Phospholipids, a major component of the lipid bilayer, are classified as amphipathic molecules because:
(A) They contain hydrophilic fatty acid residues and hydrophobic phosphates
B) They contain polar heads and non-polar tails
(C) They are able to interact with water
D) All of the above
The correct answer and explanation is:
The correct answer is (B) They contain polar heads and non-polar tails.
Phospholipids are classified as amphipathic molecules because they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions within the same molecule. This dual characteristic makes them essential components of the lipid bilayer in cell membranes.
Phospholipids consist of a glycerol backbone, two fatty acid tails, and a phosphate group attached to a polar head. The fatty acid tails are hydrophobic, meaning they do not interact well with water. These non-polar tails are usually made up of long chains of hydrocarbons, which form the interior of the lipid bilayer.
On the other hand, the phosphate group attached to the glycerol backbone is hydrophilic. This polar head interacts favorably with water, allowing the phospholipids to form stable structures in aqueous environments.
In a membrane, the hydrophilic heads face outward, towards the aqueous environments inside and outside the cell, while the hydrophobic tails face inward, away from the water. This arrangement creates a bilayer structure, with the hydrophobic regions protected from water in the interior, and the hydrophilic regions interacting with the water on the outside of the membrane. This bilayer structure is fundamental to the integrity and function of cell membranes, allowing for selective permeability and creating a barrier that separates the cell’s interior from the external environment.
Therefore, the key feature of phospholipids that makes them amphipathic is their combination of polar (hydrophilic) and non-polar (hydrophobic) regions, which is accurately described in option (B).