Final answer:
Phospholipid movement from one leaflet of the bilayer to the other is facilitated by enzymes known as flippases, rather than occurring routinely, being aided by cholesterol, or being impossible.
Step-by-step explanation:
The movement of phospholipids from one leaflet of the bilayer to the other, also known as flip-flop, is not a routine process because the hydrophilic head of a phospholipid must pass through the hydrophobic center of the membrane, which is energetically unfavorable. Instead, this movement is facilitated by specialized proteins called flippases. Flippases are a type of enzyme that catalyzes the flipping of phospholipids to the opposite leaflet, maintaining the asymmetry of the plasma membrane which is essential for various cellular functions.
Active transport, on the other hand, is the process that involves the movement of substances across the membrane using energy from ATP. Passive transport refers to the movement of substances across a membrane without the use of cellular energy. Considering the question, we can deduce that cholesterol does not aid in flip-flop movements of phospholipids and it is not impossible for phospholipids to flip sides, though it rarely happens spontaneously.
Therefore, the correct answer is that movement of phospholipids from one leaflet to the other occurs with the help of flippases.