## **Core Concept**
Membrane fluidity refers to the viscosity of the plasma membrane and its ability to change shape and allow certain molecules to pass through while restricting others. It is crucial for various cellular functions, including transport, signaling, and cell division. The fluidity is influenced by the composition of the membrane, particularly the types of lipids and proteins present.

## **Why the Correct Answer is Right**
The correct answer, , increases membrane fluidity by introducing unsaturated fatty acids into the membrane. Unsaturated fatty acids have one or more double bonds in their hydrocarbon tails, which create kinks in the tails and prevent them from packing tightly together. This results in increased space between the phospholipid molecules, making the membrane more fluid.

## **Why Each Wrong Option is Incorrect**
* **Option A:** - This option would actually decrease membrane fluidity. Cholesterol can fill the spaces between phospholipid molecules, making the membrane less fluid and more rigid, especially at lower temperatures.
* **Option B:** - While certain proteins can affect membrane fluidity, the general statement does not specifically address how proteins increase fluidity. Integral and peripheral proteins can influence fluidity, but their effect is more complex and not as straightforward as that of unsaturated fatty acids.
* **Option C:** - This option might seem related but is not directly relevant to the increase in membrane fluidity in the context provided.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **unsaturated fats** increase membrane fluidity, which is important in understanding cellular functions and adaptations, especially in changing environmental conditions. This concept is also relevant in the context of certain diseases and conditions where membrane fluidity is altered.

## **Correct Answer:** . Unsaturated fatty acids