Phospholipids (College Board AP Biology)

Phospholipids

• Phospholipids are a type of lipid; therefore they are formed from the monomer glycerol and fatty acids
• Unlike triglycerides, there are only 2 fatty acids bonded to a glycerol molecule in a phospholipid as one has been replaced by a phosphate ion (PO₄^3-)
• As the phosphate is polar it is soluble in water (hydrophilic)
• The fatty acid "tails" are nonpolar and, therefore, insoluble in water (hydrophobic)

Formation of Phospholipids Diagram

Phospholipids are the major components of cell surface membranes. They have fatty acid tails that are hydrophobic and a phosphate head that is hydrophilic; both are attached to a glycerol molecule.

The Bipolar Nature of Phospholipids

• Phospholipids have a dual character, with both hydrophobic and hydrophilic parts
  • They have different parts at different ends, so they can be regarded as bipolar
• As a result of having hydrophobic and hydrophilic parts, phospholipid molecules form monolayers or bilayers in water

In the presence of water due to the hydrophobic and hydrophilic parts, phospholipids will form monolayers or bilayers.

Role of Phospholipids

• The main component (building block) of cell membranes

Due to the presence of hydrophobic fatty acid tails, a hydrophobic core is created when a phospholipid bilayer forms

• This acts as a barrier to water-soluble molecules
• The hydrophilic phosphate heads form H-bonds with water allowing the cell membrane to be used to compartmentalize
• This enables the cells to organize specific roles into organelles helping with efficiency

Composition of phospholipids contributes to the fluidity of the cell membrane

• If there are mainly saturated fatty acid tails, the membrane will be less fluid
• If there are mainly unsaturated fatty acid tails, the membrane will be more fluid

Phospholipids control membrane protein orientation

• Weak hydrophobic interactions between the phospholipids and membrane proteins hold the proteins within the membrane but still allow movement within the layer

Phospholipids vs Triglycerides Table

	Triglyceride	Phospholipid
Number of fatty acid tails	3	2
Presence of phosphate	No	Yes
Polar/Nonpolar	Nonpolar	Polar phosphate head
Number of water molecules	3	3
Function	Energy storage	Cell membrane component