Final answer:
The true statements about membrane fluidity are that it varies with membrane composition, phospholipids move laterally within the membrane, and unanchored proteins can also move laterally. Membranes do not all have the same composition and fluidity.
Step-by-step explanation:
The correct statements concerning membrane fluidity are A) The degree of membrane fluidity varies depending on the composition of the membrane, and B) Phospholipids are relatively free to move around laterally within the membrane. As for D), unanchored proteins are indeed relatively free to move laterally within the membrane. However, statement C) is incorrect because not all membranes have the same composition and degree of fluidity.
Membrane fluidity is influenced by the chemical composition of the membrane, such as the saturation level of fatty acids in phospholipids and the presence of cholesterol. More unsaturated fatty acids with their kinked tails increase fluidity, while cholesterol tends to decrease it by filling up spaces between fatty acids, thus reducing the mobility of membrane components. The fluid mosaic model describes how proteins and lipids can move within the membrane, contributing to its fluid nature. Environmental factors like temperature also play a significant role in membrane fluidity.