Final answer:
The true statements are B) The degree of membrane fluidity varies with membrane composition and D) Lipids and proteins move freely within the lipid bilayer. The surface of the phospholipid bilayer is hydrophilic, and membrane composition and fluidity are not uniform across all cell membranes.
Step-by-step explanation:
The true statements about cell membranes are B) The degree of membrane fluidity varies depending on the composition of the membrane, and D) Individual lipids and unanchored proteins are relatively free to move around laterally within the lipid bilayer. The surface of the phospholipid bilayer is not hydrophobic; it's hydrophilic since the hydrophilic heads (phosphates) are attracted to the water of the ICF and ECF. Also, it is not true that all membranes have exactly the same composition or fluidity; this varies by many factors, including the presence of cholesterol and types of fatty acids.
In more detail, phospholipids in the cell membrane are arranged in a bilayer, with hydrophilic heads facing outward towards the watery environments of the ICF and ECF, and hydrophobic tails pointing inward. This structure is crucial as it forms a barrier that is selectively permeable. Fluidity of the membrane is essential to function, and this is afforded by the lipids and proteins which are not fixed rigidly in place, allowing them to move laterally within the layer. The composition of the membrane, including the presence of cholesterol, fatty acids, and proteins, can affect this fluidity. Membranes are hence described as following the fluid mosaic model.