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

Short answer

Option (a) is correct: two carboxylic acid residues and one phosphate group.

Step-by-step solution

Understanding Phospholipid Structure

Phospholipids are a specific type of lipid molecules that are fundamental components of cell membranes. Each phospholipid molecule is composed of a glycerol backbone, to which two fatty acid chains (carboxylic acid residues) and one phosphate group are attached. This structure creates a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails.

Review Each Option

Review the given options by comparing them to the phospholipid structure framework: (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. We should choose the option that matches the actual phospholipid structure.

Choosing the Correct Option

Analyze the given options: - Option (a) matches the described structure of phospholipids with two fatty acid residues and one phosphate group. - Option (b) does not match as it mentions one carboxylic acid residue and two phosphate groups. - Option (c) is incorrect as it states three phosphate groups. - Option (d) is inaccurate as it indicates three carboxylic acid residues.

Conclusion

From the review, option (a) "two carboxylic acid residues and one phosphate group" perfectly aligns with the phospholipid structure, confirming it's the correct answer.

Key concepts

Lipids

Lipids are a diverse group of molecules that are essential for numerous biological functions. They are largely non-polar, making them hydrophobic or water-repelling. This characteristic is crucial for their role in forming membranes that separate the interior of cells from the external environment.
Lipids are not just limited to phospholipids; they also include fats, oils, waxes, and certain vitamins. Among their key functions, lipids store energy, serve as structural components of cell membranes, and act as important signaling molecules in the body.

• Energy Storage: Lipids are an efficient way of storing energy, offering more than twice the energy content per gram compared to carbohydrates and proteins.
• Membrane Structure: Phospholipids, a subtype of lipids, are critical in forming the bilayer structure of cell membranes.
• Signaling: Some lipids can function as hormones, such as steroid hormones, which regulate various physiological processes.

These functions highlight the importance of lipids in both maintaining the integrity of cells and facilitating communication within and between cells.

Glycerol

Glycerol is a simple polyol compound that serves as a backbone in lipid molecules. Its molecular structure consists of three carbon atoms, each bonded to a hydroxyl (OH) group. This structure makes glycerol hydrophilic, meaning it attracts water.
In the context of phospholipids, glycerol plays a crucial role by connecting the hydrophobic fatty acid chains to the hydrophilic phosphate group, allowing the phospholipid molecule to establish the unique dual nature essential for biological membranes.

• Structure: Glycerol's three hydroxyl groups enable the formation of ester bonds with fatty acids and phosphate groups, forming a stable phospholipid.
• Function: By acting as a bridge, glycerol helps maintain the integrity and fluidity of cell membranes.

The presence of glycerol in phospholipids is vital in maintaining the delicate balance between hydrophilic and hydrophobic properties in cell membranes, facilitating selective permeability and signaling across the membrane.

Fatty Acid Chains

Fatty acid chains are long hydrocarbon tails that are attached to the glycerol backbone in phospholipids. These chains are composed of straight chains of carbon atoms, which vary in length and degree of saturation.
Fatty acids are hydrophobic, meaning they repel water, and as part of phospholipids, they form the hydrophobic tail region in the membrane bilayer.

• Saturation: The presence of double bonds in fatty acid chains determines their saturation. Saturated fatty acids have no double bonds, while unsaturated fatty acids have one or more.
• Membrane Fluidity: The saturation level of fatty acids affects the fluidity of the phospholipid bilayer. Unsaturated fatty acids contribute to more flexible and permeable membranes.

These characteristics of fatty acid chains are essential for understanding how cell membranes function under various physiological conditions, impacting everything from cell mobility to the passage of substances across the membrane.

Phosphate Group

The phosphate group is a critical component of phospholipids, constituting the polar "head" of these molecules. It is negatively charged, which facilitates interactions with positively charged molecules and makes it hydrophilic.
In the phospholipid structure, the phosphate group is what allows the molecule to associate with water, orienting the hydrophilic head towards the aqueous environment inside and outside the cell.

• Charge: The negative charge of the phosphate group is crucial for the formation and function of cell membranes, aiding in the self-assembly of the lipid bilayer.
• Anchoring Potential: This group can also bind to various other molecules, providing anchoring points for additional structures or signaling molecules.

The involvement of the phosphate group in phospholipids underpins many of their functional roles in cellular compartmentalization, signaling, and interactions, highlighting the complexity and versatility of cellular processes.