The presence of protein molecules in the lipid bilayer can significantly affect the surface tension and fluidity of biomembranes. Here's how:1. Surface tension: Protein molecules can alter the surface tension of the lipid bilayer by disrupting the regular arrangement of lipid molecules. The hydrophobic and hydrophilic interactions between the protein and lipid molecules can create local changes in the surface tension, which can affect the overall stability and integrity of the membrane. This can also influence the formation of membrane curvature and the ability of the membrane to undergo shape changes, which are essential for various cellular processes.2. Fluidity: The fluidity of a biomembrane is determined by the ease with which lipid molecules can move within the bilayer. The presence of protein molecules can either increase or decrease the fluidity of the membrane, depending on the size, shape, and properties of the protein. Integral membrane proteins, which span the entire lipid bilayer, can restrict the movement of lipid molecules, leading to a decrease in fluidity. On the other hand, peripheral membrane proteins, which are only associated with one leaflet of the bilayer, can have a lesser impact on fluidity.Moreover, some proteins can act as "fluidity buffers" by preferentially interacting with specific lipid species, such as cholesterol, which can modulate membrane fluidity. Additionally, the presence of membrane proteins can lead to the formation of lipid "microdomains" or "rafts" with distinct fluidity properties, which can play a crucial role in cellular signaling and membrane trafficking.In summary, the presence of protein molecules in the lipid bilayer can significantly affect the surface tension and fluidity of biomembranes. The specific effects depend on the properties of the protein, its interactions with lipid molecules, and its localization within the membrane. These changes in surface tension and fluidity can have important implications for the function and stability of the biomembrane and the cellular processes it mediates.