IB Biology Practice Exam 2026 - Free Practice Questions and Study Guide

Gorter and Grendel's proposal about plasma membranes is centered on the structure consisting of a bilayer of phospholipid molecules. Their experiments involved extracting lipids from red blood cells and found that the area covered by the lipids was twice that of the cell's surface area. This observation led them to suggest that the lipids formed a bilayer, with hydrophilic (water-attracting) heads facing outward towards the aqueous environment and hydrophobic (water-repelling) tails facing inward, away from the water. This bilayer arrangement is fundamental to the structure of all biological membranes, providing both a barrier and a medium through which certain substances can move.

Understanding this model is crucial because it establishes the basis for the fluid mosaic model of the cell membrane. This model incorporates not only phospholipids but also various proteins, carbohydrates, and cholesterol, highlighting the dynamic and complex nature of membranes. The suggestion that membranes consist solely of carbohydrate layers or only proteins does not account for the essential role of lipids in membrane structure and function. Similarly, the focus on cholesterol and integral proteins only fails to recognize the significance of the phospholipid bilayer as a primary structural component.