The graph shows the effect of temperature on the membrane fluidity of two artificial lipid bilayers, Membrane X (without cholesterol) and Membrane Y (with cholesterol).
Which of the following statements correctly explains the differences in fluidity between Membrane X and Membrane Y at the specified temperatures?
At 10∘C10^\circ\text{C}10∘C, Membrane Y is more fluid than Membrane X because the rigid steroid rings of cholesterol form ionic bonds with the hydrophilic phosphate heads of phospholipids, pushing them apart.
At 40∘C40^\circ\text{C}40∘C, Membrane Y is less fluid than Membrane X because cholesterol molecules bind directly to the glycerol backbones, causing the hydrophobic fatty acid tails to become completely saturated.
At 10∘C10^\circ\text{C}10∘C, Membrane Y is more fluid than Membrane X because cholesterol prevents the hydrocarbon tails of phospholipids from packing closely together and crystallising.
At 40∘C40^\circ\text{C}40∘C, Membrane Y is less fluid than Membrane X because the polar hydroxyl group of cholesterol extends fully into the aqueous environment, creating a rigid hydration shell around the bilayer.
