Problem 63 - A Complete Resource Book in Chemistry for JEE Main

Question

Phospholipids are esters of glycerol with (a) two carboxylic acid residues and one phosphate group (b) one carboxylic acid residue and two phosphate groups (c) three phosphate groups (d) three carboxylic acid residues

Step-by-Step Solution

Answer

Option (a) is correct: phospholipids are esters of glycerol with two carboxylic acid residues and one phosphate group.

Step 1: Understand Phospholipids

Phospholipids are a type of lipid molecule that are a major component of all cell membranes. They can form bilayers because of their unique structure, which includes glycerol, fatty acids, and a phosphate group.

Step 2: Identify Glycerol Components

The structure of glycerol is involved in forming phospholipids, and it serves as a backbone onto which other components attach. Glycerol has three hydroxyl (OH) groups:

Step 3: Attach Carboxylic Acid Residues

Phospholipids typically have two fatty acid molecules attached to the glycerol. These fatty acids are usually attached through an ester linkage to two of the hydroxyl groups on glycerol.

Step 4: Attach the Phosphate Group

The third hydroxyl group on glycerol is typically linked to a phosphate group. This phosphate group is often further attached to other structures or molecules, but in the simplest form, it is just a phosphate group.

Step 5: Analyze the Options

Given the composition of phospholipids, the correct choice is based on this structure: two fatty acids (esters of carboxylic acids) and a phosphate group. Compare this with the given options.

Key Concepts

Glycerol

Glycerol is a simple yet essential chemical compound often referred to as the backbone of lipids, especially phospholipids. Its structure consists of three carbon atoms, each bonded to a hydroxyl group (OH). This trihydroxy alcohol appears as a clear, colorless, and viscous liquid at room temperature, with a sweet taste and non-toxic nature.
Glycerol's three hydroxyl groups make it a versatile compound, capable of forming ester bonds with fatty acids. These ester bonds are critical because they enable glycerol to act as a scaffold for building more complex lipid molecules.

• The first and second hydroxyl groups typically link to fatty acids, forming what chemists refer to as ester linkages.
• The third hydroxyl group usually attaches to a phosphate group, another vital component of phospholipids. This unique capacity to bond with different chemical groups is what allows glycerol to play such a crucial role in forming cell membrane structures.

Fatty Acids

Fatty acids are long chains of hydrocarbons terminating in a carboxyl group (COOH). In phospholipids, two fatty acids usually bond to a glycerol backbone via ester linkages. These fatty acids can be saturated—meaning they lack double bonds—or unsaturated, containing one or more double bonds in the carbon chain.
The combination of these two types of fatty acids in phospholipids is what gives the molecule its amphipathic properties—having both hydrophilic (water-attracting) and hydrophobic (water-repelling) qualities.

• The hydrophobic tail regions of phospholipids are comprised of these fatty acid chains. They tend to avoid water and align themselves within the interior of the membrane bilayer, away from the aqueous environment.
• Because of this configuration, fatty acids are fundamental in maintaining the structural integrity and fluidity of cell membranes, allowing for essential cellular functions such as signaling and transport.

Understanding the role of fatty acids is key to comprehending how cell membranes function and how they form a selective barrier around cells.

Phosphate Group

The phosphate group is a vital component of phospholipids, contributing to their overall structure and function. It is a negatively charged group, made up of a phosphorus atom bonded to four oxygen atoms. In the context of a phospholipid, the phosphate group is attached to the third hydroxyl group of glycerol.
What makes the phosphate group particularly important is its hydrophilic nature.

• This hydrophilic head is oriented to face the external and internal watery environments of the cell, while the hydrophobic tails of fatty acids turn inward.
• The phosphate group's negative charge can interact with water and other polar molecules, stabilizing the lipid bilayer structure.
• Additionally, the phosphate group can bind to various other molecules, forming phospholipids with different functions and properties.

Thus, the phosphate group not only dictates membrane properties but also assists in cell signaling and energy transfer within cells.