In fluid mosaic model of plasma membrane :

Question

Accepted Answer

In the fluid mosaic model of the plasma membrane, Option 3 is the correct answer: "Phospholipids form a bilayer that provides fluidity." Here's a detailed explanation supporting this:

1. **Explanation for Option 3:**
   - The plasma membrane is primarily composed of a phospholipid bilayer. Each phospholipid molecule has a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails." In an aqueous environment, these molecules arrange themselves into a bilayer with the hydrophobic tails facing inward, shielded from water, while the hydrophilic heads face outward towards the water.
   - This bilayer structure is fluid, allowing the lipids and proteins within the membrane to move laterally. This fluidity is crucial for the functioning of the membrane, enabling it to be flexible, self-healing, and dynamic in its interactions with the environment and other cells.

2. **Explanation for why other options are incorrect:**
   - **Option 1: "Upper layer is non-polar and hydrophilic."**
     - This statement is incorrect because the "upper layer" of the phospholipid bilayer is not non-polar and hydrophilic. Instead, the outer surface of the bilayer is polar and hydrophilic due to the presence of the phospholipid heads, which interact with the aqueous environment.
   
   - **Option 2: "Polar layer is hydrophobic."**
     - This is incorrect because the polar regions of the phospholipids (the heads) are hydrophilic, not hydrophobic. The hydrophobic regions are the non-polar tails, which face inward, away from water.
   
   - **Option 4: "Proteins form a middle layer."**
     - This statement is incorrect because proteins do not form a distinct middle layer in the plasma membrane. Instead, proteins are interspersed throughout the phospholipid bilayer, either embedded within it or attached to its surfaces. These proteins serve various functions, such as transport, signaling, and structural support, but they do not constitute a separate layer.

In summary, Option 3 accurately describes the fundamental structure of the plasma membrane as a phospholipid bilayer that provides fluidity, which is essential for its function. The other options misrepresent the arrangement and properties of the components within the membrane.