Final answer:
Cell membrane fluidity is increased by higher temperatures and the incorporation of phospholipids with unsaturated fatty acids, which have double bonds that cause kinks and increase membrane fluidity.
Step-by-step explanation:
Two factors that can increase cell membrane fluidity are higher temperatures and the presence of more unsaturated fatty acids within the phospholipid bilayer. The increase in temperature makes the phospholipid and protein components of the membrane more fluid, similar to how heating a solution increases the movement of dissolved particles. Conversely, phospholipids with unsaturated fatty acids contain double bonds that cause bends or kinks in their tails, thereby increasing the space between the fatty acid tails and allowing the components to move more freely. This enhances the fluidity of the cell membrane, which is necessary for the proper function and dynamic growth of the cell.