How does the structure of a phospholipid differ from a triglyceride? Describe the arrangement of phospholipids in the plasma membrane. (page 33 )

To understand how phospholipids differ from triglycerides, we first have to discuss their molecular components. A triglyceride is composed of a glycerol molecule (with three carbon atoms) and three fatty acid chains. The fatty acid chains can be either saturated or unsaturated, and they are attached to the glycerol molecule by ester bonds. On the other hand, a phospholipid also starts with a glycerol backbone and has two fatty acid chains. The key difference is that instead of the third fatty acid chain, a phospholipid has a phosphate group attached to the glycerol molecule. This phosphate group is often connected to another small polar or charged molecule, forming the polar head of the phospholipid.

Triglycerides are neutral molecules, meaning they are hydrophobic and will not dissolve in water. In contrast, phospholipids have both hydrophilic (water-loving) and hydrophobic (water-hating) properties due to the polar head (phosphate group) and the nonpolar hydrocarbon tails (fatty acid chains). This characteristic makes phospholipids amphipathic.

In the plasma membrane, phospholipids are arranged in a bilayer structure. The polar heads (phosphate groups) face the aqueous environments on both the inside and outside of the cell, while the hydrophobic fatty acid tails face each other in the middle of the bilayer. This specific arrangement allows the plasma membrane to selectively control the entrance and exit of substances and ensure structural stability for the cell. In conclusion, the structure of a phospholipid differs from a triglyceride mainly because it has a phosphate group instead of the third fatty acid chain. Additionally, phospholipids have hydrophilic and hydrophobic properties, which allows them to form the plasma membrane bilayer, providing a barrier for cellular contents and regulating the passage of substances into and out of the cell.