Final answer:
Steroids like cholesterol affect the fluidity of the bilayer by acting as a buffer: reducing fluidity at high temperatures to prevent loss of integrity and preventing solidification at low temperatures to maintain fluidity. Cholesterol's unique structure modulates membrane properties and is essential for cellular homeostasis.
Step-by-step explanation:
The presence of steroids, such as cholesterol, in the bilayer impacts its fluidity. Cholesterol molecules integrate within the phospholipid bilayer, and they have a dual role depending on the temperature. At higher temperatures, they restrict the movement of phospholipids, thereby reducing fluidity, which helps maintain membrane integrity. Conversely, at lower temperatures, cholesterol prevents the membrane from solidifying, thus aiding in keeping it fluid. This is due to the steroid's structure — a hydrophobic core with a small hydrophilic component — allowing it to interact with the fatty acid tails of membrane lipids to modulate the physical state of the membrane.
In essence, cholesterol acts as a buffer for the cell membrane, maintaining appropriate levels of fluidity and functionality across a range of temperatures. Moreover, the presence of unsaturated fatty acids, with their kinks caused by double bonds, also increases the fluidity of the membrane by creating more space within the bilayer. Therefore, cholesterol's influence on membrane fluidity plays a crucial role in cellular homeostasis and the physiological function of membranes.