21. In the context of Hemoglobin (Hb) allostery, the 'Perutz Mechanism' describes the transition from the T-state to the R-state.Which molecular event triggers the movement of the F-helix?(A) The formation of salt bridges between carbamylated N-termini.(B) The release of 2,3-BPG from the central cavity.(C)The dissociation of the alpha-beta dimers into monomers.(D) The transition of the heme iron from high-spin to low-spin state upon oxygen binding.(E) The expulsion ofwater molecules from the distal heme pocket.

9. Phospholipids are the primary structural components of the cell membrane bilayer. Which of the following descriptions regarding the function of phospholipids in the membrane bilayer is the most accurate? (A) Phospholipids, through their amphipathic nature, spontaneously form a barrier that prevents the free movement of water-soluble substances in and out of the cell. (B) The primary function of phospholipids is to act as active transport pumps, directly consuming ATP to move ions across the membrane. (C) Phospholipids remain in a fixed, static state within the membrane to provide mechanical rigidity and replace the need for a cytoskeleton. (D) The main function of phospholipids is to act as enzymes that catalyze the translation and modification of membrane (E) proteins. Phospholipids use their hydrophilic tails to create channels in the membrane core to facilitate the simple diffusion of large polar molecules.

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