Final answer:
The movement of phospholipids across different layers of the plasma membrane requires specialized enzymes called flippases and does not occur routinely.
Step-by-step explanation:
The movement of phospholipids from one leaflet of the plasma membrane to the other is not a process that occurs routinely. Instead, this process, known as 'flip-flop', is an energetically unfavorable event because the polar head of the phospholipid must traverse the hydrophobic core of the membrane bilayer. However, the movement can be facilitated by specific enzymes called flippases. These enzymes use energy to transport phospholipids from the outer to the inner leaflet and vice versa. Therefore, the correct answer to the question is (c) requires flippases.
The plasma membranes are composed of phospholipid bilayers, which have hydrophilic heads facing the aqueous environments on either side and hydrophobic tails facing inward, away from water. This arrangement forms a barrier that affects the movement of substances into and out of the cell. While cholesterol is present in some plasma membranes and affects fluidity and other properties, it is not directly involved in the flip-flop movement of phospholipids. Lastly, the movement of phospholipids is possible and not impossible, though it requires the assistance of flippases, hence, (d) is incorrect.