The plasma membrane is said to be a “bilayer” because it consists of two layers
ID: 269771 • Letter: T
Question
The plasma membrane is said to be a “bilayer” because it consists of two layers of primarily lipids. Can you explain why the phospholipid molecules have orientated themselves as shown is figure 17.6 Figure 17.6 Schematic Diagram of a Cell Membrane Extracellular Fluid Globular protein Glycoprotein Carbohydrate Protein channe (transport protein) Hydrophilic heads Phospholipid bilayer Cholesterol Integral protein globular protein) Phospholipid molecule Glycolipid Surface protein Peripherial protein Hydrophobic tails Alpha-Helix protein (integral protein) CytoplasmExplanation / Answer
Plasma membrane defines the boundary of a cell and forms a semipermeable barrier between its internal environment (cytoplasm) and the extracellular space. It consists of two sheets of phospholipid molecules, thus called the BI-layer. The hydrophobic tail of the phospholipid molecules in one sheet faces the hydrophobic tails of the phospholipid molecules in another sheet forming a thermodynamically stable hydrophobic core. The polar (hydrophilic) head of the phospholipid molecules in each layer (sheet/ leaflet) faces opposite to the hydrophobic core. Formation of hydrophobic core minimizes the exposure of the hydrophobic tails to the aqueous surrounding, thus increases the thermodynamic stability of the membrane by eliminating the chances of repulsion between hydrophobic tails and hydrophilic aqueous surroundings. The polar heads in inner sheet of phospholipids face the aqueous cytoplasm whereas those of outer sheet faces the extracellular environment. Being polar in nature, the polar heads at the surface of lipid bilyer favorably interact with the polar, aqueous surrounding – and aid maintaining the lipid bilayer as a stable entity as a whole.