IB Biology Practice Exam 2025 – Complete Study Resource

The statement about plasma membranes that is true is that they have a bilayer of phospholipids with embedded proteins. This structure is fundamental to their function and is often referred to as the fluid mosaic model. In this model, the phospholipid bilayer provides a semi-permeable barrier that separates the cell's interior from the external environment. The hydrophilic (water-attracting) heads of the phospholipids face outward towards the aqueous environments inside and outside the cell, while the hydrophobic (water-repelling) tails orient inward, away from water.

Embedded within this bilayer are various types of proteins that serve multiple functions including transport, acting as enzymes, or serving as receptors for signaling molecules. This arrangement allows for dynamic interactions and movements within the membrane, which are critical for cellular communication and transport processes.

In contrast, the other statements are inaccurate: plasma membranes are not composed only of lipids as proteins and carbohydrates are also integral components. They are not static; rather, they exhibit fluidity, allowing molecules to move laterally within the membrane. Additionally, cholesterol is an important component present in many plasma membranes, contributing to membrane fluidity and stability.