Composition of the Lipid Bilayer

1. Phospholipids

A phospholipid is a special molecule with two parts:

• Phosphate head: Hydrophilic (water-loving)
• Fatty acid tails: Hydrophobic (water-fearing)

In the membrane, these molecules arrange themselves in two layers (bilayer):

• Hydrophilic heads face outward, towards the water (both inside and outside the cell)
• Hydrophobic tails face inward, away from water

Real-Life Example: Think of a sandwich:

• Bread = hydrophilic heads
• Filling = hydrophobic tails

Mnemonic: “Head Loves Water, Tails Run from Water”

2. Cholesterol

• Found only in eukaryotic cells.
• Cholesterol fits between phospholipids.
• It helps maintain fluidity (flexibility) of the membrane, especially during cold conditions.

Example: Like oil in dough makes it flexible, cholesterol keeps the membrane soft and not stiff.

Mnemonic: “C for Cold & Cholesterol” – Cholesterol helps in cold conditions.