails of phospholipids.

For a polar compound to enter the hydrophobic space of the phospholipid tails, the cell usually uses other transport systems other than passive transport such as transport by specialized proteins. Water, despite being a polar molecule, due to its small size it can pass through the membrane at low speed.

The bonds formed with water do not break in aqueous medium. the "lack of attraction" does not exist between the molecules, there is always attraction although it can be of different intensity.

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