The ability of protein to move within the membrane is measured as its:

Answer

To solve the question regarding the ability of proteins to move within the membrane, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of the Cell Membrane**:
- The cell membrane, also known as the plasma membrane, is primarily composed of lipids and proteins. The major lipids are phospholipids, which are arranged in a bilayer.

2. **Phospholipid Bilayer Arrangement**:
- In the phospholipid bilayer, the hydrophilic (polar) heads of the phospholipids face outward towards the aqueous environment, while the hydrophobic (non-polar) tails face inward, away from water. This arrangement creates a semi-permeable membrane.

3. **Fluid Mosaic Model**:
- The structure of the cell membrane is explained by the Fluid Mosaic Model proposed by Singer and Nicholson. According to this model, the membrane is not static; it is fluid, allowing lipids and proteins to move laterally within the layer.

4. **Movement of Proteins**:
- The ability of proteins to move within the membrane is a key feature of the fluid mosaic model. This movement is essential for various cellular functions, including signaling, transport, and cell recognition.

5. **Measurement of Protein Movement**:
- The ability of proteins to move within the membrane is referred to as "fluidity." Fluidity indicates how freely proteins and lipids can move laterally within the membrane.

6. **Conclusion**:
- Therefore, the answer to the question is that the ability of protein to move within the membrane is measured as its **fluidity**.

### Final Answer:
The ability of protein to move within the membrane is measured as its **fluidity**.

---