Which of the following statements about plasma lipoproteins is CORRECT?
They have a hydrophobic core of triacylglycerol and cholesterol esters
They have a hydrophobic core of phospholipids and free cholesterol
They have a hydrophilic core of triacylglycerol and cholesterol esters
They have a hydrophobic core of phospholipid and apolipoproteins
The Correct Answer and Explanation is :
The correct statement is: “They have a hydrophobic core of triacylglycerol and cholesterol esters.”
Explanation:
Plasma lipoproteins are complex particles composed of lipids and proteins that are essential for the transport of hydrophobic lipids, such as cholesterol and triglycerides, through the aqueous environment of the bloodstream. These lipoproteins play a crucial role in lipid metabolism and are categorized into several classes based on their density, including chylomicrons, very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL).
- Core of Triacylglycerol and Cholesterol Esters:
- The core of plasma lipoproteins is primarily made up of triacylglycerols (triglycerides) and cholesterol esters, both of which are hydrophobic. The hydrophobic nature of these lipids allows them to form a core that is shielded from the surrounding aqueous environment. These lipids are insoluble in water, and their presence in the core enables the lipoprotein particle to transport them through the bloodstream.
- Cholesterol esters are formed when fatty acids esterify cholesterol, making them less polar than free cholesterol, which is more readily integrated into the hydrophobic core.
- Triacylglycerols serve as a major form of energy storage and are also hydrophobic, contributing to the core of lipoproteins.
- Phospholipid Monolayer and Apolipoproteins:
- Surrounding this hydrophobic core is a phospholipid monolayer, which is amphipathic (having both hydrophilic and hydrophobic regions). The hydrophilic heads of phospholipids face the aqueous environment, while the hydrophobic tails are oriented inward, creating a barrier that stabilizes the structure.
- Apolipoproteins (proteins associated with lipoproteins) are embedded in or associated with the phospholipid monolayer. These proteins help in the recognition, binding, and metabolism of lipoproteins and also serve as ligands for receptors on cells.
Why the other options are incorrect:
- “They have a hydrophobic core of phospholipids and free cholesterol”: This is incorrect because phospholipids are a part of the outer layer (not the core) and free cholesterol is not primarily part of the core. It is more commonly found in the surface layer.
- “They have a hydrophilic core of triacylglycerol and cholesterol esters”: This is incorrect because both triacylglycerols and cholesterol esters are hydrophobic, so they cannot form a hydrophilic core.
- “They have a hydrophobic core of phospholipid and apolipoproteins”: This is incorrect as phospholipids and apolipoproteins form the outer surface, not the hydrophobic core.
Thus, the correct configuration of a lipoprotein includes a hydrophobic core composed of triacylglycerols and cholesterol esters, surrounded by a phospholipid monolayer with embedded apolipoproteins.